How does the SSH option CheckHostIP=yes really help me?

Question

There are many discussions about this option and most people argue that "it improves security", "it protects you from MiTM attacks/DNS spoofing", etc., but I fail to see how that is true. ssh_config(5) says that the option

allows ssh to detect if a host key changed due to DNS spoofing

but it still seems vague or even misleading, because

1. The option doesn't detect if a "host key has changed" but only if a host's IP has changed, and
2. It doesn't elaborate on what it does when it detects such a thing (for example, does it stop the connection from happening?).

Because of the vagueness of the documentation, other people even seem to believe that CheckHostIP=yes will not let you connect to a host that has changed its IP (i.e. it has a different IP than the one that is already recorded in known_hosts) which is totally untrue, at least as far as I can understand through my own tests.

• When I connect with CheckHostIP=yes (the default) to a remote server whose host key is already stored in known_hosts, but has changed its IP, the connection proceeds normally but I get a warning:

  Permanently added the ECDSA host key for IP address [......]

  Also a new line is appended into known_hosts, containing the server's host key (which has not changed) and the new IP address (which is different). If the host later changes its IP again, the next time I connect to it with CheckHostIP=yes, a new line will be appended in known_hosts and so on.

• If I connect with CheckHostIP=no to a server that has changed its IP address, the connection proceeds (which is what also happens with CheckHostIP=yes), but this time without a warning or appending a new line in known_hosts.

So the only difference between yes and no seems to be a simple one-line warning upon connecting, plus the recording of new addresses in known_hosts every time the host changes its IP (effectively keeping a history record of a server's different IPs through time).

If that's the case, I'm not even sure that CheckHostIP=yes is better than CheckHostIP=no, because if the client machine was compromised the attacker would be able to deduce from known_hosts that

1. The server with the host key X has a dynamic IP, and
2. It has changed its IP address Y times.

Obviously it's not much and I doubt the attacker could actually do something with that bit of information, but it is leakage of information that would not happen with CheckHostIP=no.

I have tried the above with two separate SSH servers that have dynamic IPs. (I use a free DDNS service to connect to them).

Can an SSH guru confirm to me that this is all the protection CheckHostIP=yes offers (a warning that doesn't even ask for user interaction), or correct me if I am terribly wrong about something? Should I just always keep CheckHostIP=no for connections to servers that I know will be changing their IPs through time?

Answer 1

Summary

In my view, CheckHostIP just reassures you that an attacker who has already compromised your remote server's private key hasn't also poisoned your local DNS.

Leak instructions

If you found this post because of a remote server's host key leaking, follow the instructions from the service provider to cycle the relevant known_hosts entries. You may also want to remove the matching lines from known_hosts that CheckHostIP has added.

Disclosure Date	Provider	Instructions
2023-03-24	GitHub	Blog post
2023-05-15	Bitbucket	Blog post

Expert opinion

An "SSH guru," specifically an OpenSSH maintainer, has affirmed your position.

I think we're more likely to turn off CheckHostIP (the thing that spams addresses into known_hosts) in the short term, as nobody has satisfactorily explained what problem it solves to me.

@DamienMiller (Twitter, 2020-12-11)

Three months later, Damien followed through in the release of OpenSSH v8.5 on 2021-03-03, where CheckHostIP was turned off with this changelog message:

• ssh(1): disable CheckHostIP by default. It provides insignificant benefits while making key rotation significantly more difficult, especially for hosts behind IP-based load-balancers.

Bringing back CheckHostIP

If you really want to keep CheckHostIP, you can turn it back on and disable it only for hosts with dynamic IPs to skip the warning and known_hosts pollution. Don't do this, though. It's not that helpful.

It is so improbable that CheckHostIP is a meaningful measure for dealing with a threat actor who can both…

• Steal the private key of the remote SSH server, and
• Manipulate your client's DNS resolution.

In the face of such a foe, it seems ludicrous that somehow you'll be saved by the "Warning: Permanently added host key" message when connecting to a host, especially when it shows you the warning¹ every single time.

The known_hosts trick

Some people proactively dump all the known IPs for a service into a known_hosts line. I think they're doing this just to avoid the warning message. It front-loads all the nonsense into known_hosts that you'd gradually accumulate otherwise. But frankly, the important part of known_hosts is not the IP, it's the host key.

In order to cut down on the super-long known_hosts entries, other people simulate IP ranges with wildcards, which may appear clever, but it's actually finicky and it only works because CheckHostIP works against IPs and ignores hostnames.²

_{¹ That is to say: "when it cries 'Wolf!'"
² For example, if you try to allow 127.0.0.* for your dynamic-IP SSH server, you technically also match 127.0.0.evilsite.example because IPs and hostnames are not distinguished. You can limit wildcards to single characters with ?, but be careful: 127.0.0.?? will match 127.0.0.cz. Maybe it seems unlikely that .cz would allow 0 as a second-level domain, but we're talking about an adversary who is poisoning your DNS!}