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Asked: 2026-05-23T04:28:45+00:00

I have 2 linux based systems – a client with 2 interfaces (1 LAN,

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I have 2 linux based systems – a client with 2 interfaces (1 LAN, 1 modem) and a server.
I open 2 UDP sockets, and use setsockopt with SO_BINDTODEVICE to bind each socket to it’s interface.
Then I send a message from client to server through each of those sockets.
Both of them reach server. Server socket reads them, and sends a reply to each of them.
Then I try to read server’s reply on the client.
BUT, there is only 1 reply.

Also if I run tcpdump, I see that both of the replies are received on their relevant interfaces, on the same port that they left. Yet only one of them reaches socket. The other is lost?

The “lost” packet is not random, it’s the “non” default one. If my routing table is empty, the modem one is lost. If I add a route to server ip from modem interface, the lost packet will be the lan one.

Yet, they always reach server, always return back, always seen in tcpdump, but 1 never reaches socket. How can that be?

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1 Answer

  1. Editorial Team

    There is an ipv4 network configuration parameter called rp_filter (reversed path validation filter). Basically, if the reply to a packet wouldn’t go out the interface this packet came in, then this is a bogus packet and should be ignored. Which is why while I saw the packet on the tcpdump, it never reached socket. Disabling it did the trick.

    sysctl -w net.ipv4.conf.all.rp_filter=0
sysctl -w net.ipv4.conf.eth0.rp_filter=0
sysctl -w net.ipv4.conf.ppp0.rp_filter=0
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