HostKeyAlgorithms should also be set in sshd_config

[pinysuse]

Issue

Section Client authentication advises to add

    HostKeyAlgorithms ssh-ed25519-cert-v01@openssh.com,ssh-rsa-cert-v01@openssh.com,ssh-ed25519,ssh-rsa

to section Host * of /etc/ssh/ssh_config.

At least in OpenSuse Leap 15.3 the command

systemctl restart sshd.service

triggers

ssh-keygen -A

(for good or for bad) and those deleted keys are recreated.

man sshd_config

    [...]
    HostKey
        [...]
        Note that [...] that the HostKeyAlgorithms option restricts which of the keys are actually used by sshd(8).
        [...]

Proposal

Add

HostKeyAlgorithms ssh-ed25519-cert-v01@openssh.com,ssh-rsa-cert-v01@openssh.com,ssh-ed25519,ssh-rsa

to /etc/ssh/sshd_config as well.

[manfred-kaiser]

Hello, I came across this issue ticket by searching for Terrapin attack.

Setting HostKeyAlgorithms is not recommended by the OpenSSH devs, because this can lead to more mitm warnings. this was discussed with CVE-2020-14145 (ssh fingerprint information leak):

• https://bugzilla.mindrot.org/show_bug.cgi?id=3313
• https://docs.ssh-mitm.at/vulnerabilities/CVE-2020-14145.html