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SO_BINDTODEVICE socket option for Linux 2.0.30+
by Elliot Poger (elliot@poger.com)
of Stanford's MosquitoNet project (http://mosquitonet.stanford.edu)

Using the SO_BINDTODEVICE socket option allows your user-level Berkeley
sockets code to explicitly select which network interface is used for
both input and output on a per-socket basis.  I originally wrote it to 
allow the Internet Software Consortium DHCP server 
(http://www.fugue.com/dhcp/) to run on Linux machines with multiple 
interfaces.  It has been tested with UDP and TCP sockets.

Usage is as follows:


        int skfd;
        struct ifreq interface;

        skfd = socket(AF_INET, SOCK_DGRAM, 0);
        strncpy(interface.ifr_ifrn.ifrn_name, "eth1", IFNAMSIZ);
        if (setsockopt(skfd, SOL_SOCKET, SO_BINDTODEVICE,
                       (char *)&interface, sizeof(interface)) < 0) {
                perror("sendpacket: setting SO_BINDTODEVICE");
                exit(1);
        }


Once the BINDTODEVICE socket option has been set for a socket, as above,
any data sent over this socket is guaranteed to go out of the "eth1" 
interface, and any data received through the socket is guaranteed to 
have arrived on eth1.  If you want to send and receive over multiple
interfaces, keeping them separate, you can open several sockets and bind
each one to a different interface with SO_BINDTODEVICE.  (You _can_ call
BINDTODEVICE more than once for a socket to change the interface it's 
bound to, but results may be unpredictable because of caching effects
in the kernel...)

Note that the routing table is still consulted when packets are transmitted.
Basically, routing proceeds as usual, except that any routes which go
through a network interface other than the one specified in the BINDTODEVICE
call are ignored.  If you attempt to send a packet to a certain IP address 
through an interface which provides no route to that IP address, you'll get
a "network unreachable" error.  Here is an example of a routing table which
will allow you to send packets to any IP address through either eth0 or
eth1:

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
171.64.69.0     0.0.0.0         255.255.255.192 U     0      0       37 eth0
171.64.69.192   0.0.0.0         255.255.255.192 U     0      0      677 eth1
127.0.0.0       0.0.0.0         255.0.0.0       U     0      0        4 lo
0.0.0.0         171.64.69.1     0.0.0.0         UG    0      0       45 eth0
0.0.0.0         171.64.69.193   0.0.0.0         UG    1      0        5 eth1

Note that there are actually TWO default routes.  The routing table is
searched from top to bottom, so every time you send out a packet, the first
(uppermost) matching route which the kernel routing function finds which
matches the destination IP address is used.  In this case, packets sent to
the IP address 152.2.128.159 will normally be sent through eth0 and gateway
171.64.69.1; if the socket is bound to the eth1 device, the packets will be
sent through eth1 and gateway 171.64.69.193; if the socket is bound to some
other device, you will get a "network unreachable" error. 

By the way, you can add multiple default routes and set the order of
preference as follows:

route add default gateway 171.64.69.1
route add default gateway 171.64.69.193 metric 1

Routes with a higher "metric" are put lower in the table and thus have a 
lower preference.