At 3:46p -0400 18 Apr 2007, Chuck Swiger wrote:
On Apr 18, 2007, at 12:17 PM, Kevin Hunter wrote:
At 2:42p -0400 18 Apr 2007, Bill Moran wrote:
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?


You should consider just how difficult it is to rename a malicious program to, say, "ssh" in order to get around such checking. (Answer: trivial.) If you really want to control traffic in this fashion, you should look towards what the industry calls "deep packet inspection" or mandatory usage of proxies for all permitted protocols, instead.

Hrm. I was assuming that if I got into the nitty gritty, I could do more than just check the name of the binary, but perhaps not? Thanks for the warning.

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

So perhaps our specific example might be prudent:
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?

The main approaches are to use a stateful firewall ruleset, to explicitly permit return traffic via additional rules, or to simply permit established connections through. These options are arranged in rough order of how secure they are.

I have been given to understand that this approach does not lend itself to fine-tuning down the road, if such -- for /some/ reason -- were needed. . . .?

I suspect that you are encountering a steep learning curve,

You are /probably/ (read: definitely) correct.  I am. :-)

and that some additional reading will help you make much better decisions about how to configure a firewall.

Consider getting either or both of:

"Building Internet Firewalls", ISBN-10: 1565928717

"Firewalls and Internet Security: Repelling the Wily Hacker", ISBN-10: 020163466X,1144,020163466X, 00.html

Will look into those.  Thank you again.

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