Subject: Re: RFC: New BGF 'LOOP' instruction

From: Paul LeoNerd Evans <[email-masked]>
Date: Tue, 3 Aug 2010 14:40:44 +0100

On Tue, Aug 03, 2010 at 11:10:28AM +0200, Hagen Paul Pfeifer wrote:
> >> Rightnow, BPF is all but useless for parsing, say, IPv6. I only pick
> >> IPv6 as one example, I'm sure there must exist a great number more
> >> packet-based protocols that use a "linked-list" style approach to
> >> headers. None of those are currently filterable on the current set of
> >> instructions. LOOP would allow these.
> > 
> > It's not meant for detailed packet protocol header analysis,
> > it's for stateless straight line matching of masked values
> > in packet headers.
> 
> David is right, BPF cannot - and will not - keep with any high level
> connection tracking packet filter. There is an processing trade-off between
> packet classification and packet storage with post processing analysis.

This has nothing to do with high-level connection tracking.

I want to accept all (IPv4 or IPv6) TCP packets concerning port 80.
That's all. No connection tracking. Simply a "stateless straight line
matching of masked values in packet headers". Namely, the TCP source or
destination ports, being 80. 

Should BPF be allowed to implement such a filter?

This is the core question.

If yes, then we either need LOOP, or alternatively my SKF_AD_TRANSPROTO
/ SKF_TRANS_OFF idea (see the other thread fork). Without either LOOP or
TRANSPROTO, it becomes next-to-impossible to -find- the TCP header in an
IPv6 packet, and hence make filtering decisions based on it.

If no, please justify what BPF -is- for then, given that right now
applications like tcpdump/libpcap already use it for this very purpose.
Please further justify why BPF has the "LDX MSH" instruction

-- 
Paul "LeoNerd" Evans

[email-masked]
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http://www.leonerd.org.uk/