Design Guide
Cisco IOS®
Firewall is a stateful security software component of Cisco IOS
Software. Firewall integration in Cisco IOS routers augments a router's
inherent capabilities: multitopology interfaces, industry-standard
routing protocols, and a broad range of services, as well as an
expanding group of other security features such as virtual private
network (VPN) and intrusion prevention system (IPS) features. Cisco IOS
Firewall interoperates with other Cisco IOS Software technologies,
including Network Address Translation (NAT), quality of service (QoS),
and IP Security (IPsec) and Secure Sockets Layer (SSL) VPN, to become a
vital component of an end-to-end network security infrastructure.
Cisco IOS Firewall includes multiple security features:
•
Cisco IOS Firewall stateful packet inspection provides true firewall
capabilities to protect networks against unauthorized traffic and
control legitimate business-critical data.
•
Authentication proxy controls access to hosts or networks based on user
credentials stored in an authentication, authorization, and accounting
(AAA) server.
•
Multi-VRF firewall offers firewall services on virtual routers with
virtual routing and forwarding (VRF), accommodating overlapping address
space to provide multiple isolated private route spaces with a full
range of security services.
• Transparent firewall adds stateful inspection without time-consuming, disruptive IP addressing modifications.
•
Application inspection controls application activity to provide
granular policy enforcement of application usage, protecting legitimate
application protocols from rogue applications and malicious activity.
Cisco
IOS Firewall is primarily supported in Cisco IOS Software mainline and
technology release trains. The service provider release train
incorporates limited firewall feature set capabilities, and is not as
current as the mainline and technology releases for integration of new
features.
This
document offers technical discussion of most Cisco IOS Firewall
features and provides deployment scenarios in typical network
infrastructures, using features supported up to Cisco IOS Software
Release 12.4(2)T. The scenarios can act as a guide for the deployment of
features that are useful when securing a range of home, small business,
branch office, extranet, and enterprise networks.
CISCO IOS FIREWALL BENEFITS
Cisco
IOS Firewall has been tested for compliance with several industry
certifications, offering third-party validation that Cisco IOS Firewall
provides ample firewall protection to meet business security
requirements. ICSA Laboratories certified Cisco IOS Firewall under the
Modular Firewall Certification Criteria Version 4.1.
Cisco
IOS Firewall offers stateful inspection capability on par with
competing firewall products, and several benefits when compared to
dedicated firewall appliances. A Cisco IOS router with the Firewall
Feature set offers additional functions and benefits integrated with the
firewall's capabilities:
• Integrated Routing Capabilities-Cisco
IOS Firewall provides integrated, inline security services. These
enhance current Cisco IOS Software capabilities: secure IP routing,
multitopology interfaces, industry-standard routing protocols, NAT, and
voice and video services.
• Industry-Leading VPN-Cisco
IOS Software offers secure VPN capabilities to address almost any
secure network requirement. EasyVPN, DMVPN, traditional IPsec
site-to-site, and Web-based SSL VPN support capabilities to securely
connect remote-access users and remote sites over the public Internet,
or to offer added security to existing private-network connections.
• Full-Featured Firewall-Stateful
Packet Inspection provides stateful security and control for both
common and user-defined network services, and configurable protection
from denial of service (DoS) attacks.
• Authentication Proxy-The
Authentication Proxy (Auth Proxy) offers per-user authenticated access
control to network resources. Users' authorization policy may be
provided to the Auth Proxy device by an AAA server.
• Scalability-Available
on a wide variety of Cisco IOS platforms, Cisco IOS Firewall scales to
meet any network's bandwidth and performance requirements.
• Security in the Infrastructure-Enables
sophisticated security and policy enforcement for connections within an
organization (intranet), from central sites to remote offices and
telecommuters in the home, between an organization and its partner
networks, and between the organization and the Internet.
• Integrates with Existing Investment in Cisco-IT managers can enhance security without additional cost and complexity of adding standalone security appliances.
TECHNICAL HIGHLIGHTS
Cisco IOS Firewall consists of several major subsystems:
• Stateful Packet Inspection provides a granular firewall engine
• Authentication Proxy offers a per-host access control mechanism
• Application Inspection features add protocol conformance checking and network use policy control
Enhancements
to these features extend these capabilities to VRF instances to support
multiple virtual routers per device, and to Cisco Integrated
Route-Bridging features to allow greater deployment flexibility, reduce
implementation timelines, and ease requirements to add security to
existing networks.
This portion of the document introduces the individual subsystems and discusses their benefits.
Cisco IOS Software Stateful Packet Inspection
Stateful
Packet Inspection (SPI) is at the heart of Cisco IOS Firewall,
providing a per-application control mechanism across network perimeters,
as well as within networks through the Transparent Firewall capability.
Stateful Packet Inspection was known as Context-Based Access Control
(CBAC) in early versions of Cisco IOS Firewall, but the name was changed
as the feature set was enhanced and augmented far beyond the original
CBAC capability. SPI enhances security for TCP and UDP applications by
scrutinizing several attributes of data connection. The inspection
engine tracks the state and context of network connections to secure
traffic flow. SPI provides support for several complex, advanced
services such as streaming protocols, IP voice, and other complex
services that require detailed scrutiny to support additional data and
media channels.
Protection Against Attack
Cisco
IOS Firewall SPI provides DoS detection and prevention against some
popular attack modes, such as SYN (synchronize/start) flooding, port
scans, and packet injection. When the router detects unusually high
rates of new connections, it issues an alert message, and resets
excessive half-open TCP connections to prevent system resource
depletion. Cisco IOS Firewall tracks connections by destination address
and port pairs to control undesired activity and reduce impact on hosts
on the protected network that are under attack from malicious activity
originating outside the firewall.
Cisco
IOS Firewall SPI protects against packet-injection attacks by checking
several components of TCP and UDP sessions. Source and destination IP
address and port numbers must match, as well as TCP sequence number.
Other attributes are checked as well, such as TCP window size, reducing
the likelihood of buffer overrun attacks.
Alerts and Audit Trails
Cisco
routers generate real-time alerts and audit trails based on inputs from
the SPI engine. Enhanced audit trail features use syslog to track
all network transactions for advanced, session-based reporting-recording
time stamps, source host, destination host, ports used, and the total
number of transmitted bytes.
Real-time
alerts send syslog error messages to central management consoles upon
detecting suspicious activity. Using the firewall engine inspection
rules, alerts and audit trail information can be configured on a
per-application protocol basis. These configurable real-time alerts,
audit trail, and logging events allow administrators to track potential
security breaches and other nonstandard activities in real time.
Authentication Proxy
Network
administrators can create specific security policies for each user with
the Cisco IOS Firewall per-user authentication and authorization.
Previously, user identity and related authorized access was determined
by a user's fixed IP address, or a single security policy had to be
applied to an entire user group or subnet. Now, per-user policy can be
downloaded dynamically to the router from a TACACS+ or RADIUS
authentication server.
Users
log into the network resources or onto the Internet via HTTP, HTTPS,
FTP, and Telnet authentication interfaces, and their specific access
profiles are downloaded to Cisco IOS Firewall routers with
Authentication Proxy upon successful authentication. Authentication and
authorization can be applied for inbound and/or outbound traffic, which
means that auth proxy can support Internet, intranet, and extranet
configurations.
Synergy with NAT and Port-to-Application Mapping (PAM)
The
combination of Cisco IOS Firewall and NAT enable the firewall to
perform stateful inspection, while hiding the internal IP addresses from
the outside world and minimizing public Internet address space
requirements. Flexible port-application mapping (PAM) supports
applications running on nonstandard ports, customizing access control
for specific applications and services to meet the requirements of the
network.
Application Inspection
Cisco
IOS Firewall offers deeper, more detailed inspection of certain
application protocols to prevent abuse and malicious activity that may
be transmitted over service ports generally used for more desirable
traffic, such as HTTP, Simple Mail Transfer Protocol (SMTP)/Extended
SMTP (ESMTP), Post Office Protocol (POP), and Internet Mail Access
Protocol (IMAP). Application Inspection capabilities vary by service,
from checking authentication to ensure login credentials are encrypted,
to performing granular control over types and quantities of content that
may be carried over a controlled service.
CISCO IOS FIREWALL TECHNICAL DISCUSSION AND APPLICATION EXAMPLES
Stateful Packet Inspection
Stateful
Packet Inspection (SPI), the foundation of Cisco IOS Firewall, was
introduced in Cisco IOS Software Release 11.2P, and found its way into
an early release of 12.0. SPI has been enhanced several times to improve
performance, capability, and flexibility. This document focuses on the
most recent implementation of Cisco IOS SPI, but offers some
evolutionary background where appropriate, specifically when relevant to
improvements in function, monitoring, and configuration.
SPI
was introduced as a feature called Context-Based Access Control (CBAC).
Prior to CBAC, Cisco IOS Software's only packet-filtering mechanism was
the access control list (ACL). CBAC greatly enhanced the packet
filtering capability of ACLs by introducing stateful filtering
capability. The early Cisco IOS Firewall capability was occasionally
perceived as a "glorified" ACL. This misconception is partly due to the
fact that ACL monitoring commands were used to monitor CBAC activity, as
well as the fact that inspection used (and still uses) ACLs to filter
traffic, permitting desired traffic, while blocking unwanted,
potentially harmful traffic. However, CBAC substantially augments an
ACL's capability for restricting traffic. CBAC monitors several
attributes in TCP connections, UDP sessions, and Internet Control
Message Protocol (ICMP) dialogue to ensure that the only traffic allowed
through a firewall ACL is the return traffic for dialogue that was
originated on the private side of the firewall.
Cisco
IOS SPI can be explained most simply as being a mechanism to discover
"good" connections that originate on the secure side of the firewall,
and watch for and allow the return traffic that correlates with these
connections. Connections originating on the unsecure side of the
firewall are not allowed to reach the secure network, as controlled by
an ACL facing the unsecure network (Figure 1).
Figure 1. Cisco IOS Firewall Stateful Inspection
Many
changes have been made to CBAC to enhance its capability and increase
performance. Inspection of some protocols has been enhanced to ensure
protocol compliance or offer application-level service filtering. Cisco
IOS Software Release 12.3(4)T's ACL Bypass feature introduced
substantial improvements in performance and significant changes to the
stateful inspection architecture. CBAC had outgrown its original, basic
function and was renamed Cisco IOS Stateful Packet Inspection to more
accurately reflect the feature's capability. CBAC is frequently still
used synonymously with Stateful Packet Inspection, but the CBAC name
does not reflect the full feature set offered by Cisco IOS SPI.
SPI inspects the packet after it passes the inbound ACL of an input interface if ip inspect in is applied, or after the outbound ACL of output interface if ip inspect out is used. Thus, outbound traffic must be permitted by input ACLs facing the source, and outbound ACLs facing the destination.
Figure 2. Access List Action on Traffic
SPI
monitors connections from a secured network to an unsecured network,
and anticipates the traffic returning to the secure host from the
unsecure network. The mechanism for anticipating and allowing the return
traffic changed slightly as Cisco IOS Firewall changed from CBAC to
SPI. Prior to Cisco IOS Software Release 12.3(4)T, CBAC placed dynamic
access control entries (ACEs) in ACLs in the return path for internally
originated connections, as indicated in "show access-list" for a simple
"deny all" ACL:
sdp-ezvpn#show access-lists
Extended IP access list 111
permit tcp host 172.16.105.1 eq telnet host 172.16.105.10 eq
1176 (30 matches)
10 deny ip any any (708 matches)
With
the introduction of Cisco IOS Software Release 12.3(4)T, the ACL Bypass
feature modified SPI's infrastructure so dynamic ACEs are no longer
used. Instead, SPI maintains a session table listing all of the
firewall's active sessions. The contents of the session table can be
viewed with the "show ip inspect sessions" command:
yourname#sh ip insp sessions
Established Sessions
Session 63D9A9E0 (192.168.110.10:1038)=>(172.16.110.1:23) telnet
SIS_OPEN
SIS_OPEN
ACL
Bypass improves firewall performance for two reasons. SPI is able to
maintain a more efficient list to track active sessions, reducing the
time required for session setup and verification. Also, return traffic
is not subjected to ACLs on the return path, so when return traffic
finds a matching entry in the session table, it is shunted past the ACLs
in the packet path, reducing the CPU overhead the packet incurs as it
moves through the router's processing.
Stateful Packet Inspection Memory Consumption
The
router allocates a small amount of memory for every session it must
track. Basic SPI functions require roughly 700 bytes of the router's
memory to record source address and port number, destination address and
port number, and protocol. If the router is configured for multi-VRF
capability, the source and destination VRF for each connection are
maintained in the session information as well. Firewall inspection using
additional, deeper application-layer inspection requires more memory
per session. The router allocates a 500-session block of memory when SPI
is configured. As more sessions are tracked by the firewall, the router
allocates additional 500-session blocks from the router's unused
memory. When the router has no remaining free memory, new session
allocation requests will fail.
Stateful Inspection Design and Configuration Tasks
Deployment
scenarios toward the end of this document offer several examples for
SPI configuration, but two basic discussions of firewall configurations
are offered here. One configuration is the least complex, offering the
easiest configuration, requiring little knowledge of network usage
patterns, but offering little network use control. The other
configuration is the preferred application of Cisco IOS SPI, offering
better network policy control and tighter security, but requiring a
better grasp of network protocol usage.
Least Complex Cisco IOS Firewall
The
least complex SPI configuration uses a "deny any" ACL facing the
unsecure network, and offers limited capability to restrict network
usage to a limited application list. This prevents hosts on the unsecure
network from sending traffic to the secure network, but blocks return
traffic on legitimate, internally originated connections. To facilitate
the return of legitimate traffic, you will need to configure a simple
inspection set and apply it to inbound traffic on the internal
interface, or to the outbound traffic on the external interface.
Least complex Cisco IOS SPI configuration tasks:
• Configure ACLs to block traffic from the unsecure network.
• Be sure ACLs permit legitimate traffic connections from the secure network to the unsecure network.
• Create inspection rules. Apply the rules inbound to the secure-side interface or outbound to the unsecure-side interface.
• Verify firewall function.
Consider this example:
Figure 3. Simple Network Diagram
Configure
and apply the "deny any" ACL on the public-facing interface,
fastethernet 0/0, to block requests from the unsecure network.
access-list 101 deny ip any any
interface fastethernet 0/0
ip access-group 101 in
Configure
a basic inspection policy. Most Internet traffic can be inspected by
"inspect tcp", "inspect udp", and "inspect icmp". This permits the
most common Internet traffic, including Web browsing, e-mail
applications, file transfer, remote-console and remote-desktop
applications, instant messaging, and peer-to-peer file transfer
applications. Certain applications that use a secondary data channel,
such as voice applications or streaming media applications, may require
that you configure the protocol-specific inspection for that particular
service, such as "inspect ftp", "inspect skinny", or "inspect h.323". If
you set up Cisco IOS Stateful Inspection and find that one of your
network applications that must traverse the firewall stops working, you
should consult the product's documentation or knowledgebase and
determine if the software vendor offers documentation specific to
setting up a Cisco IOS Firewall.
Define the inspection set:
ip inspect name myfw tcp
ip inspect name myfw udp
ip inspect name myfw icmp
Apply the inspection inbound to the inside interface:
interface fastethernet 0/1
ip inspect myfw in
Or, apply inspection outbound to the outside interface:
interface fastethernet 0/0
ip inspect myfw out
This completes the configuration of the least complex Cisco IOS SPI.
Inspecting Complex Services
Several
Internet services use multiple channels to handle the service control
and data communications. For instance, FTP uses one channel to open
initial communications from the client to the server, and the server
opens a separate channel back to the client to send the actual file
transfer traffic. Similarly, H.323 uses one channel for initial call
setup, and other channels are negotiated from the initial connection to
carry the actual streaming media, such as audio traffic in an IP
telephony connection.
The
Stateful Inspection engine only needs to see the initiating connection
for these complex services. Subsequent connections for the session are
dynamically opened for the session, based on SPI's scrutiny of the
connection setup. This is usually known as "fixup". If an outbound ACL
is configured on an interface to restrict network access policy, it must
only account for the initiating port. The task of accommodating the
media channels will be handled by Stateful iInspection's fixup. The
following command lists the complex services and their initiating ports
that Cisco IOS Stateful Inspection can handle:
FWRouter# sh ip port-map
Default mapping: vdolive port 7000 system defined
Default mapping: sunrpc port 111 system defined
Default mapping: netshow port 1755 system defined
Default mapping: cuseeme port 7648 system defined
Default mapping: rtsp port 8554 system defined
Default mapping: realmedia port 7070 system defined
Default mapping: streamworks port 1558 system defined
Default mapping: ftp port 21 system defined
Default mapping: rtsp port 554 system defined
Default mapping: h323 port 1720 system defined
Default mapping: sip port 5060 system defined
Default mapping: mgcp port 2427 system defined
Granular Inspection
Granular
Protocol Inspection (GPI), introduced in Cisco IOS Software Release
12.3(14)T, offered complete integration with PAM. Prior to GPI,
a firewall policy was defined by configuring inspection for outbound
TCP, UDP, and ICMP traffic. Inspection was explicitly configured for
specific protocols, such as FTP, H.323, Skinny, Session Initiation
Protocol (SIP) and others that required fixup to watch for and allow
protocol-specific media channels. Common single-connection services such
as POP, Telnet, Microsoft RPC, and other simple protocols were
inspected by the generic capability of TCP, UDP, and ICMP inspection.
Using these generic inspection capabilities is simple to configure, but
it limits Stateful Packet Inspection's granularity-any traffic that was
allowed to leave through a firewall was allowed to return because
inspection created an ACL Bypass entry for that traffic.
GPI
allows creation of specific ACL Bypass for only the desired traffic, as
defined by an inspection list consisting of only the protocols that are
explicitly permitted by an organization's Internet/security access
policy.
A
complete list of the default services that GPI can inspect is contained
in the appendix at the enof the Stateful Inspection section.
More Secure Cisco IOS Firewall
A
more secure firewall bears a slight resemblance to the least complex
firewall, at least from the standpoint of the unsecured public network's
access to the secured network. However, the stateful inspection policy
is much more focused on the specific services that will be allowed
through the firewall. This means that you must have a better
understanding of the network's requirement for service use. Your
organization should have a documented list of services that are
appropriate for use on the network. If you maintain a list of acceptable
network services, you can follow a clear course of action should it
become necessary to address employee violation of policy or abuse of
network resources.
GPI
allows the user to specify PAM-defined services for firewall
permission. Prior to GPI, if you wished to restrict your firewall
policy, you simply used "inspect tcp/udp/icmp" as in the least complex
firewall example, but you placed an ACL in the outbound packet path to
block access to specific services, or to restrict the list to a specific
few. If you use GPI, you must explicitly state the list of protocols,
but you will be able to use the user-configurable PAM names for the
allowed services, instead of using specific port numbers or ACL service
names, which cannot be modified to reflect network requirements.
More secure Cisco IOS Stateful Packet Inspection configuration tasks:
• Identify traffic that will be allowed out through the firewall.
• Configure ACLs to block traffic from the unsecure network.
• Be sure ACLs permit legitimate traffic from the secure network to the unsecure network.
• Create inspection rules. Apply the rules inbound to the secure-side interface or outbound to the unsecure-side interface.
• Verify firewall function.
Consider this example:
Figure 4. Simple Network Diagram
Network policy in this example allows users to access these Internet services:
• Web and secure Web (HTTP/HTTPS)
• Mail (POP3, IMAP, SMTP)
• Secure terminal (SSH)
• Internet name resolution (DNS)
• File transfer (FTP)
Apply the "deny any" ACL on the public-facing interface to block requests from the unsecure network:
access-list 101 deny ip any any
interface fastethernet 0/0
ip access-group 101 in
Configure
the inspection policy for the protocols listed in the network use
policy. Every protocol that will be allowed through the firewall must
be specifically named. If you wish to allow additional services, add the
services to the inspection policy. If you set up Cisco IOS Stateful
Inspection and find that one of your network applications that must
traverse the firewall stops working, you should consult the product's
documentation or knowledgebase and determine if the software vendor
offers documentation specific to setting up a Cisco IOS Firewall.
Define the inspection set:
ip inspect name myfw http
ip inspect name myfw https
ip inspect name myfw pop3
ip inspect name myfw esmtp
ip inspect name myfw imap
ip inspect name myfw ssh
ip inspect name myfw dns
ip inspect name myfw ftp
ip inspect name myfw icmp
"Inspect
http" adds capability to inspect returned content for java applets,
offering the option to block potentially malicious java content.
However, java filtering incurs a substantial performance penalty. To
configure an http inspection policy that does not inspect for embedded
java content, define an ACL exempting network address ranges from java
inspection and associate the ACL with "inspect http":
access-list 102 permit ip any any
ip inspect name myfw http java-list 102
Apply the inspection inbound to the inside interface:
interface fastethernet 0/1
ip inspect myfw in
Or, apply inspection outbound to the outside interface:
interface fastethernet 0/0
ip inspect myfw out
This
completes the configuration of the most secure Cisco IOS Stateful
Packet Inspection. Cisco.com's reference for Granular Protocol
Inspection is available at: http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_feature_guide09186a008040afd7.html
Note:
In general, network administrators should account for their specific
local topology (which interfaces are considered protected and
unprotected). Additional time should be allowed for testing and access
considerations for ongoing management of the routers before configuring
the firewall. To prevent traffic via the firewall, it is critical to
understand how extended access lists function.
Denial of Service
Cisco
IOS Stateful Packet Inspection maintains counters of the number of
"half-open" TCP connections, as well as the total connection rate
through the firewall and IPS software. These half-open connections are
TCP connections that have not completed the SYN-SYN/ACK-ACK handshake
that is always used by TCP peers to negotiate the parameters of their
mutual connection. Cisco IOS Firewall also regards UDP sessions with
traffic in only one direction as "half-open", as nearly all applications
that use UDP for transport will acknowledge reception of data. UDP
sessions without acknowledgement are likely indicative of DoS activity,
or attempts to connect between two hosts where one of the hosts has
become unresponsive. Some malicious individuals write worms or viruses
that infect multiple hosts on the Internet, then attempt to overwhelm
specific Internet servers with a SYN attack, in which large numbers of
SYN connections are sent to a server by multiple hosts on the public
Internet or within an organization's private network. SYN attacks
represent a hazard to Internet servers, as servers' connection table can
be loaded with "bogus" SYN connection attempts that arrive faster than
the server can deal with the new connections. This is called a
"Denial-of-Service" attack, as the large number of connections in the
victim server's TCP connection list prevents legitimate users from
gaining access to the victim Internet servers.
Cisco IOS Stateful Packet Inspection provides protection from DoS attack as a default
when an inspection rule is applied. The DoS protection is enabled on
the interface, in the direction in which the firewall is applied, for
the protocols that the firewall policy is configured to inspect. DoS
protection is only enabled on network traffic if the traffic enters or
leaves an interface with inspection applied in the same direction of the
traffic's initial movement. Cisco IOS Firewall inspection provides
several adjustable values to protect against DoS attacks. These settings
have default values that may interfere with proper network operation if
they are not configured for the appropriate level of network activity
in networks where connection rates will exceed the defaults:
• ip inspect max-incomplete high value (default 500)
• ip inspect max-incomplete low value (default 400)
• ip inspect one-minute high value (default 500)
• ip inspect one-minute low value (default 400)
• ip inspect tcp max-incomplete host value (default 50) [block-time minutes (default 0)]
These
parameters allow you to configure the points at which your firewall
router's DoS protection begins to take effect. When your router's
DoS counters exceed the default or configured values, the router will
reset one old half-open connection for every new connection that exceeds
the configured max-incomplete or one-minute high values, until the number of half-open sessions drops below the max-incomplete low
values. The router will send a syslog message if logging is enabled,
and if Intrusion Protection System (IPS) is configured on the router,
the firewall router will send a DoS signature message via SDEE. If the
DoS parameters are not adjusted to your network's normal behavior,
normal network activity may trigger the DoS protection mechanism,
causing application failures, poor network performance, and high CPU
utilization on the Cisco IOS Firewall router.
While
you cannot "disable" your firewall's DoS protection, you can adjust the
DoS protection so that it will not take effect unless a very large
number of half-open connections are present in your firewall router's
Stateful Inspection session table.
Follow this procedure to tune your firewall's DoS Protection to your network's activity:
Step 1.
Be sure your network is not infected with viruses or worms that could
lead to erroneously large half-open connection values and attempted
connection rates. If your network is not a "clean slate", there is no
way to properly adjust your firewall's DoS protection.
Step 2. Set the max-incomplete high values to very high values:
ip inspect max-incomplete high 20000000
ip inspect one-minute high 100000000
ip inspect tcp max-incomplete host 100000 block-time 0
This
will prevent the router from providing DoS protection for the time
being while you observe your network's connection patterns. If you wish
to leave DoS protection disabled, stop following this procedure now.
Step 3. Clear the IOS Firewall statistics, using the following command:
show ip inspect statistics reset
Step 4.
Leave the router configured in this state for some time, perhaps as
long as 24-48 hours, so you can observe the network's pattern over
a full day's activity cycle. While the values are adjusted to very high levels, your network will not benefit from Cisco IOS Firewall or IPS DoS protection.
Step 5.
After waiting for some observation period, check the DoS counters with
the following command. The parameters you must observe to tune your DoS
protection are highlighted in bold:
router#show ip inspect statistics
Packet inspection statistics [process switch:fast switch]
tcp packets: [528:22519]
udp packets: [318:0]
Interfaces configured for inspection 1
Session creations since subsystem startup or last reset 766
Current session counts (estab/half-open/terminating) [1:0:0]
Maxever session counts (estab/half-open/terminating) [48:12:5]
Last session created 00:12:21
Last statistic reset never
Last session creation rate 0
Last half-open session total 0
Step 6. Configure "ip inspect max-incomplete high" to a value twenty-five percent higher than your router's indicated maxever session count half-open value.
For example:
Maxever session counts (estab/half-open/terminating) [48:12:5]
65 * 1.25 = 81.25, thus, configure:
router(config)#ip inspect max-incomplete high 818
Step 7. Configure "ip inspect max-incomplete low" to the value your router displayed for its maxever session count half-open value.
For example:
Maxever session counts (estab/half-open/terminating) [48:12:5]
Thus, configure:
router(config)#ip inspect max-incomplete low 65
Step 8.
The counter for "ip inspect one-minute high" and "one-minute low"
maintains a sum of all TCP, UDP, and ICMP connection attempts during the
preceding minute of the router's operation, whether the connections
have been successful or not. A rising connection rate could be
indicative of a worm infection on a private network, or an attempted DoS
against a server. IOS does not maintain a value of the maxever
one-minute connection rate, so you must calculate the value you will
apply based on observed maxever values. While the maximum indicated
values for established, half-open, and terminating sessions are unlikely
to occur in the same instant, the calculated values used for the
one-minute settings have been observed to be reasonably accurate. To
calculate the ip inspect one-minute low value, add the indicated established, half-open, and terminating values, then multiply the sum by three.
For example:
Maxever session counts (estab/half-open/terminating) [48:12:5]
(48+12+5) * 3 = 195, thus, configure:
ip inspect one-minute low 195
Step 9. Calculate and configure "ip inspect max-incomplete high." The ip inspect one-minute high value should be twenty-five percent greater than the calculated one-minute low value.
For example:
ip inspect one-minute low (195) * 1.25 = 244, thus, configure:
ip inspect one-minute high 244
Step 10.
You will need to define a value for "ip inspect tcp max-incomplete
host" according to your understanding of your servers' capability.
Step 11.
Monitor your network's DoS protection activity. Ideally, you should use
a syslog server and record occurrences of DoS attack detection. If
detection happens very frequently, you may need to monitor and adjust
your DoS protection parameters.
For more information about TCP SYN DoS attacks, please visit: http://www.cisco.com/en/US/tech/tk828/technologies_tech_note09186a00800f67d5.shtml
Logging and Audit-Trail
Real-time
alerts send syslog error messages to central management consoles upon
the detection of suspicious activity. Enhanced audit trail features use
syslog to track all transactions and to record time stamps, source host ,
destination host, ports used, session duration, and the total number of
transmitted bytes for advanced, session-based reporting.
To enable logging and send messages to a syslog server:
FWRouter(config)# logging on
FWRouter(config)# logging 192.168.1.11
To enable audit-trail of firewall messages:
FWRouter(config)# ip inspect audit-trail
Audit-trail can be enabled or disabled per protocol in the firewall rules to control the amount of audit-trail messages.
Packet Path for Cisco IOS Firewall Inspection
Understanding
the inspection process can be important when configuring Cisco IOS
Firewall. When an outbound packet arrives at an interface, it will be
processed sequentially:
• The inbound ACL of the input interface is applied
• The NAT inbound is applied
• The NAT outbound is applied
• The outbound ACL of the output interface is applied
• Advanced firewall inspection processing occurs
• The IP packet goes through the output interface
Cisco
IOS Firewall inspects packets after input and output ACL checks. When
inspecting, the advanced firewall engine may insert or remove the ACL
items associated with a session, depending upon its state and context.
The following explains the process that the packet undertakes for many
of the components within the router.
Troubleshooting CBAC: http://www.cisco.com/en/US/products/sw/secursw/ps1018/products_tech_note09186a0080094112.shtml
Stateful Inspection Enhancements
Some
protocol inspection, such as inspection for HTTP, SMTP, ESMTP, and Sun
RPC, offers additional, deeper scrutiny into application activity to
ensure that malicious or unauthorized activity is not occurring.
Appendix 1: Complete List of Granular Protocol Inspection-Supported Services
Configuring Cisco IOS Transparent Firewall
Many
networks have IP address flexibility limitations or may require a
firewall to temporarily augment security or assist in diagnosing a
network security issue. In these circumstances, a simple "drop-in"
firewall can be placed between two physical network segments to protect
hosts in one segment from the hosts in the other segment, while
maintaining existing IP addressing. The drop-in firewall requires no
network addressing changes and offers a short implementation period.
Cisco IOS Transparent Firewall answers this need by integrating Layer 2
bridging with Cisco IOS Firewall packet inspection.
This section describes two applications for Cisco IOS Transparent Firewall.
Background
Most
firewall applications require TCP/IP Layer 3 routing, where the network
that a firewall protects must be in a completely separate subnet
from the potentially hostile network. The firewall must forward the
packets from one subnet to the other, inspecting the traffic for
firewall policy compliance during the routing operation. This is
impractical for some applications, where networking or operational
requirements do not allow sufficient address space or downtime
flexibility to reconfigure a network to accommodate a traditional
"routing" firewall.
Cisco
IOS Transparent Firewall uses Layer 2 bridging to apply Cisco IOS
Firewall capabilities within a single IP subnet. Cisco IOS Firewall
inspection is applied as the router bridges the traffic from one segment
to the other, according to the policy. Cisco IOS Transparent Firewall
only inspects the traffic moving between the segments of the bridge
group. Traffic to other subnets requires inspection as it traverses
Layer 3 interfaces. Appendix A provides links to more Cisco IOS bridging
information.
The
Transparent Firewall feature was introduced in Cisco IOS Software
Release 12.3(7)T. Table 1 indicates which releases added support for
particular platforms.
Table 1. Cisco IOS Transparent Firewall Release and Platform Support
The
simplest application of Cisco IOS Transparent Firewall involves bridges
between two Ethernet ports on a router, while inspecting all traffic
in one direction (client HTTP traffic sending requests to the server,
and denying all connections from the server toward the client, for
example), shown in Figure 5.
Figure 5. Cisco IOS Transparent Firewall Network Example
The bridging- and firewall-relevant configuration for this simple example is as follows:
! generic ip inspection policy
ip inspect name 1-fw tcp audit-trail off
ip inspect name 1-fw udp
ip inspect name 1-fw icmp
!
! set up bridging
bridge irb
!
! interfaces in a bridge group become Layer 2 interfaces, thus they
! have no IP address and must be associated with a bridge group.
interface FastEthernet0/0
no ip address
ip access-group 111 in
bridge-group 1
!
! interfaces in a bridge group become Layer 2 interfaces, thus they
! have no IP address and must be associated with a bridge group.
interface FastEthernet0/1
no ip address
ip inspect 1-fw in
bridge-group 1
!
! bridge interface for bridge group
interface BVI1
ip address 192.168.1.254 255.255.255.0
!
! define ACL to block connections from "hostile" net
access-list 111 remark private net in-acl
access-list 111 deny ip any any
!
! define bridge behavior
bridge 1 protocol ieee
bridge 1 route ip
Practical Applications
This
document explores two different practical applications of Cisco IOS
Transparent Firewall. The first example is a scenario where two physical
network segments in the same subnet are offered similar policies to the
public Internet, but one segment is firewalled from the other. The
second example is a configuration for limiting public access to a DMZ,
and blocking all outbound DMZ access except Extended Simple Mail
Transport Protocol (ESMTP) traffic.
Firewall Separating Two Network Segments in Same Subnet
The
first scenario illustrates a public LAN segment, a private LAN segment,
and a public Internet connection, as might be seen on a retail
network with a public Wi-Fi hotspot. This application is particularly
suited for a Cisco integrated services router, such as the Cisco 871W or
1811W, which include built-in Wi-Fi and Ethernet switch interfaces. One
LAN segment is a wireless segment for Wi-Fi clients; the other is a
group of hosts connected to switch ports on a fixed-configuration router
(or a Cisco EtherSwitch®
module on a modular router). The transparent firewall will deny access
from the Wi-Fi hosts to the hosts on the wired LAN, but hosts on the
Ethernet LAN will be able to connect to hosts on the Wi-Fi LAN. All
hosts will have the same firewall policy for access to the public
Internet.
Both segments will use Port Address Translation (PAT) to access the public Internet, to conserve IP addresses.
Figure 6. Cisco IOS Transparent Firewall Applying Dissimilar Policy to Two LAN Segments in One Subnet
Interfaces
assigned to a bridge group do not have an IP address. The interfaces
share the address on the bridge virtual interface (BVI), as mentioned in
the Transparent Firewall background.
Define the bridge:
bridge irb
bridge 1 protocol ieee
bridge 1 route ip
Configure the BVI. The BVI will act as the PAT inside interface:
interface BVI 1
ip add 192.168.1.254 255.255.255.0
ip nat inside
Configure
the outside interface. This instance will use a static IP address for
Ethernet WAN connectivity through a DSL or cable modem:
interface FastEthernet 4
ip address 171.71.58.67 255.255.255.0
ip nat outside
Set up PAT:
ip nat inside source list 101 interface FastEthernet 4 overload
access-list 101 permit ip 192.168.1.0 0.0.0.255 any
Configure
the ACL to restrict access from the wireless LAN to the wired LAN. This
ACL will only affect traffic switched to other segments within the
bridge group. If you wish to permit access from the Wi-Fi network to the
Ethernet network, you must edit this ACL:
access-list 102 deny ip any any
Apply the ACL to the wireless interface and assign the interface to the bridge group:
interface Dot11Radio0
no ip address
ip access-group 102 in
bridge-group 1
Define the firewall policy for access from the Ethernet segment to the Wi-Fi segment:
ip inspect name transparent tcp
ip inspect name transparent udp
ip inspect name transparent icmp
Assign
the VLAN 1 interface for the Ethernet LAN to the bridge group and apply
the inspection policy. This inspection policy will only inspect traffic
moving between interfaces in the bridge group:
interface VLAN 1
no ip address
ip inspect transparent in
bridge-group 1
Define the firewall policy for access from the Ethernet and Wi-Fi segments to the public Internet:
ip inspect name internet tcp
ip inspect name internet udp
ip inspect name internet icmp
Apply the Internet access policy to the BVI. This inspection policy only inspects traffic leaving the bridge group:
interface BVI 1
ip inspect internet in
Define the ACL to block traffic from the Internet:
access-list 103 deny ip any any
interface FastEthernet 4
ip access-group 103 in
This
completes the Cisco IOS Transparent Firewall configuration for
separating two network segments. The complete configuration is available
in Appendix B.
Several
commands are useful for troubleshooting firewall activity and viewing
the configuration. "Show" and "Debug" commands are discussed in the
"Transparent Firewall Troubleshooting and Management" section of this
document.
DMZ in Same Subnet as Protected Hosts
The
second scenario examines an application to split an IP subnet into a
"private" network and a DMZ. This example illustrates an application
using a Cisco 1841 Integrated Services Router, in which an ISP has
granted a routable subnet of 32 numbers (30 usable addresses) for a
small network, so that all hosts on the LAN can be assigned a routable
address. Some hosts will be exposed to the Internet to offer e-mail,
Web, and other services to hosts on the public Internet.
Exposing
a service to the Internet opens a vulnerability to compromise by worms
and malicious activity. If the exposed host is compromised, a firewall
between the infected host and other hosts is desired to contain as much
of the infection as possible. A traditional routing-type firewall
would cause segmentation of the available address space and waste IP
addresses. A transparent firewall minimizes address loss, improving
efficiency of address use. This example could also be viewed as an
enterprise assigning a small subnet to a remote location, and needing to
restrict headquarters' access to a portion of the subnet.
Figure 7. Cisco IOS Firewall Separating DMZ and Protected Private LAN
This
network's configuration does not differ appreciably from the previous
example, except for the omission of the NAT policy and addition of some
ACL entries to minimize service accessibility between network segments. A
common practice in use with most Internet service DMZs is to disallow
any connections from the DMZ to any host, so that a compromised host
will not act as an attack "zombie" or offer a stepping-stone to attack
other hosts and mask the true source of an attack.
Define the bridge:
bridge irb
bridge 1 protocol ieee
bridge 1 route ip
Configure the BVI. The BVI will act as the PAT inside interface:
interface BVI 1
ip add 192.168.1.254 255.255.255.0
Configure
the outside interface. This instance will use a static IP address for
Ethernet WAN connectivity through a DSL or cable modem:
interface FastEthernet 0/0
ip address 171.71.58.67 255.255.255.0
Configure the ACL to restrict access from the DMZ LAN to the protected LAN:
access-list 102 deny ip any any
Apply the ACL to the wireless interface and assign the interface to the bridge group:
interface Dot11Radio0
no ip address
ip access-group 102 in
bridge-group 1
Define the firewall policy for access from the Ethernet segment to the Wi-Fi segment:
ip inspect name transparent tcp
ip inspect name transparent udp
ip inspect name transparent icmp
Assign the VLAN 1 interface for the Ethernet LAN to the bridge group and apply the inspection policy:
interface VLAN 1
no ip address
ip inspect transparent in
bridge-group 1
Define
the firewall policy for access from the private and DMZ segments to the
public Internet. The ACL will block all connections from the DMZ except
ESMTP, which will be checked for protocol conformance by "inspect
ESMTP". The mail protocol must be allowed to pass so that the Internet
server can send outbound mail:
ip inspect name internet tcp
ip inspect name internet udp
ip inspect name internet icmp
ip inspect name internet esmtp
Apply the Internet access policy to the BVI:
interface BVI 1
ip inspect internet in
Define
the ACL to block traffic from the Internet, but to allow access to
services on hosts in the DMZ. For this example, we will permit requests
for Web (HTTP), secure Web (HTTPS), mail (SMTP), Internet name
resolution (DNS), and file transfer (FTP) to reach the Internet server.
We will permit ICMP echo requests to the entire subnet:
access-list 103 permit tcp any host 198.133.219.1 eq 80
access-list 103 permit tcp any host 198.133.219.1 eq 443
access-list 103 permit tcp any host 198.133.219.1 eq 25
access-list 103 permit tcp any host 198.133.219.1 eq 53
access-list 103 permit udp any host 198.133.219.1 eq 53
access-list 103 permit tcp any host 198.133.219.1 eq 25
access-list 103 permit icmp any 198.133.219.0 0.0.0.31
access-list 103 deny ip any any
Apply the ACL to the public interface:
interface FastEthernet 0/0
ip access-group 103 in
Transparent Firewall Troubleshooting and Management
• show ip inspect Sessions:
Displays established firewall connections, and firewall connections
that are still opening. There is no difference in this output between
Transparent Firewall and Layer 3 Firewall.
• show ip inspect Statistics:
Displays firewall policy activity with regard to type and number of
packets handled, as well as switching path in which the packets were
handled.
• show ip inspect name [name]: Displays inspect policy configuration by policy name.
• show ip inspect Interfaces: Displays inspection policies and access lists (inbound and outbound) applied per interface.
• show ip inspect config: Displays Cisco IOS Firewall inspection denial-of-service policy, and protocol policies per firewall inspection policy.
Appendix A: Additional Reading for Transparent Firewall
Transparent Firewall Command Reference: http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_feature_guide09186a00801ee193.html
Configuring Transparent Bridging: http://www.cisco.com/en/US/products/ps6350/products_configuration_guide_chapter09186a00800ca767.html
Authentication Proxy
Authentication
Proxy (Auth Proxy) offers a mechanism to authenticate and authorize
users' access to network resources. Two common applications for Auth
Proxy are authenticating users' access from a secure network to the
public Internet to avoid network resource abuse, and authenticating user
access to sensitive network resources, such as network management
resources or human resources and payroll servers. Authentication proxy
requires that users provide a valid user name and password before they
can access these resources. The user names and passwords may be stored
locally on the router, or held on a authentication, accounting, and
authorization (AAA) server. AAA servers offer the benefits of offering
service to multiple routers, so every router that uses Auth Proxy or
other authenticated services does not need to be configured with a new
user name any time a user is added on the network. AAA also provides
authorization policy information when users provide their credentials,
so the users' access to authenticated resources can be filtered on a
granular, user-specific basis. Auth Proxy provides HTTP, HTTPS, Telnet,
and FTP interfaces to authenticate user access. HTTP and HTTPS provide
the benefits of launching a separate browser window for the users'
credentials when they try to access HTTP or HTTPS resources protected by
the Auth Proxy-enabled router.
Auth
Proxy is configured on an interface without direction, as access
authentication is always inbound, intercepting the packet before it
reaches the inbound ACL. Therefore, an inbound ACL can be configured to
block all traffic (deny ip any any). Once a user is authenticated by Auth Proxy, ACL Bypass is applied to shunt legitimate traffic around the inbound ACL.
This document does not include AAA server configurations. For more information, please visit: http://www.cisco.com/en/US/products/ps6350/products_configuration_guide_chapter09186a00804ad9bc.html
HTTP, POP/IMAP, and SMTP/ESMTP Application Inspection
Firewalls
and IPSs are becoming increasingly effective at detecting and blocking
unauthorized or malicious traffic. Developers of malicious software are
making such detection and blocking more difficult by disguising their
applications' unwanted traffic as desirable protocols such as HTTP,
POP3, IMAP, or SMTP. The unwanted traffic is usually still recognizable
as fraudulent, but only after additional, deeper inspection into the
data packet to detect indications that the traffic is not legitimate.
Cisco IOS Software has introduced several application inspection engine
features to address the requirement for deeper packet firewall
inspection to block malicious and prohibited application traffic.
This
document describes use cases and application backgrounds for Web
(HTTP), mail client (POP3 and IMAP), and mail server (SMTP and ESMTP)
application inspection services.
Background
Cisco
IOS Software Release 12.3(14)T introduced new application inspection
engines for three protocols, augmenting the existing ESMTP RFC
conformance capability. Most well-known Internet services, such as HTTP,
POP3, IMAP, and SMTP/ESMTP are described by RFCs, a step in
the Internet Engineering Task Force (IETF) standardization process. RFCs
define how Internet services must conduct their activities to ensure
compatibility and interoperability in the multivendor environment of the
public Internet.
The
HTTP application inspection engine offers the greatest range of
capabilities by offering the capability to inspect packets for RFC
conformance as well as checking various parameters within the content to
detect malicious or unauthorized network traffic.
POP
and IMAP inspection monitors connection setup to help ensure a secure
connection and block unwanted traffic on mail-client ports.
ESMTP
inspection has been available since Cisco IOS Software Release
12.3(7)T, augmenting existing SMTP inspection support. ESMTP/SMTP
inspection offers protocol compliance checking to block various
malicious activities directed at e-mail servers.
HTTP Application Inspection Engine
HTTP
is the most commonly used application-layer protocol on the Internet.
HTTP offers a flexible, extensible mechanism to support numerous
networked applications. Businesses, educational institutions, and
government offices that rely on the Internet must allow HTTP traffic
through their firewalls to accommodate most Web-based applications.
Unfortunately, the pervasive nature of HTTP support has contributed to
TCP port 80 being a transmission vector for malicious software such as
worms and viruses, as well as offering an effective conduit for
concealing other traffic generated by undesirable software such as
instant messaging (IM) applications and peer-to-peer (P2P) file-sharing
tools.
The
Cisco IOS Software HTTP application inspection engine offers flexible
application-layer inspection to examine network traffic to detect and
take action against malicious or unwanted HTTP traffic. The HTTP
application inspection engine offers three fundamental capabilities:
protecting servers from malicious clients, protecting clients from
malicious or compromised content, and enforcing organizational
information systems policies. This document examines three respective
examples of some of the capabilities available with HTTP application
inspection: HTTP method control, HTTP content verification, and IM and
P2P blocking.
All
of the HTTP application inspection engine functions offer the option to
allow or reset the offending traffic. Furthermore, syslog can send an
alarm concerning the violation to a monitoring station. Thus,
application inspection can monitor traffic if allow is used with a rule, or it can filter out unwanted traffic using the reset option.
RFC
conformance checking plays an important role in the capability of the
HTTP application inspection engine. HTTP was originally defined in RFC
2068, which was superseded by RFC 2616. These RFCs describe the
methodology for establishing HTTP sessions and transferring hypertext
content. A Web browser and Web server employ a somewhat limited set of
requests and responses to carry out their communications over the course
of the session.
When
malicious traffic is directed at a Web client or Web server, or when
non-HTTP applications disguise their communications with a TCP port
80 header, the malicious traffic frequently violates the protocol
specification or uses commands outside of the usual command set. Such
behavior offers a clear indication that the traffic is somehow atypical
and should probably be blocked before entering or leaving the protected
network. HTTP application engine RFC conformance checking is an
effective solution to stop the most obvious policy-violating traffic,
and should be applied with any HTTP application policy.
The HTTP application inspection engine offers several options (Table 2).
Table 2. HTTP Application Inspection Options
The following examples combine several of these options to illustrate application-specific HTTP policy enforcement.
Protecting Servers from Malicious Traffic
Most
Web servers on the public Internet are constantly subjected to
reconnaissance activity probing for weaknesses in the Web server
software or the Web applications they offer. Some of this activity is
more effectively addressed with Cisco IPS capabilities, using either a
sensor or the IPS features in a Cisco IOS Software router. Examples of
inspection more suited to an IPS include:
• Searching for a specific text string (for example, worm traffic)
• Watching for activity on multiple ports (for example, port scanning)
• Blocking specific crafted-packet attacks
Web
browsers use a standardized group of requests to request and transmit
data to and from Web servers. This request is carried in the HTTP header
inside the TCP packet, inside the IP packet. By examining the contents
of the Web browser's request to the Web server, application inspection
can permit or allow specific types of requests, based on the perceived
threat of the different types of requests.
Many
attacks against Web servers use specific requests that are fairly
uncommon for ordinary Web transactions. An HTTP inspection engine can
restrict the particular HTTP methods that Web clients use to communicate
with Web servers by checking the HTTP request type field in the HTTP
header of the IP packet.
In a simple network, a Web server is separated from the Web client by a Cisco IOS router (Figure 8).
Figure 8. Protect Web Servers with HTTP Application Inspection
To apply the server protection just discussed to this simple network, configure an HTTP application inspection policy:
appfw policy-name method-control
application http
strict-http action reset alarm
request-method rfc put action reset alarm
This
application inspection policy must then be applied to an inspection
policy, either an existing inspection set or a minimal set that will
only employ application inspection:
ip inspect name my-fw appfw method-control
Finally, the policy must be applied in the direction that the inspection occurs:
interface fastethernet 1/0
ip inspect my-fw out
To see the HTTP application inspection engine configuration, issue this command at the privileged EXEC prompt:
sh ip inspect config
Protecting Clients from Malicious Content
Web
browser clients are vulnerable to malicious Web content that they might
mistakenly download through a misleading link or from legitimate
Websites that have had their content replaced with malicious material.
As with server protection, some client protection tasks are best left to
an IPS. Searching for complex worm code strings in Web content is not
an appropriate task for HTTP application inspection.
An
appropriate use of the HTTP application inspection engine for client
protection applies the content-type-verification feature. Several
documented Web browser vulnerabilities involve a browser's
susceptibility to intentionally mislabeled content. When a Web browser
requests content from a Web server, the Web server indicates to the Web
browser the type of embedded content in its reply. The Web browser
usually passes the content to an application on the workstation that is
associated with the type of content that the Web server indicated in its
reply. However, if the content is mislabeled, the application that
opens the content might try to implement the content according to the
content's file header information or the file name extension. As an
example, consider an attack that takes advantage of this weakness to
distribute a Microsoft Visual Basic script file with an .mpg or .avi
extension on the file name (for instance, an attacker applies the name
"attack.mpg" to the attack.vbs file). This causes a default installation
of Microsoft Internet Explorer to pass the file to Microsoft Windows
Media Player. Media Player receives the file from Internet Explorer and
recognizes that the content does not have the appropriate attributes to
be a genuine .mpg or .avi file, but it makes its best effort to open the
file nonetheless. Media Player might recognize the file as a Visual
Basic script and execute it accordingly. If the Visual Basic script
contains the appropriate content, the workstation will be compromised.
The
content-type-verification feature of the HTTP application engine checks
Web content replies to protected browsers to verify that the embedded
content matches the content type indicated by the Web server. Like most
HTTP application inspection features, content-type-verification can be
configured to allow or reset traffic that doesn't pass the content-type
check. Obviously, the only appropriate action for invalid traffic is to
reset the Web client connection so that the invalid traffic is not
allowed to pass through to the Web client.
Once
again, this exercise assumes a simple network consisting of a Web
server separated from the Web client by a Cisco IOS router (Figure 9).
Figure 9. Protect Web Clients with HTTP Application Inspection
To apply the server protection just discussed to this simple network, configure an HTTP application inspection policy:
appfw policy-name chk-content
application http
strict-http action reset alarm
request-method rfc put action reset alarm
This
application inspection policy must then be applied to an inspection
policy, either an existing inspection set or a minimal set that will
only employ application inspection:
ip inspect name my-fw appfw chk-content
Finally, the policy must be applied in the direction that connection was initiated:
interface fastethernet 1/0
ip inspect chk-content in
To see the HTTP application inspection engine configuration, issue this command at the privileged EXEC prompt:
sh ip inspect config
Blocking Instant Messaging Traffic
Instant
messaging applications offer substantial productivity gains when they
are used for business applications such as business-focused discussion
among colleagues. Unfortunately, they can cause substantial productivity
losses if employees spend a great deal of time using public IM services
such as Yahoo! Messenger or AOL Instant Messenger. P2P file-sharing
networks can consume a large portion of an organization's Internet
connectivity bandwidth and can place a substantial liability burden on
an organization's shoulders if company resources are used to share
copyrighted media such as music or films.
Some
implementations of IM applications and P2P file sharing software that
offer the capability to conceal their traffic within a TCP port 80
(HTTP) header do not implement the complete RFC 2616 dialogue
methodology. The Application Inspection Engine's "strict-rfc" option
recognizes these applications' traffic as it is clearly not HTTP
traffic. However, some IM and P2P applications implement their TCP port
80 traffic with a sufficiently high degree of fidelity to RFC 2616 to
make the traffic indistinguishable from legitimate HTTP traffic. The
Application Inspection Engine can detect this traffic by enabling the
"port-misuse" option, which currently recognizes Yahoo! Messenger IM,
KaZaa and Gnutella P2P file sharing, and TCP port 80-based tunneling by
HTTPPort/HTTPHost, GNU Httptunnel, GotoMyPC, Firethru, and the
Http-tunnel.com client. Applying the port-misuse feature coupled with
strict HTTP RFC compliance checking helps assure that valid HTTP
dialogue is conducted according to the RFC's specification, and allows
recognition of known IM and P2P traffic that closely emulates legitimate
HTTP traffic.
This exercise assumes a simple network consisting of a Web browser accessing the Web using a Cisco IOS router (Figure 10).
Figure 10. Blocking Instant Messaging Traffic on a Simple Network
To apply instant messaging and P2P blocking on this simple network, configure an HTTP application inspection policy:
appfw policy-name no-abuse
application http
strict-http action reset alarm
port-misuse default action reset alarm
This
application inspection policy must then be applied to an inspection
policy, either an existing inspection set or a minimal set that will
only employ application inspection:
ip inspect name my-fw appfw no-abuse
Finally, the policy must be applied in the direction that the inspection occurs:
interface fastethernet 0/1
ip inspect my-fw in
To see the HTTP application inspection engine configuration, issue this command at the privileged EXEC prompt:
sh ip inspect config
Securing E-Mail Client Service
Cisco
IOS Software offers application inspection services for two common
Internet e-mail client protocols: IMAP (TCP port 143) and POP3 (TCP port
110). E-mail client application inspection helps ensure that clients
negotiate a valid client connection with the server and, if necessary,
use secure authentication. E-mail client application inspection
addresses two concerns regarding traffic on the e-mail client ports.
The
first concern is that the e-mail client ports, which are frequently
left open to the public Internet to allow employees to send and receive
e-mail when they are away from the office, are not used for other
applications such as a "back door" or other unauthorized service on the
permitted ports.
The
second concern regarding remote e-mail client access is when users have
configured their e-mail client to secure their authentication
credential exchange. Most POP3 and IMAP client applications default to
passing the user's user name and password to the server as "cleartext,"
meaning this information is not encrypted to conceal its credentials. If
an attacker intercepts a user name and password being transmitted as
cleartext, the attacker could read the user name and password and could
masquerade as the authorized user. Such a situation is especially
problematic if the credentials are used for multiple services.
To
help ensure security on POP3 and IMAP ports and maximize connection
security between mail clients and servers, e-mail client traffic
inspection monitors the clients' negotiations with servers to help
ensure that the session setup follows the requirements of the respective
RFCs. RFC 1939 defines the POP3 client negotiation, and RFC 3501
defines the IMAP client negotiation (Figure 15).
E-mail
client inspection can be configured to log and/or reset client
negotiations that violate the RFC requirements. If neither log nor reset is specified, e-mail client application inspection not be useful. Furthermore, if secure login is required, the secure-login
switch will require that clients ask for a secure negotiation. If the
client does not ask for a secure login, the client will be logged out or
reset according to the configuration.
Table 3 shows the options that are available in e-mail application inspection configuration.
Table 3. E-Mail Application Inspection Configuration Options
This
example assumes a simple network consisting of an e-mail client
accessing an e-mail server using a Cisco IOS router (Figure 11).
Figure 11. Simple Network Diagram
To
apply the e-mail client traffic protection just discussed to this
simple network, define an IP inspection statement in an existing
inspection policy or add the statement to an existing inspection set:
ip inspect name mail-clients pop3 log reset secure-login
ip inspect name mail-clients imap log reset secure-login
Finally, the policy must be applied in the direction that the inspection occurs:
interface fastethernet 0/1
ip inspect mail-client in
To show the e-mail client application inspection configuration, issue the following show command:
show ip inspect config
To see e-mail client application inspection engine messages for individual events, enable debug for the appropriate protocol:
debug ip inspect pop3
debug ip inspect imap
Securing E-Mail Server Traffic
Cisco
IOS Software introduced SMTP inspection in Release 12.0(1)T and
augmented the feature to support ESMTP in Release 12.3(7)T. SMTP
inspection and ESMTP inspection monitor the connection between the
client and server to help ensure that only valid commands that are
specified by the RFCs are allowed between the two participants in SMTP
and ESMTP dialogues. This restriction prevents unauthorized use of the
SMTP and ESMTP port (TCP port 25) so that mail servers are protected
from invalid, possibly malicious traffic, and so that exploit software
such as back doors and rootkits is not allowed to use TCP port 25.
This
example assumes a simple network consisting of an e-mail server
connecting to the public Internet using a Cisco IOS router (Figure 12).
Figure 12. Simple Network Diagram
To
apply the e-mail server traffic protection to this simple network,
define an SMTP or ESMTP inspection statement for use by itself or add
the statement to an existing inspection set:
ip inspect name mail-server [ smtp | esmtp ]
SMTP
traffic inspection configuration and ESMTP traffic inspection
configuration in any given inspection set contain mutually exclusive
commands. ESMTP inspection is a superset of SMTP inspection. SMTP
inspection should only be applied if the mail server is to be limited
to supporting only SMTP. Otherwise, ESMTP inspection should be applied.
Apply the policy in the direction that the inspection occurs:
interface fastethernet 0/1
ip inspect mail-server in
To show SMTP and ESMTP application inspection configuration, issue the following show command:
show ip inspect config
References
ESMTP inspection engine: http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_feature_guide09186a00801ed6ee.html
POP and IMAP inspection engine: http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_feature_guide09186a00803f85bd.html
HTTP inspection engine: http://www.cisco.com/en/US/partner/products/sw/iosswrel/ps5207/products_feature_guide09186a0080420260.html
Blocking Instant Messaging and Peer-to-Peer File Sharing Applications with Cisco IOS Software Release 12.3(14)T
Most
organizations view IM and P2P applications as frivolous consumers of
expensive resources-employee time and network bandwidth. Furthermore,
some P2P networks can act as a conduit for malicious software such as
worms, offering an easy path around firewalls into an organization to
compromise desktop computing resources.
Cisco
IOS Software Release 12.3(14)T introduced application inspection
engines and granular inspection, two critical new features that allow
Cisco IOS Firewall to control IM and P2P applications on networks. This
document offers some sample configurations to use these features to
monitor and block IM and P2P file sharing traffic.
Background
P2P
and IM traffic generally offer two modes of operation-a native mode,
where the application runs on a uniquely defined set of TCP or UDP
ports, and "HTTP cloaked" mode, in which the application masquerades as
HTTP (TCP port 80) traffic in order to gain passage through firewalls
and other network policy controls. Some of the more advanced P2P and IM
applications implement sufficient RFC 2616 dialogue to appear as a
legitimate conversation between a Web browser and a Web server.
Prior
to Release 12.3(14)T, Cisco IOS Software was bound by two major
restrictions in the control of P2P and IM applications-a limited list of
applications that were supported in Cisco IOS Firewall Stateful
Inspection (formerly known as Context-Based Access Control [CBAC]), and
lack of application inspection capability.
Cisco
IOS Firewall Stateful Inspection is the basis of the Cisco IOS Firewall
feature set. If a specific application was not built into Cisco IOS
Firewall (see the list of original supported protocols in Appendix 1),
the inspect tcp or inspect udp
commands were used to watch for any outbound connection activity
through a firewall, and anticipated return traffic was subsequently
allowed through firewall blocking policies with ACL bypass capability.
Unfortunately, these commands allow all traffic that is not specifically
filtered out to make a connection with the appropriate server through a
firewall, and return traffic is allowed back in. This mode of operation
offers little control in allowing or disallowing specific protocols.
From
an application inspection standpoint, HTTP inspection was one of the
more thorough protocol inspections that Cisco IOS Firewall offered.
However, even if an extremely restrictive Cisco IOS Firewall policy
allowing only "HTTP out" was applied, users might still be able to use
P2P and IM applications that offered HTTP cloaking.
Cisco
IOS Software Release 12.3(14)T introduced application inspection and
granular inspection capabilities to address both of these shortcomings.
Example Network
We
can examine a simple network to build an example of an effective
inspection policy that will prohibit P2P and IM traffic, and that will
offer control over cloaked applications that try to exploit TCP port 80
to gain access though the firewall (Figure 13). This network consists of
one or more client PCs in a private network, connected to the public
Internet through a Cisco IOS router running Cisco IOS Software Release
12.3(14)T.
Figure 13. Example Network
This
sample network needs standard services, such as Web access (HTTP and
Secure HTTP [HTTPS]), Internet e-mail (SMTP, POP3, and IMAP), packet
voice (H.323), DNS lookup, FTP, Network Time Protocol (NTP), and ICMP.
Furthermore, the network users employ Virtual Network Computing (VNC),
an open-source remote console application that runs by default on TCP port 5900,
and they need HTTP access on atypical ports (TCP port 81 and 8080) for
connectivity to vendor or customer e-commerce Webpages.
Background
Cisco
IOS Firewall uses Cisco IOS Firewall Stateful Inspection to restrict a
public network's access to protected networks, while maintaining the
private network's ability to access resources located in the public
network (Figure 14).
Figure 14. Cisco IOS Firewall Stateful Inspection
Cisco
IOS Firewall Stateful Inspection protects networks with two basic
components. ACLs restrict inbound connections, and stateful inspection
examines activity traversing the Cisco IOS Firewall from the protected
network to the public network and anticipates the return traffic.
Stateful inspection is a mechanism that observes the initiation,
maintenance, and closure of network data connections.
Cisco
IOS Firewall Stateful Inspection with granular inspection supports
several of the specific application protocols listed in Appendix 1. Some
of these protocols are common, simple protocols such as HTTP and
Telnet, which only use one connection between client and server (or
peers) to request and return application data. More complex supported
protocols, such as FTP and H.323, employ a control channel to establish
communications and a secondary data channel to transmit application
data.
Some common protocols are not specifically predefined in IP inspection. Prior to Cisco IOS Software Release 12.3(14)T, ip inspect tcp and ip inspect udp
were used as universal options for any services not covered by specific
inspection services to inspect outgoing traffic and allow return
traffic through a Cisco IOS Firewall's inbound permission ACL.
Unfortunately, the inspect tcp
option's capability to allow any return traffic is problematic in
circumstances where specific protocols must be disallowed, particularly
when a complex application such as IM or P2P employs unpredictable port
numbers and other mechanisms that make the traffic difficult to detect
and block as it leaves the network. Undesired complex applications can
be blocked by denying all return traffic except the traffic allowed by
specific inspection services.
Controlling P2P and Instant Messaging Applications
Cisco
IOS Software Release 12.3(14)T introduced granular protocol inspection,
which offers the capability to use PAM protocol definitions with Cisco
IOS Firewall Stateful Inspection. PAM allows users to define specific,
named protocols. This significantly changes the older paradigm of
employing specific inspection statements for advanced protocols that
required comprehensive inspection to allow return access back through a
firewall (commonly called "fixup" on Cisco PIX® products), then using inspect tcp
to cover simpler protocols that do not require close scrutiny to allow
additional data connections. Granular inspection uses PAM with Cisco IOS
Firewall Stateful Inspection to associate user-defined application
labels to traffic on specific ports, in order to define the list of
desired traffic to be inspected so the return traffic "pinholes" are
allowed in the inbound ACL; this protects the private network from
unwanted access from the public network. Since the complete list of
desired traffic can be specified, there is no need for inspect tcp to offer coverage for previously unrecognized application traffic.
Granular
inspection is an effective solution for blocking applications using
port-hopping techniques that defy ACLs attempting to block the traffic,
because the only traffic that is allowed to return through the firewall
is running on the specific desired ports that the user allows with
existing, predefined inspection capabilities, as well as user-specific,
PAM-defined granular inspection policies. inspect tcp does not offer this application-specific mechanism to permit traffic-it simply anticipates all traffic running over TCP.
Application Inspection
Granular
inspection leaves some openings that advanced P2P and IM applications
may exploit. Most networks allow HTTP traffic through their firewalls,
as it is the standard transport of many business applications, including
ordinary Web traffic. Many IM and P2P applications have developed
mechanisms to disguise their traffic within TCP port 80 (HTTP) traffic,
thus offering their application an additional mechanism to work around
restrictive firewalls. To address this issue, Cisco IOS Software Release
12.3(14)T introduced application inspection. The HTTP Application
Inspection Engine offers the port-misuse
option to scan traffic for specific known applications that disguise
their undesired traffic as legitimate HTTP traffic. Presently, HTTP
inspection can recognize Yahoo! Messenger traffic, Gnutella and KaZaA
file sharing activity, and some applications that can tunnel other
traffic through TCP port 80 to avoid an otherwise restrictive firewall.
By
combining granular inspection with the HTTP Application Inspection
Engine, network engineers can allow desired protocols' traffic to return
to their networks through access lists that protect the private network
from unwanted public Internet traffic. Application inspection can
control specific unwanted application traffic that has been concealed
inside legitimate HTTP traffic.
Configuring Cisco IOS Firewall
Consider
a simple network consisting of a Cisco IOS router with two Fast
Ethernet ports. Port 0/0 is connected to the public Internet through a
broadband connection, and Port 0/1 is connected to an Ethernet switch in
the private network (Figure 15).
Figure 15. Example Network
The
first configuration step restricts hosts on the public Internet from
reaching the protected network with an ACL blocking all traffic from the
public Internet, and applying the list to an interface:
access-list 101 deny ip any any
interface Fastethernet 0/0
ip access-group 101 in
In
the next step, specific inspection statements are configured based on
the acceptable traffic that the router will allow out through the
firewall, and on the expected return traffic:
ip inspect name my-ios-fw http
ip inspect name my-ios-fw https
ip inspect name my-ios-fw esmtp
ip inspect name my-ios-fw pop3
ip inspect name my-ios-fw imap3
ip inspect name my-ios-fw dns
ip inspect name my-ios-fw ftp
ip inspect name my-ios-fw ntp
ip inspect name my-ios-fw icmp
Cisco
IOS Software supports the most popular Internet protocols, as well as
several protocols that require additional effort to accommodate
secondary data connections (Appendix 1). This example requires support
for VNC, which is not supported by default IP inspection capability;
VNC runs on TCP port 5900 by default. Granular protocol inspection
provides the capability to configure inspection for specific protocols
that are not natively supported by IP inspection. Configure inspection
for VNC by defining the PAM entry for the protocol. Note: User-defined
protocol labels must begin with "user-":
ip port-map user-vnc port tcp 5900
Next, apply the new protocol to the stateful inspection set:
ip inspect name my-ios-fw user-vnc
Now
that the IP inspection set is complete, apply the inspection policy to
the outbound traffic. Since this example protects traffic sourced on the
private side of the router, ip inspect in
is applied to the private interface. The router will inspect traffic
passing from the private network to the public Internet, and the
appropriate ACL bypass entries will be entered on the public side of the
router to allow desired return traffic from the public Internet to pass
back to the private network.
interface fastethernet 0/1
ip inspect my-ios-fw in
The
Cisco IOS Firewall Stateful Inspection configuration that you have
defined blocks unwanted connections from the public Internet and allows
return traffic for desired applications. Some P2P and IM applications
may be able to carry their traffic over TCP port 80, so we'll use the
HTTP Application Inspection Engine to inspect further into TCP port 80
packets, and look for indications of the unwanted P2P and IM traffic.
First,
define the application inspection policy name, then configure HTTP
inspection. Next, set up the policy. This policy will only inspect TCP
port 80 traffic for misuse by non-HTTP traffic; you may wish to define
other application inspection features. Check the configuration reference
list at the end of this document for details on using other HTTP
application inspection features:
appfw policy-name abuse-control
application http
port-misuse default action reset alarm
Apply the application inspection policy to the existing inspection set:
ip inspect name my-ios-fw appfw abuse-control
This completes the configuration for Cisco IOS Firewall with granular inspection and application inspection.
Verifying Cisco IOS Firewall Capability
You can check the Cisco IOS Firewall configuration and activity with several show commands:
References
Configuring Cisco IOS Firewall Stateful Inspection: http://www.cisco.com/en/US/partner/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800ca7c5.html
Configuring HTTP application inspection: http://www.cisco.com/en/US/partner/products/sw/iosswrel/ps5207/products_feature_guide09186a0080420260.html
Configuring granular protocol inspection: http://www.cisco.com/en/US/partner/products/sw/iosswrel/ps5207/products_feature_guide09186a008040afd7.html
Configuring PAM: http://www.cisco.com/en/US/partner/products/sw/iosswrel/ps1831/products_configuration_guide_chapter09186a00800d981c.html
CAVEATS
• This document does not illustrate a security policy that must be followed; rather, it explains how to initiate such a policy.
• Separate documentation outlines IDS design.
•
Cisco IOS Firewall does not currently protect ICMP and it is not
covered. For the purposes of troubleshooting, ICMP can be allowed.
However, as a security measure, it is not advisable to permit ICMP to
pass. It is advisable to use ACLs to control the various types of
ICMP packets.
•
ACLs can be used for IP Spoofing Defense, so that traffic is denied if
it comes from outside and has spoofed the inside IP address.
•
All firewall states are internal to a single router; therefore, there
is currently no provisioning available for redundant firewall routers.
Configurations with asymmetric routing (only one direction of each
session passes through the router) cause limitations for dynamic
modification of ACLs.
•
Due to the connectionless nature of UDP traffic, the advanced firewall
engine relies on idle timers for closing client-server channels. These
timers are not specifically covered in this document, but can be
configured for certain types of UDP traffic (i.e. DNS), which can limit
very large numbers of open UDP channels.
•
A separate document explains Advanced Firewall
Inspection/Authentication Proxy with IPSec VPN. Although IPSec VPNs can
affect a security policy, Cisco IOS Firewall addresses IPSec type
packets directly in the packet flow process.
DEPLOYMENT SCENARIOS
Corporations today can implement the Cisco IOS Firewall to secure the following perimeters (Figure 21):
• Internet Perimeter-Connect to the Internet via WAN
• Corporate Intranet Perimeter-Segmentation of corporate intranet by departments
• Branch Office Intranet-Connect to the branch office via WAN, such as Frame Relay
• Extranet Perimeter/Partner-Connect to the partners or suppliers via WAN, such as Frame Relay
• Home Office/Telecommuter-Connect to the Internet and corporate intranet via IPSec secure LAN-LAN tunnel
Figure
16 details several key components of a typical enterprise network. Each
component is broken down into sections to better understand the
configuration tasks and security implications for each segment within
the network. Security policies and intruder concerns vary widely in each
scenario. These implications often involve "political" relationships
among departments and/or partners, which affects the technical
implementation of the defined security policies for the firewall. Each
policy should be reviewed periodically to best address access and
security concerns.
Figure 16. Corporate Network
Internet Perimeter
Cisco IOS Firewall used as corporate Internet firewall is shown in Figure 17.
Figure 17. Internet Perimeter Firewall
•
The Web server is on the DMZ network to protect the inside network from
outside access through the Web server, in the event the Web server were
compromised.
• Cisco IOS Firewall provides real-time log messages, including alerts, to the syslog server
• Auth Proxy with an AAA server is used to control Internet and DMZ access per user
• Simple example: Static NAT and PAT
Internet Firewall Policy
This
policy outlines a typical set of protocols that might be used within a
corporate network when accessing the Internet. Care has been taken to
specifically limit traffic from the general inbound Internet. A DMZ is
one method to segment off general corporate users from
Internet-accessible servers. This method limits the liabilities that can
be involved with any one attack to any specific part of the network.
The
company assigns everyone a user ID and password for authentication. The
protocols the company wants to secure are SMTP, FTP, H.323,
DNS, Telnet, SQLnet, and RealAudio. The protocols-HTTP, SSL, FTP,
VDOlive, NetShow, and H.323-are allowed for outside to access the Web
server in DMZ. As a policy, no one is allowed to initiate any connection
from DMZ.
• Authentication proxy is enabled for outbound access from inside.
•
Allow the firewall to inspect and secure the following protocols for
all authenticated inside users that go out to the Internet and DMZ.
– Protocols to be inspected from inside: SMTP, FTP, H.323, TCP, UDP, SQLnet, RealAudio.
– HTTP
need not be specified unless Java blocking is desired. Specifying `TCP'
allows inspections for single channel TCP protocols that include HTTP
and Telnet. `UDP' is specified for DNS.
• All of the following protocol traffic should be allowed into the DMZ from the outside; it should not be allowed inside:
– HTTP, FTP, VDOlive, NetShow, and H.323.
• Cisco IOS Firewall does not currently inspect SSL. TCP port 443 will be opened to permit SSL.
• No traffic initiated from the DMZ is to be allowed outside of the DMZ.
Internet Firewall Sample Configuration
FWRouter#sh run
Building configuration...
Current configuration:
!
version 12.0
service timestamps debug uptime
service timestamps log uptime
service password-encryption
!
hostname FWRouter
!
! Enable authentication proxy globally.
!
boot system flash
aaa new-model
aaa authentication login default group tacacs+
aaa authentication login console_line line none
aaa authentication login vty_line line none
aaa authorization exec vty_line none
aaa authorization auth-proxy default group tacacs+
enable secret 5 $1$VZGK$Els0ipcmt8Pe0R98RMEds0
enable password 7 0822455D0A16544541
!
username cisco password 7 121A0C041104
!
ip subnet-zero
no ip source-route
no ip finger
!
! INFIRE is configured for traffic destined for the internet or the DMZ. Inspection
! is configured inbound on the inside interface (e0/0)
!
ip inspect name INFIRE smtp
ip inspect name INFIRE ftp
ip inspect name INFIRE tcp
ip inspect name INFIRE udp
ip inspect name INFIRE sqlnet
ip inspect name INFIRE realaudio
ip inspect name INFIRE h323
!
! OUTFIRE is setup for traffic heading from the internet. This traffic is can go
! ONLY to the DMZ and is applied to inbound traffic on the outside interface (s0/0)
!
ip inspect name OUTFIRE tcp
ip inspect name OUTFIRE ftp
ip inspect name OUTFIRE vdolive
ip inspect name OUTFIRE netshow
ip inspect name OUTFIRE h323
ip auth-proxy name PROXY http ! Enables authentication proxy for http traffic.
!
! e0/0 is the inside interface to the Corp network.
!
interface Ethernet0/0
ip address 192.168.1.1 255.255.255.0
ip access-group 101 in ! TACACS+ traffic
ip access-group 102 out ! lock-down acl
no ip directed-broadcast
no ip proxy-arp
ip nat inside
ip inspect INFIRE in ! firewall inspection for inbound traffic.
ip auth-proxy PROXY ! Associates authentication proxy on the inside interface.
no cdp enable
!
! s0/0 is the interface closest to the internet. The outside interface.
!
interface Serial0/0
ip address 200.1.1.1 255.255.255.0
ip access-group 121 in ! allows internet initiated traffic.
no ip directed-broadcast
ip nat outside
ip inspect OUTFIRE in ! firewall inspection for traffic coming from the internet.
no cdp enable
!
! this is the DMZ interface.
!
interface Ethernet0/1
ip address 200.1.2.1 255.255.255.0
ip access-group 111 in ! DNS traffic.
ip access-group 112 out ! allows specific traffic to the DMZ server.
no ip directed-broadcast
no ip proxy-arp
no cdp enable
!
! an unsed interface.
!
interface Serial0/1
no ip address
no ip directed-broadcast
shutdown
no cdp enable
!
! Inside addresses 192.168.1.21-25 will be translated to 200.1.1.21-25 respectively.
! static nat is used as a simple example. Dynamic nat or a combination can be used
! depending on the address requirements.
!
ip nat inside source static 192.168.1.25 200.1.1.25
ip nat inside source static 192.168.1.24 200.1.1.24
ip nat inside source static 192.168.1.23 200.1.1.23
ip nat inside source static 192.168.1.22 200.1.1.22
ip nat inside source static 192.168.1.21 200.1.1.21
ip classless
! configures the HTTP server to allow AAA authentication.
ip http server
ip http authentication aaa
ip http access-class 10
!
! syslog is configured for reporting etc.
!
logging facility syslog
logging 192.168.1.11
access-list 10 deny any
!
! acl 101 blocks all traffic except TACACS+ communications with the AAA server.
! With successful user authentication, Authentication Proxy will override this ACL
! and open connections based on the AAA ACL policy. This ACL is applied to the
! inbound on the inside interface (e0/0)
access-list 101 permit tcp host 192.168.1.12 eq tacacs host 192.168.1.1
! acl 102 locks down traffic heading to the inside net. It is applied to the
! inside interface for outbound traffic.
access-list 102 deny ip any any
! acl 111 permits DNS requests from the HTTP server on the DMZ net. (e0/1)
access-list 111 permit udp host 200.1.2.11 any eq domain
! acl 112 permits internet traffic inspected by the firewall destined to the DMZ.
access-list 112 permit tcp any host 200.1.2.11 eq www
access-list 112 permit tcp any host 200.1.2.11 eq ftp
access-list 112 permit tcp any host 200.1.2.11 eq 7000
access-list 112 permit tcp any host 200.1.2.11 eq 1755
access-list 112 permit tcp any host 200.1.2.11 eq 1720
! acl 121 corresponds to acl 112. it allows internet traffic inspected by the
! firewall to the server on the DMZ. It is applied to inbound traffic on the
! outside interface (s0/0)
access-list 121 permit tcp any host 200.1.2.11 eq www
access-list 121 permit tcp any host 200.1.2.11 eq ftp
access-list 121 permit tcp any host 200.1.2.11 eq 7000
access-list 121 permit tcp any host 200.1.2.11 eq 1755
access-list 121 permit tcp any host 200.1.2.11 eq 1720
no cdp run
! configure the TACACS server's address and key.
tacacs-server host 192.168.1.12
tacacs-server key cisco
!
! The following is down in order to secure access to the router via the console
! before enter AAA configuration commands. note: AAA commands take effect before
! saving out of config mode.
!
line con 0
exec-timeout 0 0
password 7 0822455D0A16
login authentication console_line
transport input none
line aux 0
line vty 0 4
password 7 01100F175804
login authentication vty_line
!
!
!
End
Corporate Intranet Perimeter
Cisco IOS Firewall used as a corporate intranet firewall is shown in Figure 18.
Figure 18. Corporate Intranet Perimeter Firewall
• Each interface can be configured so that each subnet has its own policy
• Sub-interfaces of WAN protocols are supported (i.e. ATM and Frame Relay)
Corporate Intranet Firewall Policy
Each
department has an individual policy of permissible protocols, including
the standard protocols Telnet, FTP, and UDP. As a policy, no one
will be allowed to initiate any session from the server subnet.
The
administrative network has Web servers that need to be accessed by all
of the company and permit only HTTP in. Marketing maintains servers and
needs to allow the Telnet, FTP, SMTP, UDP, and HTTP inbound protocols.
Similar to the Internet perimeter scenario, only specific protocols that
are necessary for each department are allowed. This approach not only
limits the number of resources that can be attacked at any one time, but
also layers security into administrative areas. The server subnet has
only servers that will require Telnet, FTP, and UDP.
• No users will reside on the server subnet
• Administrative subnet will permit HTTP inbound only
• Marketing subnet will permit Telnet, FTP, SMTP, UDP, and HTTP inbound
• Engineering subnet will permit Telnet, FTP, and HTTP inbound
•
Everyone on the administrative, marketing, and engineering subnet is
permitted all outbound Telnet, SMTP, FTP, H.323, DNS (TCP and UDP),
SQLnet, and RealAudio
Corporate Intranet Firewall Sample Configuration
FWRouter#sh run
Building configuration...
Current configuration:
!
version 12.0
service timestamps debug uptime
service timestamps log uptime
service password-encryption
!
hostname FWRouter
!
boot system flash
enable secret 5 $1$VZGK$Els0ipcmt8Pe0R98RMEds0
enable password 7 0822455D0A16544541
!
username cisco password 7 121A0C041104
!
!
no ip source-route
no ip finger
!
! inspection is configured per interface. Traffic traversing interfaces depends on
! the configuration of the outbound ACLs.
!
ip inspect audit-trail
! inspection for the admin, eng and mktng subnets. This traffic can go anywhere and
! is applied to traffic heading into the interface. Note: tcp inspection will
! account for TELNET traffic and HTTP which are single channel protocols. UDP
! inspection takes care of DNS.
ip inspect name CorpFIREin tcp
ip inspect name CorpFIREin udp
ip inspect name CorpFIREin ftp
ip inspect name CorpFIREin smtp
ip inspect name CorpFIREin realaudio
ip inspect name CorpFIREin h323
ip audit notify log
ip audit po max-events 100
!
! e0/0 is the server subnet. Note: no inspection rules are applied due to
! inspection on all the other interfaces. All other traffic accessing this network
! is therefore "secure".
!
interface Ethernet0/0
ip address 192.168.10.1 255.255.255.0
ip access-group 101 in ! no initiated traffic from the server subnet.
ip access-group 102 out ! allows SPECIFIC traffic to the server subnet.
no ip directed-broadcast
no ip proxy-arp
no cdp enable
!
! e0/1 is the administrative subnet.
!
interface Ethernet0/1
ip address 192.168.20.1 255.255.255.0
ip access-group 112 out ! allows web only to the Admin subnet.
no ip directed-broadcast
no ip proxy-arp
ip inspect CorpFIREin in ! firewall inspection applied to inbound traffic
! according to the Corp. security policy.
no cdp enable
!
! e1/0 is the engineering subnet.
!
interface Ethernet1/0
ip address 192.168.30.1 255.255.255.0
ip access-group 122 out ! allows SPECIFIC traffic to the Eng subnet.
no ip directed-broadcast
no ip proxy-arp
ip inspect CorpFIREin in ! firewall inspection applied to inbound traffic
! according to the Corp. security policy.
no cdp enable
!
! e1/1 is the marketing subnet.
!
interface Ethernet1/1
ip address 192.168.40.1 255.255.255.0
ip access-group 132 out ! allows SPECIFIC traffic to the Mkt subnet.
no ip directed-broadcast
no ip proxy-arp
ip inspect CorpFIREin in ! firewall inspection applied to inbound traffic
! according to the Corp. security policy.
no cdp enable
!
ip classless
!
logging facility syslog
logging 192.168.1.11
! acl 101 rejects any traffic initiated FROM the server subnet. It's applied
! to ther Server interface (e0/0) for inbound traffic.
access-list 101 deny ip any any
! acl 102 allows specific traffic to the Server subnet. It's applied to outbound
! traffic on the server interface. (e0/0)
access-list 102 permit tcp any any eq telnet
access-list 102 permit tcp any any eq ftp
access-list 102 permit udp any any
! acl 112 allows only HTTP traffic to the Admin subnet. (e0/1)
! Note: outbound ACLs 112, 122 and 132 setup implicit denies to the Admin., Eng.,
! and Mkt. interfaces. Firewall inspection inbound on the interfaces permits return
! traffic through the acls. This accounts for telnet, smtp, ftp, h323, dns, sql and
! realaudio for users within the Admin, Eng, and Mkt subnets. (see firewall
! policies)
access-list 112 permit tcp any any eq www
! acl 122 allows only specific traffic to the Eng. subnet. (e1/0)
access-list 122 permit tcp any any eq telnet
access-list 122 permit tcp any any eq ftp
access-list 122 permit tcp any any eq www
! acl 132 allows only specific traffic to the Mkt. Subnet. (e1/1)
access-list 132 permit tcp any any eq telnet
access-list 132 permit tcp any any eq ftp
access-list 132 permit udp any any
access-list 132 permit tcp any any eq www
access-list 132 permit tcp any any eq smtp
no cdp run
!
line con 0
exec-timeout 0 0
password 7 14141B180F0B
login
transport input none
line aux 0
line vty 0 4
password 7 045802150C2E
login
!
End
Branch Office Intranet Perimeter
Cisco IOS Firewall used as a branch office intranet firewall is shown in Figure 19.
Figure 19. Branch Office Intranet Firewall
• Static NAT is used with the private addresses
• Similar to the extranet perimeter firewall, but without the Auth Proxy
Branch Office Intranet Firewall Policy
The
protocols allowed on the branch office intranet firewall are Telnet,
FTP, and HTTP for both inside and outside traffic. Only the branch
office subnet, 200.2.4.0/24, is allowed inside.
• Only the subnet 200.2.4.0/24 is permitted for Telnet, FTP, and HTTP into inside.
• Inside has Telnet, FTP, and HTTP access to the branch office.
The
branch office policy limited not only protocols, but also the source
address of the network. This limit also restricts the types of attacks
that can be generated.
Note
the placement of this router relative to the corporate intranet
perimeter. This placement, along with the security policy defined per
department, helps to limit liabilities throughout the whole network.
This builds a layered security approach.
Branch Office Intranet Firewall Sample Configuration
FWRouter#sh run
Building configuration...
Current configuration:
!
version 12.0
service timestamps debug uptime
service timestamps log uptime
service password-encryption
!
hostname FWRouter
!
boot system flash
enable secret 5 $1$VZGK$Els0ipcmt8Pe0R98RMEds0
enable password 7 0822455D0A16544541
!
username cisco password 7 121A0C041104
!
!
!
!
no ip source-route
no ip finger
!
! Firewall inspection is setup for bi-directionally for traffic to/from the Corp.
! and Branch networks. Note: tcp inspection will account for TELNET traffic and
! HTTP which are single channel protocols.
!
!
ip inspect name BranchFIRE ftp
ip inspect name BranchFIRE tcp
ip audit notify log
ip audit po max-events 100
!
!
! e0/0 is the corporate subnet.
!
interface Ethernet0/0
ip address 192.168.1.3 255.255.255.0
ip access-group 101 in ! allows specific traffic from the corp subnet. Also implicitly
denies unwanted traffic to the branch.
no ip directed-broadcast
no ip proxy-arp
ip nat inside
ip inspect BranchFIRE in ! firewall inspection for traffic FROM the corp.
! subnet.
no cdp enable
!
! s0/0 is the serial interface to the branch network.
!
interface Serial0/0
ip address 200.2.2.1 255.255.255.0
ip access-group 121 in ! allows specific traffic from the branch office. Also implicitly
denies unwanted traffic to the corp. network.
no ip directed-broadcast
no ip proxy-arp
ip nat outside
ip inspect BranchFIRE in ! firewall inspection for traffic from the branch
! office.
no cdp enable
!
! Inside addresses 192.168.1.41-45 will be translated to 200.2.2.41-45 respectively.
! static nat is used as a simple example. Dynamic nat or a combination can be used
! depending on the address requirements.
!
ip nat inside source static 192.168.1.45 200.2.2.45
ip nat inside source static 192.168.1.44 200.2.2.44
ip nat inside source static 192.168.1.43 200.2.2.43
ip nat inside source static 192.168.1.42 200.2.2.42
ip nat inside source static 192.168.1.41 200.2.2.41
ip classless
!
! syslog is configured for reporting etc.
!
logging facility syslog
logging 192.168.1.11
! acl 101 allows the initial packets sourced from the Corporate subnet. Packets are
! then inspected by the firewall rules. The inspection engine allows for the
! dynamic acl to be built and added to the input acl associated with interface to
! the branch network (s0/0). Implicit denies prevent other unwanted traffic from
! traversing the router.
access-list 101 permit tcp 192.168.1.0 0.0.0.255 any eq telnet
access-list 101 permit tcp 192.168.1.0 0.0.0.255 any eq ftp
access-list 101 permit tcp 192.168.1.0 0.0.0.255 any eq www
! similar to acl 101, acl 121 allows the initial packets sourced from the Branch
! office to be inspected. The firewall engine thus allows return traffic to get
! through acl 101.
access-list 121 permit tcp 200.2.2.0 0.0.0.255 any eq telnet
access-list 121 permit tcp 200.2.2.0 0.0.0.255 any eq ftp
access-list 121 permit tcp 200.2.2.0 0.0.0.255 any eq www
no cdp run
!
line con 0
exec-timeout 0 0
password 7 0822455D0A16
login
transport input none
line aux 0
line vty 0 4
password 7 01100F175804
login
!
!
end
Extranet Perimeter
Cisco IOS Firewall used as a corporate extranet firewall is shown in Figure 20.
Figure 20. Extranet Perimeter Firewall
• Static NAT is used with the private addresses
• Auth Proxy will be used for inside access from business partner
Extranet Firewall Policy
Corporate
establishes tighter security control by authenticating the partners
before granting them access. Only one subnet, 200.3.4.0/24, is allowed
to enter. The partner allows Telnet and FTP to their network.
• Auth Proxy is enabled on serial 0/0 for inbound access
• Only the subnet 200.3.4.0/24 is permitted for Telnet and FTP
• Inside has Telnet and FTP access to the business partner
Much
like the branch office, the extranet security policy further limits
protocol access. It adds user-level authentication to further verify
access to these limited resources. Extranet policies are often the most
restrictive parts to a corporate network. Topology placement helps to
build the overall corporate security policy, as defined in the corporate
Internet perimeter router.
Extranet Firewall Sample Configuration
FWRouter#sh run
Building configuration...
Current configuration:
!
version 12.0
service timestamps debug uptime
service timestamps log uptime
service password-encryption
!
hostname FWRouter
!
boot system flash
!
! Enable authentication proxy globally.
!
aaa new-model
aaa authentication login default group tacacs+
aaa authentication login console_line line none
aaa authentication login vty_line line none
aaa authorization exec vty_line none
aaa authorization auth-proxy default group tacacs+
enable secret 5 $1$VZGK$Els0ipcmt8Pe0R98RMEds0
enable password 7 0822455D0A16544541
!
username cisco password 7 121A0C041104
!
!
!
!
no ip source-route
no ip finger
!
ip inspect name CorpFire ftp
ip inspect name CorpFire tcp
ip auth-proxy name PROXY http
ip audit notify log
ip audit po max-events 100
!
! e0/0 is the inside interface to the corporate subnet.
!
interface Ethernet0/0
ip address 192.168.1.4 255.255.255.0
ip access-group 101 in
no ip directed-broadcast
no ip proxy-arp
ip nat inside
ip inspect CorpFire in
no cdp enable
!
! s0/0 is the interface to the extranet partner.
!
interface Serial0/0
ip address 200.3.3.1 255.255.255.0
ip access-group 111 in
no ip directed-broadcast
no ip proxy-arp
ip nat outside
ip inspect CorpFire in
ip auth-proxy PROXY ! Associates authentication proxy on the interface to the
! extranet partner.
no cdp enable
!
! Inside addresses 192.168.1.31-35 will be translated to 200.3.3.31-35 respectively.
! static nat is used as a simple example. Dynamic nat or a combination can be used
! depending on the address requirements.
!
ip nat inside source static 192.168.1.35 200.3.3.35
ip nat inside source static 192.168.1.34 200.3.3.34
ip nat inside source static 192.168.1.33 200.3.3.33
ip nat inside source static 192.168.1.32 200.3.3.32
ip nat inside source static 192.168.1.31 200.3.3.31
ip classless
! configures the HTTP server to allow AAA authentication.
ip http server
ip http authentication aaa
ip http access-class 10
!
! sys log is configured for reporting etc.
!
logging facility syslog
logging 192.168.1.11
access-list 10 deny any
!
! acl 101 blocks all traffic except TACACS+ communications with the AAA server.
! With successful user authentication, Authentication Proxy will override this ACL
! and open connections based on the AAA ACL policy. This ACL is applied to the
! inbound on the inside interface (e0/0)
access-list 101 permit tcp host 192.168.1.12 eq tacacs host 192.168.1.4
access-list 101 permit tcp 192.168.1.0 0.0.0.255 any eq telnet
access-list 101 permit tcp 192.168.1.0 0.0.0.255 any eq ftp
!
! acl 111 denies all initiated traffic from the extranet partner that is not
! authenticated. Note: IOS Firewall's order of operations checks authentication
! before acls and inspection.!
access-list 111 deny ip any any
no cdp run
! configure the TACACS server's address and key.
tacacs-server host 192.168.1.12
tacacs-server key key
!
! The following is done in order to secure access to the router via the console
! before enter AAA configuration commands. note: AAA commands take effect before
! saving out of config mode.
!
line con 0
exec-timeout 0 0
password 7 0822455D0A16
login authentication console_line
transport input none
line aux 0
line vty 0 4
password 7 01100F175804
login authentication vty_line
!
!
end
Telecommuter/Home Office
Cisco IOS Firewall used as a home office perimeter firewall is shown in Figure 21.
Figure 21. Telecommuter/Home Office Firewall with VPN
• NAT overload/PAT is used with private addresses
• Cisco IOS Firewall is used to protect the home network
• An IPSec LAN-LAN tunnel is configured to the corporate network
Internet Firewall Policy
The
telecommuter is granted secure access to the corporate network, using
Inspect tunneling. Security to the home network is accomplished through
firewall inspection. The protocols that are allowed are TCP, UDP, RTSP,
H.323, NetShow, FTP, and SQLnet. There are no servers on the home
network, so no traffic is allowed that is initiated from outside. IPSec
tunneling secures the connection from the home LAN to the corporate
network.
Like
the Internet Firewall policy, HTTP need not be specified, as Java
blocking is not necessary. Specifying TCP inspection allows for
single-channel protocols like Telnet and HTTP. UDP is specified for DNS.
Telecommuter/Home Office Sample Configuration
!
hostname telecommuter-fwvpn
!
enable secret 0 xxxx
!
ip subnet-zero
no ip domain lookup
ip domain name telcom.com
ip name-server 210.110.100.1 ! Provides DNS to for the router
ip dhcp excluded-address 10.0.0.1 ! Excludes the router interface in the DHCP pool.
!
ip dhcp pool Client ! Provides DHCP assignments to all local workstations
import all
network 10.0.0.0 255.255.255.0
default-router 10.0.0.1
dns-server 210.110.100.1
domain-name telecom.com
!
! Firewall inspection is setup for all tcp and udp traffic as well as specific application
protocols as defined by the security policy.
ip inspect name firewall tcp
ip inspect name firewall udp
ip inspect name firewall rtsp
ip inspect name firewall h323
ip inspect name firewall netshow
ip inspect name firewall ftp
ip inspect name firewall sqlnet
!
crypto isakmp policy 1 ! defines the key association and authentication for ipsec tunnel.
hash md5
authentication pre-share
crypto isakmp key cisco123 address 200.1.1.1
!
!
crypto ipsec transform-set set1 esp-3des esp-md5-hmac ! defines encryption and transform
set for the ipsec tunnel.
!
crypto map to_corporate 1 ipsec-isakmp ! associates all crypto values and peering address
for the ipsec tunnel.
set peer 200.1.1.1
set transform-set set1
match address 105
!
!!
interface Ethernet0 ! e0 is the internal home network
ip address 10.1.1.1 255.255.255.0
ip nat inside
ip inspect firewall in ! inspection examines outbound traffic
no cdp enable
!
interface Ethernet1 ! e1 is the outside or internet exposed interface.
ip address 210.110.101.21 255.255.255.0
ip access-group 103 in ! acl 103 permits ipsec traffic from the corp. router as well as
denies internet initiated traffic inbound.
ip nat outside
no cdp enable
crypto map to_corporate ! applies the ipsec tunnel to the outside interface.
!
ip nat inside source list 102 interface Ethernet1 overload ! utilize nat overload in order
to make best use of the single address provided by the isp.
ip classless
ip route 0.0.0.0 0.0.0.0 210.110.101.1
no ip http server
!
!
! acl 102 associated addresses used for nat.
access-list 102 permit ip 10.1.1.0 0.0.0.255 any
! acl 103 defines traffic allowed from the peer for the ipsec tunnel.
access-list 103 permit udp host 200.1.1.1 any eq isakmp
access-list 103 permit udp host 200.1.1.1 eq isakmp any
access-list 103 permit esp host 200.1.1.1 any
access-list 103 permit icmp any any ! allow icmp for debugging but should be disabled due
to security implications.
access-list 103 deny ip any any ! prevents internet initiated traffic inbound.
! acl 105 matches addresses for the ipsec tunnel to/from the corporate network.
access-list 105 permit ip 10.1.1.0 0.0.0.255 192.168.0.0 0.0.255.255
no cdp run
!
line con 0
exec-timeout 120 0
stopbits 1
line vty 0 4
exec-timeout 0 0
password 0 xxxx
login local
end
GUIDELINES FOR SECURING CISCO IOS FIREWALL
As
with all networking devices, controlling access into the firewall is
essential. This section highlights some techniques that can secure
it. Password and privilege setup is described in "Configuring Passwords
and Privileges". Additionally, AAA can be used to set up user
authentication, authorization, and accounting (see Cisco documentation
for more information).
Access
• Keep the firewall in a secured (locked) room.
• Think about access control before connecting a console port to the network in any way, including attaching a modem to the port. Be aware that a break character on the console port might give total control of the firewall, even with access control configured.
•
Apply access lists and password protection to all virtual terminal
ports. Use access lists to restrict network addresses that are allowed
to Telnet into the router.
– When setting passwords for privileged access to the firewall, use the enable secret command rather than the enable password command, which does not have as strong an encryption algorithm.
– Put
a password on the console port. In AAA environments, use the same
authentication for the console as for elsewhere. In a non-AAA
environment, at a minimum configure the login and password commands.
• Configure the no ip proxy-arp
command to prevent internal addresses from being revealed. (This is
important to do if you do not already have NAT configured to prevent
internal addresses from being revealed.)
Services
Any
enabled service could present a potential security risk. A determined,
hostile party might be able to find creative ways to misuse the enabled
services to access the firewall or the network. Do not enable any local
service (such as SNMP or NTP) that will not be used.
• Cisco Discovery Protocol (CDP) is enabled by default. To turn off CDP, enter the no cdp run global configuration command, or apply no cdp enable on public or vulnerable interfaces if CDP is required for specific interfaces.
•
For local services that are enabled, protect against misuse by
configuring the services to communicate only with specific peers, and
protect by configuring access lists to deny packets for the services at
specific interfaces.
• Disable minor services. For IP, enter the no service tcp-small-servers and no service udp-small-servers global configuration commands.
•
If NTP is essential, configure it only on required interfaces to listen
only to certain peers, and use authenticated NTP if NTP poisoning is a
concern. To disable the NTP server on specific interfaces, enter the ntp disable interface configuration command on each interface where NTP will not be served.
•
Disable all HTTP services except those required for network function.
Service restriction is configured with "ip http active-session-modules"
command, as documented on Cisco.com: http://www.cisco.com/en/US/products/ps6350/products_configuration_guide_chapter09186a0080455929.html
Cisco
IOS offers two different options to "lock down" routers. AutoSecure is
available at the command line by typing "auto secure" at the router's
enable prompt, and Cisco Router and Security Device Manager (SDM) offers
the Router Security Audit tool in a GUI format. Both of these tools
can lead the network engineer through a dialogue to disable services
that may increase the router's vulnerability profile, set up stronger
authentication, and configure other settings to improve router security.
Routing and Spoofing
Cisco
IOS Software includes many features to prevent malicious application of
various IP features. These features can protect the router and the
network against activity that is trying to exploit an alternative route
around devices where network policy may be applied, or to:
•
Protect against spoofing: protect the networks on both sides of the
firewall from being spoofed from the other side. You could protect
against spoofing by configuring input access lists at all interfaces to
pass only traffic from expected source addresses, and to deny all other
traffic. Many network attacks (SYN attacks in particular) use a "bogus"
source address, originating from one of the reserved address ranges. The
reserved address space list is somewhat dynamic, as the Internet
Assigned Numbers Authority grants and revokes address space to Internet
network service providers. The list of reserved or unassigned IP
addresses is known as the "bogon" list. A current bogon list is
available at: http://www.cymru.org
•
IP Source Routing allows a traffic source to specify that a packet must
pass through certain hosts on the Internet on the way to its
destination. Source routing rarely has legitimate use on the public
Internet, and is more frequently used for network attack or abuse.
Under most circumstances, all routers in a network should have source
routing disabled. Disable source routing. For IP, enter the no ip source-route global configuration command.
Directed Broadcast
•
Directed broadcasts are rarely required by IP networks. Directed
broadcasts should be disabled for all applicable protocols on your
firewall and on all your other routers. To disable it for IP, use the no ip directed-broadcast command.
•
Directed broadcasts can be misused to multiply the power of DoS
attacks, because every DoS packet sent is broadcast to every host on a
subnet. Furthermore, some hosts have other intrinsic security risks
present when handling broadcasts.
CONFIGURING AND MANAGING THE CISCO IOS FIREWALL
Configuration
Most small to medium-sized networks can employ element-based
configuration and management, where a network engineer/administrator
individually configures and monitors devices on the network.
Element-based configuration and monitoring/management is accomplished
with the router's CLI, or with Cisco SDM if an easier-to-use graphical
interface is desired. The CLI and Cisco SDM can only configure one
device at a time, and do not provide any multiple-device correlation to
compile networkwide rules' impact on traffic.
As the number of devices on the network increases, determination of the network's policy for a given traffic flow becomes more complex. Larger networks usually benefit from application of network-based
configuration and management systems, such as Cisco Security Manager to
integrate networkwide provisioning, configuration, and management into
one system, and Cisco Security Monitoring, Analysis, and Response System
(MARS) for network monitoring and security event correlation.
Management and Monitoring
Cisco
IOS Firewall monitoring is accomplished by syslog messages for alarms,
and CLI interaction to view statistics of firewall activity.
Consult product documentation for relevant "show" and "debug" commands
for monitoring and troubleshooting IOS Firewall activity.
REFERENCES
• SDM 2.2 User's Guide , including several pages for configuring IOS Firewall with SDM
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