手頭有臺linux服務器ssh登錄時超級慢，需要幾十秒。其它服務器均沒有這個問題。平時登錄操作都默默忍了。今天終于忍不住想搞清楚到底什么原因。搜索了一下發現了很多關于ssh登錄慢的資料，于是自己也學著來分析、印證一下ssh登錄慢的原因。
出現ssh登錄慢一般有兩個原因：DNS反向解析的問題和ssh的gssapi認證
1：ssh的gssapi認證問題
GSSAPI ( Generic Security Services application PRogramming Interface) 是一套類似Kerberos 5 的通用網絡安全系統接口。該接口是對各種不同的客戶端服務器安全機制的封裝，以消除安全接口的不同，降低編程難度。但該接口在目標機器無域名解析時會有問題
默認情況下，GSSAPIAuthentication在服務器端和客戶端都激活的。如果DNS服務出現問題，那么登錄過程要等到DNS查詢超時后才能繼續，這就是為什么SSH登錄提示符要等很久才出現的原因。 為什么ssh登錄過程中要用到DNS解析服務呢？這個是GSSAPI認證方式需要的緣故。
所以在配置文件/etc/ssh/sshd_config(服務器)或/etc/ssh/ssh_config（客戶端）將參數GSSAPIAuthentication設置為no可以解決ssh登錄慢的問題。
2：DNS反向解析的問題
OpenSSH在用戶登錄的時候會驗證ip，它根據用戶的IP使用反向DNS找到主機名，再使用DNS找到IP地址，最后匹配一下登錄的IP是否合法。如果客戶機的IP沒有域名，或者DNS服務器很慢或不通，那么登錄就會很花時間。
問題分析：
首先可以在ssh命令后面加上“-v“ 參數，輸出debug信息定位問題。 具體操作為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 ~]# 
從上面輸出信息看到有關于Unspecified GSS failure，于是我將/etc/ssh/sshd_config(服務器)或/etc/ssh/ssh_config（客戶端）將參數GSSAPIAuthentication設置為no，重啟了sshd服務，測試發現ssh登錄還是很慢。
[root@localhost ~]# service sshd statusopenssh-daemon (pid 3594) is running...
[root@localhost ~]# service sshd restartStopping sshd: [ OK ]
Starting sshd: [ OK ]
那么原因應該是DNS反向解析的問題，關于DNS反向解析的問題有幾個解決方法：
1：在server上/etc/hosts文件中把常用的ip和hostname加入，然后在/etc/nsswitch.conf看看程序是否先查詢hosts文件
2：在server上/etc/ssh/sshd_config文件中修改或加入UseDNS=no。然后重啟sshd服務
我在/etc/ssh/sshd_config上將UseDNS設置為no,重啟sshd服務后，然后測試ssh連接速度。果然飛快連接上。看來主要還是DNS反向解析的問題。
參考資料：
http://www.linuxidc.com/Linux/2012-12/77144.htm
http://blog.chinaunix.net/uid-16728139-id-3435980.html
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