How does the OS resolve a request to connect to 0.0.0.0?

Question

As I understand 0.0.0.0 means all network interfaces of this host (including 127.0.0.1).

Suppose I have three interfaces 192.0.2.40, 203.0.113.150 and 127.0.0.1 on some host (OS linux).

On 192.0.2.40:777, I have ServerA. On 203.0.113.150:777, I have ServerB. How does the OS resolve a request to connect to 0.0.0.0? I mean when the same port (777) is listening on different interfaces what server(ServerA or serverB) and why will be connected when I will do in terminal of this host telnet 0.0.0.0 777

Answer 1

It only means that as a source address (i.e. when binding a socket to a local interface). As the destination address it doesn't have this meaning – it's technically an illegal address.

However it seems that originally BSD treated it as a loopback connection and Linux carries it on.

For IPv4, an all-zeros destination is handled in net/ipv4/route.c at route lookup time:

struct rtable *ip_route_output_key_hash_rcu(...) {
    ...
    if (!fl4->daddr) {
        fl4->daddr = fl4->saddr;
        if (!fl4->daddr)
            fl4->daddr = fl4->saddr = htonl(INADDR_LOOPBACK);
        dev_out = net->loopback_dev;
        fl4->flowi4_oif = LOOPBACK_IFINDEX;
        res->type = RTN_LOCAL;
        flags |= RTCF_LOCAL;
        goto make_route;
    }
    ...
}

Which translates to "If destination is empty, fill it in with 127.0.0.1 and route via the lo interface". The OS pretends that you tried to connect to localhost.

The same in IPv6 is handled at protocol level (individually by TCP and UDP). For example, net/ipv6/tcp_ipv6.c contains:

static int tcp_v6_connect(...) {
    ...
    /*
     *  connect() to INADDR_ANY means loopback (BSD'ism).
     */
    if (ipv6_addr_any(&usin->sin6_addr)) {
        if (ipv6_addr_v4mapped(&sk->sk_v6_rcv_saddr))
            ipv6_addr_set_v4mapped(htonl(INADDR_LOOPBACK),
                           &usin->sin6_addr);
        else
            usin->sin6_addr = in6addr_loopback;
    }
    ...
}

Meanwhile in net/ipv6/udp.c:

int udpv6_sendmsg(...) {
    ...
    if (!ipv6_addr_any(daddr))
        fl6.daddr = *daddr;
    else
        fl6.daddr.s6_addr[15] = 0x1; /* :: means loopback (BSD'ism) */
    ...
}