ClientAliveInterval：指定了服务器端向客户端请求消息的时间间隔，默认值是0；
ClientAliveCountMax：则指定这种请求服务器端发送后，客户端最多的无响应次数（但网上一般是说服务器端最多向客户端发送这种消息多少次，我觉得不太对），默认值是3。

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 session. It is important to note that the use of client alive messages is very different from TCPKeepAlive (below). The client alive messages are sent through the encrypted channel and therefore will not be spoofable. The TCP keepalive option enabled by TCPKeepAlive is spoofable. The client alive mechanism is valuable when the client or server depend on knowing when a connection has become inactive. The default value is 3. If ClientAliveInterval (see below) is set to 15, and ClientAliveCountMax is left at the default, unresponsive 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 version 2 only.

/* Wait for something to happen, or the timeout to expire. */
	ret = select((*maxfdp)+1, *readsetp, *writesetp, NULL, tvp);

	if (ret == -1) {
		memset(*readsetp, 0, *nallocp);
		memset(*writesetp, 0, *nallocp);
		if (errno != EINTR)
			error("select: %.100s", strerror(errno));
	} else {
		if (ret == 0 && client_alive_scheduled)
			client_alive_check();
		if (!compat20 && program_alive_scheduled && fdin_is_tty) {
			if (!fdout_eof)
				FD_SET(fdout, *readsetp);
			if (!fderr_eof)
				FD_SET(fderr, *readsetp);
		}
	}

int select(int maxfdp,fd_set *readfds,fd_set *writefds,fd_set *errorfds,struct timeval*timeout); 

if (compat20 &&
	 max_time_milliseconds == 0 && options.client_alive_interval) {
		client_alive_scheduled = 1;
		max_time_milliseconds = options.client_alive_interval * 1000;
	}

static void
client_alive_check(void)
{
	int channel_id;

	/* timeout, check to see how many we have had */
	if (packet_inc_alive_timeouts() > options.client_alive_count_max) {
		logit("Timeout, client not responding.");
		cleanup_exit(255);
	}

	/*
	 * send a bogus global/channel request with "wantreply",
	 * we should get back a failure
	 */
	if ((channel_id = channel_find_open()) == -1) {
		packet_start(SSH2_MSG_GLOBAL_REQUEST);
		packet_put_cstring("keepalive@openssh.com");
		packet_put_char(1);	/* boolean: want reply */
	} else {
		channel_request_start(channel_id, "keepalive@openssh.com", 1);
	}
	packet_send();
}

int
packet_inc_alive_timeouts(void)
{
	return ++active_state->keep_alive_timeouts;
}

channel_request_start(channel_id, "keepalive@openssh.com", 1);

packet_start(SSH2_MSG_CHANNEL_REQUEST);

dispatch_set(SSH2_MSG_CHANNEL_REQUEST, &server_input_channel_req); 

reply = packet_get_char();

…………


if (reply) {
		packet_start(success "htmlcode">

dispatch_set(SSH2_MSG_CHANNEL_SUCCESS, &server_input_keep_alive);
dispatch_set(SSH2_MSG_CHANNEL_FAILURE, &server_input_keep_alive);
dispatch_set(SSH2_MSG_REQUEST_SUCCESS, &server_input_keep_alive);
dispatch_set(SSH2_MSG_REQUEST_FAILURE, &server_input_keep_alive);


都是server_input_keep_alive函数，这个函数的意思就很简单了：


static void
server_input_keep_alive(int type, u_int32_t seq, void *ctxt)
{
	debug("Got %d/%u for keepalive", type, seq);
	/*
	 * reset timeout, since we got a sane answer from the client.
	 * even if this was generated by something other than
	 * the bogus CHANNEL_REQUEST we send for keepalives.
	 */
	packet_set_alive_timeouts(0);
}


packet_set_alive_timeouts(0)就是把active_state->keep_alive_timeouts的值设为0，也就是重置了这个计数。
所以饶了一圈，这些代码的意思大概应该是这样，如果客户端在规定时间内没有响应，就先判断未响应次数是否超过设置的值，如果超过就结束。
如果没超过，就把这个次数加1，然后发送一个请求，看看客户端还在不在，如果客户端网络正常（应该会自动回复），就会得到客户端的回复，于是就重置这个未响应计数。
四、两个方式的不同
TMOUT方式可以针对所有用户通过本地tty或远程ssh登录时起作用，而修改sshd_config只针对使用ssh登录的用户。
另外一点就是TMOUT判断你有没有在操作，好像是看你有没有输入什么字符然后敲回车执行，你输入正确的命令敲回车执行命令，就算。输入无意义的字符敲回车没有找到可执行的命令，那也算。光输入字符不敲回车就不算，而判断你处于空闲状态，超时就会登出。
而ssh是基于网络来判断，只要客户端对服务器有发送信息，那就算有在操作。
这也是两者的一些细微的不同之处。
总结
以上所述是小编给大家介绍的等保测评：Centos超时退出,希望对大家有所帮助，如果大家有任何疑问请给我留言，小编会及时回复大家的。在此也非常感谢大家对网站的支持！

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