getting the host tables right


mar@ATHENA.MIT.EDU
Tue, 16 Feb 88 16:53:28 EST


Recent discussion about mail problems indicates that some people have
been having problems coordinating host configuration changes. Here
are some hints from an address change I performed a while back.

The goal was to change the address on every host in our installation,
including 2 listed in the hosttable, our domain nameserver, and our
mail hub. We planned this a couple of months in advance. The steps
we followed were:

1. Turn on both sets of addresses in our gateways, and use a PC
configured on the new network address to make sure we had connectivity
with the rest of the internet.

2. Notified the NIC that we were planning the change for about a month
later, and got back confirmation that they could change the hosttables
on the day we chose. Also notified the system administrator at the
machine serving as secondary nameserver for our domain, and got
confirmation that they would be able to make the change on the correct
day.

3. Changed the timeouts in our name domain to 1 hour so that resolvers
wouldn't cache our old address for 3 days after the change.

4. A couple of days before the change we changed the sendmail.cf on
our mailhub to put:
        Notice: foo.arpa's address will be 1.2.3.4 after April 1
lines in the headers of all mail passing through.

5. We also started changing the addresses of PC's and lesser known
workstations a day early, just because it would take too long to
change everything at once.

6. On the flag day, we first checked with the NIC to see that they had
the new information. We would have called off the change if they
hadn't done it yet. We then changed the addresses of our well known
machines, and swapped in the new domain database. We changed the text
of the extra header line in the message slightly.

Since our secondary nameserver is run by a different organization at a
different site, as suggested in the RFC, hosts that had cached the old
information could still get to us. They would be unable to contact
our primarhy nameserver at the old address, so try the secondary
nameserver (which hadn't moved) and get our new addresses from there.

7. I then monitored our gateway log for the next week to catch hosts
attempting to contact our old address. When I spotted these, I sent
mail to the postmaster at those sites informing them of our move and
that they should get new hosttables.

The whole thing was relatively painless, with not much disruption of
service. The flag day was on a Friday, so that caches would have a
better chance of timing out by Monday when people wanted to do work
again. The key to the operation was planning it a couple of months in
advance, and not going from one stage to the next until we were sure
it was going to work. If you are forced to make a change like this on
short notice, I imagine that there would be many more problems.
                                        -Mark

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