How to stop behaviour: C++ Socket sendto changes interface

Question

I run Ubuntu 12.04 and I am currently writing C++ code to create a UDP socket, which sends packets to various destinations with sendto. Now it happens that my laptop has both a wlan0 and a eth0 interface. If I bind it to either one of these, using the IP address, and the SO_BINDTODEVICE option, depending on the destination address, sendto will still decide to use the other interface if it suits him.

Specifically, if I bind a UDP socket to the eth0 interface, with its ip address and some port, and I send a packet to another laptop (locally, with only wifi access), it will decide to use my wlan0 interface.

I understand that this behaviour has pros, but I would like to be able to turn it off, i.e. I want to be able to say to the socket that it should only use the one interface I assigned it.

Suggestions?

EDIT:

struct sockaddr_storage sa = address;
fd = socket(address.get_family(), SOCK_DGRAM, 0);
char *devname = "wlan0";
setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE, devname, strlen(devname));
bind(fd, (sockaddr*)&sa, len);

"If I bind it to either one of these, using the IP address, and the SO_BINDTODEVICE option" I always thought that SO_BINDTODEVICE gets an interface name (at least thats how I use it for multicast, and it works fine) — PlasmaHH
You're using bind() with the address of the interface, and that doesn't work? (I assume they have distinct addresses) — Hasturkun

harper harper · Accepted Answer · 2013-10-21T13:56:21

Binding a socket to an interface with SO_BINDTODEVICE or bind defines a filter for received packets. When a packet has not been received using the specified interface it is not passed to the socket's receive queue. See: http://linux.die.net/man/7/socket

But binding a socket to an interface does not affect the normal IP routing process. When you send a packet it's the responsibility of the network IP stack to find the best route and to send the IP packet over a hardware adapter. This can be an Ethernet adpter but it's not controller with and bind operation.

When you want to send a packet at a specific interface you need raw sockets. You construct the complete packet content including IP and hardware layer (probably Ethernet) and send it using the raw socket.

How to stop behaviour: C++ Socket sendto changes interface

1 Answers