Don't Block ICMP at Boarder: ICMP Type 3 Code 3 and DNS
When first starting to do centralized logging for a network is surprizing to see how much cleaning has to be done: from poorly disigned apps, to old and miscofiguration, etc.

One of the most common is to see DNS denied traffic on firewall logs. And most of it is actually ICMP Destination Unreachable, Port Unreachable.
What happens in this case is that we are performing DNS queries to IPs that are not actually DNS servers (don't have port 53 opened), or were and have been decomissioned. As we are guilty of not always cleaning up the coonfigurations after this king of changes, we end up with seeing many (many many depending on the network size) logs like this:

Apr 22 2018 17:36:12: %ASA-4-313005: No matching connection for ICMP error message: icmp src OUTSIDE: dst INSIDE: (type 3, code 3) on OUTSIDE interface. Original IP payload: udp src dst

What this actually means is that does a DNS query to on UDP port 53, and as the port is not open, it replies with a ICMP type 3 code 3 message. This is normal host behavior, to reply with ICMP Port Unreachable when receiving a request to a closed port. (

As it can be seen in the Wireshark capture (with different IP, as it's in a lab environement), in the ICMP reply we have the initial DNS request, as it's also showed in the firewall log.

[Image: ICMP33DNS_wireshark.png]

Our firewalls block this, and the No matching connection message shows that it doesn't relate the DNS request with the ICMP reply. And as the "best practice" says to block all ICMP traffic, here we have it.

Another detected case we need to take into account is that sometimes when the reply arrives the port can be already closed (delay, etc.), so the DNS server will also reply with Port Unreachable.

[Image: ICMP33DNS_wireshark2.png]

So in this case, the solution is:
  • track the source IP, check if a DNS server (based on the high number of requests) and change is configuration

  • identifying what is the destination and why was it though a DNS server

  • understand why was this configuration perfomed in the first place

Perfect for spring/summer cleaning dashboards and logs ?

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