netdev - ipv4: ip unreachable with SO

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Message-ID: <87egfxjd9o.fsf@pingu.sky.yk.fujitsu.co.jp>
Date:	Wed, 11 Nov 2015 14:49:57 +0900
From:	Kouya Shimura <kouya@...fujitsu.com>
To:	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
CC:	"David S. Miller" <davem@...emloft.net>
Subject: ipv4: ip unreachable with SO_BINDTODEVICE socket

Hi

When both server and client are on the same machine and each their
socket option is set to SO_BINDTODEVICE, sometimes a packet doesn't
reach to the server.

The reproducible test program is attached. (modify "IF_ADDR=, IP_ADDR=,
PORT=" lines appropriately).  Please try 'taskset -c 1 python test.py'
since per cpu data (rt_cache) affects results.  Also 'tcpdump -i lo'
is helpful for testing.  you can see "ICMP udp port unreachable".

In this test program, a packet doesn't pass through the bound
interface but 'lo' interface. So, it might be granted that local
communication with SO_BINDTODEVICE socket fails. However, dnsmasq and
dhcp_release commands rely on it (Actually I've found this issue on
the OpenStack envirionment) and the test program works well on
linux-2.6.32 but doesn't work on linux-3.10.0 and 4.3.0.

I'd like to know whether this is a kernel bug or the specification of
SO_BINDTODEVICE.

The attached patch fixes this issue, but no confidence this is a
right modification.

Thanks,
Kouya

View attachment "test.py" of type "text/plain" (1783 bytes)

View attachment "0001-PATCH-net-ipv4-re-create-rt_dst-when-rt_iif-doesn-t-.patch" of type "text/plain" (811 bytes)