Cisco Systems Pings and PIX messages 302020: Built ICMP - 302021: Teardown ICMP Lots of them....

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Subject Author Date
Pings and PIX messages 302020: Built ICMP - 302021: Teardown ICMP Lots of them.... Scott Townsend 05-01-06
Posted by Scott Townsend on May 1, 2006, 2:40 pm
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On my Edge Router I have an Access list for ICMP as follows:

access-list 103 permit icmp any any time-exceeded
access-list 103 permit icmp any any port-unreachable
access-list 103 deny   icmp any any
access-list 103 deny   icmp any 0.0.0.0 255.255.255.0
access-list 103 deny   icmp any 0.0.0.255 255.255.255.0
access-list 103 deny   icmp any any redirect


On the PIX Firewall, I have the Following:

access-list acl_outside extended permit icmp any any echo-reply
access-list acl_outside extended permit icmp any any time-exceeded
access-list acl_outside extended permit icmp any any unreachable

On my PIX log I get hundreds of the Following

%PIX-6-302020: Built ICMP connection for faddr 82.160.189.125/0 gaddr
A.B.C.D/0 laddr 10.10.3.10/0
%PIX-6-302021: Teardown ICMP connection for faddr 83.79.179.113/0 gaddr
A.B.C.D/0 laddr 10.10.3.10/0

The Address A.B.C.D/0 laddr 10.10.3.10/0 has been caught using a Sharing
program. I've turned off Port 6346/6347 on the Edge Router, but I'm still
getting the Built and Teardowns.

I would like to be able to Initiate a Ping out from the 10.1.1.0/24, but not
a from anywhere else and would like to not allow anyone to Ping us.

What should I change?

Thanks,
  Scott<-



Posted by Walter Roberson on May 2, 2006, 12:21 pm
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Which direction is that applied on?


In the ACL applied out,

permit icmp 10.1.1.0 0.0.0.255 any echo

In the ACL applied in,

permit icmp any PUBLIC_IP PUBLIC_MASK echo-reply

[PUBLIC_IP PUBLIC_MASK for the case where you are NAT'ing, which you
need to be doing because RFC1918 does not allow you to source packets
in any of the reserved IP ranges past the edge of your network.]

Posted by Scott Townsend on May 4, 2006, 10:31 am
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interface MFR0.672 point-to-point
 description WAN to SBC Internet Service
 ip access-group 103 in


So should I be applying this to the MFR0 or Ethernet Interface??


I think I have a Few Issues.

I guess I Have to assign a Static NAT IP to the Users I want to be able to
Ping so the Edge Router knows who to let have the Ping Replies.

Since the Edge router is not doing the NAT, I have a PIX behind it, it cant
know which of the Public IPs is in the 10.1.1.0/24 network.

Hmmm...

Thank you!



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