RE: Port 113 requests?

[Chris Keladis <Chris.Keladis () cmc cwo net au>]

The only bad thing about 'rejecting' i can think of, is TCP/IP stack 
fingerprinting of the returned RST packet.

Well, also the ability to enumerate that tcp/113 returns an RST which will 
show up in nmap.

I think the best way to deal with these things is to tune the firewalls 
TCP/IP stack to obscure fingerprinting attempts and configure the firewall 
to return an RST on behalf of the protected host, to work around the 
extended timeout problem.

At least this way if someone enables ident on their machine for whatever 
reason, the firewall continues to send RSTs on behalf of the host, unless 
the firewall admin specifically allows ident into the protected host.

Sure someone can enumerate you have blocked tcp/113 a different way than 
the other ports, but if it's blocked, it's blocked.

You can defeat (at least, obfuscate) nmap enumeration by making all ports 
return RSTs, if enumeration is a concern.

Coupled with a unique fingerprint, you can have the best of both worlds.

I guess it's a balance of performance vs security factors your willing to 
live with.




Regards,

Chris.

At 01:51 PM 6/12/2001 -0700, Slighter, Tim wrote:

> you really should try and specify that the rule "drops" instead of reject so
> that the potential intruder is not provided with any information about their
> attempted connection.
>
> -----Original Message-----
> From: Chris Wilkes [mailto:cwilkes () ladro com]
> Sent: Thursday, December 06, 2001 1:05 PM
> To: incidents () securityfocus com
> Subject: Re: Port 113 requests?
>
>
> On Thu, Dec 06, 2001 at 01:51:57PM -0500, Michael Ward wrote:
> > I have been receiving the following entries at my firewall for since
> > noon US Eastern Time (-5:00) on 12/4/01.
> >
> > They have been coming every 15 minutes since then.  I notified the owner
> > of the IP's and he hasn't responded yet.
> >
> > 12/04/2001 11:59:30.336 - TCP connection dropped -
> > Source:mail.domain-i-edited.com, 40454, WAN -
> > Destination:my.mail.server, 113, LAN - 'Authentication' - Rule 32
>
> Its the SMTP AUTH protocol where a mail server tries to do an
> authenication check on who is sending it mail.  I've turned this off on
> my mail server as it really doesn't do any good.  I think some IRC
> servers use this feature.
>
> In my firewall I've setup this rule to handle these requests:
>         -p tcp --dport 113 -j REJECT --reject-with icmp-port-unreachable
>
> In short, nothing to be concerned about.
>
> Chris
>
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