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I would like to see what hosts are in my known_hosts file but it doesn't appear to be human readable. Is it possible to read it?

More specifically there is a host that I can connect to via several names and I want to find out what the fingerprint I expect for it from my known hosts file.

Update: I'm using OpenSSH_5.3p1 Debian-3ubuntu7, OpenSSL 0.9.8k 25 Mar 2009

A line from my known_hosts file looks something like this,

|1|guO7PbLLb5FWIpxNZHF03ESTTKg=|r002DA8L2JUYRVykUh7jcVUHeYE= ssh-rsa AAAAB3NzaC1yc2EAAFADAQABAAABAQDWp73ulfigmbbzif051okmDMh5yZt/DlZnsx3DEOYHu3Nu/+THJnUAfkfEc1XkOFiFgbUyK/08Ty0K6ExUaffb1ERfXXyyp63rpCTHOPonSrnK7adl7YoPDd4BcIUZd1Dk7HtuShMmuk4l83X623cr9exbfm+DRaeyFNMFSEkMzztBYIkhpA2DWlDkd90OfVAvyoOrJPxztmIZR82qu/5t2z58sJ6Jm2xdp2ckySgXulq6S4k+hnnGuz2p1klviYCWGJMZfyAB+V+MTjGGD/cj0SkL5v/sa/Fie1zcv1SLs466x3H0kMllz6gAk0/FMi7eULspwnIp65g45qUAL3Oj
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4 Answers 4

up vote 52 down vote accepted

You've got HashKnownHosts set to "yes" in your ssh_config file, so the hostnames aren't available in plaintext.

If you know the hostname you're looking for ahead of time, you can search for it with:

    ssh-keygen -H -F hostname

Here's the relevant section from the ssh-keygen(1) man page:

 -F hostname
         Search for the specified hostname in a known_hosts file, listing
         any occurrences found.  This option is useful to find hashed host
         names or addresses and may also be used in conjunction with the
         -H option to print found keys in a hashed format.
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Does hash known hosts essentially mean it's not possible? i.e. I need to know the name of the host in order to look at it's info? –  Colin Newell Feb 13 '12 at 15:54
@ColinNewell That's right, you need to know the hostname(s). It's a security measure to keep an attacker from harvesting hostnames/IP addresses of other boxes you frequent if your machine is compromised. –  pdo Feb 13 '12 at 15:58
Don't be scared when there is no output at all, but try not the full qualified hostname but the hostname instead. –  math May 7 '13 at 17:58
Not sshd_config, but ssh_config. –  fossilet Nov 21 '13 at 10:31
@pdo - your command does not always work. If the host has SSH on port other than 22, then the format in known_hosts is different. Then you have to use the following command: ssh-keygen -H -F []:2222 –  Martin Vegter Jul 13 '14 at 15:56

There is host string/ip at the beginning of each "known_hosts" line (before the "ssh-dss" or "ssh-rsa" string):

hostgn6 ssh-dss AAAB3NzaC1kc3MAAACBAIfGV4+/28Zr+dT/i+ifydUBS0dMRUjCtExIThOj3Yexynu+wSRGjMm4GfF+og2kAljZyUjhBFeM+WYbJzcDSDB [...] == ssh-dss AAAAB3NzaC1kc3MAAACBAIfGV4+/28Zr+dT/i+ifydUBS0dMRUjCtExITh [...] ==
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...and thus awk '{print $1}' known_hosts does the trick. Be aware that servers listening on non-standard ports end up as, say, [some-server]:5555 in known_hosts. –  sr_ Feb 13 '12 at 14:35
Unfortunately not in mine. Mine looks more like base64 encoded data. The data is also pipe delimited in my file. –  Colin Newell Feb 13 '12 at 14:40
Which ssh software are you using then (we were assuming that you were using the almost-standart openssh software) ? –  Ouki Feb 13 '12 at 14:49
I've added more details to my question. –  Colin Newell Feb 13 '12 at 15:11
As stated by @pdo, your sshd as "HashKnownHosts" set to on, which may appear a bit anal, but security concern it is. And of course there is no way to simply reverse the hashes and get the hostnames from your "known_hosts". –  Ouki Feb 13 '12 at 15:41

Does ssh-keygen -l -f ~/.ssh/known_hosts help? (Using -vyou also get nice little treasure maps, e.g.

+--[ RSA 2048]----+
|        .        |
|       + .       |
|      . B .      |
|     o * +       |
|    X * S        |
|   + O o . .     |
|    .   E . o    |
|       . . o     |
|        . .      |
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Not in the strictest sense no, it's all still base64. pdo's explanation of it being hashed suggests it's a one way thing so I guess I'm stuck unless I know the hostname. –  Colin Newell Feb 13 '12 at 15:57
That helped me. What I wanted specifically was a combination. To find the previously verified ssh fingerprint, you can run: ssh-keygen -l -f ~/.ssh/known_hosts -F <hostname> –  isaaclw Sep 22 at 21:30
uh, ssh-keygen -l -F <hostname> is even simplier –  isaaclw Sep 24 at 15:08

For future searchers, this article (non-disclaimer: I'm not affilated) has a relatively simple Perl script to brute-force hashed IPs and hostnames in known_hosts.

It allows starting from a particular IP address. It could also easily be modified to use a dictionary.

Also, in June 2014, the John the Ripper project added support for known_hosts cracking, which can take advantage of multiple CPU cores, GPUs, dictionary mangling, etc.

You can also use this nmap snippet to generate a dictionary of all RFC1918 IP addresses to use as a dictionary:

nmap -sL -Pn -n |\
    grep '^Nmap scan report for' | cut -d\  -f5 >ips.list
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There is also a python implementation with similar features, can be found at: –  Lars Nordin Jun 12 '14 at 14:44
Lars Nordin, thanks for the tip. I submitted Remily's info to the John the Ripper team, and they added support for this in bleeding-jumbo: –  Royce Williams Jul 13 '14 at 15:00

protected by slm Jun 11 '14 at 13:23

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