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Is it possible to remove a particular host key from SSH's known_hosts file?

I usually end up deleting the entire known_hosts file, which I have no problems with doing, but just out of curiosity, is it possible to remove just a single entry?

I opened the known_hosts file, but I am struggling to understand its contents.

Below is the message I faced, which led me to ask this question:

Add correct host key in /home/wissen16/.ssh/known_hosts to get rid of this message.
Offending key in /home/wissen16/.ssh/known_hosts:1
RSA host key for foo.com has changed and you have requested strict checking.
Host key verification failed.
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11 Answers 11

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642

Use this command to remove entries from known_hosts:

ssh-keygen -R hostname

or if the host IP is available

ssh-keygen -R <Host_IP>
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  • 30
    It works with an IP address as well. For instance, I have a DNS host shortcut for my Web server. To remove a conflict I had between the keys for the custom hostname and the IP address, I had to remove the entries for both.So ssh-keygen -R xxx.xxx.xxx.xxx.
    – StrangeElement
    Nov 13, 2012 at 21:10
  • 1
    As @StrangeElement says, sometime is posible that you have to remove also IP host apart from hostname.
    – Gonzalo Cao
    Feb 10, 2015 at 9:05
  • 6
    Plus, it keeps a backup automatically (on my osx machine : Original contents retained as /Users/nha/.ssh/known_hosts.old, assuming it is the same for Ubuntu).
    – nha
    Nov 24, 2015 at 13:37
  • 28
    Plus, if you are using non-standard ssh port, use this format ssh-keygen -R [ssh.sssshh.com]:1234
    – Shiji.J
    Dec 10, 2015 at 0:04
  • 2
    The best answer ever for this question. Works like a charm
    – bademba
    Mar 10, 2017 at 9:31
36

Yes, you can remove just one key. Just open it in an editor and delete the offending line. The number after the colon in the error message is the line number, so that's the line to delete -- line 1 in your example..

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  • 1
    I had no idea it identified the line number, that is incredibly helpful.
    – deltree
    Feb 25, 2015 at 17:07
18

I have only recently started using host key's, but when I have messed with them it is generally one key per line so backup the file and remove them one at a time until you find the right one. Then add the others back. Bit of a long way to do it, but should work.

Also based on that error, and with no idea what so ever, it could be the first host key in the file that is the problem so open up the file with vim

vim ~/.ssh/known_hosts

and hit

dd

then save it.

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2
  • You can use :set nu in vim to show the line numbers. Takkat answer is the best one anyway.
    – Javier Rivera
    Jan 10, 2011 at 10:46
  • I agree didn't know you could do that. Am going to use it in the future. I kept meaning to lookup the line numbers setting for vim. thanks.
    – Buddy Lindsey
    Jan 10, 2011 at 21:17
5

This one worked for me. This is the default method shown in the display when tried to add a known host.

ssh-keygen -f "/home/noob/.ssh/known_hosts" -R "172.16.132.132"

Just you have to change the IP address and host file path here.

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4

Using ssh-keygen -R hostname will not always work. If you have a newer version of SSH that is "hiding" the hostnames to prevent ssh-agent hijacking, apparently ssh-keygen is unable to unhash the hostname.

For example, I have a host called build-node-01 and I have connected to it and accepted the key. I then rebuild it from scratch, getting a new host fingerprint and I try to reconnect, I will get a warning that there is a conflict on line X (say 3). I run ssh-keygen -R hostname, but the next time I try to connect I still get a warning that there is a conflict. I examined the file only to discover that the hostname was hashed and showed up as [1] Bu4Ch@R@4D0M57uFF instead of a readable hostname.

In this case the only way to successfully get the offending host removed was to use

sed -i 'xd' ~/.ssh/known_hosts

To take this sed one step further, you may wish to make a backup of the known_hosts in case you delete the wrong line, in this case just add a .bak (or any extension) to the -i option to create a backup with that extension. Using ssh-keygen does this automatically.

sed -i.bak 'xd' ~/.ssh/known_hosts
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  • 7
    This is not correct. ssh-keygen -R {hostname} will work, even when hostnames are 'hidden' (hashed). Still, yes, one can delete the entry by number (e.g., 10th entry via sed -i.bak 10d ~/.ssh/known_hosts), but that's not typically necessary. It could be that a non-standard port was used, in which case, you may have to format the command as (note quotes): ssh-keygen -R '[hostname]:2222'
    – michael
    Sep 20, 2016 at 9:13
  • Good to know @michael_n, it's very likely that it was a non-standard port that was affecting my ability to remove the entry. I also should note that if you have accepted multiple fingerprints for a host, I'm not sure if it removes ALL entries at once, or just one at a time.
    – dragon788
    Sep 20, 2016 at 15:06
2

Just to share another clean and easy answer I just found. Removing the hostname is out for me, as the known_hosts file is hashed. However, I COULD manually edit out the host entry based on the line number in the error message. As noted by Mike Scott previously, the offending hostname line number is in the error message.

Or, I can do this. From here: how to fix offending key in ssh known_hosts file

I got this bit of cli magic

sed -i 'xd' ~/.ssh/known_hosts

Replace the x with the line number, and voila. He also offers a perl answer if the sed will not work.

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2

If you want to remove host key for certain ip, then this will help:

ssh-keygen -f "~/.ssh/known_hosts" -R "49.12.89.136"

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1

To remove the offending entry is this

sed -i '/<ipaddr|hostname>/d' ~/.ssh/known_hosts

I cobbled together the below scipt. Just save it as ~/.rmhost

#!/bin/bash

rmhost_help()
{
        echo "Usage rmhost {ipaddr|hostname}"
        echo "          -h      Print this help message"
        exit 0
}
if [ -z $1 ] || [ $1 == "-h" ]; then rmhost_help

fi

sed -i "/$1/d" ~/.ssh/known_hosts

echo "host $1 removed"
exit 0

Then put this alias in ~/.bashrc

alias rmhost=~/.rmhost

When you get the error just type in 'rmhost {ipaddr|hostname}'

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sed '/10\.20\.120\.211/d' ~/.ssh/known_hosts > temp && mv temp ~/.ssh/known_hosts

In this case, 10.20.120.211 is the host I want to delete from my known_hosts file, make sure you escape the special characters like (.)

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  • IP addresses & hostnames are no longer stored verbatim (clear text) in the known hosts file, so this won't work. One should either use ssh-keygen -R ... (preferably); or, sed with the specific line number to delete. Also, to sed in-place, use -i' option; e.g., sed -i.bak 10d ~/.ssh/known_hosts` to delete the 10th line, and (optionally) keep the original in a backup file suffixed by .bak.
    – michael
    Sep 20, 2016 at 9:18
0

You can avoid removing the particular host by updating it:

ssh-keyscan -t ecdsa hostname >> ~/.ssh/known_hosts

This way you don't have to connect again to the host.

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0

If you are getting fingerprint option while adding rsa token use that. Once you choose fingerprint option it will add the host name of the stash repo to the known_host

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