Question

Using socket programming APIs (e.g., socket(), connect(), accept() …), how can I know if a TCP connection is between two processes on the same machine? Say, I have the socket file descriptor, and the remote ip. Can I simply inspect if the remote ip is 127.0.0.1?

Answer 1

Here is the approach I have used. The idea is to attempt to bind a listener to that IP address and use the failure/success codes to decide whether the address is local.

I am not claiming this is particularly efficient, but it should be fairly reliable, and for my application it was appropriate.

#include <sys/socket.h>
#include <errno.h>
/* ...probably need some other headers I am forgetting... */

int
is_local(const struct sockaddr *addr, socklen_t addr_len)
{
    const char *func = "is_local()";
    int result = 0;

    int tmp = socket(addr->sa_family, SOCK_STREAM, 0);
    if (tmp < 0) {
        printf("%s: socket(%d,SOCK_STREAM,0) failed, errno %d\n",
               func, addr->sa_family);

        goto out;
    }

    /* If bind() succeeds, or if it fails because the address is in
       use, then the address must be local to this system. */
    if (bind(tmp, addr, addr_len) < 0) {
        if (errno == EADDRINUSE)
            result = 1;
        else if (errno == EADDRNOTAVAIL)
            ; /* do nothing; address is remote */
        else
            printf("%s: bind() unexpected error %d\n", func, errno);
    }
    else {
        result = 1;
    }

    close(tmp);

 out:
    return result;
}

You call it like this:

struct sockaddr_storage client_addr;
socklen_t client_addr_len = sizeof(client_addr);
int fd = accept(listener, &client_addr, &client_addr_len);

if (is_local((struct sockaddr *)&client_addr, client_addr_len))
    /* peer is local */