netdev - Problem with ICMP rate limiting and redirects

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Date:	Wed, 30 Sep 2015 13:10:16 -0300
From:	Hugo Vasconcelos Saldanha <hugovs@...il.com>
To:	netdev@...r.kernel.org
Subject: Problem with ICMP rate limiting and redirects

Hi,

While updating the kernel from v3.2 to v3.14, I started to see a
different behavior concerning ICMP redirects sent by this updated
server. The network is somewhat configured like this:

               ---|firewall|----{Internet}
|client|------|
              |
               ---|router|------|172.16/12 network|

The client's default gateway is 'firewall', which is the updated
server. It has a static route to 172.16 network by 'router'. If
'client' wants to talk to a server in that network, 'firewall' sends a
ICMP redirect pointing to router as the gateway.

This worked fine with v3.2. But after the upgrade, if an ICMP message
that is rate-limited (by the sysctl_icmp_ratelimit mask) is sent to
'client', ICMP redirects stop being sent to the same client. This
happens, for example, when traceroute'ing from the client to the
server inside the mentioned network. In this situation, a ICMP Time
Exceeded message is sent in response to traceroute's first packet, but
then the following packets never generate any ICMP redirect messages
in 'firewall'.

Debugging the code, I was able to see that the problem is being caused
by the fact that ip_rt_send_redirect() started to use the inetpeer
cache and the fields used to rate limit ICMP redirects (rate_tokens
and rate_last) are now being shared with the algorithm applied in
inet_peer_xrlim_allow(). This never happened with v3.2 because
apparently inet_peer_xrlim_allow() and ip_rt_send_redirect() used
different inetpeer objects.

The reason why this breaks the functionality is that, while
inet_peer_xrlim_allow() uses a time bucket, ip_rt_send_redirect() uses
rate_tokens as a packet counter. Not to mention the fact that these
are two completely different policies which should be controlled by
different buckets, counters, flags, etc. Because of this,
ip_rt_redirect_silence, ip_rt_redirect_number and ip_rt_redirect_load
/proc files are broken also.

The easiest solution would be to create new fields in 'struct
inetpeer' to control ICMP redirects only, but I'm not able to measure
its convenience.

Any thoughts?

PS: Apparently, a similar problem was reported here:
http://marc.info/?l=linux-netdev&m=139696540600985

PS2: I could try to reproduce the problem with the latest code if this
is really necessary.

        Hugo
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