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I would like to disallow certain ciphers for OpenSSH in Fedora.
I've read that Fedora overrides settings in sshd_config with its crypto policies.
I'd like to edit those.

My first idea would be to edit /etc/crypto-policies/back-ends/opensshserver.config, but I wonder if that is the recommended way of doing this.

2 Answers 2

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You can control settings like encryption ciphers and quite a few other parameters (key exchange algorithms, host-key algorithms, MAC algorithms) under /etc/ssh/sshd_config respectively using parameters Ciphers, KexAlgorithms, HostKeyAlgorithms and MACs. See https://man.openbsd.org/sshd_config for more information.

Software like ssh-audit (https://github.com/arthepsy/ssh-audit.git) can help you work out what could do with changing.

Disallowing a cipher happens if you don't list it as an option.

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    I've read that Fedora overrides this with its crypto policies. I'd like to edit those.
    – markonius
    Apr 29, 2020 at 15:55
  • Thanks for ssh-audit, it seems useful
    – markonius
    Apr 29, 2020 at 15:57
  • Adjustment to e.g. KexAlgorithms via /etc/ssh/sshd_config don't work on my Fedora 33 host.
    – HorstKevin
    Mar 27, 2021 at 13:35
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Editing KexAlgorithms in /etc/ssh/sshd.config won't show any effect, at least not on my Fedora 33 system.

But as you pointed out yourself, you can edit /etc/crypto-policies/back-ends/opensshserver.config and systemctl restart sshd afterwards. That works perfectly fine.

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