"Bob Puff@NLE" wrote:
>
> Hello,
>
> I've got a problem with one of my ISPs that I'm 95% sure is on their end, but I want
>to get some opinions of the experts here, and know what exactly to tell them.
My understanding of the topology:
64.65.206.24/?? 64.65.210.162/?? IPob/??
/---(DSL)----ISP#1----(DSL)----otherBox
ethA |
LM7.1 |
ethB (default) |
\----(T1)----ISP#2
IPb/?? IP2/??
Can you fill in the other IP addresses (including netmasks)...?
> Scenerio:
> Mandrake 7.1 Box with two ethernet cards, one on ISP #1 (DSL), another one on ISP #2
>(T1). Default route is set to go out to the T1.
>
> Situation: When I ping the DSL IP address of my box, the reply gets sent back thru
>the T1. It works everywhere BUT within ISP #1's system.
So... from say 64.65.210.162, "ping 64.65.206.24" is seen by LM7.1 and reply
goes out T1; but reply is not seen by 64.65.210.162... if so, read on...
> In other words, if I try to access my box's DSL ip address from another DSL line, it
>fails. When looking at my box, I can see the packet coming in, and the reply going
>out to the T1, but that reply packet never gets back to the other DSL line. It's
>like the DSL provider is filtering the return packet, because it isn't coming from
>the address it was initially sending to.
> This holds true for all services - http, mail, etc. The DSL line has NO firewalling
>or NAT on it, so they say. The kicker is that everything works fine on everyone
>else's network, just not the DSL's system!
>
> Also FWIW, if I make the DSL line the default route, all is fine, including within
>the DSL network.
>
> Is there a way for return packets to be handled differently?
I'll repeat a statement I made in previous posts: EACH direction is treated
independently. When a packet is originated (request OR reply), it will follow
the best LEGAL path as understood by the intervening routers.
> How can I explain this to my DSL ISP so that they won't dismiss the problem as a
>"Weird configuration on the user end"?
Without specific addresses, I can only suspect you are experiencing the rule
which states: "a packet from netX cannot be routed through netY and back to
netX" [EXCEPT if the final destination is netZ]*
* the exception is not written up anywhere that I know of; but I did discover it
circa 1989.
This means that a packet going from S to D where:
S in D in routable via netX
---- ---- -----------------
netA netA NO (classless(?) & host routing excepted)
netA netC yes
This will take quite some explaining for those not intimately familiar with
routing; but this leads me to the possibility that your LM7.1 box and ISP#1 have
conflicting netmasks. Without specific IP addresses and netmasks at each
interface involved, it is really complicated to come up with a matching scenario
to explain the problem.
> Bob
>
> P.S. the IP address in question is: 64.65.206.24
WAY too little info... and don't even try to use bogus addresses; it is very
unlikely you could come up with replacements which could explain what is
happening to the real ones... :>
Pierre
