program/binary ip filtering

Kevin Hunter hunteke at
Wed Apr 18 19:18:32 UTC 2007

At 2:42p -0400 18 Apr 2007, Bill Moran wrote:
>> We are in the process of setting up a bastion host.  One of the  
>> things we'd like to do is to filter packets not only at the ip  
>> layer, but by what program is listening on a particular port.  Is  
>> this a possibility?
> Are you saying that you want to have the packet filter check to see  
> what application is listening on a particular port, then allow/deny  
> access based on the name of the application?


> Do you not have control over what is run on this system?

So perhaps our specific example might be prudent:

kevin $: ssh bastion
bastion $: ssh internalserver

Relevant part of log:

Apr 18 09:35:23 kappia ipmon[405]: 09:35:22.695348 fxp0 \
	@0:4 b internalserver,22 -> bastion,53136 PR tcp \
	len 20 52 -AS IN

It's blocking because we are dropping all packets not destined for  
port 22.  Since ssh /from/ the bastion picks a random high port, it's  
dropping all the return packets to that random high port.

How have others handled this type of scenario, where a hardening of a  
bastion host has been desired/necessary?

> However, you might be able to accomplish this by using a pf table,  
> then having a secondary script update the table based on the output  
> of sockstat or some other similar hack.

We did not know about this utility.  We'll check out your idea.   
Thank you for the pointer.  We'll let the list know how it  
goes.  :-)  (Of course, if there's a standard, cut and dry solution,  
that'd be ideal!)

Thanks Bill,


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