SSH 连接超时(ssh timeout) 解决办法

原来的ssh是不会超时的, 但是自从上次给Ubuntu Server 系统升级后, 出现了ssh 超时现象:只要putty连续3分钟左右没有输入, 就自动断开, 然后必须重新登陆, 很麻烦.
 
在网上查了很多资料, 发现原因有多种, 环境变量TMOUT引起,ClientAliveCountMax和ClientAliveInterval设置问题或者甚至是防火墙的设置问题. 所以可以这么尝试:
 
1, echo $TMOUT
 如果显示空白,表示没有设置, 等于使用默认值0, 一般情况下应该是不超时. 如果大于0, 可以在如/etc/profile之类文件中设置它为0.
 Definition: TMOUT: If set to a value greater than zero, the value is interpreted as the number of seconds to wait for input after issuing the primary prompt. Bash terminates after waiting for that number of seconds if input does not arrive.

 2. ClientAliveInterval 60
 在/etc/ssh/sshd_config中增加ClientAliveInterval 60, ClientAliveInterval指定了服务器端向客户端请求消息的时间间隔, 默认是0, 不发送.而ClientAliveInterval 60表示每分钟发送一次, 然后客户端响应, 这样就保持长连接了.这里比较怪的地方是:不是客户端主动发起保持连接的请求(如FTerm, CTerm等),而是需要服务器先主动.
 
另外,至于ClientAliveCountMax, 使用默认值3即可.ClientAliveCountMax表示服务器发出请求后客户端没有响应的次数达到一定值, 就自动断开. 正常情况下, 客户端不会不响应.
 
ClientAliveCountMax
 Sets the number of client alive messages (see below) which may be
 sent without sshd(8) receiving any messages back from the client.
 If this threshold is reached while client alive messages are
 being sent, sshd will disconnect the client, terminating the ses-
 sion. It is important to note that the use of client alive mes-
 sages is very different from TCPKeepAlive (below). The client
 alive messages are sent through the encrypted channel and there-
 fore will not be spoofable. The TCP keepalive option enabled by
 TCPKeepAlive is spoofable. The client alive mechanism is valu-
 able when the client or server depend on knowing when a connec-
 tion has become inactive.
 
The default value is 3. If ClientAliveInterval (see below) is
 set to 15, and ClientAliveCountMax is left at the default, unre-
 sponsive SSH clients will be disconnected after approximately 45
 seconds. This option applies to protocol version 2 only.
 
ClientAliveInterval
 Sets a timeout interval in seconds after which if no data has
 been received from the client, sshd(8) will send a message
 through the encrypted channel to request a response from the
 client. The default is 0, indicating that these messages will
 not be sent to the client. This option applies to protocol ver-
 sion 2 only.
 
3. 启用putty keepalive
 putty -> Connection -> Seconds between keepalives ( 0 to turn off ), 默认为0, 改为60.
 
4. 上述3个方法应该能解决大部分问题, 如果不行, 请 man sshd_config, 然后尝试更改其他设置吧.