Troubleshooting Cisco IOS Firewall Configurations

Available Languages

Updated:August 15, 2006

Document ID:13897

Bias-Free Language

The documentation set for this product strives to use bias-free language. For the purposes of this documentation set, bias-free is defined as language that does not imply discrimination based on age, disability, gender, racial identity, ethnic identity, sexual orientation, socioeconomic status, and intersectionality. Exceptions may be present in the documentation due to language that is hardcoded in the user interfaces of the product software, language used based on RFP documentation, or language that is used by a referenced third-party product. Learn more about how Cisco is using Inclusive Language.

Introduction

Prerequisites

Requirements

Components Used

Conventions

Troubleshoot

Related Information

In order to reverse (remove) an access list, put a "no" in front of the access-group command in interface configuration mode:
```
int <interface>
no ip access-group # in|out
```
If too much traffic is denied, study the logic of your list or try to define an additional broader list, and then apply it instead. For example:
```
access-list # permit tcp any any
access-list # permit udp any any
access-list # permit icmp any any
int <interface>
ip access-group # in|out
```
The show ip access-lists command shows which access lists are applied and what traffic is denied by them. If you look at the packet count denied before and after the failed operation with the source and destination IP address, this number increases if the access list blocks traffic.
If the router is not heavily loaded, debugging can be done at a packet level on the extended or ip inspect access list. If the router is heavily loaded, traffic is slowed through the router. Use discretion with debugging commands.

Temporarily add the no ip route-cache command to the interface:
```
int <interface>
no ip route-cache
```
Then, in enable (but not config) mode:
```
term mon
debug ip packet # det
```
produces output similar to this:
```
*Mar 1 04:38:28.078: IP: s=10.31.1.161 (Serial0), d=171.68.118.100 (Ethernet0), 
   g=10.31.1.21, len 100, forward
*Mar 1 04:38:28.086: IP: s=171.68.118.100 (Ethernet0), d=9.9.9.9 (Serial0), g=9.9.9.9, 
   len 100, forward
```

Extended access lists can also be used with the "log" option at the end of the various statements:

access-list 101 deny ip host 171.68.118.100 host 10.31.1.161 log
access-list 101 permit ip any any

You therefore see messages on the screen for permitted and denied traffic:

*Mar 1 04:44:19.446: %SEC-6-IPACCESSLOGDP: list 111 permitted icmp 171.68.118.100 
   -> 10.31.1.161 (0/0), 15 packets
*Mar  1 03:27:13.295: %SEC-6-IPACCESSLOGP: list 118 denied tcp 171.68.118.100(0) 
   -> 10.31.1.161(0), 1 packet

If the ip inspect list is suspect, the debug ip inspect <type_of_traffic> command produces output such as this output:

Feb 14 12:41:17 10.31.1.52 56: 3d05h: CBAC* sis 258488 pak 16D0DC TCP P ack 3195751223 
   seq 3659219376(2) (10.31.1.5:11109) => (12.34.56.79:23)
Feb 14 12:41:17 10.31.1.52 57: 3d05h: CBAC* sis 258488 pak 17CE30 TCP P ack 3659219378 
   seq 3195751223(12) (10.31.1.5:11109) <= (12.34.56.79:23)

For these commands, along with other troubleshooting information, refer to Troubleshooting Authentication Proxy.