How to remove invalid entires from known hosts file

There are rare cases when known hosts file becomes corrupted, so basic ssh utilities fail within custom built shell scripts due to ~/.ssh/known_hosts is not a valid known_hosts file error.

This simple solution will in-place remove lines marked as missing key, invalid key and too long. It will not verify host keys by any means, as this is different kind of operation.

One-liner solution

The following command will remove invalid entries from user known hosts file.

$ ssh-keygen -F non-existent-address 2>&1 1>&- | awk '/^line .* missing|invalid key|too/ {if (!seen[$2]++)  printf $2"d;"}' | xargs  -I % -r sed -i -e '%' ~/.ssh/known_hosts

Brief description of the above mentioned command.

  1. Use ssh-keyscan to search for non-existent address and capture only standard error.
  2. Use awk to generate sed delete command, discard duplicated lines.
  3. Use xargs to execute sed provided command to edit ~/.ssh/known_hosts file in-place.

Step by step example

Create ~/.ssh/known_hosts file. This file will be used in the following example. Remember to make a backup.

$ cat <<EOF | tee -a ~/.ssh/known_hosts
example.org ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAvd4U77Q2yK0NjwBrAAVUeRTEvjH/2rnm1Mi3FgX5Y/g9/5h4wZ+FfqtdcSnbBnKN1uSnwqLS8kO3ti6MNf3/nhhXBO5XYBt1zpMggg2KAN63ThJNk62UK9QFjDdilJktOhSfMMhsZmTifX53Mpc44PdTLB53GTg57HD91E74e3UjWtjHnK1ZEjc4czVyfNdLRq9z6JoorSKqPPz9kiHiitwaKENY4kaLM4KN0CGK421G6G7uYDs3t1WyCe6DiiwJJ7r10WnE3KcSQOiow2CSVv7Mk2Ou5MiJHgASg3y+Hre6Y87OOJP0c9Lc300yMNt28hQptRyY9SD5FNsATUuMsw==
invalid-key.example.org ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAvd4UERROR77Q2yK0NjwBrAAVUeRTEvjH/2rnm1Mi3FgX5Y/g9/5h4wZ+FfqtdcSnbBnKN1uSnwqLS8kO3ti6MNf3/nhhXBO5XYBt1zpMggg2KAN63ThJNk62UK9QFjDdilJktOhSfMMhsZmTifX53Mpc44PdTLB53GTg57HD91E74e3UjWtjHnK1ZEjc4czVyfNdLRq9z6JoorSKqPPz9kiHiitwaKENY4kaLM4KN0CGK421G6G7uYDs3t1WyCe6DiiwJJ7r10WnE3KcSQOiow2CSVv7Mk2Ou5MiJHgASg3y+Hre6Y87OOJP0c9Lc300yMNt28hQptRyY9SD5FNsATUuMsw==
empty-key.example.org
too-long.example.org 12345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890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EOF

Search for example.org. host which is present in the host file.

$ ssh-keygen -F example.org  
# Host example.org found: line 1 type RSA
example.org ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAvd4U77Q2yK0NjwBrAAVUeRTEvjH/2rnm1Mi3FgX5Y/g9/5h4wZ+FfqtdcSnbBnKN1uSnwqLS8kO3ti6MNf3/nhhXBO5XYBt1zpMggg2KAN63ThJNk62UK9QFjDdilJktOhSfMMhsZmTifX53Mpc44PdTLB53GTg57HD91E74e3UjWtjHnK1ZEjc4czVyfNdLRq9z6JoorSKqPPz9kiHiitwaKENY4kaLM4KN0CGK421G6G7uYDs3t1WyCe6DiiwJJ7r10WnE3KcSQOiow2CSVv7Mk2Ou5MiJHgASg3y+Hre6Y87OOJP0c9Lc300yMNt28hQptRyY9SD5FNsATUuMsw==
line 2 invalid key: invalid-key.example.org...
line 3 missing key: empty-key.example.org...
line 4 too long: too-long.example.org 1234567890123456789...
/home/vagrant/.ssh/known_hosts is not a valid known_hosts file.

You will get a host key on standard output if it is present in the host file, but this is not important here. It is here, so you could see the difference between output of the current, previous and following command.

$ ssh-keygen -F example.org 2>&-
# Host example.org found: line 1 type RSA
example.org ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAvd4U77Q2yK0NjwBrAAVUeRTEvjH/2rnm1Mi3FgX5Y/g9/5h4wZ+FfqtdcSnbBnKN1uSnwqLS8kO3ti6MNf3/nhhXBO5XYBt1zpMggg2KAN63ThJNk62UK9QFjDdilJktOhSfMMhsZmTifX53Mpc44PdTLB53GTg57HD91E74e3UjWtjHnK1ZEjc4czVyfNdLRq9z6JoorSKqPPz9kiHiitwaKENY4kaLM4KN0CGK421G6G7uYDs3t1WyCe6DiiwJJ7r10WnE3KcSQOiow2CSVv7Mk2Ou5MiJHgASg3y+Hre6Y87OOJP0c9Lc300yMNt28hQptRyY9SD5FNsATUuMsw==

Capture only the standard error as it potentially contains problems we want to solve.

$ ssh-keygen -F example.org 2>&1 1>&-
line 2 invalid key: invalid-key.example.org...
line 3 missing key: empty-key.example.org...
line 4 too long: too-long.example.org 1234567890123456789...
/home/vagrant/.ssh/known_hosts is not a valid known_hosts file.

Generate sed command using awk to delete lines marked as missing key, invalid key and too long. Discard duplicated lines as these can appear in the output if line is really too long.

$ ssh-keygen -F example.org 2>&1 1>&- | ssh-keygen -F example.org 2>&1 1>&- | awk '/^line .* missing|invalid key|too/ {if (!seen[$2]++)  printf $2"d;"}'
2d;3d;4d;

Feed the sed command using xargs to edit known hosts file in-place.

$ ssh-keygen -F example.org  2>&1 1>&- | awk '/^line .* missing|invalid key|too/ {if (!seen[$2]++)  printf $2"d;"}' | xargs  -I % -r sed -i -e '%' ~/.ssh/known_hosts

Verify known host file, all invalid entries were removed.

$ cat ~/.ssh/known_hosts
example.org ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAvd4U77Q2yK0NjwBrAAVUeRTEvjH/2rnm1Mi3FgX5Y/g9/5h4wZ+FfqtdcSnbBnKN1uSnwqLS8kO3ti6MNf3/nhhXBO5XYBt1zpMggg2KAN63ThJNk62UK9QFjDdilJktOhSfMMhsZmTifX53Mpc44PdTLB53GTg57HD91E74e3UjWtjHnK1ZEjc4czVyfNdLRq9z6JoorSKqPPz9kiHiitwaKENY4kaLM4KN0CGK421G6G7uYDs3t1WyCe6DiiwJJ7r10WnE3KcSQOiow2CSVv7Mk2Ou5MiJHgASg3y+Hre6Y87OOJP0c9Lc300yMNt28hQptRyY9SD5FNsATUuMsw==

Done. It wasn't that hard, was it?

Milosz Galazka's Picture

About Milosz Galazka

Milosz is a system administrator working for a successful Polish company and a long time supporter of Free Software Foundation and Debian operating system.

Gdansk, Poland https://sleeplessbeastie.eu