routing

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From - Fri Sep 28 12:24:18 2001
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From: rja.carnegie@excite.com (Robert Carnegie)
Newsgroups: comp.unix.sco.misc
Subject: Re: Routing question
Date: 28 Sep 2001 09:20:57 -0700
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"David Nash" <nashcom@btinternet.com> wrote in message news:<9p18hr$jkc$1@neptunium.btinternet.com>...
> Hi
> 
> I posted a question a couple of weeks ago about retrieving SMTP mail from an
> ISP using a Windows NT server.  Thanks for the replies.  For those that are
> interested, and before I ask another question, the ISP needs to allocate a
> fixed IP address to the WAN port of the ISDN router.  The router then has to
> have 'Port Address Transalation' so that the ISP can connect to port 25 of
> the router WAN address, and this is then translated to port 25 of your SMTP
> Exchange server.  Basically, when you deliver outgoing mail to the ISP using
> SMTP, you need to send an ETRN command that requests the SMTP server at the
> ISP to send your incoming mail to your server.  Sounds complicated, but I
> thought it may be of interest/use.

Ours is different, I think; if I understand it, we have a class C
LAN which is connected to the Internet by an ISDN router (all addresses
issued by the ISP), and any server that we put on that network
(usually dual-homed) is on the Internet.  So that's where our mail
server is - we did have to tell the ISP which of our addresses we
would use for that.  Whereas, if I read right, your ISDN router is
using _one_ ISP-provided address, which addresses the ISDN side of the
router, and translating each possible port number to an appropriate
port on a server on your LAN.

> However, we have two ISPs, one for the email, and a 'free line' for
> the Internet.  If I configure the Port Address Translation so that
> the email can work, the call to the Internet ISP doesn't work
> properly.  I'm thinking about maybe adding a second router and
> splitting the Internet/Email, but I'm not sure how to route
> Internet through one router and email through the other.
> The default gateway is set to the existing router.  I've tried
> configuring the router so that traffic destined for the Mail host
> brings up one 'autocall', and the Internet traffic brings up the
> free autocall, but as I say, it seems the the NAT for the fixed
> WAN IP address is causing problems.

ISDN is designed to be shared, so certainly you should be able to
use one router just to get to specified hosts on your e-mail provider
and another router to get to the 'free line'.  In this case you'd
have default route to the second router, for the free line,
and you'd set an explicit route to router one for the e-mail provider's
hosts.

(If the 'free line' provider provides e-mail, you might be able to
send it out in that direction with your e-mail provider's addresses
on it, but then again you might not - and anyway, if your configuration's
like ours in another detail, you don't get e-mail _from_ them by ISDN
until you place a call _to_ them by ISDN.  Ours doesn't call us,
we call them.)

A detail of NAT that I don't appreciate is whether the ISP's e-mail
will appear, from your server's point of view, to be coming from your
e-mail ISP's server or directly from your router.  This is relevant
because SCO OpenServer, at least, and with MMDF at least, and for me
at least, has been fussy about which servers it does and doesn't
accept SMTP e-mail from. (This is reasonable, since the alternative
is an open relay which can be used for spam.)

Our router is a fairly basic model, but it can place calls to
multiple remote ISDN devices at once, so I don't see why yours
shouldn't do this.  And you say that it does, _until_ you set
the router to accept e-mail from _that_ ISP and pass it on to
your mail server.  Then you can't use your Internet connection.

That appears to be a defect in the router, and not a deficiency in
your understanding of the network.  Perhaps a defect of design, in
which case you might as well buy another one - or buy a different one.

One suggestion, though: perhaps your router is unhappy that
one autocall is supposed to connect you to the e-mail ISP,
whereas another autocall is supposed to connect you to the
entire Internet, _including_ the e-mail ISP's servers on the
Net.  In that case, perhaps you could get around that by
setting the router to use the free line to connect you,
not to the whole Internet, but only to the 'free line'
provider's local network - which should include a proxy
server.  You would then set Internet applications to use
the proxy server to get to the rest of the Internet.  If
the router doesn't know that that connection goes to the
whole Internet then it may not get confused ;-)

You of course are using a "private" network address space as
recommended by RFC 1918 for your own network.
 

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