一个TCP套接口创建后默认是没有开启keepalive选项的，要使用setsockopt()函数来开启。
keepalive选项开启后，tcp_keepalive_time、tcp_keepalive_interval、tcp_keepalive_probes三个值都采用系统的默认值，使用getsockopt()函数可以查看这些值。改变这些值有两种方法：一是改变系统默认值，这样的话创建的socket默认都采用该值，二是对socket使用setsockopt()函数改变某个socket的这三个值。
查看和改变系统默认值的方法：
1.
# cat /proc/sys/net/ipv4/tcp_keepalive_time
7200
# cat /proc/sys/net/ipv4/tcp_keepalive_intvl
75
# cat /proc/sys/net/ipv4/tcp_keepalive_probes
9
# echo 600 > /proc/sys/net/ipv4/tcp_keepalive_time
# echo 60 > /proc/sys/net/ipv4/tcp_keepalive_intvl
# echo 20 > /proc/sys/net/ipv4/tcp_keepalive_probes
2.
# sysctl \
> net.ipv4.tcp_keepalive_time \
> net.ipv4.tcp_keepalive_intvl \
> net.ipv4.tcp_keepalive_probes
net.ipv4.tcp_keepalive_time = 7200
net.ipv4.tcp_keepalive_intvl = 75
net.ipv4.tcp_keepalive_probes = 9
# sysctl -w \
> net.ipv4.tcp_keepalive_time=600 \
> net.ipv4.tcp_keepalive_intvl=60 \
> net.ipv4.tcp_keepalive_probes=20
net.ipv4.tcp_keepalive_time = 600
net.ipv4.tcp_keepalive_intvl = 60
net.ipv4.tcp_keepalive_probes = 20
使用函数查看和改变某个socket的keepalive相关值：
//tcp_keepalive.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
//#define EXIT_FAILURE -1
//#define EXIT_SUCCESS 0
#define perror printf
int main()
{
int s,t;
int optval;
int tcp_keepalive_time;
int tcp_keepalive_interval;
int tcp_keepalive_probes;
socklen_t optlen = sizeof(optval);
/* Create the socket */
if((s = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0) {
perror("socket()");
exit(EXIT_FAILURE);
}
/* Check the status for the keepalive option */
if(getsockopt(s, SOL_SOCKET, SO_KEEPALIVE, &optval, &optlen) < 0) {
perror("getsockopt()");
close(s);
exit(EXIT_FAILURE);
}
printf("SO_KEEPALIVE is %s\n", (optval ? "ON" : "OFF"));
/* Check keepalive parameters */
getsockopt(s,SOL_TCP,TCP_KEEPIDLE,&tcp_keepalive_time,&optlen);
getsockopt(s,SOL_TCP,TCP_KEEPINTVL,&tcp_keepalive_interval,&optlen);
getsockopt(s,SOL_TCP,TCP_KEEPCNT,&tcp_keepalive_probes,&optlen);
printf("tcp_keepalive_time: %d,tcp_keepalive_interval: %d,tcp_keepalive_probes: %d\n",
tcp_keepalive_time,tcp_keepalive_interval,tcp_keepalive_probes);
/* Set the option active */
optval = 1;
optlen = sizeof(optval);
if(setsockopt(s, SOL_SOCKET, SO_KEEPALIVE, &optval, optlen) < 0) {
perror("setsockopt()");
close(s);
exit(EXIT_FAILURE);
}
printf("SO_KEEPALIVE set on socket\n");
/* Check the status again */
if(getsockopt(s, SOL_SOCKET, SO_KEEPALIVE, &optval, &optlen) < 0) {
perror("getsockopt()");
close(s);
exit(EXIT_FAILURE);
}
printf("SO_KEEPALIVE is %s\n", (optval ? "ON" : "OFF"));
/* Check keepalive parameters */
getsockopt(s,SOL_TCP,TCP_KEEPIDLE,&tcp_keepalive_time,&optlen);
getsockopt(s,SOL_TCP,TCP_KEEPINTVL,&tcp_keepalive_interval,&optlen);
getsockopt(s,SOL_TCP,TCP_KEEPCNT,&tcp_keepalive_probes,&optlen);
printf("tcp_keepalive_time: %d,tcp_keepalive_interval: %d,tcp_keepalive_probes: %d\n",
tcp_keepalive_time,tcp_keepalive_interval,tcp_keepalive_probes);
/* Set keepalive parameters */
tcp_keepalive_time=180;
tcp_keepalive_interval=60;
tcp_keepalive_probes=20;
optlen=sizeof(tcp_keepalive_time);
setsockopt(s,SOL_TCP,TCP_KEEPIDLE,&tcp_keepalive_time,optlen);
optlen=sizeof(tcp_keepalive_interval);
setsockopt(s,SOL_TCP,TCP_KEEPINTVL,&tcp_keepalive_interval,optlen);
optlen=sizeof(tcp_keepalive_probes);
setsockopt(s,SOL_TCP,TCP_KEEPCNT,&tcp_keepalive_probes,optlen);
/* Check keepalive parameters */
getsockopt(s,SOL_TCP,TCP_KEEPIDLE,&tcp_keepalive_time,&optlen);
getsockopt(s,SOL_TCP,TCP_KEEPINTVL,&tcp_keepalive_interval,&optlen);
getsockopt(s,SOL_TCP,TCP_KEEPCNT,&tcp_keepalive_probes,&optlen);
printf("tcp_keepalive_time: %d,tcp_keepalive_interval: %d,tcp_keepalive_probes: %d\n",
tcp_keepalive_time,tcp_keepalive_interval,tcp_keepalive_probes);
/* Create another socket */
if((t = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0) {
perror("socket()");
exit(EXIT_FAILURE);
}
/* Check the status again */
if(getsockopt(t, SOL_SOCKET, SO_KEEPALIVE, &optval, &optlen) < 0) {
perror("getsockopt()");
close(s);
exit(EXIT_FAILURE);
}
printf("SO_KEEPALIVE is %s\n", (optval ? "ON" : "OFF"));
/* Check keepalive parameters */
getsockopt(t,SOL_TCP,TCP_KEEPIDLE,&tcp_keepalive_time,&optlen);
getsockopt(t,SOL_TCP,TCP_KEEPINTVL,&tcp_keepalive_interval,&optlen);
getsockopt(t,SOL_TCP,TCP_KEEPCNT,&tcp_keepalive_probes,&optlen);
printf("tcp_keepalive_time: %d,tcp_keepalive_interval: %d,tcp_keepalive_probes: %d\n",
tcp_keepalive_time,tcp_keepalive_interval,tcp_keepalive_probes);
close(s);
close(t);
exit(EXIT_SUCCESS);
}
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