打开 SSH 调试跟踪显示 ssh 正在寻找 known_hosts 文件两次，一次是在密钥交换之前，一次是在密钥交换期间，这是为什么？
我理解，作为密钥交换的一部分，ssh 会验证 known_hosts 文件中的密钥，但我不明白为什么它要尝试打开它们两次？
我在下面将一个调试 ssh 示例连接到 github，并注意到debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts: No such file or directory发生了两次。
ssh -T [email protected] -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: [email protected] MAC: <implicit> compression: none
debug1: kex: client->server cipher: [email protected] 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=<[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],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 [email protected] 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
答案1
什么时候UpdateHostKeys启用后，在用户通过身份验证后，将验证是否有需要更新的主机密钥~/.ssh/known_hosts。
因此第一次~/.ssh/known_hosts是在身份验证期间读取的。
第二次是检查是否有任何主机密钥需要更新。
在提供的输出中我可以看到[email protected] want_reply- 从这一点开始~/.ssh/known_hosts第二次访问。