Is it a bad idea for a firewall to block ICMP?

  • This question was inspired by this answer which states in part:

    The generic firewall manifest file finishes off by dropping everything I didn't otherwise allow (besides ICMP. Don't turn off ICMP).

    But, is it truly a good practice for a firewall to allow ICMP? What are the security implications, and are there cases where ICMP should be turned off?

    I'm sure there are plenty of other reasons, but for one - it makes remote administration a nightmare.

    It's one of those "Unless you're a networking god and really know what you're doing, don't mess with it" sort of things.

    IMO, this rule applies to the entire firewall, not just ICMP.

    RFC 4890 : and this draft RFC: ... both provide detailed advice on how to filter both ICMPv4 and ICMPv6 packets.

    I blocked all incoming traffic of my local network with iptables to avoid unauthorized access to services running on my computer. However sometimes I noticed logging in to websites like GitHub was freezing (e.g. the first log in on a clean Firefox profile). Wireshark displayed ICMP packets with the message "Destination unreacheable (Fragmentation needed)". It looks like the problem came from my firewall rules blocking ICMP which was needed by the router because it was fixed when allowing ICMP.

  • Scott Pack

    Scott Pack Correct answer

    8 years ago

    Compared to other IP protocols ICMP is fairly small, but it does serve a large number of disparate functions. At its core ICMP was designed as the debugging, troubleshooting, and error reporting mechanism for IP. This makes it insanely valuable so a lot of thought needs to into shutting it down. It would be a bit like tacking >/dev/null 2>&1 to the end of all your cron entries.

    Most of the time when I talk to people about blocking ICMP they're really talking about ping and traceroute. This translates into 3 types

    • 0 - Echo Reply (ping response)
    • 8 - Echo Request (ping request)
    • 11 - Time Exceeded

    That's 3 types out of 16. Let's look at a couple of the other ICMP type that are available.

    • 4 - Source Quench (send by a router to ask a host to slow down its transmissions)
    • 3 - Destination Unreachable (consists of 16 different kinds of messages ranging from reporting a fragmentation problem up to a firewall reporting that a port is closed)

    Both of which can be invaluable for keeping non-malicious hosts operating properly on a network. In fact there are two (probably more but these are the most obvious to me) very good cases where you don't want to restrict ICMP.

    • Path MTU Discovery - We use a combination of the Don't Fragment flag and type 3 code 4 (Destination Unreachable - Fragmentation required, and DF flag set) to determine the smallest MTU on the path between the hosts. This way we avoid fragmentation during the transmission.
    • Active Directory requires clients ping the domain controllers in order to pull down GPOs. They use ping to determine the "closest" controller and if none respond, then it is assumed that none are close enough. So the policy update doesn't happen.

    That's not to say that we should necessarily leave everything open for all the world to see. Reconnaissance is possible with ICMP and that is generally the reason given for blocking. One can use pings to determine if a host is actually on, or Time Exceededs (as part of a traceroute) to map out network architectures, or Rory forbid a Redirect (type 5 code 0) to change the default route of a host.

    Given all that, my advice is, as always, take a measured and thoughtful approach to your protections. Blocking ICMP in its entirety is probably not the best idea, but picking and choosing what you block and to/from where probably will get you what you want.

    small detail: While ICMP is optional in IPv4, it is required by IPv6 to operate normally. The role of ICMP has changed a lot. Lightweight read about it:

    @Mike Oh sure, I guess I wasn't clear but I was specifically talking about v4. IPv6 is a different enough beast that we really need to treat it as completely different protocol when designing and protecting v6 networks.

    +1 "*... or Rory forbid...*" I Actually laughed out loud.

    @tylerl: I laughed writing it too. Granted I had had some wine and it was 1.5 hours past my bedtime.

    Source Quench has been formally deprecated (RFC 6633). And has hardly ever been seen on the Internet for decades.

    ICMP Redirect is also pretty well abandoned as well, but it happens sometimes. It makes for a good example of how wife the users of ICMP are. The linked RFC is interesting, but also very new. I would expect a very large number of network stacks in use haven't been updated since that was released.

    RFC 5927 section 6.2 notes that Source Quench has been removed from many operating systems since 2005 or before, based on its uselessness for congestion control and its potential to be used for a blind throughput-reduction attack.

    FYI: Don't block ping requests on IPv6 as its used for incoming connection from people still on IPv4 networks using teredo to access the IPv6 network

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Content dated before 6/26/2020 9:53 AM