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Message-ID: <BN3PR0201MB10592F17F9CC717E46A738B0F9D80@BN3PR0201MB1059.namprd02.prod.outlook.com> Date: Tue, 26 Jan 2016 17:29:14 +0000 From: Alan Davey <Alan.Davey@...aswitch.com> To: "David S. Miller" <davem@...emloft.net>, Alexey Kuznetsov <kuznet@....inr.ac.ru>, James Morris <jmorris@...ei.org>, Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>, Patrick McHardy <kaber@...sh.net> CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org> Subject: 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 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
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