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Date:      Fri, 5 Jun 2009 15:54:04 +1000 (EST)
From:      Ian Smith <smithi@nimnet.asn.au>
To:        Freddie Cash <fjwcash@gmail.com>
Cc:        freebsd-ipfw@freebsd.org
Subject:   Re: Rules processing in ipfw: processing ends with rule 65535 or first  match?
Message-ID:  <20090605152016.N38006@sola.nimnet.asn.au>
In-Reply-To: <b269bc570906041523v2076ec83y90a9b8474199f457@mail.gmail.com>
References:  <b269bc570906041523v2076ec83y90a9b8474199f457@mail.gmail.com>

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On Thu, 4 Jun 2009, Freddie Cash wrote:
 > Over the years, various how-tos and docs that I've read comparing ipfw
 > to ipf and pf have categorised them as such:
 > 
 >   - ipf/pf compares the packet against every rule in the ruleset, and
 > the last matching action is used once the end of the ruleset is
 > reached (last-match-wins)
 > 
 >   - ipfw compares the packet against the rules, and stops processing
 > the rulesset once a rule matches (first-match-wins)

That's true for terminal actions (allow, deny) but not for non-terminal 
actions (esp. skipto), but also pipe, queue, divert, nat .. which may 
continue rule processing, modulo net.inet.ip.fw.one_pass, until a match 
with a terminal action occurs.  worst case, rule 65535 always matches.

 > And, if one wants to get the ipfw behaviour in ipf/pf, they can use
 > the "quick" keyword, which stops processing of the ruleset as soon as
 > one of those rules matches.
 > 
 > IOW, for a ruleset with 1000 rules, ipf/pf will scan every single rule
 > for every single packet; and ipfw will only scan the ruleset up to the
 > first matching rule.  In theory, the ipfw method would be a lot
 > faster, and less intensive.

I can't comment on ipf or pf or their relative efficiencies.

 > However, reading through the man page for ipfw(8) on FreeBSD 7.2, it
 > lists the following (Description section):
 >     The packet passed to the firewall is compared against each
 >     of the rules in the firewall ruleset.  When a match is found, the action
 >     corresponding to the matching rule is performed.

Yes, but noting the following para:

     Depending on the action and certain system settings, packets can be rein-
     jected into the firewall at some rule after the matching one for further
     processing.

 > And, later, in the Packet Flow section:
 >     Also note that each packet is always checked against the complete rule-
 >     set, irrespective of the place where the check occurs, or the source of
 >     the packet.

That's referring to where ipfw is invoked from on each pass, eg from 
ip_input or ip_output at layer 3, or ether_demux or ether_output_frame 
if filtering at layer 2 for routing, or bdg_forward if bridging.  In 
each such ipfw pass invoked on a packet, that indicates rule scanning 
starts at the beginning of the ruleset and ends on a (terminal) match.

 > These make it sound like ifpw processes the entire ruleset for every
 > packet, regardless of when a match occurs.
 > 
 > So, which is it?  Is ipfw a first-match-wins and rule processing ends
 > setup?  Or does it check every single rule for every single packet?

First _terminal_ match terminates the search.  Just saw Julian's post 
arrive .. dynamic rules matching state indeed short-circuit the search 
at the first encountered check-state or keep-state rule, though even in 
that case the action may be a skipto rather than a terminal action.

cheers, Ian



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