在服务器端启用 TCP keepalive

这是一个客户端 - 服务器示例。该进程在子进程中的父进程和服务器中分叉并运行客户端：

• 客户端连接到服务器并等待服务器退出;
• 服务器接受来自客户端的连接，启用 keepalive，并等待任何信号。

使用 socket(7) 和 tcp(7) 手册页中描述的以下选项配置 Keepalive ：

• SO_KEEPALIVE - 允许发送保持活动消息
• TCP_KEEPIDLE- TCP 开始发送 keepalive 探测之前连接需要保持空闲的时间（以秒为单位）
• TCP_KEEPINTVL - 各个 keepalive 探测器之间的时间（以秒为单位）
• TCP_KEEPCNT - TCP 在丢弃连接之前应发送的最大 keepalive 探测数

源代码：

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/signal.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <netinet/tcp.h>

#define check(expr) if (!(expr)) { perror(#expr); kill(0, SIGTERM); }

void enable_keepalive(int sock) {
    int yes = 1;
    check(setsockopt(
        sock, SOL_SOCKET, SO_KEEPALIVE, &yes, sizeof(int)) != -1);

    int idle = 1;
    check(setsockopt(
        sock, IPPROTO_TCP, TCP_KEEPIDLE, &idle, sizeof(int)) != -1);

    int interval = 1;
    check(setsockopt(
        sock, IPPROTO_TCP, TCP_KEEPINTVL, &interval, sizeof(int)) != -1);

    int maxpkt = 10;
    check(setsockopt(
        sock, IPPROTO_TCP, TCP_KEEPCNT, &maxpkt, sizeof(int)) != -1);
}

int main(int argc, char** argv) {
    check(argc == 2);

    struct sockaddr_in addr;
    addr.sin_family = AF_INET;
    addr.sin_port = htons(12345);
    check(inet_pton(AF_INET, argv[1], &addr.sin_addr) != -1);

    int server = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    check(server != -1);

    int yes = 1;
    check(setsockopt(server, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int)) != -1);

    check(bind(server, (struct sockaddr*)&addr, sizeof(addr)) != -1);
    check(listen(server, 1) != -1);

    if (fork() == 0) {
        int client = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
        check(client != -1);
        check(connect(client, (struct sockaddr*)&addr, sizeof(addr)) != -1);
        printf("connected\n");
        pause();
    }
    else {
        int client = accept(server, NULL, NULL);
        check(client != -1);
        enable_keepalive(client);
        printf("accepted\n");
        wait(NULL);
    }

    return 0;
}

可以使用 tcpdump 监控 Keepalive 数据包。

用法示例：

$ ./a.out 127.0.0.1 &
[1] 14010
connected
accepted

$ tcpdump -n -c4 -ilo port 12345
dropped privs to tcpdump
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on lo, link-type EN10MB (Ethernet), capture size 262144 bytes
18:00:35.173892 IP 127.0.0.1.12345 > 127.0.0.1.60998: Flags [.], ack 510307430, win 342, options [nop,nop,TS val 389745775 ecr 389745675], length 0
18:00:35.173903 IP 127.0.0.1.60998 > 127.0.0.1.12345: Flags [.], ack 1, win 342, options [nop,nop,TS val 389745775 ecr 389745075], length 0
18:00:36.173886 IP 127.0.0.1.12345 > 127.0.0.1.60998: Flags [.], ack 1, win 342, options [nop,nop,TS val 389745875 ecr 389745775], length 0
18:00:36.173898 IP 127.0.0.1.60998 > 127.0.0.1.12345: Flags [.], ack 1, win 342, options [nop,nop,TS val 389745875 ecr 389745075], length 0
4 packets captured
8 packets received by filter
0 packets dropped by kernel