NATD and Address Redirection

Jim Durham durham at
Sat Jul 26 19:13:09 PDT 2003

On Saturday 26 July 2003 03:13 am, you wrote:
> On Sat, Jul 26, 2003 at 02:22:05AM +0200, Clement Laforet wrote:
> > for incoming traffic, you must use -redirect_address, but for
> > outgoing you have to set -alias_address.
> > If you want to use a specific public IP to map incoming AND
> > outgoing packets, you need to run 2 natd, using ipfw matching.
> I'm afraid this is not exactly correct.
> IIRC when 5 years ago I was hacking natd and libalias to use them
> for transparent HTTP proxying, their internals looked rather clear.
> In a nutshell, they were as follows.
> There was a translation table inside libalias with 3 columns in it:
> the internal connection point (IP&port), alias point, and external
> point.
> When a packet was heading outside, its source IP&port were matched
> against the "internal" column, and its destination IP&port against
> the "external" column.  If an entry were found, the packet's source
> IP&port would be replaced with the values from the "alias" column.
> When a packet was going in the opposite direction, inside, its
> source IP&port were matched against the "external" column, and its
> destination IP&port against the "alias" column.  Then the packet's
> destination IP&port were replaced with the values from the
> "internal" column of the entry found.
> By specifying a redirect_address rule, just an entry was inserted
> to that table with a wildcard value for all the ports and for the
> external IP address.  Upon matching, such an entry would clone into
> a new one containing the information specific for a particular
> session.  Thus the solution was symmetric by design, without
> requiring 2 natd's or additional ipfw rules.
> P.S. As I can see, today's libalias still uses the same approach.

That's a great explanation! Thanks. I knew that NAT worked by 
establishing a "session" when an inside machine initiated a 
connection to the outside world and used that info the figure out how 
packets going back to that inside machine from the outside address 
got routed. I didn't know the internals. 

So redirect_address apparently just forces a permanent entry in the 
table, which would be symmetrical.  Hmmmm... OK.

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