Allow RSA key size below 2048 bit to successfully establish SSH connections to legacy systems

The crypto-policy that comes with Red Hat Enterprise Linux 9 and AlmaLinux 9 does not allow RSA key sizes below 2048 bit. Some legacy SSH servers are configured with 1024 bit key sizes though which will result in connection failures:

# ssh username@servername.example.com
Bad server host key: Invalid key length

Trying to find the issue using ssh -vvv, you would get:

OpenSSH_8.8p1, OpenSSL 3.0.5 5 Jul 2022
debug1: Reading configuration data /home/geeklab/.ssh/config
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Reading configuration data /etc/ssh/ssh_config.d/50-redhat.conf
debug1: Reading configuration data /etc/crypto-policies/back-ends/openssh.config
debug1: configuration requests final Match pass
debug1: re-parsing configuration
debug1: Reading configuration data /home/geeklab/.ssh/config
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Reading configuration data /etc/ssh/ssh_config.d/50-redhat.conf
debug1: Reading configuration data /etc/crypto-policies/back-ends/openssh.config
debug1: Reading configuration data /etc/ssh/ssh_config.d/99-jxs.conf
debug1: Connecting to servername.example.com [1.2.3.4] port 22.
debug1: Connection established.
debug1: identity file /home/geeklab/.ssh/id_rsa type 0
debug1: identity file /home/geeklab/.ssh/id_rsa-cert type -1
debug1: identity file /home/geeklab/.ssh/id_dsa type -1
debug1: identity file /home/geeklab/.ssh/id_dsa-cert type -1
debug1: identity file /home/geeklab/.ssh/id_ecdsa type -1
debug1: identity file /home/geeklab/.ssh/id_ecdsa-cert type -1
debug1: identity file /home/geeklab/.ssh/id_ecdsa_sk type -1
debug1: identity file /home/geeklab/.ssh/id_ecdsa_sk-cert type -1
debug1: identity file /home/geeklab/.ssh/id_ed25519 type 3
debug1: identity file /home/geeklab/.ssh/id_ed25519-cert type -1
debug1: identity file /home/geeklab/.ssh/id_ed25519_sk type -1
debug1: identity file /home/geeklab/.ssh/id_ed25519_sk-cert type -1
debug1: identity file /home/geeklab/.ssh/id_xmss type -1
debug1: identity file /home/geeklab/.ssh/id_xmss-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_8.8
debug1: Remote protocol version 2.0, remote software version OpenSSH_5.3
debug1: compat_banner: match: OpenSSH_5.3 pat OpenSSH_5* compat 0x0c000002
debug1: Authenticating to servername.example.com:22 as 'root'
debug1: load_hostkeys: fopen /home/geeklab/.ssh/known_hosts2: No such file or directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts: No such file or directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts2: No such file or directory
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: algorithm: diffie-hellman-group-exchange-sha256
debug1: kex: host key algorithm: ssh-rsa
debug1: kex: server->client cipher: aes256-ctr MAC: hmac-sha2-256 compression: none
debug1: kex: client->server cipher: aes256-ctr MAC: hmac-sha2-256 compression: none
debug1: kex: diffie-hellman-group-exchange-sha256 need=32 dh_need=32
debug1: kex: diffie-hellman-group-exchange-sha256 need=32 dh_need=32
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(2048<8192<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug1: SSH2_MSG_KEX_DH_GEX_GROUP received
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: SSH2_MSG_KEX_DH_GEX_REPLY received
Bad server host key: Invalid key length

While the best solution would be to fix the key size of the target server, this is not always possible on embedded devices. For those devices, you could alternatively make a change to your ~/.ssh/config file:

Host servername.example.com
     RSAMinSize 1024

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