[Linux]: How to Hash Known Hosts Files of ~/.ssh/ directory

During connecting to a SSH server, the client stores the few details like Server’s Hostname, IP Address and Host key in a file name known_hosts. This file will be located in you ~/.ssh directory as shown below :

Techglimpse.com,192.168.10.12 ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEA01yz1a/UkkdKsqNIfALi13OmJ305weWukUtdG5WY2xKBzc3UDqBTVndbpzMEeXl/A/4SAPdc/dUUVNYJWHc8SvcFa2n+NXduq6UPmimJYxX0glHLql9rhX9X6BrpYq93J08tcdPJlS88AF86oL0HRk1l3whN8x7v62UfPSF3/apihx5PQVEYI0rL47wi6gYPRb70CiEn1MCvIJLeyBaIjvhZ+LKsXhNafahGo36Ck7Tf2iqTNuuy56U/ijt0MHg3kOwEecVVbWS3RSASQCfu345BK2a4soeIG1JpfTakz23Cb5T76wBM63uUDvFmmjn+ljZlNafN/AQLwIfYyxQ/pw==

This file contains list of all servers to which you connect in a plaintext. It poses a small security risk if the host is shared or your client gets compromised. This can be avoided by Hashing the known_hosts file. Hashing known_hosts file is easy, you just use the ssh-keygen command as shown below :

Thu Nov 21>$ ssh-keygen -H -f 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
Thu Nov 21>$ rm -rf known_hosts.old

Note : You need to delete the backup file known_hosts.old

Here is how the known_hosts file looks like after obscure :

|1|tLNDSqW6e4t2qiTRia319tcpr6s=|2vWa2a/276pFWvQWY3jPukIt7OE= ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAzhZmG33G/3FG3vm0eDdyX1u++i0ceakIkJNgDxVVy6MpodRrpwqXXQj8/OGT
Iwb4YpRXGuL3236IkGugI9GUgFd00UNjMSMt3pqob4hKsEzADl7YfZeV1X7X0b617nze0otdO7TwDMlQ/5KWUwdUoxg50VfpieTzcOpUN/G4J159iKZ41iSF7o4vI+fYisX8y5rJ1BRbt1HO0Gi7w9HZ8tN0B
0glM6JKyoE8TjvbZAeD9PWIWp9JpG1KTY4yXTV1B1CyvtxjRqTMm8mcb+gSGGvv6mSlWCNxJnlXhp91F2GtmgzKsE3FjcMUfkn3c0+P0bKaR8L3GtbyaXJmtDX4xQ==
|1|wlPQdgFoYgYsqG6ae20lYopRLPI=|p61txQKmb+Hn49dsD+v0CNuEKd4= ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAzhZmG33G/3FG3vm0eDdyX1u++i0ceakIkJNgDxVVy6MpodRrpwqXXQj8/OGT
Iwb4YpRXGuL3236IkGugI9GUgFd00UNjMSMt3pqob4hKsEzADl7YfZeV1X7X0b617nze0otdO7TwDMlQ/5KWUwdUoxg50VfpieTzcOpUN/G4J159iKZ41iSF7o4vI+fYisX8y5rJ1BRbt1HO0Gi7w9HZ8tN0B
0glM6JKyoE8TjvbZAeD9PWIWp9JpG1KTY4yXTV1B1CyvtxjRqTMm8mcb+gSGGvv6mSlWCNxJnlXhp91F2GtmgzKsE3FjcMUfkn3c0+P0bKaR8L3GtbyaXJmtDX4xQ==

While the above procedure obscure all existing host keys, any new host keys that gets added to your known_hosts file after you hash it do not get hashed by default !

To make it default, create a file called config under ~/.ssh/ directory and add the below directive :

HashKnownHosts yes

Updated on September 3, 2017

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