RE: NAT external/Public IP

["Dan Lynch" <DLynch () placer ca gov>]

Strictly speaking for address translation only, and not ACLs or firewall
rules, I believe that PAT does make a host more secure, not because it
obscures a host's native IP address, but because it is a one-way
function. PAT is dynamically created. As the client host initiates a new
connection, a new port is opened at the translating device. That port is
closed when the connection is torn down. Just as a server cannot exploit
a client's dynamically opened ephemeral port for a new connection, new
connections cannot be made through a PAT back to a client host. 

A one-to-one NAT on the other hand _can_ (not must) allow connections to
be established in both directions.

I think it's this distinction that led to the PCI requirement under
discussion.

- Dan


P.S. - That said, a firewall performing address translation services
(PAT or NAT) for a population of clients should regardless have a rule
blocking inbound access just for good measure.

P.P.S. - Please correct me if I'm wrong, but I believe inbound
connections into your LAN from your primary MTA to your internal mail
server is how most internet email gets delivered to internal users. 

Dan Lynch, CISSP
Information Technology Analyst
County of Placer
Auburn, CA


> -----Original Message-----
> From: listbounce () securityfocus com 
> [mailto:listbounce () securityfocus com] On Behalf Of Ansgar 
> -59cobalt- Wiechers
> Sent: Tuesday, October 30, 2007 10:04 AM
> To: security-basics () securityfocus com
> Subject: Re: NAT external/Public IP
>
> On 2007-10-30 Grant Donald wrote:
> > With PAT private IP addresses are hidden from the outside 
> world. This 
> > basically makes the job of hacking into a system more difficult, 
> > because the original host's IP address and source port is unknown.
>
> This is mere obscurity. It doesn't make a host any more or 
> less secure than it already is. Like I said before: either a 
> host is secure, then it doesn't matter if an attacker knows 
> the address, or it isn't secure, then you're "security" is 
> based on the hope that an attacker won't discover the host.
>
> > Depending on firewall capabilities (or lack of 
> capabilities) ports may 
> > need to be opened inbound for certain applications to work (e.g..
> > ident & pptp). A horizontal scan of such a network could produce a 
> > wealth of knowledge, if that network does not support port address 
> > translation.
>
> Ummm... wot? Why would you want to allow any inbound 
> connections into your LAN? And how would an attacker be able 
> to scan your network from the outside? For some obscure 
> reason you seem to assume that using public IP addresses in 
> your LAN means that the firewall at the perimeter magically 
> allows access from WAN to LAN. This assumption is wrong.
>
> Regards
> Ansgar Wiechers
> --
> "All vulnerabilities deserve a public fear period prior to 
> patches becoming available."
> --Jason Coombs on Bugtraq