Force SSH public key authentication for specific users

Question

Is it possible to force specific users to login with public key, while allowing other users to login with password? Since public key authentication (with passphrase) is stronger than password-only authentication, we would like to require sudoers to login with public key. However, it is less convenient to force normal users to do so. In sshd_config, I don't see any policy-related configuration.

score 14 · Accepted Answer · answered Feb 22 '11 at 16:37

You have a few options. In this answer I'm going to assume you have a sudoers group defined.

Take a look at the sshd_config man page, and look for the Match directive. This lets you specify configuration blocks that apply only to a subset of your ssh connections. You could do something like this:

Match Group sudoers
PasswordAuthentication no
ChallengeResponseAuthentication no

You could in theory accomplish something similar with a PAM configuration that would simply fail authentication attempts by people in the sudoers group. This would probably involve the pam_succeed_if module...you could add something like this to your auth config for sshd:

auth        requisite     pam_succeed_if.so user notingroup sudoers quiet

This means that only people not in the sudoers group can authentication via PAM. Note that this is untested. You could also use the pam_listfile module to do something similar.

Thanks! I must also note that the Match directive is introduced in OpenSSH 5.0. For conservative distributions like CentOS, it may not be available natively. — Reci, Feb 23 '11 at 08:44
This breaks OpenSSH 7.7p1 - I had to remove the ChallengeResponseAuthentication directive in order to get it to restart. — rbsec, Sep 12 '18 at 15:11

score 4 · Answer 2 · edited Aug 17 '21 at 19:40

Another possible answer, as @larsks, answer did not work for my version of ssh_d as my version seems to be using the documentation found here which states:

Only a subset of keywords may be used on the lines following a Match keyword. Available keywords are . . .

That list of keywords does not include: ChallengeResponseAuthentication.

A really fun way I found was to use AuthenticationMethods which in your case would work like so:

Match Group sudoers
AuthenticationMethods "publickey"

AuthenticationMethods takes a list of comma separated values which represent a series of methods a user must pass before accessing the server.

AuthenticationMethods "publickey,password" would force the user to pass with a public key and then a password.

To read more man sshd_config.

Force SSH public key authentication for specific users

2 Answers2