RE: ICMP blocking on PIX .4.4.1

[GibsonB () gruntal com]

As I said what you want to do, ideally, is allow echo requests out.
Depending on your user base you may limit this to specific machines.  By
allowing only echo requests out, and not replies or other ICMP types, you
significantly limit an outside user from gaining information.  
 
You can additionally limit external access by allowing ICMP requests of any
kind only from machines that are Single IP NATed by your Firewall.   
 
 
 

-----Original Message-----
From: David Ashwood [mailto:David.Ashwood () Rubus com]
Sent: Monday, May 08, 2000 7:50 AM
To: 'GibsonB () gruntal com'; nawk () real-secure com; dufresne () sysinfo com
Cc: firewall-wizards () nfr net; phred () pacificwest com; jseymour () LinxNet com
Subject: RE: [fw-wiz] ICMP blocking on PIX .4.4.1


 
You don't wanna just let ICMP out - this allows somebody to map your
internal network.

-----Original Message-----
From: GibsonB () gruntal com [mailto:GibsonB () gruntal com]
Sent: 05 May 2000 18:24
To: nawk () real-secure com; dufresne () sysinfo com
Cc: firewall-wizards () nfr net; phred () pacificwest com; jseymour () LinxNet com
Subject: RE: [fw-wiz] ICMP blocking on PIX .4.4.1



I don't agree with this. ICMP is an invaluable tool for diagnostics.  If you
shut it down then you are limiting your ability to troubleshoot problems.  

What you want to do is allow ICMP to go out but not to come in.  Ideally
what you want to do is allow certain types of ICMP out(ie Echo requests) and
only certain types of ICMP to come in(ie Echo Reply, Time exceeded,
unreachable).  This is not easily done in a router.  

Actually blocking connectionless protocols in general is not easy thing to
do in a router. 



-----Original Message----- 
From: User nawk [ mailto:nawk () real-secure com <mailto:nawk () real-secure com>
] 
Sent: Saturday, April 29, 2000 12:57 PM 
To: R. DuFresne 
Cc: firewall-wizards () nfr net; phred () pacificwest com; 
jseymour () LinxNet com 
Subject: Re: [fw-wiz] ICMP blocking on PIX .4.4.1 


Hi, 

    That is exactly how it should be done. You want ICMP and spoofing 
stopped on the router. Firewalls are a great device, but not perfect. 
Cisco's ACL do a much better job on blocking. Just make sure the lists are 
not to long so the CPU of the router does not get saturated. Think of it as 
what if you or someone makes a mistake on the firewall and now you opened 
yourself up. All it is are layers of defense. If you really want to be anal,

setup ACL on your border routers, then apply your rules on the firewall and 
last setup another router behind the firewall with ACL again. This way the 
attacker has to pass all three to get into your network. 


Thanks 

----- Original Message ----- 
From: "R. DuFresne" <dufresne () sysinfo com> 
To: "Jim Seymour" <jseymour () LinxNet com> 
Cc: <nawk () real-secure com>; <firewall-wizards () nfr net>; 
<phred () pacificwest com> 
Sent: Thursday, April 27, 2000 6:06 PM 
Subject: Re: [fw-wiz] ICMP blocking on PIX .4.4.1 


> It's always been our impression that veiwing security as an 'onion' on 
> pulls all the onoins skins together to form as tight a security system as 
> possible to deal with the security policy at hand.  This would include 
> ACL's in routers to deal with ICMP/UDP and spoofing there, as well as 
> backup those rules in the firewalls rule sets, just in case one device 
> barfed up and packets slipped by it. 
>
> Even the most recent issue of sysadmin mag has an article titled: 
>
> The Use of Routers in Firewall Setup 
>
> May 2000 vol 9 # 5 
>
> Thanks, 
>
> Ron DuFresne 
>
> On Thu, 27 Apr 2000, Jim Seymour wrote: 
>
> > nawk <nawk () real-secure com> wrote: 
> > > I think it's best practice to block things like icmp and spoofing 
> > > on your routers not firewall. The firewall is just to block things 
like 
> > > ports and provent access to your internal network. 
> >
> > Two schools of thought on that.  The consultant that installed our 
> > first Gauntlet firewall (TIS was offering at the time free installs and 
> > one day of training for up to three people) recommended that the router 
> > be stripped of *all* packet filtering rules so that the firewall would 
> > see everything.  His logic was that Gauntlet was much more capable at 
> > detecting and reporting activity than was the firewall router. 
> >
> > My feeling was that sufficient rules to protect the *router* itself had 
> > to remain.  So that's what I did: the router has only enough rules in 
> > it to protect *it*.  The firewall gets everything else.  (Except when I 
> > get really fed up with something.  Then I block it at the router.) 
> >
> > Note also that there is a potential problem in simply out-right 
> > blocking all ICMP at the router.  If you're running a mail gateway on 
> > the firewall (as I do [Postfix]), blocking ICMP path MTU discovery can 
> > lead to SMTP sessions timing-out on large emails.  (See, for example: 
> > http://msgs.SecurePoint.com/cgi-bin/get/postfix9904/37/1.html
<http://msgs.SecurePoint.com/cgi-bin/get/postfix9904/37/1.html> .)  And I 
> > don't see any particular reason why others shouldn't be allowed to ping 
> > my firewall. 
> >
> > Allowing ICMP (or any connection-less protocol, such as UDP) *through* 
> > the firewall is another issue entirely.  Connection-less protocols are 
> > not safe.  Cannot be made safe.  Other than perhaps allowing syslog 
> > from the router to a syslog host, specifically, I don't see any 
> > particular reason to allow any UDP through a firewall. 
> >
> > As regards the original poster's query: I don't know the PIX firewall, 
> > but wouldn't it be possible to log on to the PIX and run your pings and 
> > traceroutes from there?  Less convenient, to be sure.  But far safer 
> > than allowing UDP through it, I should think.  I'll take safety over 
> > convenience any day. 
> >
> >
> > Regards, 
> > Jim 
>
> -- 
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
>         admin & senior consultant:  darkstar.sysinfo.com 
>                   http://darkstar.sysinfo.com
<http://darkstar.sysinfo.com>  
> "Cutting the space budget really restores my faith in humanity.  It 
> eliminates dreams, goals, and ideals and lets us get straight to the 
> business of hate, debauchery, and self-annihilation." 
>                 -- Johnny Hart 
>
> testing, only testing, and damn good at it too! 



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Gruntal & Co., L.L.C.'s e-mail system is for business purposes only.  
Messages are not confidential.  All e-mail may be reviewed by 
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E-mail will be archived for at least three years and may be produced 
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