手头有台linux服务器ssh登录时超级慢，需要几十秒。其它服务器均没有这个问题。平时登录操作都默默忍了。今天终于忍不住想搞清楚到底什么原因。搜索了一下发现了很多关于ssh登录慢的资料，于是自己也学着来分析、印证一下ssh登录慢的原因。
出现ssh登录慢一般有两个原因：DNS反向解析的问题和ssh的gssapi认证
1：ssh的gssapi认证问题
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
默认情况下，GSSAPIAuthentication在服务器端和客户端都激活的。如果DNS服务出现问题，那么登录过程要等到DNS查询超时后才能继续，这就是为什么SSH登录提示符要等很久才出现的原因。 为什么ssh登录过程中要用到DNS解析服务呢？这个是GSSAPI认证方式需要的缘故。
所以在配置文件/etc/ssh/sshd_config(服务器)或/etc/ssh/ssh_config（客户端）将参数GSSAPIAuthentication设置为no可以解决ssh登录慢的问题。
2：DNS反向解析的问题
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。
问题分析：
首先可以在ssh命令后面加上“-v“ 参数，输出debug信息定位问题。 具体操作为ssh -v root@serverip
[root@localhost ~]# ssh -v root@192.168.xxx.xxxOpenSSH_4.3p2, OpenSSL 0.9.8e-fips-rhel5 01 Jul 2008
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *debug1: Connecting to 192.168.xxx.xxx [192.168.xxx.xxx] port 22.
debug1: Connection established.
debug1: permanently_set_uid: 0/0
debug1: identity file /root/.ssh/identity type -1
debug1: identity file /root/.ssh/id_rsa type -1
debug1: identity file /root/.ssh/id_dsa type -1
debug1: loaded 3 keys
debug1: Remote protocol version 2.0, remote software version OpenSSH_4.3
debug1: match: OpenSSH_4.3 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0debug1: Local version string SSH-2.0-OpenSSH_4.3
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-ctr hmac-md5 none
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
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
The authenticity of host '192.168.xxx.xxx (192.168.xxx.xxx)' can't be established.
RSA key fingerprint is 04:08:57:22:7e:8d:dc:d3:8e:91:20:d0:ba:d9:ed:78.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '192.168.xxx.xxx' (RSA) to the list of known hosts.debug1: ssh_rsa_verify: signature correct
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 can continue: publickey,gssapi-with-mic,passWord
debug1: Next authentication method: gssapi-with-mic
debug1: Unspecified GSS failure. Minor code may provide more information
No credentials cache found
debug1: Unspecified GSS failure. Minor code may provide more information
No credentials cache found
debug1: Unspecified GSS failure. Minor code may provide more information
No credentials cache found
debug1: Next authentication method: publickey
debug1: Trying private key: /root/.ssh/identity
debug1: Trying private key: /root/.ssh/id_rsa
debug1: Trying private key: /root/.ssh/id_dsa
debug1: Next authentication method: password
root@192.168.xxx.xxx's password:
debug1: Authentication succeeded (password).
debug1: channel 0: new [client-session]
debug1: Entering interactive session.
debug1: Sending environment.
debug1: Sending env LANG = en_US.UTF-8
Last login: Sun Sep 6 08:30:47 2015 from 192.168.7.222
[root@ceglnx01 ~]# 
从上面输出信息看到有关于Unspecified GSS failure，于是我将/etc/ssh/sshd_config(服务器)或/etc/ssh/ssh_config（客户端）将参数GSSAPIAuthentication设置为no，重启了sshd服务，测试发现ssh登录还是很慢。
[root@localhost ~]# service sshd statusopenssh-daemon (pid 3594) is running...
[root@localhost ~]# service sshd restartStopping sshd: [ OK ]
Starting sshd: [ OK ]
那么原因应该是DNS反向解析的问题，关于DNS反向解析的问题有几个解决方法：
1：在server上/etc/hosts文件中把常用的ip和hostname加入，然后在/etc/nsswitch.conf看看程序是否先查询hosts文件
2：在server上/etc/ssh/sshd_config文件中修改或加入UseDNS=no。然后重启sshd服务
我在/etc/ssh/sshd_config上将UseDNS设置为no,重启sshd服务后，然后测试ssh连接速度。果然飞快连接上。看来主要还是DNS反向解析的问题。
参考资料：
http://www.linuxidc.com/Linux/2012-12/77144.htm
http://blog.chinaunix.net/uid-16728139-id-3435980.html
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