ssh登錄很慢解決方法
使用ssh客戶端（如：putty）連接Linux服務器，可能會等待10-30秒才有提示輸入密碼。嚴重影響工作效率。登錄很慢，登錄上去后速度正常，這種情況主要有兩種可能的原因：
1. DNS反向解析問題
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。
解決辦法：在目標服務器上修改sshd服務器端配置,并重啟sshd
vi /etc/ssh/sshd_configUseDNS no
2. 關閉ssh的gssapi認證
用ssh -v user@server 可以看到登錄時有如下信息：
debug1: Next authentication method: gssapi-with-micdebug1: Unspecified GSS failure. Minor code may provide more information
注：ssh -vvv user@server 可以看到更細的debug信息
解決辦法：
修改sshd服務器端配置
vi /etc/ssh/ssh_configGSSAPIAuthentication no
可以使用ssh -o GSSAPIAuthentication=no user@server登錄
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
使用strace查看后發現，ssh在驗證完key之后，進行authentication gssapi-with-mic，此時先去連接DNS服務器，在這之后會進行其他操作
[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 debug1: Unspecified GSS failure. Minor code may provide more information Cannot determine realm for numeric host address debug1: Unspecified GSS failure. Minor code may provide more information debug1: Unspecified GSS failure. Minor code may provide more information Cannot determine realm for numeric host address 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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