1.通过[-v]参数，查看ssh连接的具体过程
deMacBook-Pro:~ yyq$ssh -v root@x.x.x.x -p xx
OpenSSH_6.9p1, LibreSSL 2.1.8
debug1: Reading configuration data/etc/ssh/ssh_config
debug1:/etc/ssh/ssh_config line 21: Applying optionsfor *
debug1: Connecting to x.x.x.x [x.x.x.x] port xx.
debug1: Connection established.
debug1: identityfile /Users/yyq/.ssh/id_rsa type 1
debug1: key_load_public: No suchfile or directory
debug1: identityfile /Users/yyq/.ssh/id_rsa-cert type -1
debug1: identityfile /Users/yyq/.ssh/id_dsa type 2
debug1: key_load_public: No suchfile or directory
debug1: identityfile /Users/yyq/.ssh/id_dsa-cert type -1
debug1: key_load_public: No suchfile or directory
debug1: identityfile /Users/yyq/.ssh/id_ecdsa type -1
debug1: key_load_public: No suchfile or directory
debug1: identityfile /Users/yyq/.ssh/id_ecdsa-cert type -1
debug1: key_load_public: No suchfile or directory
debug1: identityfile /Users/yyq/.ssh/id_ed25519 type -1
debug1: key_load_public: No suchfile or directory
debug1: identityfile /Users/yyq/.ssh/id_ed25519-cert type -1
debug1: Enabling compatibility modefor protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_6.9
debug1: Remote protocol version 2.0, remote software version OpenSSH_7.2p2 Ubuntu-4ubuntu2.1
debug1: match: OpenSSH_7.2p2 Ubuntu-4ubuntu2.1 pat OpenSSH* compat 0x04000000
debug1: Authenticating to x.x.x.x:xx as'root'
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client chacha20-poly1305@openssh.com <implicit> none
debug1: kex: client->server chacha20-poly1305@openssh.com <implicit> none
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: Server host key: ecdsa-sha2-nistp256 SHA256:HictrRpAs7Yv495dDLNqHsFCNKXVACpX9FWUnNVenZU
debug1: Host'[x.x.x.x]:xx' is known and matches the ECDSA host key.
debug1: Found keyin /Users/yyq/.ssh/known_hosts:38
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that cancontinue: publickey,password
debug1: Next authentication method: publickey
debug1: Offering RSA public key:/Users/yyq/.ssh/id_rsa
debug1: Authentications that cancontinue: publickey,password
debug1: Offering DSA public key:/Users/yyq/.ssh/id_dsa
debug1: Authentications that cancontinue: publickey,password
debug1: Trying private key:/Users/yyq/.ssh/id_ecdsa
debug1: Trying private key:/Users/yyq/.ssh/id_ed25519
debug1: Next authentication method: password
root@x.x.x.x's password:
首先观察有没有出现耗时的操作？
2.打开服务器的系统日志，查看错误日志
tail -f/var/log/auth.log
sshd[12642]: pam_systemd(sshd:session): Failed to create session: Connection timed out
dbus[617]: [system] Failed to activate service'org.freedesktop.login1': timed out
sshd[12642]: Received disconnect from x.x.x.x port 52856:11: disconnected by user
sshd[12642]: Disconnected from x.x.x.x port 52856
sshd[12642]: pam_unix(sshd:session): session closedfor user root
sshd[12689]: userauth_pubkey: keytype ssh-dss notin PubkeyAcceptedKeyTypes [preauth]
sshd[12689]: Accepted passwordfor root from x.x.x.x port 52866 ssh2
sshd[12689]: pam_unix(sshd:session): session openedfor user root by (uid=0)
sshd[12689]: pam_systemd(sshd:session): Failed to create session: Connection timed out
dbus[617]: [system] Failed to activate service'org.freedesktop.login1': timed out
从日志中可以看到[system] Failed to activate service 'org.freedesktop.login1': timed out的错误，大致意思如下：
dbus的服务重启后，systemd-logind服务没有重启导致，可以查看systemctl status systemd-logind的状态，解决方法就是重启该服务 systemctl restart systemd-logind
重启systemd-logind服务后，发现ssh可以秒连接了