Packet filters

[Bill Moran]

"Andy Baran" <abaran1 at depaul.edu> wrote:
> This question sounds like it has an easy answer at first but please bear
> with me.  I am going to setup a network tap to monitor network traffic
> flows.  The machine will be running FreeBSD 4.10 and has two NICs.  One
> interface will be used for management and the other will be to collect
> the flows.  Obviously, security is a concern with a machine of this
> nature so I need to setup a firewall on the management interface. 
> However, I need to be absolutely sure that the firewall will not be
> handling any of the packets on the second interface.  I am well aware
> that IPFW and IPF can both be setup to monitor only a specific
> interface.  However, I'd like verification from someone familiar with
> the code for either that the filter will not touch packets on the
> interface being used as a tap.  My apologies if I'm posing this question
> to the wrong list.  If I am please let me know whom I should be asking. 
> Thanks in advance for any replies. 

Since nobody else has answered ...

While I can't, personally, verify this "at the code level", I can say from
experience, that ALL packets go through the firewall.  Whether or not the
firewall "handles" and of the packets is simply a matter of your ruleset.
Using IPFW, if the packets do not match any rules, they'll simply pass in
one side of the packet filter, and out the other.  With the setup you
describe, you can easily ensure that the packets never get altered by
having a "via" clause in all your rules.

For example, if your sniffing interface is fxp0 and your management interface
is fxp1, then rules similar to:
ipfw add drop tcp from any to any 25 via fxp1
Will _never_ match a packet that comes in or goes out through the fxp0 card.

HTH.

-- 
Bill Moran
Potential Technologies
http://www.potentialtech.com