SO_BINDTODEVICE socket option for Linux 2.0.30+
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SO_BINDTODEVICE socket option for Linux 2.0.30+
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by Elliot Poger (elliot@poger.com)
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by Elliot Poger (elliot@poger.com)
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of Stanford's MosquitoNet project (http://mosquitonet.stanford.edu)
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of Stanford's MosquitoNet project (http://mosquitonet.stanford.edu)
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Using the SO_BINDTODEVICE socket option allows your user-level Berkeley
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Using the SO_BINDTODEVICE socket option allows your user-level Berkeley
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sockets code to explicitly select which network interface is used for
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sockets code to explicitly select which network interface is used for
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both input and output on a per-socket basis. I originally wrote it to
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both input and output on a per-socket basis. I originally wrote it to
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allow the Internet Software Consortium DHCP server
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allow the Internet Software Consortium DHCP server
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(http://www.fugue.com/dhcp/) to run on Linux machines with multiple
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(http://www.fugue.com/dhcp/) to run on Linux machines with multiple
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interfaces. It has been tested with UDP and TCP sockets.
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interfaces. It has been tested with UDP and TCP sockets.
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Usage is as follows:
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Usage is as follows:
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int skfd;
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int skfd;
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struct ifreq interface;
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struct ifreq interface;
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skfd = socket(AF_INET, SOCK_DGRAM, 0);
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skfd = socket(AF_INET, SOCK_DGRAM, 0);
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strncpy(interface.ifr_ifrn.ifrn_name, "eth1", IFNAMSIZ);
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strncpy(interface.ifr_ifrn.ifrn_name, "eth1", IFNAMSIZ);
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if (setsockopt(skfd, SOL_SOCKET, SO_BINDTODEVICE,
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if (setsockopt(skfd, SOL_SOCKET, SO_BINDTODEVICE,
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(char *)&interface, sizeof(interface)) < 0) {
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(char *)&interface, sizeof(interface)) < 0) {
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perror("sendpacket: setting SO_BINDTODEVICE");
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perror("sendpacket: setting SO_BINDTODEVICE");
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exit(1);
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exit(1);
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}
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}
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Once the BINDTODEVICE socket option has been set for a socket, as above,
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Once the BINDTODEVICE socket option has been set for a socket, as above,
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any data sent over this socket is guaranteed to go out of the "eth1"
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any data sent over this socket is guaranteed to go out of the "eth1"
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interface, and any data received through the socket is guaranteed to
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interface, and any data received through the socket is guaranteed to
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have arrived on eth1. If you want to send and receive over multiple
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have arrived on eth1. If you want to send and receive over multiple
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interfaces, keeping them separate, you can open several sockets and bind
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interfaces, keeping them separate, you can open several sockets and bind
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each one to a different interface with SO_BINDTODEVICE. (You _can_ call
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each one to a different interface with SO_BINDTODEVICE. (You _can_ call
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BINDTODEVICE more than once for a socket to change the interface it's
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BINDTODEVICE more than once for a socket to change the interface it's
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bound to, but results may be unpredictable because of caching effects
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bound to, but results may be unpredictable because of caching effects
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in the kernel...)
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in the kernel...)
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Note that the routing table is still consulted when packets are transmitted.
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Note that the routing table is still consulted when packets are transmitted.
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Basically, routing proceeds as usual, except that any routes which go
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Basically, routing proceeds as usual, except that any routes which go
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through a network interface other than the one specified in the BINDTODEVICE
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through a network interface other than the one specified in the BINDTODEVICE
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call are ignored. If you attempt to send a packet to a certain IP address
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call are ignored. If you attempt to send a packet to a certain IP address
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through an interface which provides no route to that IP address, you'll get
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through an interface which provides no route to that IP address, you'll get
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a "network unreachable" error. Here is an example of a routing table which
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a "network unreachable" error. Here is an example of a routing table which
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will allow you to send packets to any IP address through either eth0 or
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will allow you to send packets to any IP address through either eth0 or
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eth1:
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eth1:
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Destination Gateway Genmask Flags Metric Ref Use Iface
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Destination Gateway Genmask Flags Metric Ref Use Iface
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171.64.69.0 0.0.0.0 255.255.255.192 U 0 0 37 eth0
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171.64.69.0 0.0.0.0 255.255.255.192 U 0 0 37 eth0
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171.64.69.192 0.0.0.0 255.255.255.192 U 0 0 677 eth1
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171.64.69.192 0.0.0.0 255.255.255.192 U 0 0 677 eth1
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127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 4 lo
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127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 4 lo
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0.0.0.0 171.64.69.1 0.0.0.0 UG 0 0 45 eth0
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0.0.0.0 171.64.69.1 0.0.0.0 UG 0 0 45 eth0
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0.0.0.0 171.64.69.193 0.0.0.0 UG 1 0 5 eth1
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0.0.0.0 171.64.69.193 0.0.0.0 UG 1 0 5 eth1
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Note that there are actually TWO default routes. The routing table is
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Note that there are actually TWO default routes. The routing table is
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searched from top to bottom, so every time you send out a packet, the first
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searched from top to bottom, so every time you send out a packet, the first
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(uppermost) matching route which the kernel routing function finds which
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(uppermost) matching route which the kernel routing function finds which
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matches the destination IP address is used. In this case, packets sent to
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matches the destination IP address is used. In this case, packets sent to
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the IP address 152.2.128.159 will normally be sent through eth0 and gateway
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the IP address 152.2.128.159 will normally be sent through eth0 and gateway
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171.64.69.1; if the socket is bound to the eth1 device, the packets will be
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171.64.69.1; if the socket is bound to the eth1 device, the packets will be
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sent through eth1 and gateway 171.64.69.193; if the socket is bound to some
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sent through eth1 and gateway 171.64.69.193; if the socket is bound to some
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other device, you will get a "network unreachable" error.
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other device, you will get a "network unreachable" error.
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By the way, you can add multiple default routes and set the order of
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By the way, you can add multiple default routes and set the order of
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preference as follows:
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preference as follows:
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route add default gateway 171.64.69.1
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route add default gateway 171.64.69.1
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route add default gateway 171.64.69.193 metric 1
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route add default gateway 171.64.69.193 metric 1
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Routes with a higher "metric" are put lower in the table and thus have a
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Routes with a higher "metric" are put lower in the table and thus have a
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lower preference.
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lower preference.
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