لطفا منتظر باشید ...
16.1 Using CNAME Records
We talked about CNAME resource records in Chapter 4. We didn't tell you
everything about CNAME records, though; we saved
that for this chapter. When you set up your first name servers, you
didn't care about the subtle nuances of the magical
CNAME record. Maybe you didn't realize there was
more to it than we explained; maybe you didn't care.
Some of this trivia is interesting; some is arcane.
We'll let you decide which is which.
16.1.1 CNAMEs Attached to Interior Nodes
If you've ever renamed
your zone because of a company reorganization, you may have
considered creating a single CNAME record that pointed from the
zone's old domain name to the new domain name. For
instance, if the fx.movie.edu zone were renamed
to magic.movie.edu, we'd be
tempted to create a single CNAME record to map all the old names to
the new names:
fx.movie.edu. IN CNAME magic.movie.edu.
With this record in place, you'd expect a lookup of
empire.fx.movie.edu to result in a lookup of
empire.magic.movie.edu. Unfortunately, this
doesn't workyou
can't have a CNAME record
attached to an interior node like fx.movie.edu
if it owns other records. Remember that
fx.movie.edu has an SOA record and NS records,
so attaching a CNAME record to it violates the rule that a domain
name be either an alias or a canonical name, not both. So, instead of
using a single CNAME record to rename a complete zone,
we'll have to do it the old-fashioned waya
CNAME record for each individual host within the zone:
empire.fx.movie.edu. IN CNAME empire.magic.movie.edu.
bladerunner.fx.movie.edu. IN CNAME bladerunner.magic.movie.edu.
If the subdomain isn't delegated and consequently
doesn't have an SOA record and NS records attached
to it, you can create an alias for fx.movie.edu,
but it will apply only to the domain name
fx.movie.edu and not to domain names under
fx.movie.edu.
16.1.2 CNAMEs Pointing to CNAMEs
You may have wondered whether it
was possible to have an alias (CNAME record) pointing to another
alias. This might be useful in situations where an alias points from
a domain name outside of your zone to a domain name inside your zone.
You may not have any control over the alias outside of your zone.
What if you want to change the domain name to which it points? Can
you simply add another CNAME record?The answer is yes: you can chain together CNAME records. The
Microsoft DNS Server supports it, and the RFCs don't
expressly forbid it. But, while you can chain
CNAME records, is it a wise thing to do? The RFCs recommend against
it because of the possibility of creating a CNAME loop and because it
slows resolution. You may be able to do it in a pinch, but you
probably won't find much sympathy if something
breaks.
16.1.3 CNAMEs in the Resource Record Data
For any other record besides a CNAME
record, you must have the canonical domain name in the resource
record data. Applications and name servers won't
operate correctly otherwise. As we mentioned back in Chapter 5, for example, many mailers recognize only the
canonical name of the local host on the right side of an MX record.
If a mailer doesn't recognize the local host, it
won't strip out the right MX records when paring
down the MX list and may deliver mail to itself or to less-preferred
mail exchangers, causing mail to loop.
16.1.4 Looking Up CNAMEs
At times you may want to look up a
CNAME record itself, not data for the canonical name. With
nslookup, this is easy to do. You can set the
query type either to cname or to
any and then look up the name:
C:\> nslookup
Default Server: wormhole.movie.edu
Address: 192.249.249.1
> set query=cname
> bigt
Server: wormhole.movie.edu
Address: 192.249.249.1
bigt.movie.edu canonical name = terminator.movie.edu
> set query=any
> bigt
Server: wormhole.movie.edu
Address: 192.249.249.1
bigt.movie.edu canonical name = terminator.movie.edu
16.1.5 Finding Out a Host's Aliases
One thing
you can't easily do with
nslookupor any query tool, for that
matteris find out a host's aliases. With the
host table, it's easy to find both the canonical
name of a host and any aliases. No matter which you look up,
they're all there together on the same line, as
shown in the following excerpt from HOSTS:
192.249.249.3 terminator.movie.edu terminator bigt
With DNS, however, if you look up the canonical name, all you get is
the canonical name. There's no easy way for the name
server or the application to know whether aliases exist for that
canonical name:
C:\> nslookup
Default Server: wormhole.movie.edu
Address: 192.249.249.1
> terminator
Server: wormhole.movie.edu
Address: 192.249.249.1
Name: terminator.movie.edu
Address: 192.249.249.3
If you use nslookup to look up an alias,
you'll see that alias and the canonical name.
nslookup reports both the alias and the
canonical name in the packet. But you won't see any
other aliases that might point to that canonical name:
C:\> nslookup
Default Server: wormhole.movie.edu
Address: 192.249.249.1
> bigt
Server: wormhole.movie.edu
Address: 192.249.249.1
Name: terminator.movie.edu
Address: 192.249.249.3
Aliases: bigt.movie.edu
You can find out all the CNAMEs for a host in a particular
zone by transferring the whole zone and picking out the
CNAME records in which that host is the canonical name. You can have
nslookup filter on CNAME records:
C:\> nslookup
Default Server: wormhole.movie.edu
Address: 192.249.249.1
> ls -t cname movie.edu
[wormhole.movie.edu]
bigt terminator.movie.edu
wh wormhole.movie.edu
dh diehard.movie.edu
You can also do this with dnscmd:
C:\> dnscmd /enumrecords movie.edu @ /type A
This method won't show you aliases in other zones that point
to the canonical name, though.
