Monitoring Network Access Control

The show commands provided by Cisco ASA are extremely useful in checking the health and status of the hardware and in isolating network-related issues. The necessary show commands to manage network access control are discussed in the following two sections.

Monitoring ACLs

Cisco ASA provides the show access-list command to determine if the packets are passing through the configured ACLs. When a packet is matched against an ACE, the security appliance increments the hitcnt (hit count) counter by one. This is useful when you want to know if traffic is hitting a configured ACE. It is also useful to check if packets are allowed or denied if there is a new virus outbreak in the network that might be sending traffic from spoofed addresses. Example 5-28 shows the configuration of an ACL called outside_in. If object groups are used and the show access-list outside_in command is executed, Cisco ASA displays all the ACEs that are otherwise grouped into protocols, networks, and services. As evident from this example, the security appliance processed 1009 packets that were denied and logged by the ACE.

Example 5-28. Output of show access-list outside_in

Chicago(config)# show running-config access-list outside_in

access-list outside_in remark ACL to block inbound traffic on the outside interface

access-list outside_in extended permit tcp any object-group DMZ_Web_Servers eq www

access-list outside_in extended permit tcp any object-group DMZ_Email_Servers eq smtp

access-list outside_in extended deny ip any any log


Chicago(config)# show access-list outside_in

access-list outside_in; 6 elements

access-list outside_in line 1 remark ACL to block inbound traffic on the outside


access-list outside_in line 2 extended permit tcp any object-group DMZ_Web_Servers

eq www

access-list outside_in line 2 extended permit tcp any host eq www


access-list outside_in line 2 extended permit tcp any host eq www


access-list outside_in line 2 extended permit tcp any host eq www


access-list outside_in line 3 extended permit tcp any object-group DMZ_Email_Servers

eq smtp

access-list outside_in line 3 extended permit tcp any host eq smtp


access-list outside_in line 3 extended permit tcp any host eq smtp


access-list outside_in line 4 extended deny ip any any log informational interval

300 (hitcnt=1009)

To reset the hit-count counters, you can issue the clear access-list <ACL_name> counters command, as shown in Example 5-29, in which the counters for the outside_in ACL are being cleared.

Example 5-29. Resetting Hit-Count Counters with clear access-list counters

Chicago(config)# clear access-list outside_in counters

If a UDP, TCP, or, optionally an ICMP packet is allowed to pass through the security appliance, a connection entry is created, which can be shown by using the show conn command, as displayed in Example 5-30. The first column of the connection entry displays the protocol used followed by "out" to indicate IP address of the outside host and then "in" to display the inside hosts' IP addresses. It also shows the source and destination Layer 4 ports. The security appliance shows the idle timer per connection in hours, minutes, and seconds. The most important information to look at is the flags counter, which has the information about the current state of the connection. Table 5-11 lists and describes all the flags. The TCP entry has flags set to "UIO" to indicate that the connection is up and is passing traffic in both inbound and outbound directions.

Table 5-11. Description of Flags in the show conn Command Output






Awaiting outside ACK to SYN


Awaiting inside ACK to SYN


Initial SYN from outside


Computer Telephony Interface Quick Buffer Encoding (CTIQBE) media connection






Outside back connection


Inside FIN


Outside FIN


Media Gateway Control Protocol (MGCP) connection


Connection is part of a group


H.225 packet


H.323 packet


Incomplete TCP or UDP connection


Inbound data


Skinny Client Control Protocol (SCCP) media connection


SIP media connection


SMTP data


Outbound data


Inside back connection


SQL*NET data


Outside acknowledged FIN for TCP connection or UDP RPC


Inside acknowledged FIN


Awaiting inside SYN


Awaiting outside SYN


SIP connection


SIP transient connection




Example 5-30. Output of show conn

Chicago# show conn

3 in use, 17 most used

UDP out in idle 0:00:01 flags -

TCP out in idle 0:00:02 bytes 108 flags UIO

ICMP out in idle 0:00:00 bytes 72

Cisco ASA can act as a sniffer to gather information about the packets passing through the interfaces. This is important if you want to confirm that traffic from a particular host or network is reaching the interfaces. You can use an ACL to identify the type of traffic and bind it to an interface by using the capture command.

In Example 5-31, an ACL, called inside-capture, is set up to identify packets sourced from and destined for The security appliance is using this ACL to capture the identified traffic on the inside interface using a capture list named cap-inside.

To view the captured packets, use the show capture command followed by the name of the capture list. The security appliance captured 15 packets that matched the ACL on the inside interface. The highlighted entry shows that it is a TCP SYN (shown as S after the destination port) packet sourced from with a source port of 11084 and it is destined for on destination port 23. The TCP window size is 4128 while the Maximum Segment Size (MSS) is set to 536 bytes.

Example 5-31. Packet Capturing

Chicago(config)# access-list inside-capture permit ip host host

Chicago(config)# capture cap-inside access-list inside-capture interface inside

Chicago(config)# show capture cap-inside

15 packets captured

1: 02:12:47.142189 > S

433720059:433720059(0) win 4128 

 2: 02:12:47.163489 > ack 1033049551

win 4128

!Output omitted for brevity

15 packets shown


When the capture command is enabled, the security appliance allocates memory right away. The default memory allocation is 512 KB. The security appliance can overwrite content from the beginning in this buffer space when it is full. The capture command has minimal CPU impact and therefore it is one of the most important troubleshooting tools available in Cisco ASA.


The output of the capture command can be exported into pcap format, which can be imported into a sniffing tool such as Ethereal or TCPDUMP for further analysis.


Monitoring Content Filtering

If the security appliance is set up to filter traffic by inspecting the URLs, you can view the packet-filtering statistics to ensure that nonallowed traffic is denied. Use the show url-server statistics command to check how many packets have been allowed and dropped based on the responses from the URL server (such as Websense). In Example 5-32, the security appliance has denied 9000 URL (HTTP) attempts due to restricted or blocked content, whereas it has allowed 161,302 requests. The status of the Websense server is up which indicates that there is a bidirectional communication channel between the server and the security appliance.

Example 5-32. Output of show url-server statistics

Chicago# show url-server statistics

URL Server Statistics:


Vendor websense

URLs total/allowed/denied 170302/161302/9000

HTTPSs total/allowed/denied 1765/876/889

FTPs total/allowed/denied 10/8/2

URL Server Status:

------------------ UP

URL Packets Sent and Received Stats:


Message Sent Received

STATUS_REQUEST 496908 482321

LOOKUP_REQUEST 170694 170603



If URL caching is enabled, as in the case of the deployment scenario, you can collect statistics such as allocated memory for this purpose. In Example 5-33, the security appliance shows that the total maximum number of cached URLs is 171, the total number of active URLs in the cache is 100, the total lookups it performed is 456, and the number of packets that matched the cached URLs is 306.

Example 5-33. Output of show url-cache statistics

Chicago# show url-cache statistics

URL Filter Cache Stats


 Size : 100KB

 Entries : 171

 In Use : 100

 Lookups : 456

 Hits : 306

Part I: Product Overview

Introduction to Network Security

Product History

Hardware Overview

Part II: Firewall Solution

Initial Setup and System Maintenance

Network Access Control

IP Routing

Authentication, Authorization, and Accounting (AAA)

Application Inspection

Security Contexts

Transparent Firewalls

Failover and Redundancy

Quality of Service

Part III: Intrusion Prevention System (IPS) Solution

Intrusion Prevention System Integration

Configuring and Troubleshooting Cisco IPS Software via CLI

Part IV: Virtual Private Network (VPN) Solution

Site-to-Site IPSec VPNs

Remote Access VPN

Public Key Infrastructure (PKI)

Part V: Adaptive Security Device Manager

Introduction to ASDM

Firewall Management Using ASDM

IPS Management Using ASDM

VPN Management Using ASDM

Case Studies

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Cisco Asa(c) All-in-one Firewall, IPS, And VPN Adaptive Security Appliance
Cisco ASA: All-in-One Firewall, IPS, and VPN Adaptive Security Appliance
ISBN: 1587052091
EAN: 2147483647
Year: 2006
Pages: 231
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