Injected packets using libPCAP library is not seen from python socket sniffer and netcat

Question

I am using libpcap to transmit packets between two VETH pairs {veth1, veth1peer} and {veth2, veth2peer} and using each others peer to inject packets upwards to veth1 or veth2 and disabled "rp-filter" and enabled "accept_local" feature.

I have a python UDP server that listens on veth2 IP address, but when I send UDP packet from veth1 to veth2 (by injecting the packet from veth1 to veth2peer) the sniffer doesn't see the packet...also the same problem when running netcat UDP client on veth1 and server on veth2 the server doesn't see the client's packets.

I feel like the issue is related to iptables firewall but I can't determine the exact issue

Answer 1

The problem was that VETH interfaces are using checksum offload by default so the forwarded packets had a bad checksum and were ignored by the kernel. Running ethtool --offload IFACE rx off tx off on the main interfaces{VETH1, VETH2} fixed it.