🚀 go-pugleaf

In article <BvtG9B.GBo@breeze.rsre.mod.uk>
heading%hermes.mod.uk@relay.mod.uk writes:

:I'm trying to get the hang of this... Can someone explain?
:If I want to receive Internet mail, I might attach a machine
:foo.company.uk to the network, and have this as my mail address
:directly. I'd like, though, to have company.uk as my email address.
:
:Should I:
:   a) call the machine company.uk instead
:   b) have company.uk as an alias for foo.company.uk
:   c) register company.uk as a name with MX records
:        pointing to foo, then foo-backup, foo-reserve etc

I'd do b) and c).  Some mailers, like what Sun and Apollo enable by
default, don't support MX records, so it's usually a good idea for an
alias to be set up.  This is kind of in the spirit of "Be liberal in
what you accept and conservative in what you send out".  So you'd have
a machine named foo.comp.uk.  It'd have an alias to comp.uk.  Then
you'd have MX records for comp.uk pointing to foo.comp.uk,
foo-backup.comp.uk, etc.  

:When a large organisation has a "high-level" email address
:(dec.com, mitre.org, mit.edu  [though these are just guesses])
:do they usually correspond to physical machines, or to virtual
:machines with MX records?

It varies greatly.

						...Mike


-- 
 Michael J. O'Connor           |  Internet:  mjo@fmsrl7.srl.ford.com
 Ford Motor Company, OPEO      |  UUCP:      ...!{backbone}!fmsrl7!mjo
 20000 Rotunda, Bldg. 1-3001   |  Phone:     +1 (313) 248-1260
 Dearborn, MI  48121           |  Fax:       +1 (313) 323-6277

In article <BvtKy7.4qr@rahul.net> dhesi@rahul.net (Rahul Dhesi) writes:
>b: You may have trouble having a CNAME record that makes company.uk an
>   alias for foo.company.uk.  I don't think the name server software
>   (at least under SunOS) allows this.  However, you can have an
>   additional A record that gives company.uk the same IP address as
>   foo.company.uk.  If you order the A records appropriately, the
>   primary name can be foo.company.uk, but mail will still get
>   correctly delivered when addressed to company.uk.

This is also the case for HPUX.  A CNAME to create company.uk would hang the
resolver software.  I had to take the approach of creating an A record as
described above.
-- 
Mark Bixby                         Internet: markb@spock.dis.cccd.edu
Coast Community College District   1370 Adams Avenue
District Information Services      Costa Mesa, CA, USA  92626
Technical Support                  (714) 432-5064
"You can tune a file system, but you can't tune a fish." - tunefs(1M)

In <BvtKy7.4qr@rahul.net> dhesi@rahul.net (Rahul Dhesi) writes:
>c: Several MX records are often assumed to be more reliable for mail.
>   However, it's not clear to me that they buy you much.  Presumably if
>   foo.company.uk is down, mail will remain queued for it at one of the
>   MX hosts.  It might as well just remain queued at the sending host.

Multiple MX records have several advantages.
 
   1) Better use of network bandwidth because you don't get
      machines all over the Internet making repeated attempts to deliver
      mail to a dead machine.

   2) Faster delivery of backlogged mail once the dead machine is 
      back in operation since the mail is queued locally and can be
      delivered at ethernet speeds.
-- 
John Stewart -- Computing and Communications Services, Carleton University
Internet: jstewart@ccs.carleton.ca or Mr.Canoehead@algonquin.carleton.ca  
"Why can't we just forget about our constitutional problems and go canoeing?"

>>b: You may have trouble having a CNAME record that makes company.uk an
>>   alias for foo.company.uk.  I don't think the name server software
>>   (at least under SunOS) allows this.  However, you can have an
>>   additional A record that gives company.uk the same IP address as
>>   foo.company.uk.  If you order the A records appropriately, the
>>   primary name can be foo.company.uk, but mail will still get
>>   correctly delivered when addressed to company.uk.
>
>This is also the case for HPUX.  A CNAME to create company.uk would hang the
>resolver software.  I had to take the approach of creating an A record as
>described above.

Having foo.bar.edu be a CNAME for host.foo.bar.edu is illegal, e.g.
from rfc 1034

    The domain system provides such a feature using the canonical name
    (CNAME) RR.  A CNAME RR identifies its owner name as an alias, and
    specifies the corresponding canonical name in the RDATA section of the
    RR.  If a CNAME RR is present at a node, no other data should be
    present; this ensures that the data for a canonical name and its aliases
    cannot be different.  This rule also insures that a cached CNAME can be
    used without checking with an authoritative server for other RR types.
    
So you can't have an NS record for foo.bar.edu along with a CNAME record
for foo.bar.edu which points at host.foo.bar.edu

The options are to make the cannonical name of the host be
foo.bar.edu, with host.foo.bar.edu being a CNAME for foo.bar.edu, or
the additional A records.

heading@breeze.rsre.mod.uk (Anthony J.R. Heading) writes:

> If I want to receive Internet mail, I might attach a machine
> foo.company.uk to the network, and have this as my mail address

> Should I:
>    a) call the machine company.uk instead

You can do this... until you want to set up a second mailserver and share
the load between them, or replace it completely without changing its name.
Then you've got a problem.  Admittedly, the big advantage of this scheme is
that people can finger e-mail addresses and they work.  'talk', too.

>    b) have company.uk as an alias for foo.company.uk

If you mean a CNAME, then return addresses get messed up.  CNAMEs are never
supposed to appear in e-mail addresses, so your mailer will rewrite
user@company.uk to user@foo.company.uk in outgoing mail.

>    c) register company.uk as a name with MX records
>         pointing to foo, then foo-backup, foo-reserve etc

That's what we do here.  It works fine.  I've heard rumors that some
broken mailers only attempt the first MX record in a list, and then give
up, but I've never actually seen this happen.

This scheme loses a little in administration overhead (more nameserver
records), and a little more in that "finger user@company.uk" won't work.  I
really wish it was possible to have FX records.....

-- 
Tom Fitzgerald   Wang Labs       fitz@wang.com   "I went to the universe today;
1-508-967-5278   Lowell MA, USA                   It was closed...."

dhesi@rahul.net (Rahul Dhesi) writes:

>    However, you can have an
>    additional A record that gives company.uk the same IP address as
>    foo.company.uk.  If you order the A records appropriately, the
>    primary name can be foo.company.uk, but mail will still get
>    correctly delivered when addressed to company.uk.

What do you do with PTR records in this case?  Is it legal to have
multiple PTR records for a single Internet address?  If not, then mapping
company.uk to its IP address and reverse-mapping it back will give you
foo.company.uk.  I suspect some software will get unhappy about this.
(Actually, maybe not - I thought log_tcp would be one of the unhappy
ones, but it doesn't seem to mind as long as IP->name->IP gets back the
right address).

> c: Several MX records are often assumed to be more reliable for mail.
>    However, it's not clear to me that they buy you much.  Presumably if
>    foo.company.uk is down, mail will remain queued for it at one of the
>    MX hosts.  It might as well just remain queued at the sending host.

If the mail is actually delivered on foo, that's true; but if foo uses an
alias file to deliver it to the users' real mailhosts, then the backup MXs
can do the same, and get it delivered even when foo is down.

Besides, if the sending site is one that's not always up, you risk having
the mail delayed longer because the sender happens to be down when foo
comes back up.  The backup MXs are presumed to be up all the time.

>    A disadvantage is that people won't be able to telnet directly to,
>    or ftp directly from, company.uk.   Alternatives a and b don't
>    have this disadvantage.

For ftp, it seems like there's a growing tradition for having a CNAME for
ftp.company.uk pointing to the ftp server.

-- 
Tom Fitzgerald   Wang Labs       fitz@wang.com   "I went to the universe today;
1-508-967-5278   Lowell MA, USA                   It was closed...."

[this is leaving mail and heading for DNS territory.  Watch followups]
Tom> == Tom Fitzgerald <fitz@wang.com> 
RD>  == dhesi@rahul.net (Rahul Dhesi) writes:

 RD> c: Several MX records are often assumed to be more reliable for
 RD> mail.  However, it's not clear to me that they buy you much.
 RD> Presumably if foo.company.uk is down, mail will remain queued for
 RD> it at one of the MX hosts.  It might as well just remain queued at
 RD> the sending host.

We define two sorts of failures: machinewide and sitewide.  Machine wide
is level 0 backups, disk reconfigurations, blown ether MAUs, that kind
of thing.  Sitewide is usually either a power outage or our Internet
link going down (the latter mostly).

Machinewide failures of our mailhost (eff.org) get mail delivered to the
first backup MX, which is on the same ether.  Then when we bring the
main machine back up, we just /usr/lib/sendmail -v -Reff.org to flush
all mail with 'eff.org' in the recipient address.  Over the local ether
between two SS2s this is quite fast.  Much faster than waiting for all
those 1 hour (usually) queue runs to come around again.

Sitewide failures fall back to our 'last ditch' MXes at the net
provider.  This is more so that if we're expecting Real Important Mail
while we're down, we can arrange to have them flush their queue at us
when we return to life than anything else.

 Tom> For ftp, it seems like there's a growing tradition for having a
 Tom> CNAME for ftp.company.uk pointing to the ftp server.

We do this too.  The "service" CNAME is a really useful thing to have;
when we added machines, we moved the ftp server to one of the new
machines to spread out the load.  Folks who used 'ftp.eff.org' were set
as soon as the old records timed out where they were; folks who used IP#
or "true" hostname lost (there is a redirect message on the old machine
still, though).
--
Christopher K. Davis      |       NET.INSIGHTS INTO ARTISTIC CRITICISM:
<ckd@eff.org>   EFF #14   | ``[Y]ou don't cure bad art by locking it all up
System Administrator, EFF | in one museum, you cure it by throwing tomatoes
+1 617 864 0665  [CKD1]   | at bad artists.'' --Barry Shein <bzs@world.std.com>

[I am crossposting to comp.protocols.tcp-ip.domains]

In article <BvtG9B.GBo@breeze.rsre.mod.uk>,
heading%hermes.mod.uk@relay.mod.uk writes:
> I'd like, though, to have company.uk as my email address.

>  a) call the machine company.uk instead
>  b) have company.uk as an alias for foo.company.uk
>  c) register company.uk as a name with MX records
>       pointing to foo, then foo-backup, foo-reserve etc

(a) might be reasonable for a small site.  Even if you do this, I
    recommend using multiple MX records.
(b) is illegal.  If there are *any* DNS records for company.uk (and
    there will at least be SOA and NS records) then there may not be
    a CNAME record.
I recommend (c).
 
> Is there any disadvantage to the second approach, which seems
> on the face of it to be more flexible and more reliable?

Just that it is illegal (see above), less flexible and less reliable (in
that you can have several MX records pointing to several backup systems,
while you cannot have seevral CNAMEs pointing to several backup systems).

--apb
Alan Barrett, Dept. of Electronic Eng., Univ. of Natal, Durban, South Africa
RFC822: barrett@ee.und.ac.za                    Bang: m2xenix!undeed!barrett