blocked ports affecting http

On recommendation of Cisco, I have blocked ports TCP 3127-3199 out my interface "inside". Seems that these are commonly used for mydoom ports. Now, when the user's browser uses these ports as source ports they don't have until it exceeds this window. Has anyone seen elsewhere this problem and how do you work around it?

Roland,

A MyDoom attack can be launched from the outside (entering your local network) or inside (out to your local network).

IF YOU WANT TO BLOCK INCOMING OF MYDOOM ATTACKS:

If this traffic passes through a firewall, then by default, the sessions opened from the outside are blocked unless explicitly allowed entering. If you use a router with ACL, then you must configure an ACL in the INCOMING direction and apply it to the interface from the OUTSIDE as such:

--------------------------

For routers:

access-list 111 tcp refuse any any 3127 3199 Beach

interface

IP access-group 111 to

--------------------------

In this case, you'll experience the question that you're already because when a web server returns a bunch of session to the client (browser), then the destination ports match the ACL and the router will drop the session. To remedy this, we can apply an IOS Firewall to the external interface of the router. The ACL to block incoming attacks of MyDoom cannot while the router will maintain session state information in its table.

IF YOU WANT TO BLOCK OUTBOUND MYDOOM ATTACKS:

Then the ACL must be applied 'in' on the 'inside' interface

--------------------------

For the PIX Firewall:

access-list 111 tcp refuse any any 3127 3199 Beach

access ip-list 111 allow a whole

Access-group 111 in the interface inside

--------------------------

--------------------------

For the router:

access-list 111 tcp refuse any any 3127 3199 Beach

interface

IP access-group 111 to

--------------------------

This should provide you with enough information to work on your issue.

Paragraphs

Tags: Cisco Security

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