Possibility for customising host key algorithms?

[aodj]

Is your feature request related to a problem? Please describe.
I was using ssh-audit to inspect my ssh configurations, and after applying the ansible-ssh-hardening role it suggests removing the ecdsa-sha2-nistp256 algorithm from the permitted host key algorithms due to a potential for revealing the key with a weak RNG.

Describe the solution you'd like
Allow customization of host key algorithms.

Describe alternatives you've considered
Manual intervention

Additional context
Here's the output from running ssh-audit:

$ ssh-audit -24 mysshserver.local
# general
(gen) banner: SSH-2.0-OpenSSH_7.4
(gen) software: OpenSSH 7.4
(gen) compatibility: OpenSSH 7.2+, Dropbear SSH 2013.62+
(gen) compression: enabled ([email protected])

# key exchange algorithms
(kex) [email protected]          -- [info] available since OpenSSH 6.5, Dropbear SSH 2013.62
(kex) diffie-hellman-group-exchange-sha256 (2048-bit) -- [info] available since OpenSSH 4.4

# host-key algorithms
(key) ssh-rsa (2048-bit)                    -- [info] available since OpenSSH 2.5.0, Dropbear SSH 0.28
(key) rsa-sha2-512 (2048-bit)               -- [info] available since OpenSSH 7.2
(key) rsa-sha2-256 (2048-bit)               -- [info] available since OpenSSH 7.2
(key) ecdsa-sha2-nistp256                   -- [fail] using weak elliptic curves
                                            `- [warn] using weak random number generator could reveal the key
                                            `- [info] available since OpenSSH 5.7, Dropbear SSH 2013.62
(key) ssh-ed25519                           -- [info] available since OpenSSH 6.5

# encryption algorithms (ciphers)
(enc) [email protected]         -- [info] available since OpenSSH 6.5
                                            `- [info] default cipher since OpenSSH 6.9.
(enc) [email protected]                -- [info] available since OpenSSH 6.2
(enc) [email protected]                -- [info] available since OpenSSH 6.2
(enc) aes256-ctr                            -- [info] available since OpenSSH 3.7, Dropbear SSH 0.52
(enc) aes192-ctr                            -- [info] available since OpenSSH 3.7
(enc) aes128-ctr                            -- [info] available since OpenSSH 3.7, Dropbear SSH 0.52

# message authentication code algorithms
(mac) [email protected]              -- [info] available since OpenSSH 6.2
(mac) [email protected]         -- [info] available since OpenSSH 6.2
(mac) [email protected]         -- [info] available since OpenSSH 6.2

# fingerprints
(fin) ssh-ed25519: SHA256:xxxx
(fin) ssh-rsa: SHA256:xxxx

# algorithm recommendations (for OpenSSH 7.4)
(rec) -ecdsa-sha2-nistp256                  -- key algorithm to remove
(rec) +curve25519-sha256                    -- kex algorithm to append
(rec) +diffie-hellman-group14-sha256        -- kex algorithm to append
(rec) +diffie-hellman-group16-sha512        -- kex algorithm to append
(rec) +diffie-hellman-group18-sha512        -- kex algorithm to append

Of note are the recommendations at the bottom.

[rndmh3ro]

Hey @aodj,

right now we do not have the possibility to manage HostKeyAlgorithms. If you want to implement this feature, feel free to submit a PR!

[schurzi]

Hey @aodj,

with #278 merged this should be possible since release 8.1.0. Can you test this in your environment and confirm that it is working for you?