On Fri, 2005-11-04 at 12:30, Derek Martin wrote: > On Fri, Nov 04, 2005 at 12:58:10PM +0000, Paul Howarth wrote: > > Another traditional use was for providing named hosts for each service > > and then providing all services on one host, e.g. > > > > myserver A 10.0.0.1 > > ftp CNAME myserver > > www CNAME myserver > > rsync CNAME myserver > > > > If the load became too high, each service could then be split off onto > > separate machines: > > But this can also be accomplished by using an A record that points to > 10.0.0.1 instead of the CNAME... Then you have the issue of reverse lookups which have to resolve to only one of the names. Nothing enforces it, but in theory the name with the A record should be the same as the reverse lookup and the others should be CNAME aliases. > > myserver A 10.0.0.1 > > myserver2 A 10.0.0.2 > > myserver3 A 10.0.0.3 > > ftp CNAME myserver > > www CNAME myserver2 > > rsync CNAME myserver3 > > > > This would all be transparent to people using the "ftp", "www", and > > "rsync" aliases. It is all transparent. Anything that doesn't follow CNAMES when looking for a A record is broken. Note, however, that a CNAME really is an alias and thus CNAMEs get any other data associated with the A record. That is, if you have an MX pointing to a name of an A record (as it should), all of the CNAMES of that A record get the same MX (because they are really the same thing). > Likewise if one is using A records vs. CNAMES here. And it's not > totally transparent... Any changes that are made will screw up > clients that have the old record cached, until the record expires from > the name server's cache -- whether you're using CNAMEs or A records. The only quirk this introduces is that the CNAME entry may have a different time-to-live than the A record. If you keep them the same or the CNAME shorter it should never be a problem. Also, a point not obvious above is that the CNAME may point to A records in some other domain, managed by someone else. It's not necessary to know the actual IP address or when it might change to include a name in your domain via: myname IN CNAME yourname.yourdomain.com. (and by the way, the CNAMEs above are incorrect - they should use the full domain name with a trailing dot as the target even when in the same domain - if the bare names work it is an accident due to the clients adding a default during lookup.) -- Les Mikesell lesmikesell@xxxxxxxxx
