openssh unix dev - Oct 2024 - Re: Re: SSH host key rotation – known_hosts file not updated

On 2024-10-14 14:48, Damien Miller wrote:
> On Sun, 13 Oct 2024, Jan Eden via openssh-unix-dev wrote:
> > When I connect to serverA (`ssh -v -o UpdateHostKeys=yes serverA`)
> > afterwards, known_hosts on the client is not updated. The output of
the
> > ssh command contains this:
> > 
> > debug1: Host '[serverA.domain.internal]:22' is known and
matches the ED25519 host key.
> > # ...
> > debug1: client_input_hostkeys: searching /Users/snafu/.ssh/known_hosts
for [serverA.domain.internal]:22 / (none)
> > debug1: client_input_hostkeys: searching
/Users/snafu/.ssh/known_hosts2 for [serverA.domain.internal]:22 / (none)
> > debug1: client_input_hostkeys: hostkeys file
/Users/snafu/.ssh/known_hosts2 does not exist
> > debug1: client_input_hostkeys: host key found matching a different
name/address, skipping UserKnownHostsFile update
> One weird thing is this:
> 
> > debug1: Host '[serverA.domain.internal]:22' is known and
matches the ED25519 host key.
> 
> ssh doesn't usually decorate the hostname with port numbers like this
for
> the default port 22. Did you redact the output?
Yes, I redacted hostname and port ??sorry, should have mentioned that.
> Anyway, in answer to your question. The "host key found matching a
different
> name/address" is triggered when a key received from the server in an
update
> already exists under a different name. If you turn the debugging level up,
> then you'll see the name(s) that it matches too:
> 
>   2100          if (sshkey_equal(l->key, ctx->keys[i])) {
>   2101                  ctx->other_name_seen = 1;
>   2102                  debug3_f("found %s key under different "
>   2103                      "name/addr at %s:%ld",
>   2104                      sshkey_ssh_name(ctx->keys[i]),
>   2105                      l->path, l->linenum);
>   2106                  return 0;
>   2107          }
>   2108  }
Thank you! Increasing the verbosity revealed a known_hosts entry linked
to serverA's IP address (I had forgotten that I had connected to it by
IP address at some point). Deleting this entry solved the problem; the
new host key was stored in known_hosts when I connected to serverA
again.

- Jan
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 228 bytes
Desc: not available
URL:
<http://lists.mindrot.org/pipermail/openssh-unix-dev/attachments/20241014/3bde6c22/attachment.asc>

On Mon, Oct 14, 2024 at 5:33?AM Jan Eden via openssh-unix-dev
<openssh-unix-dev at mindrot.org> wrote:
redacted hostname and port ? sorry, should have mentioned
that.
>
> > Anyway, in answer to your question. The "host key found matching
a different
> > name/address" is triggered when a key received from the server in
an update
> > already exists under a different name. If you turn the debugging level
up,
> > then you'll see the name(s) that it matches too:
> >
> >   2100          if (sshkey_equal(l->key, ctx->keys[i])) {
> >   2101                  ctx->other_name_seen = 1;
> >   2102                  debug3_f("found %s key under different
"
> >   2103                      "name/addr at %s:%ld",
> >   2104                      sshkey_ssh_name(ctx->keys[i]),
> >   2105                      l->path, l->linenum);
> >   2106                  return 0;
> >   2107          }
> >   2108  }
>
> Thank you! Increasing the verbosity revealed a known_hosts entry linked
> to serverA's IP address (I had forgotten that I had connected to it by
> IP address at some point). Deleting this entry solved the problem; the
> new host key was stored in known_hosts when I connected to serverA
> again.
>
> - Jan
And... *THIS* is why so many people disable known_hosts entirely. The
chance of an IP address being reused for a distinct hostname is pretty
high in a DHCP environment without reservations, coupled with dynamic
DNS. It's also very common when servers get rebuilt from images and
fresh hostkeys generated automatically on the same hardware, even with
the same IP address. The popular solution is to simply disable
known_hosts in your ~/.ssh/config as needed:

    # Disable known_hosts to avoid IP re-use conflicts
    Host *
         UserKnownHostsFile /dev/null
          StrictHostKeyChecking no
          LogLevel ERROR