Re: problem with router---NAT and caching?

From: Phillip Helbig---remove CLOTHES to reply (helbig_at_astro.multiCLOTHESvax.de)
Date: 04/02/05 

Date: Fri, 1 Apr 2005 23:47:51 +0000 (UTC)

In article <d2klnq$akl$1@online.de>, helbig@astro.multiCLOTHESvax.de
(Phillip Helbig---remove CLOTHES to reply) writes:

> For a long time, I've had a DSL router forward incoming requests to a
> particular internal address (the cluster alias if it is a cluster, the
> address of a particular machine if that is what is behind the router).
> This works OK. I'm testing out some hardware, so I set up another
> machine with another IP address and told the router to forward incoming
> connections to that. Works OK.
>
> I then go back to the original setup. EVERYTHING works OK, as it did
> before, EXCEPT incoming stuff over port 6000 (remote application
> displaying locally). It's not a security problem; the only thing I
> changed---and changed back---was the address which the router forwards
> stuff to.

OK, it's working again now, but I don't understand why.

I had changed the IP address on the original machine, because the new
machine originally had the same IP address. I changed the latter,
though, before redirecting the incoming connections to it.

Fortunately, I remembered what the original address of the original
machine was. I changed it back to that, told the router to forward
stuff to it, and low and behold it works.

I suppose somewhere within the router there is a mapping between IP
address and MAC address. If I change the IP address, then presumably I
can't use another IP address with a given MAC address (nor perhaps vice
versa), at least for a while. Unfortunately, the router doesn't seem to
have an option to clear the cache or whatever. (It's a rebadged Zyxel
combination router/switch which also provides PPPoE connectivity,
PAP/CHAP authentication etc.

Maybe this is the cause of the similar (actually identical) problem I
had with another router (a LINKSYS) a couple of years ago. Still, I
don't understand why it affects only port 6000 and not other incoming
connections. I don't see what is strange about port 6000. OK, most
incoming connections (I'm talking about connections initiated from
outside, not "return traffic" associated with an outgoing connection) go
to ports < 1023, but not all (for example, some WWW servers run on port
8000 or whatever---this was never a problem when I had the port 6000
problem).

This is only marginally VMS related, in that perhaps other people have
VMS machines behind NAT routers---or perhaps TCPIP on VMS (or TCPIP in
general) does some caching which causes the problem here. Regardless of
what the problem is, though, I am really puzzled as to why only port
6000 is affected.

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