ClientAliveInterval：指定了服務器端向客戶端請求消息的時間間隔，默認值是0；
ClientAliveCountMax：則指定這種請求服務器端發送后，客戶端最多的無響應次數（但網上一般是說服務器端最多向客戶端發送這種消息多少次，我覺得不太對），默認值是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 ?
		  SSH2_MSG_CHANNEL_SUCCESS : SSH2_MSG_CHANNEL_FAILURE);
		packet_put_int(c->remote_id);
		packet_send();
	}

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);

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);
}