Check presence of a hostname under custom port in known_hosts

Question

I need to add a check if the hostname is already present in the known_hosts.

Normally I would do something like that:

ssh-keygen -H -F hostname

However, that does not seem to work for me in this particular case. I connect to the host using port 2202, like that:

ssh user@myhost -p 2102

I asked to add the hostname to known_hosts, I say yes. After that I run "ssh-keygen -H -F myhost" but receive empty result. To make the matter worse, the known_hosts is hashed.

That works perfectly with port 22, so if I login to "ssh user@myotherhost", save the known host and run "ssh-keygen -H -F myotherhost" I receive the exact line from the file.

So, how can I adjust the command to work with port 2102?

Solution

You can use this format: [hostname]:2121, as it is stored in the known_hosts file (note, you need to use the square brackets!):

ssh-keygen -H -F "[hostname]:2121"

Proof of concept (transcript of my minimal test case):

$ echo "[hostname]:2121 ssh-rsa AAA...==" > known_hosts
$ ssh-keygen -Hf known_hosts 
known_hosts updated.
Original contents retained as known_hosts.old
WARNING: known_hosts.old contains unhashed entries
Delete this file to ensure privacy of hostnames
$ ssh-keygen -H -F "[hostname]:2121" -f known_hosts
|1|R21497dX9jN052A92GSoVFbuTPM=|lRtIr6O564EaFG0SsIulNAWpcrM= ssh-rsa AAA...==

You might need to use IP address instead of hostname, but it should generally work.

Comments

[deimos]

Did not work for me, unfortunately. But it does not matter, I found an alternative solution and updated the post.

[Jakuje]

Just verified even with hashed known hosts and it still works. You might need to try with IP address instead of hostname, but it should work as a pie.

[deimos]

Hm... It did not work for me even with IP and even with 22.

[Jakuje]

It depends on how your known_hosts look like "before hashing" (it tells you the known line Warning: Permanently added '[localhost]:2222' (ECDSA) to the list of known hosts.). But it might slightly differ by openssh version.

[deimos]

OMG, I got it. Needed to use syntax ssh-keygen -H -F "[myserver.com]:2102" but I did not use the [brackets]. Thank you!

[Jakuje]

Gotcha :) Welcome to U&L. Let's proceed to the Tour to find out what to do with answers :)