How to manage my .ssh/known_hosts file

Question

I run an Ubuntu desktop with a bunch virtual test servers in Virtual Box to test stuff out etc. In the past I've also been connecting to other kinds of remote VPS Linux boxes. Currently my .ssh/known_hosts file has a whole bunch of keys in them, most are not being used any more.

I want to clean up my .shh/known_hosts file but how do I know which key belongs to what host? I.e. how do I know which keys I can safely remove and which ones I should leave alone?

Answer 1

With difficulty...

Ubuntu by default hashes hostnames the known_hosts file (this is not the default openssh behaviour), to make it difficult for anyone reading the file to know what systems you access.

If you really wanted to clean out the file, simplest option is probably just delete it and check the keys for servers you know as they arise, but really I'd just leave known_hosts alone.

You can stop new hosts entries from being hashed by commenting out the option in /etc/ssh/ssh_config

#HashKnownHosts yes

Answer 2

To find out which entry is for a known hostname in known_hosts:

 # ssh-keygen -H  -F <hostname or IP address>

To delete a single entry from known_hosts:

 # ssh-keygen -R <hostname or IP address>

Answer 3

If you've got a list of all your hosts, you can do something like

ssh-keyscan -t rsa,dsa -f list_of_hosts > ~/.ssh/known_hosts

That will overwrite your .ssh/known_hosts file with a newly generated one based on scanning the hosts.

And also do what theotherreceive suggests; HashKnownHosts is more annoyance than help here.