From: clewis@ferret.ocunix.on.ca (Chris Lewis)
Newsgroups: comp.mail.uucp,comp.mail.sendmail,comp.mail.misc
Subject: Re: Any easy way to filter this type of UUCP traffic?
Date: 9 Nov 92 08:11:10 GMT
Followup-To: comp.mail.uucp
Organization: Elegant Communications Inc., Ottawa, Canada

In article <1992Nov7.232211.4599@blilly.UUCP> bruce@blilly.UUCP (Bruce Lilly) writes:
>[silly redirect overridden; this is a UUCP mail issue, not a
>news administrative issue]

[The original post was x-posted to news.admin.misc.  Which is somewhat
silly, but certainly ain't my fault as you imply.

However, you also removed comp.mail.sendmail and comp.mail.misc.
Which makes it a good bet that the original poster doesn't see the
replies.  So, I've done the *proper* thing of putting comp.mail.sendmail
and comp.mail.misc back in, and then redirecting followups to
comp.mail.uucp.  Which is what you (or I, I guess) shoulda done in the
first place.]

>In article <3953@ecicrl.ocunix.on.ca> clewis@ferret.ocunix.on.ca (Chris Lewis) wrote:
>>uunet	... <{sitename}>({cost})

>>The angle brackets indicate that nothing should be routed *through*
>>"sitename" to other machines.

>Correct.

I should hope so ;-)

>>Furthermore, you shouldn't advertise the uunet link in *your* map entry
>>at all.  Otherwise, large numbers of systems may choose you to get to
>>uunet.

>I disagree with that.

>Pathalias will generate a ``backlink'' even if one half of the
>link is not explicitly listed in the maps.

pathalias will indeed generate a backlink, but a backlink with
cost of DEAD.  Even if the forward link isn't marked terminal.
Long ago pathalias considered each link bidirectional at the same
cost, but the versions that you need to operate with the current
generation of maps do not do this.

If you don't believe me, try it.  You'll want to play with the -l
and -c options to see the costs.

Here is a simple test:

    foo		uunet
    uunet	ecicrl
    boo		ecicrl

No explicit backlink from ecicrl to uunet.  (I should point out that
I do *not* have a link to uunet, private or otherwise)

Here is the output of "pathalias -c -l boo" (print costs, "local site"
is "boo"):

    0	boo	%s
    4000	ecicrl	ecicrl!%s
    100000000	uunet	ecicrl!uunet!%s
    100000000	foo	ecicrl!uunet!foo!%s

Notice that the cost for ecicrl!uunet is "DEAD" instead of merely
double the default (4000) as one would expect for an equal-cost
implicit backlink.

>A backlink may indicate other problems with the maps, e.g. a
>site whose map entry has not been updated when another site
>mentioned in its map has been removed from service, or the link
>disconnected.

This is irrelevant to this scenario.  The scenario is that
you are marked terminal to your email service, and you don't
advertise the backlink at all.  Third parties don't matter - unless
they're advertising links between you and somebody else - which
is illegal.

Furthermore, explicitly marking the backlink DEAD doesn't help
if somewhere else there's a cheaper cost for the backlink.
Pathalias chooses lowest cost for a link, *not* (forward or
reverse) presentation order.  To make a link cost *more*
than what's already in the maps you have to insert a

	dead {machinea!machineb}

in your local map override file - you're not allowed to do
this in the public maps (at least, last time we tried), so
you're screwed.

>If the link in fact exists, it should be listed in the maps.

Mentioning a link in the maps should be considered an open
advertisement for others to use it unless explicitly marked DEAD
(or terminal, but that's giving permission to use it to get
to the other site, just *not* beyond it).  Aside from issues of
unreasonably high volume (eg: MBAS usage), you can have no complaint
if someone automatically pathalias-routes through you.  Your
only recourse is to get the map entries fixed.

There's absolutely no reason to explicitly advertise a link that
you don't want people to traverse.

It's particularly tricky with for-pay links to systems like
uunet.  Any cost lower than DEAD has a tendency to suck everybody's
mail through the link because everybody is reachable *from* these
systems, but most people don't want to advertise routes *to*
these systems.

>If you're paranoid about others using the link, explicitly set the
>link cost as "(DEAD)",

This is already the default, so is unnecessary, and perhaps makes
it a little harder for someone to spot an obviously deprecated
backlink and manually insert a lower cost in their local overrides
to get around your intent.

Nor do I consider it paranoid to not want others traversing over
a link that you pay for personal purposes.  Much as I'd like to,
I don't think it appropriate that I have to pay for everybody else
nearby getting to the Internet.

>or specify the other end as a terminal node.

This doesn't help completely - it still permits other sites to route
through you to the other end.

>You can then override that in your local input to pathalias.

Nope.  You can't override a "terminal".  You have to remove it.
Try it out.
-- 
Chris Lewis; clewis@ferret.ocunix.on.ca; Phone: Canada 613 832-0541
Psroff 3.0 info: psroff-request@ferret.ocunix.on.ca
Ferret list: ferret-request@ferret.ocunix.on.ca
Newsgroups: comp.mail.uucp