EADDRNOTAVAIL even after using IP

I was under the impression that under Linux you could bind to a non-local address as long as you set the IP_FREEBIND socket option, but that's not the behavior I'm seeing:

$ sudo strace -e 'trace=%network' ...
...
socket(AF_INET, SOCK_RAW, IPPROTO_UDP)  = 5
setsockopt(5, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
setsockopt(5, SOL_SOCKET, SO_NO_CHECK, [1], 4) = 0
setsockopt(5, SOL_IP, IP_HDRINCL, [1], 4) = 0
setsockopt(5, SOL_IP, IP_FREEBIND, [1], 4) = 0
bind(5, {sa_family=AF_INET, sin_port=htons(abcd), sin_addr=inet_addr("w.x.y.z")}, 16) = -1 EADDRNOTAVAIL (Cannot assign requested address)
...

I also set the ip_nonlocal_bind setting, just to be certain, and I get the same results.

$ sysctl net.ipv4.ip_nonlocal_bind
net.ipv4.ip_nonlocal_bind = 1

CodePudding user response：

Unfortunately, it seems that it is not possible to bind a raw IP socket to a non-local, non-broadcast and non-multicast address, regargless of IP_FREEBIND. Since I see inet_addr("w.x.y.z") in your strace output, I assume that this is exactly what you're trying to do and w.x.y.z is a non-local unicast address, thus your bind syscall fails.

This seems in accordance with man 7 raw:

A raw socket can be bound to a specific local address using the bind(2) call. If it isn't bound, all packets with the specified IP protocol are received. In addition, a raw socket can be bound to a specific network device using SO_BINDTODEVICE; see socket(7).

Indeed, looking at the kernel source code, in raw_bind() we can see the following check:

    ret = -EADDRNOTAVAIL;
    if (addr->sin_addr.s_addr && chk_addr_ret != RTN_LOCAL &&
        chk_addr_ret != RTN_MULTICAST && chk_addr_ret != RTN_BROADCAST)
        goto out;

Also, note that .sin_port must be 0. The .sin_port field for raw sockets is used to select a sending/receiving IP protocol (not a port, since we are at level 3 and ports do not exist). As the manual states, from Linux 2.2 onwards you cannot select a sending protocol through .sin_port anymore, the sending protocol is the one set when creating the socket.