TCP_KEEPALIVE选项只是一个开关，Linux中默认的Keepalive的选项如下：
$sudo sysctl -a | grep keepalive net.ipv4.tcp_keepalive_time = 7200 net.ipv4.tcp_keepalive_probes = 9 net.ipv4.tcp_keepalive_intvl = 75
上文中的keepalive选项表示如果一个连接上7200s后没有任何数据发送，则设置了这个选项的本端向对端发送keepalive保活报文，它会有如下三种结果：
- 对端回复ACK。则本端TCP认为该连接依然存活。继续等7200s后再发送keepalive报文。
- 对端回复RESET。说明对端进程已经重启，本端的应用程序应该关闭该连接。
- 没有对端的任何回复。则本端做重试，如果重试9次（前后重试间隔为75秒）仍然不可达，则向应用程序返回错误信息，ETIMEOUT（无任何应答）或EHOST
如果应用程序向改变keepalive的默认行为，该怎么办呢？答案就是利用 TCP_KEEPIDLE、TCP_KEEPINTVL、TCP_KEEPCNT这几个TCP选项，首先看看如何使用：
int setKeepAlive(int fd, int interval)
{
int val = 1;
if (setsockopt(fd, SOL_SOCKET, SO_KEEPALIVE, &val, sizeof(val)) == -1) {
printf("setsockopt SO_KEEPALIVE: %s", strerror(errno));
return -1;
}
/* Send first probe after `interval' seconds. */
val = interval;
if (setsockopt(fd, IPPROTO_TCP, TCP_KEEPIDLE, &val, sizeof(val)) < 0) {
printf("setsockopt TCP_KEEPIDLE: %s
", strerror(errno));
return -1;
}
/* Send next probes after the specified interval. Note that we set the
* delay as interval / 3, as we send three probes before detecting
* an error (see the next setsockopt call). */
val = interval/3;
if (val == 0) val = 1;
if (setsockopt(fd, IPPROTO_TCP, TCP_KEEPINTVL, &val, sizeof(val)) < 0) {
printf("setsockopt TCP_KEEPINTVL: %s
", strerror(errno));
return -1;
}
/* Consider the socket in error state after three we send three ACK
* probes without getting a reply. */
val = 3;
if (setsockopt(fd, IPPROTO_TCP, TCP_KEEPCNT, &val, sizeof(val)) < 0) {
printf("setsockopt TCP_KEEPCNT: %s
", strerror(errno));
return -1;
}
return 0;
}
- TCP_KEEPDILE 设置连接上如果没有数据发送的话，多久后发送keepalive探测分组，单位是秒
- TCP_KEEPINTVL 前后两次探测之间的时间间隔，单位是秒
- TCP_KEEPCNT 关闭一个非活跃连接之前的最大重试次数
用tcpdump抓包就可以看到设置了上面选项的那端的详细行为。