[HEADS UP!] IPFW Ideas: possible SoC 2008 candidate

Julian Elischer julian at elischer.org
Wed Mar 26 10:31:14 PDT 2008

Vadim Goncharov wrote:
> Hi Julian Elischer! 
> On Mon, 24 Mar 2008 10:53:44 -0700; Julian Elischer wrote about 'Re: [HEADS UP!] IPFW Ideas: possible SoC 2008 candidate':
>> here are some of my ideas for ipfw changes:
>> 1/ redo locking so that packets do not have to get locks on the 
>> structure... I have several ideas on this
> Currently the main need for locking arises for rule byte/packet counters. The
> easiest short-term solution

The main need for locking is that the rules can be changed while a 
processor is traversing the rule set.

>> 2/ allow separate firewalls to be used at different parts of the 
>> network stack (i.e allow multiple taboe sto co-exist)

there are many places that ipfw is currently callable from.
ip_input(), ip_output(), ether_demux(), if_brige, ether_output()

it would be interesting tobe able to have differnt firewalls in these 
places (possibly per interface) so that state (e.g. keep_state)
can be kept seprately for one place then from another.

for example you may not want the result of 'keep state' on an
external interface to necessarily affect what happens to
packets from the same session when viewed traversing an internal 

Currently on my more complex ipfw rule sets I break the rule sets out
so that packets in different places traverse different rules
but it would be nice to have it explicitly supported.

> Umm, could you explain it a little?..
>> 3/ possibly keeping per CPU stats..
> How that would be represented to user?

it wouldn't.. you'd add them together before presenting them.
but every time a packet changes a counter that is shared, there is a 
chance that it is being altered by another processor, so if you have
fine grained locking in ipfw, you really should use atomic adds,
which are slow, or accept possibl collisions (which might be ok)
but still cause a lot of cross cpu TLB flushing.


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