Re: Port 113 requests?

[woods () weird com (Greg A. Woods)]

[ On Thursday, December 6, 2001 at 16:24:59 (-0800), Crist J . Clark wrote: ]
> Subject: Re: Port 113 requests?
>
> It's a trade. If you drop the auth attempts silently, you usually then
> have to wait for the attempts to time out before whatever you did to
> prompt the auth attempt can proceed. If you send a RST or
> ICMP-unreachable, you don't have to wait for the time out.

Where's the trade-off?  Clearly you don't want to force a time-out wait!

Note that returning a TCP RST is the only guaranteed-to-always-work
solution.  Whether you do that with your firewall, or with the target
host itself (eg. by simply not running identd) is mostly irrelevant.

TCP/IP stacks might not drop existing connection attempts when an ICMP
unreachable (port or host) is received (as doing so could open them up
to well timed DoS attacks).

> In this case, it's someone's mail server getting the auth connection
> attempt. Everyone knows where everybody else's mail servers are
> (receiving hubs have MX records, senders are in the mail
> headers). Sending RSTs on port 113 is just telling the world that you
> don't want their auth requests; you are not really giving anything
> away to an intruder.

Indeed!  You're not giving away anything at all by refusing port-113
connections.  Any potential attacker will already know far more about
your network than could ever be learned from the fact that some host is
not (or at least appears not to be) running identd!

Note too that if you silently drop port-113 destined packets then the
intruder may actually learn that you're not such a hot-shot firewall
administrator and that may even give them more clues than if you simply
punched a hole through your firewall for all port-113 connections!  ;-)

On the other hand accepting port-113 connections and returning a
carefully crafted and encrypted reply (eg. by running pidentd with the
'-C' flag) might actually deter an intruder because doing so might
suggest to them that you're somewhat savvy as to what the IDENT protocol
is really all about and thus you might be more security consious than
they're willing to deal with!  ;-)

-- 
                                                                Greg A. Woods

+1 416 218-0098;  <gwoods () acm org>;  <g.a.woods () ieee org>;  <woods () robohack ca>
Planix, Inc. <woods () planix com>; VE3TCP; Secrets of the Weird <woods () weird com>

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