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For more information regarding firewalls, refer to the chapter "Cisco IOS Firewall Overview."
For a complete description of the CBAC commands used in this chapter, refer to the "Context-Based Access Control Commands" chapter in the Security Command Reference. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.
This section describes:
CBAC intelligently filters TCP and UDP packets based on application-layer protocol session information and can be used for intranets, extranets and the Internet. You can configure CBAC to permit specified TCP and UDP traffic through a firewall only when the connection is initiated from within the network you want to protect. (In other words, CBAC can inspect traffic for sessions that originate from the external network.) However, while this example discusses inspecting traffic for sessions that originate from the external network, CBAC can inspect traffic for sessions that originate from either side of the firewall.
Without CBAC, traffic filtering is limited to access list implementations that examine packets at the network layer, or at most, the transport layer. However, CBAC examines not only network layer and transport layer information but also examines the application-layer protocol information (such as FTP connection information) to learn about the state of the TCP or UDP session. This allows support of protocols that involve multiple channels created as a result of negotiations in the control channel. Most of the multimedia protocols as well as some other protocols (such as FTP, RPC, and SQL*Net) involve multiple channels.
CBAC inspects traffic that travels through the firewall to discover and manage state information for TCP and UDP sessions. This state information is used to create temporary openings in the firewall's access lists to allow return traffic and additional data connections for permissible sessions (sessions that originated from within the protected internal network).
CBAC also provides the following benefits:
CBAC does not protect against attacks originating from within the protected network. CBAC only detects and protects against attacks that travel through the firewall.
CBAC protects against certain attacks but should not be considered a perfect, impenetrable defense. Determined, skilled attackers might be able to launch effective attacks. While there is no such thing as a perfect defense, CBAC detects and prevents most of the popular attacks on your network.
You should understand the material in this section before you configure CBAC. If you do not understand how CBAC works, you might inadvertently introduce security risks by configuring CBAC inappropriately.
In Figure 9, the inbound access lists at S0 and S1 are configured to block Telnet traffic, and there is no outbound access list configured at E0. When the connection request for User1's Telnet session passes through the firewall, CBAC creates a temporary opening in the inbound access list at S0 to permit returning Telnet traffic for User1's Telnet session. (If the same access list is applied to both S0 and S1, the same opening would appear at both interfaces.) If necessary, CBAC would also have created a similar opening in an outbound access list at E0 to permit return traffic.
This section describes how CBAC inspects packets and maintains state information about sessions to provide intelligent filtering.
Packets entering the firewall are inspected by CBAC only if they first pass the inbound access list at the interface. If a packet is denied by the access list, the packet is simply dropped and not inspected by CBAC.
CBAC inspects and monitors only the control channels of connections; the data channels are not inspected. For example, during FTP sessions both the control and data channels (which are created when a data file is transferred) are monitored for state changes, but only the control channel is inspected (that is, the CBAC software parses the FTP commands and responses).
CBAC inspection recognizes application-specific commands in the control channel, and detects and prevents certain application-level attacks.
Return traffic will only be permitted back through the firewall if the state table contains information indicating that the packet belongs to a permissible session. Inspection controls the traffic that belongs to a valid session and forwards the traffic it does not know. When return traffic is inspected, the state table information is updated as necessary.
The temporary access list entries are never saved to NVRAM.
Use CBAC when the firewall will be passing traffic such as:
Use CBAC for these applications if you want the application's traffic to be permitted through the firewall only when the traffic session is initiated from a particular side of the firewall (usually from the protected internal network).
In many cases, you will configure CBAC in one direction only at a single interface, which causes traffic to be permitted back into the internal network only if the traffic is part of a permissible (valid, existing) session.
In rare cases, you might want to configure CBAC in two directions at one or more interface, which is a more complex solution. CBAC is usually only configured in two directions when the networks on both sides of the firewall should be protected, such as with extranet or intranet configurations. For example, if the firewall is situated between two partner companies' networks, you might wish to restrict traffic in one direction for certain applications, and restrict traffic in the other direction for other applications.
In this example, a TCP packet exits the internal network through the firewall's external interface. The TCP packet is the first packet of a Telnet session, and Telnet is configured for CBAC inspection.
1. The packet reaches the firewall's external interface.
2. The packet is evaluated against the interface's existing outbound access list, and the packet is permitted. (A denied packet would simply be dropped at this point.)
3. The packet is inspected by CBAC to determine and record information about the state of the packet's connection. This information is recorded in a new state table entry created for the new connection.
(If the packet's application---Telnet---was not configured for CBAC inspection, the packet would simply be forwarded out the interface at this point without being inspected by CBAC. See the section "Define an Inspection Rule" for configuring CBAC inspection information.)
4. Based on the obtained state information, CBAC creates a temporary access list entry which is inserted at the beginning of the external interface's inbound extended access list. This temporary access list entry is designed to permit inbound packets that are part of the same connection as the outbound packet just inspected.
5. The outbound packet is forwarded out the interface.
6. Later, an inbound packet reaches the interface. This packet is part of the same Telnet connection previously established with the outbound packet. The inbound packet is evaluated against the inbound access list, and it is permitted because of the temporary access list entry previously created.
7. The permitted inbound packet is inspected by CBAC, and the connection's state table entry is updated as necessary. Based on the updated state information, the inbound extended access list temporary entries might be modified in order to permit only packets that are valid for the current state of the connection.
8. Any additional inbound or outbound packets that belong to the connection are inspected to update the state table entry and to modify the temporary inbound access list entries as required, and they are forwarded through the interface.
9. When the connection terminates or times out, the connection's state table entry is deleted, and the connection's temporary inbound access list entries are deleted.
In the sample process just described, the firewall access lists are configured as follows:
You can configure CBAC to inspect the following types of sessions:
You can also configure CBAC to specifically inspect certain application-layer protocols. The following application-layer protocols can all be configured for CBAC:
When a protocol is configured for CBAC, the protocol's traffic will be inspected, state information will be maintained, and in general, packets will be allowed back through the firewall only if they belong to a permissible session.
You can use CBAC together with all the other firewall features mentioned previously in the "Cisco IOS Firewall Overview" chapter.
CBAC works with fast switching and process switching.
If you reconfigure your access lists when you configure CBAC, be aware that if your access lists block TFTP traffic into an interface, you will not be able to netboot over that interface. (This is not a CBAC-specific limitation, but is part of existing access list functionality.)
Packets with the firewall as the source or destination address are not inspected by CBAC or evaluated by access lists.
CBAC ignores ICMP Unreachable messages.
With FTP, CBAC does not allow third-party connections (three-way FTP transfer).
When CBAC inspects FTP traffic, it only allows data channels with the destination port in the range of 1024 to 65535.
CBAC will not open a data channel if the FTP client-server authentication fails.
Also, if both encryption and CBAC are configured at the same firewall, CBAC will not work for certain protocols. In this case, CBAC will work with single-channel TCP and UDP, except for Java and SMTP. But CBAC will not work with multichannel protocols, except for StreamWorks and CU-SeeMe. So if you configure encryption at the firewall, you should configure CBAC for only the following protocols:
If the router is not an IPSec endpoint, but the packet is an IPSec packet, then CBAC will not inspect the packets because the protocol number in the IP header of the IPSec packet is not TCP or UDP. CBAC only inspects UDP and TCP packets.
Sometimes CBAC must evaluate long access lists, which might have presented a negative impact to performance. However, this impact is avoided, because CBAC evaluates access lists using an accelerated method (CBAC hashes access lists and evaluates the hash).
To configure CBAC, complete the tasks described in the following sections:
You can also perform the tasks described in the following sections. These tasks are optional.
For CBAC configuration examples, refer to the "CBAC Configuration Example" section located at the end of this chapter.
You must decide whether to configure CBAC on an internal or external interface of your firewall.
"Internal" refers to the side where sessions must originate for their traffic to be permitted through the firewall. "External" refers to the side where sessions cannot originate (sessions originating from the external side will be blocked).
If you will be configuring CBAC in two directions, you should configure CBAC in one direction first, using the appropriate "internal" and "external" interface designations. When you configure CBAC in the other direction, the interface designations will be swapped. (CBAC is rarely configured in two directions, and usually only when the firewall is between two networks that need protection from each other, such as with two partners' networks connected by the firewall.)
The firewall is most commonly used with one of two basic network topologies. Determining which of these topologies is most like your own can help you decide whether to configure CBAC on an internal interface or on an external interface.
The first topology is shown in Figure 10. In this simple topology, CBAC is configured for the external interface Serial 1. This prevents specified protocol traffic from entering the firewall and the internal network, unless the traffic is part of a session initiated from within the internal network.
---CBAC Configured at the External Interface
The second topology is shown in Figure 11. In this topology, CBAC is configured for the internal interface Ethernet 0. This allows external traffic to access the services in the Demilitarized Zone (DMZ), such as DNS services, but prevents specified protocol traffic from entering your internal network---unless the traffic is part of a session initiated from within the internal network.
---CBAC Configured at the Internal Interface
Using these two sample topologies, decide whether to configure CBAC on an internal or external interface.
Follow these two general rules when evaluating your IP access lists at the firewall:
Here are some tips for your access lists when you will be configuring CBAC on an external interface:
Here are some tips for your access lists when you will be configuring CBAC on an internal interface:
You can use the default timeout and threshold values, or you can change to values more suitable to your security requirements. You should make any changes to the timeout and threshold values before you continue configuring CBAC. Note that if you want to enable the more aggressive TCP host-specific denial-of-service prevention that includes the blocking of connection initiation to a host, you must set the block-time specified in the ip inspect tcp max-incomplete host command (see the last row in the following table).
All the available CBAC timeouts and thresholds are listed in the following table, along with the corresponding command and default value.
| Timeout or Threshold Value to Change | Command | Default |
|---|---|---|
The length of time the software waits for a TCP session to reach the established state before dropping the session. | 30 seconds | |
The length of time a TCP session will still be managed after the firewall detects a FIN-exchange. | 5 seconds | |
The length of time a TCP session will still be managed after no activity (the TCP idle timeout).1 | 3600 seconds (1 hour) | |
The length of time a UDP session will still be managed after no activity (the UDP idle timeout).1 | 30 seconds | |
The length of time a DNS name lookup session will still be managed after no activity. | 5 seconds | |
The number of existing half-open sessions that will cause the software to start deleting half-open sessions.2 | 500 existing half-open sessions | |
The number of existing half-open sessions that will cause the software to stop deleting half-open sessions.2 | 400 existing half-open sessions | |
The rate of new unestablished sessions that will cause the software to start deleting half-open sessions.2 | 500 half-open sessions per minute | |
The rate of new unestablished sessions that will cause the software to stop deleting half-open sessions.2 | 400 half-open sessions per minute | |
The number of existing half-open TCP sessions with the same destination host address that will cause the software to start dropping half-open sessions to the same destination host address.3 | ip inspect tcp max-incomplete host number block-time minutes | 50 existing half-open TCP sessions; 0 minutes |
| 1The global TCP and UDP idle timeouts can be overridden for specified application-layer protocols' sessions as described in the ip inspect name (global configuration) command description, found in the "Context-Based Access Control Commands" chapter of the Security Command Reference. 2See the following section, "Half-Open Sessions," for more information. 3Whenever the max-incomplete host threshold is exceeded, the software will drop half-open sessions differently depending on whether the block-time timeout is zero or a positive non-zero number. If the block-time timeout is zero, the software will delete the oldest existing half-open session for the host for every new connection request to the host and will let the SYN packet through. If the block-time timeout is greater than zero, the software will delete all existing half-open sessions for the host, and then block all new connection requests to the host. The software will continue to block all new connection requests until the block-time expires. |
To return any threshold or timeout to the default value, use the no form of the command in the preceding table.
CBAC measures both the total number of existing half-open sessions and the rate of session establishment attempts. Both TCP and UDP half-open sessions are counted in the total number and rate measurements. Measurements are made once a minute.
When the number of existing half-open sessions rises above a threshold (the max-incomplete high number), the software will delete half-open sessions as required to accommodate new connection requests. The software will continue to delete half-open requests as necessary, until the number of existing half-open sessions drops below another threshold (the max-incomplete low number).
When the rate of new connection attempts rises above a threshold (the one-minute high number), the software will delete half-open sessions as required to accommodate new connection attempts. The software will continue to delete half-open sessions as necessary, until the rate of new connection attempts drops below another threshold (the one-minute low number). The rate thresholds are measured as the number of new session connection attempts detected in the last one-minute sample period. (The rate is calculated as an exponentially-decayed rate.)
An inspection rule should specify each desired application-layer protocol as well as generic TCP or generic UDP if desired. The inspection rule consists of a series of statements each listing a protocol and specifying the same inspection rule name.
To define an inspection rule, follow the instructions in the following sections:
| Command | Purpose |
|---|---|
Configure CBAC inspection for an application-layer protocol (except for RPC and Java). Use one of the protocol keywords defined in Table 18, following. Repeat this command for each desired protocol. Use the same inspection-name to create a single inspection rule. | |
ip inspect name inspection-name rpc program-number number [wait-time minutes] [timeout seconds] | Enable CBAC inspection for the RPC application-layer protocol. You can specify multiple RPC program numbers by repeating this command for each program number. Use the same inspection-name to create a single inspection rule. |
Refer to the description of the ip inspect name (global configuration) command in the "Context-Based Access Control Commands" chapter in the Security Command Reference for complete information about how the command works with each application-layer protocol.
To enable CBAC inspection for Java, see the following section, "Configure Java Inspection."
Table 18 identifies application protocol keywords.
| Application Protocol | protocol Keyword |
|---|---|
CU-SeeMe | cuseeme |
FTP | ftp |
H.323 | h323 |
UNIX R commands (rlogin, rexec, rsh) | rcmd |
RealAudio | realaudio |
SMTP | smtp |
SQL*Net | sqlnet |
StreamWorks | streamworks |
TFTP | tftp |
VDOLive | vdolive |
Java applet filtering distinguishes between trusted and untrusted applets by relying on a list of external sites that you designate as "friendly." If an applet is from a friendly site, the firewall allows the applet through. If the applet is not from a friendly site, the applet will be blocked. (Alternately, you could permit applets from all external sites except for those you specifically designate as hostile.)
| Step | Command | Purpose | ||
|---|---|---|---|---|
| ip access-list standard name or access-list access-list-number {deny | permit} source [source-wildcard] | Create a standard access list that permits traffic only from friendly sites, and denies traffic from hostile sites. If you want all internal users to be able to download friendly applets, use the any keyword for the destination as appropriate---but be careful to not misuse the any keyword to inadvertently allow all applets through. | ||
| ip inspect name inspection-name http [java-list access-list] [timeout seconds] | Block all Java applets except for applets from the friendly sites defined previously in the access list. Java blocking only works with standard access lists. Use the same inspection-name as when you specified other protocols, to create a single inspection rule. |
Any application-layer protocol that is inspected will take precedence over the TCP or UDP packet inspection. For example, if inspection is configured for FTP, all control channel information will be recorded in the state table, and all FTP traffic will be permitted back through the firewall if the control channel information is valid for the state of the FTP session. The fact that TCP inspection is configured is irrelevant to the FTP state information.
With TCP and UDP inspection, packets entering the network must exactly match the corresponding packet that previously exited the network. The entering packets must have the same source/destination addresses and source/destination port numbers as the exiting packet (but reversed); otherwise, the entering packets will be blocked at the interface. Also, all TCP packets with a sequence number outside of the window are dropped.
With UDP inspection configured, replies will only be permitted back in through the firewall if they are received within a configurable time after the last request was sent out. (This time is configured with the ip inspect udp idle-time command.)
To configure CBAC inspection for TCP or UDP packets, use one or both of the following global configuration commands:
| Command | Purpose |
|---|---|
ip inspect name inspection-name tcp [timeout seconds] | Enable CBAC inspection for TCP packets. Use the same inspection-name as when you specified other protocols, to create a single inspection rule. |
ip inspect name inspection-name udp [timeout seconds] | Enable CBAC inspection for UDP packets. Use the same inspection-name as when you specified other protocols, to create a single inspection rule. |
After you define an inspection rule, you apply this rule to an interface.
Normally, you apply only one inspection rule to one interface. The only exception might occur if you want to enable CBAC in two directions as described earlier in the section "When and Where to Configure CBAC." For CBAC configured in both directions at a single firewall interface, you should apply two rules, one for each direction.
If you are configuring CBAC on an external interface, apply the rule to outbound traffic.
If you are configuring CBAC on an internal interface, apply the rule to inbound traffic.
To apply an inspection rule to an interface, use the following interface configuration command:
| Command | Purpose |
|---|---|
Apply an inspection rule to an interface. |
You can view certain CBAC information by using one or more of the following EXEC commands:
| Command | Purpose |
|---|---|
show ip inspect name inspection-name | |
show ip inspect config | |
show ip inspect interfaces | Show interface configuration with regards to applied inspection rules and access lists. |
show ip inspect session [detail] | Show existing sessions that are currently being tracked and inspected by CBAC. |
show ip inspect all | Show all CBAC configuration and all existing sessions that are currently being tracked and inspected by CBAC. |
To turn on audit trail messages, use the following global configuration command:
| Command | Purpose |
|---|---|
Turn on CBAC audit trail messages. |
If required, you can also use the CBAC debug commands listed in this section. (Debugging can be turned off for each of the commands in this section by using the no form of the command. To disable all debugging, use the privileged EXEC commands no debug all or undebug all.)
The available debug commands are listed in the following categories:
For a complete description of the debug commands, refer to the Debug Command Reference.
You can use the following generic debug commands, entered in privileged EXEC mode:
| Command | Purpose |
|---|---|
debug ip inspect function-trace | Display messages about software functions called by CBAC. |
debug ip inspect object-creation | Display messages about software objects being created by CBAC. Object creation corresponds to the beginning of CBAC-inspected sessions. |
debug ip inspect object-deletion | Display messages about software objects being deleted by CBAC. Object deletion corresponds to the closing of CBAC-inspected sessions. |
debug ip inspect events | Display messages about CBAC software events, including information about CBAC packet processing. |
debug ip inspect timers | Display messages about CBAC timer events such as when a CBAC idle timeout is reached. |
debug ip inspect detail | Enable the detailed option, which can be used in combination with other options to get additional information. |
You can use the following transport-level debug commands, entered in privileged EXEC mode:
| Command | Purpose |
|---|---|
debug ip inspect tcp | Display messages about CBAC-inspected TCP events, including details about TCP packets. |
debug ip inspect udp | Display messages about CBAC-inspected UDP events, including details about UDP packets. |
You can use the following application protocol debug command, entered in privileged EXEC mode:
| Command | Purpose |
|---|---|
debug ip inspect protocol | Display messages about CBAC-inspected protocol events, including details about the protocol's packets. Refer to Table 19 to determine the protocol keyword. |
Table 19 identifies application protocol keywords for the debug ip inspect command.
| Application Protocol | protocol keyword |
|---|---|
CU-SeeMe | cuseeme |
FTP commands and responses | ftp-cmd |
FTP tokens (enables tracing of the FTP tokens parsed) | ftp-tokens |
H.323 | h323 |
Java applets | http |
UNIX R commands (rlogin, rexec, rsh) | rcmd |
RealAudio | realaudio |
RPC | rpc |
SMTP | smtp |
SQL*Net | sqlnet |
StreamWorks | streamworks |
TFTP | tftp |
VDOLive | vdolive |
CBAC provides syslog messages, console alert messages and audit trail messages. These messages are useful because they can alert you to network attacks and because they provide an audit trail that provides details about sessions inspected by CBAC. While they are generally referred to as error messages, not all error messages indicate problems with your system.
The following types of error messages can be generated by CBAC:
For explanations and recommended actions related to the error messages mentioned in this chapter, refer to the Cisco IOS Software System Error Messages.
%FW-4-ALERT_ON: getting aggressive, count (550/500) current 1-min rate: 250 %FW-4-ALERT_OFF: calming down, count (0/400) current 1-min rate: 0
When %FW-4-ALERT_ON and %FW-4-ALERT_OFF error messages appear together, each "aggressive/calming" pair of messages indicates a separate attack. The above example shows one separate attack.
Error messages such as the following may indicate that a denial-of-service attack has occurred on a specific TCP host:
%FW-4-HOST_TCP_ALERT_ON: Max tcp half-open connections (50) exceeded for host 172.21.127.242. %FW-4-BLOCK_HOST: Blocking new TCP connections to host 172.21.127.242 for 2 minutes (half-open count 50 exceeded) %FW-4-UNBLOCK_HOST: New TCP connections to host 172.21.127.242 no longer blocked
%FW-4-SMTP_INVALID_COMMAND: Invalid SMTP command from initiator (192.168.12.3:52419)
%FW-4-HTTP_JAVA_BLOCK: JAVA applet is blocked from (172.21.127.218:80) to (172.16.57.30:44673).
%FW-3-FTP_PRIV_PORT: Privileged port 1000 used in PORT command -- FTP client 10.0.0.1 FTP server 10.1.0.1 %FW-3-FTP_SESSION_NOT_AUTHENTICATED: Command issued before the session is authenticated -- FTP client 10.0.0.1 %FW-3-FTP_NON_MATCHING_IP_ADDR: Non-matching address 172.19.148.154 used in PORT command -- FTP client 172.19.54.143 FTP server 172.16.127.242
%FW-6-SESS_AUDIT_TRAIL: tcp session initiator (192.168.1.13:33192) sent 22 bytes -- responder (192.168.129.11:25) sent 208 bytes %FW-6-SESS_AUDIT_TRAIL: http session initiator (172.16.57.30:44673) sent 1599 bytes -- responder (172.21.127.218:80) sent 93124 bytes
You can turn off CBAC, with the no ip inspect global configuration command.
In most situations, turning off CBAC has no negative security impact because CBAC creates "permit" access lists. Without CBAC configured, no "permit" access lists are maintained. Therefore, no derived traffic (returning traffic or traffic from the data channels) can go through the firewall. The exception is SMTP and Java blocking. With CBAC turned off, unacceptable SMTP commands or Java applets may go through the firewall.
The firewall has two interfaces configured:
!---------------------------------------------------------------------- ! This first section contains some configuration that is not required for CBAC, ! but illustrates good security practices. Note that there are no services ! on the Ethernet side. Email is picked up via POP from a server on the corporate ! side. !---------------------------------------------------------------------- ! version 11.2 ! ! The following three commands should appear in almost every config ! service password-encryption service udp-small-servers no service tcp-small-servers ! hostname fred-examplecorp-fr ! boot system flash c1600-fw1600-l enable secret 5 <elided> ! username fred password <elided> ip subnet-zero no ip source-route ip domain-name example.com ip name-server 172.19.2.132 ip name-server 198.92.30.32 ! ! !---------------------------------------------------------------------- !The next section includes configuration required specifically for CBAC !---------------------------------------------------------------------- ! !The following commands define the inspection rule "myfw", allowing ! the specified protocols to be inspected. Note that Java applets will be permitted ! according to access list 51, defined later in this configuration. ! ip inspect name myfw cuseeme timeout 3600 ip inspect name myfw ftp timeout 3600 ip inspect name myfw http java-list 51 timeout 3600 ip inspect name myfw rcmd timeout 3600 ip inspect name myfw realaudio timeout 3600 ip inspect name myfw smtp timeout 3600 ip inspect name myfw tftp timeout 30 ip inspect name myfw udp timeout 15 ip inspect name myfw tcp timeout 3600 ! !The following interface configuration applies the "myfw" inspection rule to ! inbound traffic at Ethernet 0. Since this interface is on the internal network ! side of the firewall, traffic entering Ethernet 0 is actually exiting the ! internal network. !Applying the inspection rule to this interface causes inbound traffic (which is ! exiting the network) to be inspected; return traffic will only be permitted back ! through the firewall if part of a session which began from within the network. !Also note that access list 101 is applied to inbound traffic at Ethernet 0. ! Any traffic that passes the access list will be inspected by CBAC. ! (Traffic blocked by the access list will not be inspected.) ! interface Ethernet0 description ExampleCorp Ethernet chez fred ip address 172.19.139.1 255.255.255.248 ip broadcast-address 172.19.131.7 no ip directed-broadcast no ip proxy-arp ip inspect myfw in ip access-group 101 in no ip route-cache no cdp enable ! interface Serial0 description Frame Relay (Telco ID 22RTQQ062438-001) to ExampleCorp HQ no ip address ip broadcast-address 0.0.0.0 encapsulation frame-relay IETF no ip route-cache no arp frame-relay bandwidth 56 service-module 56k clock source line service-module 56k network-type dds frame-relay lmi-type ansi ! !Note that the following interface configuration applies access list 111 to ! inbound traffic at the external serial interface. (Inbound traffic is ! entering the network.) When CBAC inspection occurs on traffic exiting the ! network, temporary openings will be added to access list 111 to allow returning ! traffic that is part of existing sessions. ! interface Serial0.1 point-to-point ip unnumbered Ethernet0 ip access-group 111 in no ip route-cache bandwidth 56 no cdp enable frame-relay interface-dlci 16 ! ip classless ip route 0.0.0.0 0.0.0.0 Serial0.1 ! !The following access list defines "friendly" and "hostile" sites for Java ! applet blocking. Because Java applet blocking is defined in the inspection ! rule "myfw" and references access list 51, applets will be actively denied ! if they are from any of the "deny" addresses and allowed only if they are from ! either of the two "permit" networks. ! access-list 51 deny 172.19.1.203 access-list 51 deny 172.19.2.147 access-list 51 permit 172.18.0.0 0.1.255.255 access-list 51 permit 192.168.1.0 0.0.0.255 access-list 51 deny any ! !The following access list 101 is applied to interface Ethernet 0 above. ! This access list permits all traffic that should be CBAC inspected, and also ! provides anti-spoofing. The access list is deliberately set up to deny unknown ! IP protocols, because no such unknown protocols will be in legitimate use. ! access-list 101 permit tcp 172.19.139.0 0.0.0.7 any access-list 101 permit udp 172.19.139.0 0.0.0.7 any access-list 101 permit icmp 172.19.139.0 0.0.0.7 any access-list 101 deny ip any any ! !The following access list 111 is applied to interface Serial 0.1 above. ! This access list filters traffic coming in from the external side. When ! CBAC inspection occurs, temporary openings will be added to the beginning of ! this access list to allow return traffic back into the internal network. !This access list should restrict traffic that will be inspected by ! CBAC. (Remember that CBAC will open holes as necessary to permit returning traffic.) !Comments precede each access list entry. These entries aren't all specifically related ! to CBAC, but are created to provide general good security. ! ! Anti-spoofing. access-list 111 deny ip 172.19.139.0 0.0.0.7 any ! Port 22 is SSH... encrypted, RSA-authenticated remote login. Can be used to get to
! field office host from ExampleCorp headquarters. access-list 111 permit tcp any host 172.19.139.2 eq 22 ! Sometimes EIGRP is run on the Frame Relay link. When you use an ! input access list, you have to explicitly allow even control traffic. ! This could be more restrictive, but there would have to be entries ! for the EIGRP multicast as well as for the office's own unicast address. access-list 111 permit igrp any any ! These are the ICMP types actually used... ! administratively-prohibited is useful when you're trying to figure out why ! you can't reach something you think you should be able to reach. access-list 111 permit icmp any 172.19.139.0 0.0.0.7 administratively-prohibited ! This allows network admins at headquarters to ping hosts at the field office: access-list 111 permit icmp any 172.19.139.0 0.0.0.7 echo ! This allows the field office to do outgoing pings access-list 111 permit icmp any 172.19.139.0 0.0.0.7 echo-reply ! Path MTU discovery requires too-big messages access-list 111 permit icmp any 172.19.139.0 0.0.0.7 packet-too-big ! Outgoing traceroute requires time-exceeded messages to come back access-list 111 permit icmp any 172.19.139.0 0.0.0.7 time-exceeded ! Incoming traceroute access-list 111 permit icmp any 172.19.139.0 0.0.0.7 traceroute ! Permits all unreachables because if you are trying to debug ! things from the remote office, you want to see them. If nobody ever did ! any debugging from the network, it would be more appropriate to permit only ! port unreachables or no unreachables at all. access-list 111 permit icmp any 172.19.139.0 0.0.0.7 unreachable ! These next two entries permit users on most ExampleCorp networks to telnet to ! a host in the field office. This is for remote administration by the network admins. access-list 111 permit tcp 172.18.0.0 0.1.255.255 host 172.19.139.1 eq telnet access-list 111 permit tcp 192.168.1.0 0.0.0.255 host 172.19.139.1 eq telnet ! Final deny for explicitness access-list 111 deny ip any any ! no cdp run snmp-server community <elided> RO ! line con 0 exec-timeout 0 0 password <elided> login local line vty 0 exec-timeout 0 0 password <elided> login local length 35 line vty 1 exec-timeout 0 0 password 7 <elided> login local line vty 2 exec-timeout 0 0 password 7 <elided> login local line vty 3 exec-timeout 0 0 password 7 <elided> login local line vty 4 exec-timeout 0 0 password 7 <elided> login local ! scheduler interval 500 end
Posted: Tue May 4 07:52:29 PDT 1999
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