OpenSSH tips

This page provides some tips on OpenSSH, which is one of the most common SSH client and server implementation.

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OpenSSH tips

Restricting public key authentication by client IP address in authorized_keys

When we use public key authentication for SSH login, there exists a problem with private key leaks. Generally, in order to prevent the second accident caused by cracking on other machines that store private keys, we set a passphrase to each private key. However, we would sometimes like to use empty passphrase, for example, in case loging in servers via batch script to skip entering passphrase. A main problem with this empty passphrase is that any users can login servers using the corresponding public key by using a leaked private key once the private key leaks. One solution for this problem is to use IP address-based authentication.

In authorized_keys, we can use IP address-based authentication like following:

from="" [SSH-PUBLIC-KEY]

In case you permit IP addresses in a subnet, you can use the slash (/) notation:

from="" [SSH-PUBLIC-KEY]

When you want to add two or more premitted IP addresses or ranges, you can use commas (,) for the separator like this:

from="," [SSH-PUBLIC-KEY]

Note that you can also use this authentication for IPv6 addresses.

Changing the passphrase of a private key

To change the passphrase of a private key, you can use ssh-keygen with -p option:

$ ssh-keygen -p [-f /path/to/your/private/key]

Note that you can specify the path to the private key with -f option.

Checking fingerprint of a public key

If you want to check fingerprint of a public key, you can use ssh-keygen command with option -l.

$ ssh-keygen -l -f /path/to/your/public/key