Email has been down a while
gerard at linuxfromscratch.org
Fri Aug 25 14:34:03 PDT 2000
Email has been down for a while apparently and I didn't notice it really
(I've been out most of the day). Earlier today I made a small DNS
change. I changed the A record for linuxfromscratch.org and
shadowfax.linuxfromscratch.org to point at a different IP address while
having the old IP address still available on the box using IP aliassing,
so all should just be ok. The box could be contacted on both IP's during
the DNS propagation.
Then at around 5pm today Paul Jensen called me asking if the mail server
was down and it was then that I found out that
the shadowfax.linuxfromscratch.org (the address the MX record points at)
host did not exist. That host is in my /etc/hosts file so I don't have
to wait for my ISP's DNS server to get back to me since I already know
it's IP address.
So I checked out the DNS at register.com and the shadowfax entry was
missing. I know I clicked on "Edit IP" since I had to type in the new IP
address and then confirm the new IP address. If I had clicked on "delete
IP" I wouldn't been asked to type in the new IP address. Perhaps I did
something wrong anyways, I don't know and I can't track it. So I just
re-added the entry and now waiting for the DNS server at register.com to
update (around 6pm).
Did I not check whether email still worked after the change? Yes, I did.
I send an email through ISP SMTP server to linuxfromscratch.org. It
worked fine. When Paul called it still worked fine. Then it hit me: some
SMTP daemons, when the MX host can't be resolved, will try the domain in
the To: field which still worked. Let me explain:
mail to: lfs-discuss at linuxfromscratch.org
SMTP daemons contacts DNS, finds MX shadowfax.linuxfromscratch.org
try to contact shadowfax.
no reply or not valid? Try bypassing MX record and connect to the domain
To: field (linuxfromscratch.org in this case). That worked, and email
Sendmail, however, does not do this by default and you have tell it by
adding the option O TryNulLMXList to the sendmail.cf file
So that has quite the suprise.
And yes Paul, you do have the Texas-cowboy accent ;)
-*- If Linux doesn't have the solution, you have the wrong problem -*-
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