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Date:	Mon, 30 Nov 2009 15:15:27 -0800
From:	Ashwani Wason <>

It appears that SO_BINDTODEVICE is not working when used along with
IP_TRANSPARENT of TPROXY. The use case is for a transparent proxy that
has to handle clients with overlapping IP addresses coming in on
different interfaces (VLANs or physical). Each such interface has a
unique next hop gateway.

Setup: Kernel built with required TPROXY configuration. A
system with three interfaces, eth0 (, eth0.1
(, and eth1 (, each with its own default
router and a fairly standard set of TPROXY rules:

default via dev eth1
default via dev eth0
default via dev eth0.1

iptables -t mangle -N DIVERT
iptables -t mangle -A DIVERT -j MARK --set-mark 1
iptables -t mangle -A DIVERT -j ACCEPT
iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT
iptables -t mangle -A PREROUTING -p tcp --dport 80 -j TPROXY
--tproxy-mark 0x1/0x1 --on-port 4002

ip rule add fwmark 1 iif eth0 lookup 101
ip rule add fwmark 1 iif eth1 lookup 101

ip route add local dev lo table 101

Additional information:

* The proxy has a separate listening socket for each interface.

* On each listening socket the proxy sets the IP_TRANSPARENT option.
It also sets the SO_BINDTODEVICE option (on each socket) to the
associated interface. (I have experimented with the ordering of these
options and setting one before the other has no change in behavior.)

* The proxy works perfectly fine in "transparent mode" when
SO_BINDTODEVICE is not used.

* The proxy works perfectly fine in "explicit mode" when

Now my test is very simple with the expectation that when a SYN comes
in on, say, interface eth0 then the SYN/ACK must go out back on that
interface to the MAC address of the designated next hop router. That
does not seem to happen however. The SYN/ACK is not being sent out at
all, from any interface. It seems that either the SYN or the SYN/ACK
is being eaten up somewhere. No one 'netstat -s' seems to be updated
either. (Since even 'netstat -ant' does not show the TCP connection
state, I suspect that the SYN is being eaten up.)

When I reconfigure the proxy to be in explicit mode, i.e. not set the
IP_TRANSPARENT option, and have the clients connect directly to the IP
address and port of the proxy then the system works as expected, i.e.
the SYN/ACK (and all subsequent packets) go out on the associated

Thank you,
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