by I have a linux box (Red Hat 5.0, kernel version 2.0.32) that is acting
as an internet gateway using IP masquerading to connect an NT PC to
the internet. Almost all my applications work fine across this, but I
have one specific application that uses UDP and needs to have it's
source port number perserved when the packet is forwarded out to the
internet. Here's the idea:
<----- internal network ----> | <---- outside world -->
|
----+ +----------+
NT | A.A.A.A B.B.B.B | linux | C.C.C.C
box |--------------------+ box +----------> internet
| | |
----+ +----------+
I have an application on the NT box that needs to connect via UDP to a
box on the internet. It uses UDP, and sends over a particular port,
call it "N". It uses the same port for source and destination, and
the server responds using the same ports. The problem is that the
server won't respond if the source port on the packet is not equal to
"N", and since IP masquerading alters the source port on the outgoing
packet the application doesn't work (the server ignores it).
If the server were at address "D.D.D.D", then I'd like to see this:
A.A.A.A (port N) -> D.D.D.D (port N) on local network
Linux box then "masquerades" only the IP address (it preserves the
port number)
C.C.C.C (port N) -> D.D.D.D (port N) to internet
Server responds to a request with:
D.D.D.D (port N) -> C.C.C.C (port N) back to linux box
Linux box then forwards to NT box:
D.D.D.D (port N) -> A.A.A.A (port N) on local network
I think I can get the UDP redirection on the incoming packets working,
but I don't see how I can preserve the port number through the linux
box on the outgoing packets. This is the only host on the internal
network that will use this port number, so any hard-coding in either
direction is OK. Can this be done? If so, how?
I would appreciate it if any replies could get copied to my e-mail
address as well...
Thanks,
Steve Chinatti