Open SSH debug trace shows that ssh is looking for the known_hosts file(s) twice, once before the key exchange and once during, why is that?
I understand that as part of the key exchange ssh verifies the key in the known_hosts file, but I don't understand why it is trying to open them twice?
I've put a debug ssh example connection to github below and notice that debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts: No such file or directory occurs twice.
ssh -T git@github.com -v
OpenSSH_8.6p1, LibreSSL 3.3.6
debug1: Reading configuration data /Users/foo_user/.ssh/config
debug1: /Users/foo_user/.ssh/config line 14: Applying options for *
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 21: include /etc/ssh/ssh_config.d/* matched no files
debug1: /etc/ssh/ssh_config line 54: Applying options for *
debug1: Authenticator provider $SSH_SK_PROVIDER did not resolve; disabling
debug1: Connecting to github.com port 22.
debug1: Connection established.
debug1: identity file /Users/foo_user/.ssh/id_rsa type 0
debug1: identity file /Users/foo_user/.ssh/id_rsa-cert type -1
debug1: identity file /Users/foo_user/.ssh/id_ed25519 type 3
debug1: identity file /Users/foo_user/.ssh/id_ed25519-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_8.6
debug1: Remote protocol version 2.0, remote software version babeld-b9c2a189
debug1: compat_banner: no match: babeld-b9c2a189
debug1: Authenticating to github.com:22 as 'git'
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: curve25519-sha256
debug1: kex: host key algorithm: rsa-sha2-512
debug1: kex: server->client cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: none
debug1: kex: client->server cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: none
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: SSH2_MSG_KEX_ECDH_REPLY received
debug1: Server host key: ssh-rsa SHA256:mooSHmooSHmooSHmooSHmooSHmooSHmooSHmooSHmoo
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: Host 'github.com' is known and matches the RSA host key.
debug1: Found key in /Users/foo_user/.ssh/known_hosts:1
debug1: rekey out after 134217728 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: rekey in after 134217728 blocks
debug1: Will attempt key: /Users/foo_user/.ssh/id_rsa RSA SHA256:fooIVfooIVfooIVfooIVfooIVfooIVfooIVfooIVfoo explicit agent
debug1: Will attempt key: /Users/foo_user/.ssh/id_ed25519 ED25519 SHA256:barEDbarEDbarEDbarEDbarEDbarEDbarEDbarEDbar explicit
debug1: SSH2_MSG_EXT_INFO received
debug1: kex_input_ext_info: server-sig-algs=<ssh-ed25519-cert-v01@openssh.com,ecdsa-sha2-nistp521-cert-v01@openssh.com,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp256-cert-v01@openssh.com,sk-ssh-ed25519-cert-v01@openssh.com,sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,ssh-rsa-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,sk-ecdsa-sha2-nistp256@openssh.com,ssh-ed25519,ecdsa-sha2-nistp521,ecdsa-sha2-nistp384,ecdsa-sha2-nistp256,rsa-sha2-512,rsa-sha2-256,ssh-rsa>
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey
debug1: Next authentication method: publickey
debug1: Offering public key: /Users/foo_user/.ssh/id_rsa RSA SHA256:fooIVfooIVfooIVfooIVfooIVfooIVfooIVfooIVfoo explicit agent
debug1: Server accepts key: /Users/foo_user/.ssh/id_rsa RSA SHA256:fooIVfooIVfooIVfooIVfooIVfooIVfooIVfooIVfoo explicit agent
debug1: Authentication succeeded (publickey).
Authenticated to github.com ([20.248.137.48]:22).
debug1: channel 0: new [client-session]
debug1: Entering interactive session.
debug1: pledge: filesystem full
debug1: client_input_global_request: rtype hostkeys-00@openssh.com want_reply 0
debug1: client_input_hostkeys: searching /Users/foo_user/.ssh/known_hosts for github.com / (none)
debug1: client_input_hostkeys: searching /Users/foo_user/.ssh/known_hosts2 for github.com / (none)
debug1: client_input_hostkeys: host key found matching a different name/address, skipping UserKnownHostsFile update
debug1: Sending environment.
debug1: channel 0: setting env LANG = "en_AU.UTF-8"
debug1: client_input_channel_req: channel 0 rtype exit-status reply 0
