Folks I have a doubt about the behaviour of a Linux IPv4 raw socket, with the IP_HDRINCL option set, rejecting a packet bigger than the outgoing interface’s MTU size with EMSGSIZE. Please take a look at the following and let me know if I should make a kernel code change to remove this behaviour and fragment the packet instead.
- raw_send_hdrinc() (net/ipv4/raw.c) has an explicit check for (length > rt->dst.dev->mtu), returning -EMSGSIZE if this is true. - I do not understand the reason for this restriction. If a packet was received on one interface and routed out through another interface with a smaller MTU then the packet would be fragmented. Why is a locally originated packet treated differently? - I note that the “raw(7) - Linux man page” lists “When the IP_HDRINCL option is set, datagrams will not be fragmented and are limited to the interface MTU” as a known bug. Please let me know if you have any questions. Regards Alan Davey
