Re: [RFD] iptables: mangle table obsoletes filter table

[Bill Davidsen <davidsen@tmr.com>]

Al Boldi wrote:
> Valdis.Kletnieks@vt.edu wrote:
> > On Sat, 20 Oct 2007 06:40:02 +0300, Al Boldi said:
> > > Sure, the idea was to mark the filter table obsolete as to make people
> > > start using the mangle table to do their filtering for new setups.  The
> > > filter table would then still be available for legacy/special setups. 
> > > But this would only be possible if we at least ported the REJECT target
> > > to mangle.
> > That's *half* the battle.  The other half is explaining why I should move
> > from a perfectly functional setup that uses the filter table.  What gains
> > do I get from doing so?  What isn't working that I don't know about? etc?
> >
> > In other words - why do I want to move from filter to mangle?
>
> This has already been explained in this thread; here it is again:
>
> Al Boldi wrote:
> > > > The problem is that people think they are safe with the filter table,
> > > > when in fact they need the prerouting chain to seal things.  Right now
> > > > this is only possible in the mangle table.
> > > Why do they need PREROUTING?
> > Well, for example to stop any transient packets being forwarded.  You could 
> > probably hack around this using mark's, but you can't stop the implied
> > route lookup, unless you stop it in prerouting.
>
> Basically, you have one big unintended gaping whole in your firewall, that 
> could easily be exploited for DoS attacks at the least, unless you put in 
> specific rules to limit this.
Well... true enough on a small firewall machine with a really fast link, 
maybe. I like your point about efficiency better, it's more logical, 
like putting an ACCEPT of established connections before a lot of low 
probability rules. The only time I have seen rules actually bog a 
machine was when a major ISP sent out a customer "upgrade" with a bug 
which caused certain connections to be SYN-SYN/ACK-RST leaving half open 
sockets. They sent out 160k of them and the blocking list became very 
long as blocking rules were added.

> Plus, it's outrageously incorrect to accept invalid packets, just because 
> your filtering infrastructure can only reject packets after they have been 
> prerouted.
As long as the filter table doesn't go away, sometimes things change 
after PREROUTING, like NAT, and additional rules must be used.

--
Bill Davidsen <davidsen@tmr.com>
  "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot
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