使用ssh客户端（如：putty）连接Linux服务器，可能会等待10-30秒才有提示输入密码。严重影响工作效率。登录很慢，登录上去后速度正常，这种情况主要有两种可能的原因：
1. DNS反向解析问题
OpenSSH在用户登录的时候会验证IP，它根据用户的IP使用反向DNS找到主机名，再使用DNS找到IP地址，最后匹配一下登录的IP是否合法。如果客户机的IP没有域名，或者DNS服务器很慢或不通，那么登录就会很花时间。
解决办法：在目标服务器上修改sshd服务器端配置,并重启sshd
- vi /etc/ssh/sshd_config
- UseDNS no
2. 关闭ssh的gssapi认证
用ssh -v user@server 可以看到登录时有如下信息：
- debug1: Next authentication method: gssapi-with-mic
- debug1: Unspecified GSS failure. Minor code may provide more information
注：ssh -vvv user@server 可以看到更细的debug信息
解决办法：
修改sshd服务器端配置
- vi /etc/ssh/ssh_config
- GSSAPIAuthentication no
可以使用ssh -o GSSAPIAuthentication=no user@server登录
GSSAPI ( Generic Security Services Application Programming Interface) 是一套类似Kerberos 5的通用网络安全系统接口。该接口是对各种不同的客户端服务器安全机制的封装，以消除安全接口的不同，降低编程难度。但该接口在目标机器无域名解析时会有问题
使用strace查看后发现，ssh在验证完key之后，进行authentication gssapi-with-mic，此时先去连接DNS服务器，在这之后会进行其他操作
OpenSSH_6.6.1, OpenSSL 1.0.1e-fips 11 Feb 2013
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 56: Applying options for *
debug1: Connecting to kr.worthcloud.cn [107.150.126.57] port 36000.
debug1: Connection established.
debug1: identity file /home/fredzeng/.ssh/id_rsa type -1
debug1: identity file /home/fredzeng/.ssh/id_rsa-cert type -1
debug1: identity file /home/fredzeng/.ssh/id_dsa type -1
debug1: identity file /home/fredzeng/.ssh/id_dsa-cert type -1
debug1: identity file /home/fredzeng/.ssh/id_ecdsa type -1
debug1: identity file /home/fredzeng/.ssh/id_ecdsa-cert type -1
debug1: identity file /home/fredzeng/.ssh/id_ed25519 type -1
debug1: identity file /home/fredzeng/.ssh/id_ed25519-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_6.6.1
debug1: Remote protocol version 2.0, remote software version OpenSSH_7.4
debug1: match: OpenSSH_7.4 pat OpenSSH* compat 0x04000000
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-ctr hmac-sha1-etm@openssh.com none
debug1: kex: client->server aes128-ctr hmac-sha1-etm@openssh.com none
debug1: kex: curve25519-sha256@libssh.org need=20 dh_need=20
debug1: kex: curve25519-sha256@libssh.org need=20 dh_need=20
debug1: sending SSH2_MSG_KEX_ECDH_INIT
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: Server host key: ECDSA 66:cc:4d:4a:e0:88:3d:77:5f:a4:fd:5f:ff:05:5c:94
debug1: Host ‘[kr.worthcloud.cn]:36000’ is known and matches the ECDSA host key.
debug1: Found key in /home/fredzeng/.ssh/known_hosts:3
debug1: ssh_ecdsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: Roaming not allowed by server
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,password
debug1: Next authentication method: publickey
debug1: Trying private key: /home/fredzeng/.ssh/id_rsa
debug1: Trying private key: /home/fredzeng/.ssh/id_dsa
debug1: Trying private key: /home/fredzeng/.ssh/id_ecdsa
debug1: Trying private key: /home/fredzeng/.ssh/id_ed25519
debug1: Next authentication method: password