Question

I have a problem where I create two UDP sockets, bind them to the loopback address with port 0 (requesting the stack to assign an ephemeral port). My understanding is that both sockets should be on different ports. In the code example below, both sockets are reported to be on the same IP address and port.

#include <stdio.h>
#include <arpa/inet.h>

int main(int, char**)
{
    int fd1 = ::socket(AF_INET, SOCK_DGRAM, 0);
    if (fd1 < 0)
    {
        perror("fd1 socket()");
        return -1;
    }
    int fd2 = ::socket(AF_INET, SOCK_DGRAM, 0);
    if (fd2 < 0)
    {
        perror("fd2 socket()");
        return -1;
    }

    // Set SO_REUSEADDR for both sockets
    int reuse = 1;
    if (::setsockopt(fd1, SOL_SOCKET, SO_REUSEADDR, &reuse, sizeof(reuse)) < 0)
    {
        perror("fd1 SO_REUSEADDR failed");
        return -1;
    }
    if (::setsockopt(fd2, SOL_SOCKET, SO_REUSEADDR, &reuse, sizeof(reuse)) < 0)
    {
        perror("fd2 SO_REUSEADDR failed");
        return -1;
    }

    sockaddr_storage storage;
    socklen_t addrlen = sizeof(storage);
    sockaddr_in& addr = reinterpret_cast<sockaddr_in&>(storage);
    addr.sin_family = AF_INET;
    addr.sin_port = 1234;
    addr.sin_port = 0;
    if (::inet_pton(AF_INET, "127.0.0.1", &addr.sin_addr) <= 0)
    {
        perror("Failed to create address 127.0.0.1");
        return -1;
    }
    sockaddr* pAddr = reinterpret_cast<sockaddr*>(&storage);

    if (::bind(fd1, pAddr, addrlen) < 0)
    {
        perror("bind fd1 failed");
        return -1;
    }

    // Get the local address for fd1
    addrlen = sizeof(storage);
    if (::getsockname(fd1, pAddr, &addrlen))
    {
        perror("getsockname for fd1 failed");
        return -1;
    }
    char straddr[INET_ADDRSTRLEN];
    if (!inet_ntop(AF_INET, &addr.sin_addr, straddr, sizeof(straddr)))
    {
        perror("inet_ntop for fd1 failed");
        return -1;
    }
    printf("fd1=%d addr=%s:%d\n", fd1, straddr, addr.sin_port);

    if (::bind(fd2, pAddr, addrlen) < 0)
    {
        perror("bind fd2 failed");
        return -1;
    }

    // Get the local address for fd2
    addrlen = sizeof(storage);
    if (::getsockname(fd2, pAddr, &addrlen))
    {
        perror("getsockname for fd2 failed");
        return -1;
    }
    if (!inet_ntop(AF_INET, &addr.sin_addr, straddr, sizeof(straddr)))
    {
        perror("inet_ntop for fd2 failed");
        return -1;
    }
    printf("fd2=%d addr=%s:%d\n", fd2, straddr, addr.sin_port);

    return 0;
}

This code gives the following output …

fd1=4 addr=127.0.0.1:1933
fd2=5 addr=127.0.0.1:1933

I need both sockets on the same (local) IP address, but different ports. Can anyone explain why both sockets share the same port? Can anyone suggest a fix?

Answer 1

That is the expected behavior for SO_REUSEADDR on a UDP socket. Remove that setting to return to normal allocation rules.