Subject: Re: Linux vs FreeBSD Which is correct.

From: Chris Friesen <[email-masked]>
Date: Fri, 19 Aug 2011 10:18:54 -0600

On 08/18/2011 06:42 AM, Stephen Clark wrote:

> I guess I don't really understand what reverse path filter stuff is all
> about, much less making it weaker.
> But using 2 made the pings responses be seen.

It's described in RFC3704.  The idea is to block spoofed packets.

 From Documentation/networking/ip-sysctl.txt:

rp_filter - INTEGER
0 - No source validation.
1 - Strict mode as defined in RFC3704 Strict Reverse Path
     Each incoming packet is tested against the FIB and if the interface
     is not the best reverse path the packet check will fail.
     By default failed packets are discarded.
2 - Loose mode as defined in RFC3704 Loose Reverse Path
     Each incoming packet's source address is also tested against the FIB
     and if the source address is not reachable via any interface
     the packet check will fail.

    Current recommended practice in RFC3704 is to enable strict mode
    to prevent IP spoofing from DDos attacks. If using asymmetric routing
    or other complicated routing, then loose mode is recommended.

    The max value from conf/{all,interface}/rp_filter is used
    when doing source validation on the {interface}.

    Default value is 0. Note that some distributions enable it
    in startup scripts.

-- 
Chris Friesen
Software Developer
GENBAND
[email-masked]
www.genband.com