ubuntu无法ssh到远程服务器

谁践踏了优雅 2022-06-04 01:11 266阅读 0赞

# 1 问题 #

openstack环境下创建了一个虚拟机，为虚拟机分配了浮动IP: 192.168.50.61。但是从ubuntu14.04服务器192.168.50.5上无法ssh到虚拟机服务器，SSH xxx@ip 就没有反应，也没有报错，可以PING通对方，TELNET对方的22端口也是通的。

使用ssh -vv看卡在哪里，出现错误：

root@node5:~# ssh -vv ubuntu@192.168.50.61
    OpenSSH_6.6.1, OpenSSL 1.0.1f 6 Jan 2014
    debug1: Reading configuration data /etc/ssh/ssh_config
    debug1: /etc/ssh/ssh_config line 19: Applying options for *
    debug2: ssh_connect: needpriv 0
    debug1: Connecting to 192.168.50.61 [192.168.50.61] port 22.
    debug1: Connection established.
    debug1: permanently_set_uid: 0/0
    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: identity file /root/.ssh/id_ed25519 type -1
    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_6.6.1p1 Ubuntu-2ubuntu2.8
    debug1: Remote protocol version 2.0, remote software version OpenSSH_6.6.1p1 Ubuntu-2ubuntu2.8
    debug1: match: OpenSSH_6.6.1p1 Ubuntu-2ubuntu2.8 pat OpenSSH_6.6.1* compat 0x04000000
    debug2: fd 3 setting O_NONBLOCK
    debug1: SSH2_MSG_KEXINIT sent
    ...
    debug1: sending SSH2_MSG_KEX_ECDH_INIT
    debug1: expecting SSH2_MSG_KEX_ECDH_REPLY

然后就一直卡住不动了。

# 2 方法 #

查看原因是在192.168.50.5服务器上[mtu设置太大][mtu]：http://www.snailbook.com/faq/mtu-mismatch.auto.html

Change the network interface MTU to solve it. This is a bug for ubuntu 14.04.
    This worked for me:
    
    sudo ip li set mtu 1200 dev wlan0
    Or:
    sudo ifconfig wlan0 mtu 1200

参考： [https://superuser.com/questions/568891/ssh-works-in-putty-but-not-terminal][https_superuser.com_questions_568891_ssh-works-in-putty-but-not-terminal]

查看本机：

root@node5:~# ip l
    1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1
        link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    2: p5p1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
        link/ether ac:16:2d:0b:a8:9b brd ff:ff:ff:ff:ff:ff

修改网卡mtu后问题解决。

root@node5:~# ip li set mtu 1200 dev p5p1
    root@node5:~# ssh ubuntu@192.168.50.61
    ubuntu@192.168.50.61's password: 
    root@node5:~# ip l
    1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1
        link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    2: p5p1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1200 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
        link/ether ac:16:2d:0b:a8:9b brd ff:ff:ff:ff:ff:ff

# 3 其它方法 #

## 3.1 echo "1200" > /sys/class/net/eth0/mtu ##

现象：主机间互通正常且可以判断对方ssh端口是开放的，但是用ssh xxx.xxx.xxx.xxx -v这种方式连接的时候会卡在debug1: SSH2\_MSG\_KEXINIT sent这步

解决方法：echo "1200" > /sys/class/net/eth0/mtu

原因：详情参考http://techbackground.blogspot.com/2013/06/path-mtu-discovery-and-gre.html   简单解释就是IPV4报头与GRE报头结构不同，导致GRE数据包最大内容载荷只有1454，默认mtu如果是1500的话，就会有46字节的内容无法处理导致错误

## 3.2 修改/etc/ssh/ssh\_config ##

The solution was found here: [SSH works in putty but not terminal][]

in Ubuntu 13.10/12.10, login and gain sudo access.

Edit `/etc/ssh/ssh_config`, uncomment the following lines

Ciphers aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc
    GSSAPIAuthentication yes
    GSSAPIDelegateCredentials no
    MACs hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-ripemd160

Add the following line

HostKeyAlgorithms ssh-rsa,ssh-dss

You should end up with your `/etc/ssh/ssh_config` file looking like this

Host *
    Ciphers aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc
    MACs hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-ripemd160
    SendEnv LANG LC_*
    HashKnownHosts yes
    GSSAPIAuthentication yes
    GSSAPIDelegateCredentials no
    HostKeyAlgorithms ssh-rsa,ssh-dss

Now run `ssh -T -v git@github.com` and it will ask you to add the server to your known hosts file. Hit yes, and then it should welcome you to the server.

3.3 修改内核

SSH xxx@ip 就没有反应了，也没有报错

可以PING通对方，TELNET对方的22端口也是通到  
局域网里到WINDOWS使用PUTTY是可以链接到远程主机到  
真是奇怪呀  
  
uncoffee@suncoffee:~$ ssh -v IP  
OpenSSH\_5.5p1 Debian-4ubuntu4, OpenSSL 0.9.8o 01 Jun 2010  
debug1: Reading configuration data /etc/ssh/ssh\_config  
debug1: Applying options for \*  
debug1: Connecting to IP \[IP\] port 22.  
debug1: Connection established.  
debug1: identity file /home/suncoffee/.ssh/id\_rsa type -1  
debug1: identity file /home/suncoffee/.ssh/id\_rsa-cert type -1  
debug1: identity file /home/suncoffee/.ssh/id\_dsa type -1  
debug1: identity file /home/suncoffee/.ssh/id\_dsa-cert type -1  
debug1: Remote protocol version 1.99, remote software version OpenSSH\_3.9p1  
debug1: match: OpenSSH\_3.9p1 pat OpenSSH\_3.\*  
debug1: Enabling compatibility mode for protocol 2.0  
debug1: Local version string SSH-2.0-OpenSSH\_5.5p1 Debian-4ubuntu4  
debug1: SSH2\_MSG\_KEXINIT sent  
  
到这里就停了，没有反应了

解决办法：修改内核参数  
net.ipv4.tcp\_rmem = 4096 87380 207520

# 4 如何修改内核 #

查看当前系统参数sysctl -a  
手动修改（当前有效，重启后无效）joy@joy:~$ cat /proc/sys/net/ipv4/icmp\_echo\_ignore\_all0joy@joy:~$ echo "1" >  /proc/sys/net/ipv4/icmp\_echo\_ignore\_all  
命令修改（当前有效，重启后无效）sysctl -w net.ipv4.icmp\_echo\_ignore\_all=1   （参数名与=文件路径去掉/proc/sys/，且"/ "改成"."）  
配置文件中修改（当前无效，重启永久生效）vim /etc/sysctl.conf添加net.ipv4.icmp\_echo\_ignore\_all=1  
让其修改后立刻生效 sysctl -p  
参数迁移1、当初当前主机所有配置sysctl -a > mysys.conf  
2、拷贝到其他主机并执行sysctl -p -f mysys.conf  
3、如果使用2套参数文件后性能大不相同，可对比2个参数文件

[mtu]: http://www.snailbook.com/faq/mtu-mismatch.auto.html
[https_superuser.com_questions_568891_ssh-works-in-putty-but-not-terminal]: https://superuser.com/questions/568891/ssh-works-in-putty-but-not-terminal
[SSH works in putty but not terminal]: http://superuser.com/questions/568891/ssh-works-in-putty-but-not-terminal