Date: Sat, 9 Jun 2001 11:19:37 +0700 From: Igor Podlesny <poige@morning.ru> To: Ryan <ryanpek@swbell.net> Cc: freebsd-security@FreeBSD.ORG Subject: Re: IPFILTER and flags S/SA Message-ID: <19592974009.20010609111937@morning.ru> In-Reply-To: <000601c0f08f$566f53e0$01000001@mhx800> References: <9C643FE251025246BF8CE3ADFA3765954873@hydrogen.tmolp.com> <3B215D6A.9E968BAE@globalstar.com> <000601c0f08f$566f53e0$01000001@mhx800>
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> from the IPF howto
> -
> Some examples use flags S/SA instead of flags S.
> flags S actually equates to flags S/AUPRFS and
> matches against only the SYN packet out of all six
> possible flags, while flags S/SA will allow pack-
> ets that may or may not have the URG, PSH, FIN, or
> RST flags set. Some protocols demand the URG or
> PSH flags, and S/SAFR would be a better choice for
> these, however we feel that it is less secure to
> blindly use S/SA when it isn't required. But it's
> your firewall.
> -
> I was wondering if any1 could maybe explain more in detail why S/SA is
> unsafe?
English isn't my native language, but it seems to me that the
quotation from IPF-howto does answer your question clearly. so I just
expand it to you:
S/SA means check for S looking at S and A, other flags don't matter
so it will select packets with SYN set, even if it also has RST
set.
In order to avoid such behavior, they suggest using S/SAFR which would
mean the next: Check if packet has SYN set, and none of (ACK, FIN,
RST).
> example:
> pass in quick on xl0 proto tcp from any to 64.219.216.65/32 port = 80 flags
> S keep state
> pass in quick on xl0 proto tcp from any to 64.219.216.65/32 port = 80 flags
> S/SA keep state
> ryanpek@swbell.net
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--
Igor http://poige.nm.ru
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