Cisco ASA :: Verifying ICMP Reachability on the ASA

If you’re a firewall engineer or work closely with the Cisco ASA then no doubt you will often find yourself troubleshooting and verifying reachability of packets on a network. One great feature that the ASA has to test reachability is the ‘packet-tracer’ command which when given an input will provide you with a very handy output that shows how the packet would be processed through the ASA.

In this article, I will show you how we can use the packet-tracer command to verify ICMP reachability and we will also take a look at the process in which the ASA uses. For this demonstration, I am using an ASAv version (9.8) code.

By default, ICMP is not inspected on the ASA and therefore all ICMP traffic will be dropped. In order to the allow ICMP, you need to inspect it and to do this we can add the following command to the ‘global_policy’ policy-map;

class inspection_defaultinspect icmp

Once you have configured the policy-map you can then configure ACL’s to permit ICMP traffic as you desire. In this example, we will permit ICMP from a host behind the ASA firewall to any destination and another ICMP rule that permits ICMP from the second site’s public IP address to the main site’s host address.

access-list INSIDE extended permit icmp object MAIN-SITE-HOST any access-list OUTSIDE extended permit icmp object BRANCH-PUBIP object MAIN-SITE-HOSTaccess-group INSIDE in interface INSIDE1 access-group OUTSIDE in interface OUTSIDE//pagead2.googlesyndication.com/pagead/js/adsbygoogle.js (adsbygoogle = window.adsbygoogle || []).push({});

Now that we have configured the correct parameters we can either run a ping test or we can use the packet-tracer feature. In this article, we will use the packet-tracer feature to demonstrate its capabilities. Below you will see the packet-tracer command followed by a detailed output.

ciscoasa/act/pri(config)# packet-tracer input INSIDE1 icmp 192.168.10.10 8 0 209.165.100.18 detailed    Phase: 1 Type: CAPTURE Subtype: Result: ALLOW Config: Additional Information: Forward Flow based lookup yields rule: in id=0x7fdbeda3b0b0, priority=13, domain=capture, deny=false hits=1805497, user_data=0x7fdbed256740, cs_id=0x0, l3_type=0x0 src mac=0000.0000.0000, mask=0000.0000.0000 dst mac=0000.0000.0000, mask=0000.0000.0000 input_ifc=INSIDE1, output_ifc=any Phase: 2 Type: ACCESS-LIST Subtype: Result: ALLOW Config: Implicit Rule Additional Information: Forward Flow based lookup yields rule: in id=0x7fdbeda1c630, priority=1, domain=permit, deny=false hits=992191, user_data=0x0, cs_id=0x0, l3_type=0x8 src mac=0000.0000.0000, mask=0000.0000.0000 dst mac=0000.0000.0000, mask=0100.0000.0000 input_ifc=INSIDE1, output_ifc=any Phase: 3 Type: ROUTE-LOOKUP Subtype: Resolve Egress Interface Result: ALLOW Config: Additional Information: found next-hop 209.165.100.3 using egress ifc OUTSIDE Phase: 4 Type: ACCESS-LIST Subtype: log Result: ALLOW Config: access-group INSIDE in interface INSIDE1 access-list INSIDE extended permit icmp object MAIN-SITE-HOST any Additional Information: Forward Flow based lookup yields rule: in id=0x7fdbedb42b20, priority=13, domain=permit, deny=false hits=27, user_data=0x7fdbf92d3640, cs_id=0x0, use_real_addr, flags=0x0, protocol=1 src ip/id=192.168.10.10, mask=255.255.255.255, icmp-type=0, tag=any dst ip/id=0.0.0.0, mask=0.0.0.0, icmp-code=0, tag=any, dscp=0x0 input_ifc=INSIDE1, output_ifc=any Phase: 5 Type: NAT Subtype: Result: ALLOW Config: object network MAIN-SITE-HOST nat (INSIDE1,OUTSIDE) static MAIN-SITE-HOST-NAT Additional Information: Static translate 192.168.10.10/0 to 209.165.100.4/0 Forward Flow based lookup yields rule: in id=0x7fdbeda3b350, priority=6, domain=nat, deny=false hits=3348, user_data=0x7fdbedaa1410, cs_id=0x0, flags=0x0, protocol=0 src ip/id=192.168.10.10, mask=255.255.255.255, port=0, tag=any dst ip/id=0.0.0.0, mask=0.0.0.0, port=0, tag=any, dscp=0x0 input_ifc=INSIDE1, output_ifc=OUTSIDE Phase: 6 Type: NAT Subtype: per-session Result: ALLOW Config: Additional Information: Forward Flow based lookup yields rule: in id=0x7fdbed242ec0, priority=0, domain=nat-per-session, deny=true hits=453421, user_data=0x0, cs_id=0x0, reverse, use_real_addr, flags=0x0, protocol=0 src ip/id=0.0.0.0, mask=0.0.0.0, port=0, tag=any dst ip/id=0.0.0.0, mask=0.0.0.0, port=0, tag=any, dscp=0x0 input_ifc=any, output_ifc=any Phase: 7 Type: IP-OPTIONS Subtype: Result: ALLOW Config: Additional Information: Forward Flow based lookup yields rule: in id=0x7fdbeda25750, priority=0, domain=inspect-ip-options, deny=true hits=282115, user_data=0x0, cs_id=0x0, reverse, flags=0x0, protocol=0 src ip/id=0.0.0.0, mask=0.0.0.0, port=0, tag=any dst ip/id=0.0.0.0, mask=0.0.0.0, port=0, tag=any, dscp=0x0 input_ifc=INSIDE1, output_ifc=any Phase: 8 Type: INSPECT Subtype: np-inspect Result: ALLOW Config: class-map inspection_default match default-inspection-traffic policy-map global_policy class inspection_default inspect icmp service-policy global_policy global Additional Information: Forward Flow based lookup yields rule: in id=0x7fdbedb213d0, priority=70, domain=inspect-icmp, deny=false hits=177198, user_data=0x7fdbedb1fe10, cs_id=0x0, use_real_addr, flags=0x0, protocol=1 src ip/id=0.0.0.0, mask=0.0.0.0, icmp-type=0, tag=any dst ip/id=0.0.0.0, mask=0.0.0.0, icmp-code=0, tag=any, dscp=0x0 input_ifc=INSIDE1, output_ifc=any Phase: 9 Type: QOS Subtype: Result: ALLOW Config: Additional Information: Forward Flow based lookup yields rule: in id=0x7fdbedc60f80, priority=70, domain=qos-per-class, deny=false hits=561692, user_data=0x7fdbed9545c0, cs_id=0x0, reverse, use_real_addr, flags=0x0, protocol=0 src ip/id=0.0.0.0, mask=0.0.0.0, port=0, tag=any dst ip/id=0.0.0.0, mask=0.0.0.0, port=0, tag=any, dscp=0x0 input_ifc=any, output_ifc=any Phase: 10 Type: INSPECT Subtype: np-inspect Result: ALLOW Config: Additional Information: Forward Flow based lookup yields rule: in id=0x7fdbeda24f60, priority=66, domain=inspect-icmp-error, deny=false hits=177221, user_data=0x7fdbeda244e0, cs_id=0x0, use_real_addr, flags=0x0, protocol=1 src ip/id=0.0.0.0, mask=0.0.0.0, icmp-type=0, tag=any dst ip/id=0.0.0.0, mask=0.0.0.0, icmp-code=0, tag=any, dscp=0x0 input_ifc=INSIDE1, output_ifc=any Phase: 11 Type: QOS Subtype: Result: ALLOW Config: Additional Information: Reverse Flow based lookup yields rule: in id=0x7fdbedc60f80, priority=70, domain=qos-per-class, deny=false hits=561693, user_data=0x7fdbed9545c0, cs_id=0x0, reverse, use_real_addr, flags=0x0, protocol=0 src ip/id=0.0.0.0, mask=0.0.0.0, port=0, tag=any dst ip/id=0.0.0.0, mask=0.0.0.0, port=0, tag=any, dscp=0x0 input_ifc=any, output_ifc=any Phase: 12 Type: NAT Subtype: per-session Result: ALLOW Config: Additional Information: Reverse Flow based lookup yields rule: in id=0x7fdbed242ec0, priority=0, domain=nat-per-session, deny=true hits=453423, user_data=0x0, cs_id=0x0, reverse, use_real_addr, flags=0x0, protocol=0 src ip/id=0.0.0.0, mask=0.0.0.0, port=0, tag=any dst ip/id=0.0.0.0, mask=0.0.0.0, port=0, tag=any, dscp=0x0 input_ifc=any, output_ifc=any Phase: 13 Type: IP-OPTIONS Subtype: Result: ALLOW Config: Additional Information: Reverse Flow based lookup yields rule: in id=0x7fdbeda86ea0, priority=0, domain=inspect-ip-options, deny=true hits=103963, user_data=0x0, cs_id=0x0, reverse, flags=0x0, protocol=0 src ip/id=0.0.0.0, mask=0.0.0.0, port=0, tag=any dst ip/id=0.0.0.0, mask=0.0.0.0, port=0, tag=any, dscp=0x0 input_ifc=OUTSIDE, output_ifc=any Phase: 14 Type: FLOW-CREATION Subtype: Result: ALLOW Config: Additional Information: New flow created with id 284909, packet dispatched to next module Module information for forward flow … snp_fp_tracer_drop snp_fp_inspect_ip_options snp_fp_inspect_icmp snp_fp_translate snp_fp_adjacency snp_fp_fragment snp_ifc_stat Module information for reverse flow … snp_fp_tracer_drop snp_fp_inspect_ip_options snp_fp_translate snp_fp_inspect_icmp snp_fp_adjacency snp_fp_fragment snp_ifc_stat Result: input-interface: INSIDE1 input-status: up input-line-status: up output-interface: OUTSIDE output-status: up output-line-status: up Action: allow

You may have noticed that I specified two ICMP codes ‘8’ and ‘0’ within the packet-tracer command, these codes represent echo(8) and echo-reply(0) packets. Click here for a full list of ICMP types.

Explaining each phase in detail is beyond the scope of this article but if you take a good look at each phase you should be able to see that the ASA looks for the next hop IP,  it is then matched against the ACL’s previously created before being NAT’ed and passed through additional phases. We can then see at the end of the output in the final result that the ICMP traffic is permitted.

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