sshd遠(yuǎn)程連接時(shí)超慢

1.問題描述

在vm上新安裝的虛擬機(jī)褐墅，centos7写穴，用宿主機(jī)的xshell遠(yuǎn)程連接的時(shí)候需要等待大概10s的樣子所意，嘗試通過debug的方式追溯下原因轨域。

2.實(shí)際操作

通過ssh -v執(zhí)行debug模式

[root@localhost ssh]# ssh -v root@192.168.239.100
OpenSSH_7.4p1, OpenSSL 1.0.2k-fips  26 Jan 2017
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 58: Applying options for *
debug1: Connecting to 192.168.239.100 [192.168.239.100] port 22.
debug1: Connection established.
debug1: permanently_set_uid: 0/0
debug1: key_load_public: No such file or directory
debug1: identity file /root/.ssh/id_rsa type -1
debug1: key_load_public: No such file or directory
debug1: identity file /root/.ssh/id_rsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /root/.ssh/id_dsa type -1
debug1: key_load_public: No such file or directory
debug1: identity file /root/.ssh/id_dsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /root/.ssh/id_ecdsa type -1
debug1: key_load_public: No such file or directory
debug1: identity file /root/.ssh/id_ecdsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /root/.ssh/id_ed25519 type -1
debug1: key_load_public: No such file or directory
debug1: identity file /root/.ssh/id_ed25519-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_7.4
debug1: Remote protocol version 2.0, remote software version OpenSSH_7.4
debug1: match: OpenSSH_7.4 pat OpenSSH* compat 0x04000000
debug1: Authenticating to 192.168.239.100:22 as 'root'
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: algorithm: curve25519-sha256
debug1: kex: host key algorithm: ecdsa-sha2-nistp256
debug1: kex: server->client cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: none
debug1: kex: client->server cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: none
debug1: kex: curve25519-sha256 need=64 dh_need=64
debug1: kex: curve25519-sha256 need=64 dh_need=64
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: Server host key: ecdsa-sha2-nistp256 SHA256:4WaFL0X+hj57QX2TNbGRY5wuXYOKEezqfthABkMYsiY
debug1: Host '192.168.239.100' is known and matches the ECDSA host key.
debug1: Found key in /root/.ssh/known_hosts:1
debug1: rekey after 134217728 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: rekey after 134217728 blocks
debug1: SSH2_MSG_EXT_INFO received
debug1: kex_input_ext_info: server-sig-algs=<rsa-sha2-256,rsa-sha2-512>
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password
debug1: Next authentication method: gssapi-keyex
debug1: No valid Key exchange context
debug1: Next authentication method: gssapi-with-mic
**debug1: Unspecified GSS failure.  Minor code may provide more information
No Kerberos credentials available (default cache: KEYRING:persistent:0)**

debug1: Unspecified GSS failure.  Minor code may provide more information
No Kerberos credentials available (default cache: KEYRING:persistent:0)

從上述日志中伤为，可以發(fā)現(xiàn)我是GSS認(rèn)證未關(guān)導(dǎo)致驗(yàn)證無法通過咒循，只能等待超時(shí)后進(jìn)行下一步操作据途。
重啟服務(wù)后再次嘗試，依然連接緩慢叙甸，再次開啟debug模式查看原因

[root@localhost ssh]# ssh -v root@192.168.239.100
OpenSSH_7.4p1, OpenSSL 1.0.2k-fips  26 Jan 2017
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 58: Applying options for *
debug1: Connecting to 192.168.239.100 [192.168.239.100] port 22.
debug1: Connection established.
debug1: permanently_set_uid: 0/0
debug1: key_load_public: No such file or directory
debug1: identity file /root/.ssh/id_rsa type -1
debug1: key_load_public: No such file or directory
debug1: identity file /root/.ssh/id_rsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /root/.ssh/id_dsa type -1
debug1: key_load_public: No such file or directory
debug1: identity file /root/.ssh/id_dsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /root/.ssh/id_ecdsa type -1
debug1: key_load_public: No such file or directory
debug1: identity file /root/.ssh/id_ecdsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /root/.ssh/id_ed25519 type -1
debug1: key_load_public: No such file or directory
debug1: identity file /root/.ssh/id_ed25519-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_7.4
debug1: Remote protocol version 2.0, remote software version OpenSSH_7.4
debug1: match: OpenSSH_7.4 pat OpenSSH* compat 0x04000000
debug1: Authenticating to 192.168.239.100:22 as 'root'
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: algorithm: curve25519-sha256
debug1: kex: host key algorithm: ecdsa-sha2-nistp256
debug1: kex: server->client cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: none
debug1: kex: client->server cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: none
debug1: kex: curve25519-sha256 need=64 dh_need=64
debug1: kex: curve25519-sha256 need=64 dh_need=64
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: Server host key: ecdsa-sha2-nistp256 SHA256:4WaFL0X+hj57QX2TNbGRY5wuXYOKEezqfthABkMYsiY
debug1: Host '192.168.239.100' is known and matches the ECDSA host key.
debug1: Found key in /root/.ssh/known_hosts:1
debug1: rekey after 134217728 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: rekey after 134217728 blocks
**debug1: SSH2_MSG_EXT_INFO received**

這次是卡在debug1: SSH2_MSG_EXT_INFO received這里颖医，應(yīng)該是sshd服務(wù)自動(dòng)的參數(shù)：UseDNS默認(rèn)為yes導(dǎo)致，即開啟DNS反向查詢認(rèn)證裆蒸，
再次修改sshd的配置文件熔萧，添加以下配置

UseDNS no

systemctl restart sshd重啟服務(wù)后再次嘗試ssh遠(yuǎn)程連接，秒通僚祷。

總結(jié)

本次ssh遠(yuǎn)程連接緩慢的原因：
1.GSS認(rèn)證未關(guān)閉
2.DNS返查未關(guān)閉

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