最近使用VPN链接国外服务器的时候，突然出现ssh登录之后，進行複雜的操作后连接挂起，或者說ssh直接连接不上的情况，并且报错：
xxxxxx$ ssh xxxxxxxx@xxx.xxx.xxx.xxx -vvvOpenSSH_7.6p1 Ubuntu-4ubuntu0.5, OpenSSL 1.0.2n 7 Dec 2017debug1: Reading configuration data /etc/ssh/ssh_configdebug1: /etc/ssh/ssh_config line 19: Applying options for *debug2: resolving "xxx.xxx.xxx.xxx" port 22debug2: ssh_connect_direct: needpriv 0debug1: Connecting to xxx.xxx.xxx.xxx [xxx.xxx.xxx.xxx] port 22.debug1: Connection established.debug1: key_load_public: No such file or directorydebug1: identity file /home/xxxxxxx/.ssh/id_rsa type -1debug1: key_load_public: No such file or directorydebug1: identity file /home/xxxxxxx/.ssh/id_rsa-cert type -1debug1: key_load_public: No such file or directorydebug1: identity file /home/xxxxxxx/.ssh/id_dsa type -1debug1: key_load_public: No such file or directorydebug1: identity file /home/xxxxxxx/.ssh/id_dsa-cert type -1debug1: key_load_public: No such file or directorydebug1: identity file /home/xxxxxxx/.ssh/id_ecdsa type -1debug1: key_load_public: No such file or directorydebug1: identity file /home/xxxxxxx/.ssh/id_ecdsa-cert type -1debug1: identity file /home/xxxxxxx/.ssh/id_ed25519 type 3debug1: key_load_public: No such file or directorydebug1: identity file /home/xxxxxxx/.ssh/id_ed25519-cert type -1debug1: Local version string SSH-2.0-OpenSSH_7.6p1 Ubuntu-4ubuntu0.5debug1: Remote protocol version 2.0, remote software version OpenSSH_7.6p1 Ubuntu-4ubuntu0.7debug1: match: OpenSSH_7.6p1 Ubuntu-4ubuntu0.7 pat OpenSSH* compat 0x04000000debug2: fd 3 setting O_NONBLOCKdebug1: Authenticating to xxx.xxx.xxx.xxx:22 as 'xxxxxxxx'debug3: hostkeys_foreach: reading file "/home/xxxxxxx/.ssh/known_hosts"debug3: send packet: type 20debug1: SSH2_MSG_KEXINIT sentdebug3: receive packet: type 20debug1: SSH2_MSG_KEXINIT receiveddebug2: local client KEXINIT proposaldebug2: KEX algorithms: curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1,ext-info-cdebug2: host key algorithms: ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp521-cert-v01@openssh.com,ssh-ed25519-cert-v01@openssh.com,ssh-rsa-cert-v01@openssh.com,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsadebug2: ciphers ctos: chacha20-poly1305@openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.comdebug2: ciphers stoc: chacha20-poly1305@openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.comdebug2: MACs ctos: umac-64-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-64@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1debug2: MACs stoc: umac-64-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-64@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1debug2: compression ctos: none,zlib@openssh.com,zlibdebug2: compression stoc: none,zlib@openssh.com,zlibdebug2: languages ctos: debug2: languages stoc: debug2: first_kex_follows 0 debug2: reserved 0 debug2: peer server KEXINIT proposaldebug2: KEX algorithms: curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1debug2: host key algorithms: ssh-rsa,rsa-sha2-512,rsa-sha2-256,ecdsa-sha2-nistp256,ssh-ed25519debug2: ciphers ctos: chacha20-poly1305@openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.comdebug2: ciphers stoc: chacha20-poly1305@openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.comdebug2: MACs ctos: umac-64-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-64@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1debug2: MACs stoc: umac-64-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-64@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1debug2: compression ctos: none,zlib@openssh.comdebug2: compression stoc: none,zlib@openssh.comdebug2: languages ctos: debug2: languages stoc: debug2: first_kex_follows 0 debug2: reserved 0 debug1: kex: algorithm: curve25519-sha256debug1: kex: host key algorithm: ecdsa-sha2-nistp256debug1: kex: server->client cipher: chacha20-poly1305@openssh.com MAC: compression: nonedebug1: kex: client->server cipher: chacha20-poly1305@openssh.com MAC: compression: nonedebug3: send packet: type 30debug1: expecting SSH2_MSG_KEX_ECDH_REPLYConnection closed by xxx.xxx.xxx.xxx port 22 起初懷疑是不是vpn的問題，但是外國的同事也不能ssh直接登錄，所以説這應該是服務器主機的某些設置不合理，所以利用ping工具向服務器发送tcp包测试网络情况，发现向服务器发送不同字节长度的tcp包的时候，有很明显的传输失败的现象：
这问题跟MTU的设置有关，该值的大小影响tcp传输的拆包组包，故将服务器的mtu的值进行设置(原来1404，现在设置小点）：
sudo ifconfig ham0 mtu 1389然后再次ssh连接该服务器， 发现一切正常。
来源地址：https://blog.csdn.net/weixin_44092905/article/details/127826003
