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：
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。
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
[root@192-168-3-40 ~]# ssh -vvv root@192.168.3.44
openssh_5.3p1, openssl 1.0.1e-fips 11 feb 2013
debug1: reading configuration data /etc/ssh/ssh_config
debug1: applying options for *
debug2: ssh_connect: needpriv 0
debug1: connecting to 192.168.3.44 [192.168.3.44] port 22.
debug1: connection established.
debug1: permanently_set_uid: 0/0
debug1: identity file /root/.ssh/identity type -1
debug1: identity file /root/.ssh/identity-cert type -1
debug1: identity file /root/.ssh/id_rsa type -1
debug1: identity file /root/.ssh/id_rsa-cert type -1
debug1: identity file /root/.ssh/id_dsa type -1
debug1: identity file /root/.ssh/id_dsa-cert type -1
debug1: identity file /root/.ssh/id_ecdsa type -1
debug1: identity file /root/.ssh/id_ecdsa-cert type -1
debug1: remote protocol version 2.0, remote software version openssh_5.3
debug1: match: openssh_5.3 pat openssh*
debug1: enabling compatibility mode for protocol 2.0
debug1: local version string ssh-2.0-openssh_5.3
debug2: fd 3 setting o_nonblock
debug1: ssh2_msg_kexinit sent
debug3: wrote 960 bytes for a total of 981
debug1: ssh2_msg_kexinit received
debug2: kex_parse_kexinit: diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,ssh-rsa,ssh-dss
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,rijndael-cbc@lysator.liu.se
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,rijndael-cbc@lysator.liu.se
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: none,zlib@openssh.com,zlib
debug2: kex_parse_kexinit: none,zlib@openssh.com,zlib
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit: first_kex_follows 0
debug2: kex_parse_kexinit: reserved 0
debug2: kex_parse_kexinit: diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: ssh-rsa,ssh-dss
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,rijndael-cbc@lysator.liu.se
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,rijndael-cbc@lysator.liu.se
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: none,zlib@openssh.com
debug2: kex_parse_kexinit: none,zlib@openssh.com
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit: first_kex_follows 0
debug2: kex_parse_kexinit: reserved 0
debug2: mac_setup: found hmac-md5
debug1: kex: server->client aes128-ctr hmac-md5 none
debug2: mac_setup: found hmac-md5
debug1: kex: client->server aes128-ctr hmac-md5 none
debug1: ssh2_msg_kex_dh_gex_request(1024<1024<8192) sent
debug1: expecting ssh2_msg_kex_dh_gex_group
debug3: wrote 24 bytes for a total of 1005
debug2: dh_gen_key: priv key bits set: 120/256
debug2: bits set: 506/1024
debug1: ssh2_msg_kex_dh_gex_init sent
debug1: expecting ssh2_msg_kex_dh_gex_reply
debug3: wrote 144 bytes for a total of 1149
debug3: check_host_in_hostfile: host 192.168.3.44 filename /root/.ssh/known_hosts
debug3: check_host_in_hostfile: host 192.168.3.44 filename /root/.ssh/known_hosts
debug3: check_host_in_hostfile: match line 8
debug1: host '192.168.3.44' is known and matches the rsa host key.
debug1: found key in /root/.ssh/known_hosts:8
debug2: bits set: 527/1024
debug1: ssh_rsa_verify: signature correct
debug2: kex_derive_keys
debug2: set_newkeys: mode 1
debug1: ssh2_msg_newkeys sent
debug1: expecting ssh2_msg_newkeys
debug3: wrote 16 bytes for a total of 1165
debug2: set_newkeys: mode 0
debug1: ssh2_msg_newkeys received
debug1: ssh2_msg_service_request sent
debug3: wrote 48 bytes for a total of 1213
debug2: service_accept: ssh-userauth
debug1: ssh2_msg_service_accept received
debug2: key: /root/.ssh/identity ((nil))
debug2: key: /root/.ssh/id_rsa ((nil))
debug2: key: /root/.ssh/id_dsa ((nil))
debug2: key: /root/.ssh/id_ecdsa ((nil))
debug3: wrote 64 bytes for a total of 1277
debug1: authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password
debug3: start over, passed a different list publickey,gssapi-keyex,gssapi-with-mic,password
debug3: preferred gssapi-keyex,gssapi-with-mic,publickey,keyboard-interactive,password
debug3: authmethod_lookup gssapi-keyex
debug3: remaining preferred: gssapi-with-mic,publickey,keyboard-interactive,password
debug3: authmethod_is_enabled gssapi-keyex
debug1: next authentication method: gssapi-keyex
debug1: no valid key exchange context
debug2: we did not send a packet, disable method
debug3: authmethod_lookup gssapi-with-mic
debug3: remaining preferred: publickey,keyboard-interactive,password
debug3: authmethod_is_enabled gssapi-with-mic
debug1: next authentication method: gssapi-with-mic
debug3: trying to reverse map address 192.168.3.44.
debug1: unspecified gss failure. minor code may provide more information
cannot determine realm for numeric host address</p>
<p>debug1: unspecified gss failure. minor code may provide more information
cannot determine realm for numeric host address</p>
<p>debug1: unspecified gss failure. minor code may provide more information</p>
<p>
debug1: unspecified gss failure. minor code may provide more information
cannot determine realm for numeric host address</p>
<p>debug2: we did not send a packet, disable method
debug3: authmethod_lookup publickey
debug3: remaining preferred: keyboard-interactive,password
debug3: authmethod_is_enabled publickey
debug1: next authentication method: publickey
debug1: trying private key: /root/.ssh/identity
debug3: no such identity: /root/.ssh/identity
debug1: trying private key: /root/.ssh/id_rsa
debug3: no such identity: /root/.ssh/id_rsa
debug1: trying private key: /root/.ssh/id_dsa
debug3: no such identity: /root/.ssh/id_dsa
debug1: trying private key: /root/.ssh/id_ecdsa
debug3: no such identity: /root/.ssh/id_ecdsa
debug2: we did not send a packet, disable method
debug3: authmethod_lookup password
debug3: remaining preferred: ,password
debug3: authmethod_is_enabled password
debug1: next authentication method: password
root@192.168.3.44's password:
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