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Re: Intrusion detection question.
From: Michel Arboi <arboi () bigfoot com>
Date: 12 Feb 2000 19:35:58 +0100

[sorry if you already received this message. It bounced twice]
Tomi Ollila <Tomi.Ollila () tfi net> writes:

In ip masquerading code I think it works pretty much the same as normal
port allocation scheme. I think the port counter wraps when it hits the
upper limit and it always checks whether the wanted 5-tuple 
(source ip, port - destination ip, port - protocol) is already used and 
takes the port that satisfies a non-used tuple.

That is a fundamental question and I never found a clear answer.
RFC 793 does not explain how source ports numbers are allocated. It
just states that different programs on one machine should use
different port numbers. 

AFAIK, Unix will never allocate the same TCP port numbers for client
programs connecting to different servers, althought it could perfectly 
do it and comply to RFC 793.
As it is not in the norm, it should be a way to identify the OS,
unless everybody uses the same algorithm (first free port?)

Of course, the answer is quite simple for UDP, as it is not connected.

IMHO, this question is important for big sites : the number of
available "client ports" on the firewall (proxy or NAT) will limit the
number simultaneous connections from the internal network to wild wild 
Internet.

-- 
mailto:arboi () bigfoot com     http://www.bigfoot.com/~arboi/
PGP Public keys: http://www.bigfoot.com/~arboi/pubkey.txt



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