接受后配置 TCP keepalive

Question

在套接字上的accept() 之后，我正在尝试配置 TCP keepalive。

SockConnected = accept(SockListen, &RemoteAddr,
                   &RemoteLen);
/* A bit of checking goes here */
if (setsockopt (SockConnected , SOL_SOCKET, SO_KEEPALIVE,
                            (ST_CHAR *) &sockopt_on, sizeof (int) ) )
{
   /* Error logging */
}

if (setsockopt(SockConnected , IPPROTO_TCP, TCP_KEEPIDLE, (char*)&(sockopt_tcp_keep_idle), sizeof(sockopt_tcp_keep_idle)))
{
    /* Error logging */
}
if (setsockopt(SockConnected, IPPROTO_TCP, TCP_KEEPINTVL, (char*)&(sockopt_tcp_keep_intvl), sizeof(sockopt_tcp_keep_intvl)))
{
    /* Error logging */
}
if (setsockopt(SockConnected, IPPROTO_TCP, TCP_KEEPCNT, (char*)&(sockopt_tcp_keep_cnt), sizeof(sockopt_tcp_keep_cnt)))
{
    /* Error logging */
}

在为所有四个属性调用getsockopt() 之后，一切似乎都很好。我已经检查了 Wireshark 并且没有发送 Keep alive 数据包。我不得不将SOL_TCP 更改为IPPROTO_TCP，因为在为TCP_KEEPIDLE 调用setsockopt() 时，它返回了errno 92（找不到协议）。

在调用socket() 进行传出连接后，我也在做同样的事情，并且工作正常。

我正在使用 C 和 Linux。

有什么原因导致setsockopt 在接受后可能无法正常工作？

Answer 1

这是一个最小的工作示例。如果对你有用，你可以作为参考。

#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;
}

示例输出（tcpdump 每秒报告一次 keepalive 数据包）：

$ ./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

Answer 2

SO_KEEPALIVE 应与SOL_SOCKET 一起使用，而不是IPPROTO_TCP 或SOL_TCP。请参阅socket(7) 和tcp(7)。

所以试试这个：

int val = 1;
setsockopt(SockConnected, SOL_SOCKET, SO_KEEPALIVE, &val, sizeof(val));

如果SO_KEEPALIVE 设置不正确，我猜TCP_KEEPIDLE 和类似选项会被忽略，而SOL_TCP 是IPPROTO_TCP 的同义词。

如果没有帮助，请考虑发布一个重现问题的最小示例。