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1、在linux下，测试tcp保活，可以使用tcp自带keepalive功能。

2、几个重要参数：

•        tcp_keepalive_time：对端在指定时间内没有数据传输，则向对端发送一个keepalive packet，单位：秒

•        tcp_keepalive_intvl：向对端发送了一个keepalive packet,如果对端无响应，则等待tcp_keepalive_intvl秒后再次发送一个keepalive packet

•        tcp_keepalive_probes：向对端发送一个keepalive packet，如果对端没有响应，重发的次数,如果tcp_keepalive_probes次数后，对端都没有响应，则表示这个socket已经断开，系统会差生连接断开事件，本地就可以关闭这个socket上连接。

3、修改三个参数的系统默认值

      上述三个参数当前配置可以在 /proc/sys/net/ipv4查看   

可以使用cat查看具体配置，比如

• 全局设置:可更改/etc/sysctl.conf,加上:

     sudo vim /etc/sysctl.conf

      net.ipv4.tcp_keepalive_intvl = 20
      net.ipv4.tcp_keepalive_probes = 3
      net.ipv4.tcp_keepalive_time = 60

       保存：wq

       重启系统：sudo reboot

4、编程设置keeepalive:

#include <netinet/tcp.h>/*开始设置保活机制*/int val = 1;if (setsockopt(nc->sock, SOL_SOCKET, SO_KEEPALIVE, &val, sizeof val) != 0)
{perror("Set SO_KEEPALIVE fail");}/* Default settings are more or less garbage, with the keepalive time* set to 7200 by default on Linux. Modify settings to make the feature* actually useful. *//* Send first probe after interval. */val = 10;if (setsockopt(nc->sock, IPPROTO_TCP, TCP_KEEPIDLE, &val, sizeof(val)) < 0)
{perror("Set TCP_KEEPIDLE fail");}/* 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 = 3;if (setsockopt(nc->sock, IPPROTO_TCP, TCP_KEEPINTVL, &val, sizeof(val)) < 0)
{perror("Set TCP_KEEPINTVL fail");}/* Consider the socket in error state after three we send three ACK* probes without getting a reply. */val = 3;if (setsockopt(nc->sock, IPPROTO_TCP, TCP_KEEPCNT, &val, sizeof(val)) < 0)
{perror("Set TCP_KEEPCNT fail");}

5、查看通讯情况，打开wireshark,在过滤器输入设置keepalive的Ip格式为：ip.addr==xxx.xxx.xxx.xxx

上图可以看到，每隔10秒钟发送了一个keepalive packet.因为上述代码中，我们设置的TCP_KEEPIDLE=10