> From: Steve Kirk [mailto:[EMAIL PROTECTED] > Thanks Peter, interesting.
Internet issues in the large tend to be - you get emergent behaviour that is often unexpected :-). > I think these are > referred to as "caching" servers as opposed to "secondary". It's the > secondaries that receive the zone transfers. Yes. Note that these roles are per-zone; a given DNS server may act as a primary or secondary for some zones, and as a caching server for others. > Having said that, I'd have thought that a large ISP such as > AOL would have > secondaries, (inaccesible by joe public), but would also have caching > servers, which are the ones they make public. It would be difficult to persuade those secondaries to be effective - for what zones are they secondaries? Let's say AOL want to act as a secondary for foo.com. How do AOL contact the owners of foo.com in order to request that their secondary server is added to the list of allowed IPs for zone transfers? Other than that, AOL could then make use of those servers as forwarders from their caching servers, I accept. > Since they typically have > several caching DNS servers, in theory there is a good chance > that each of > them will get a different one of the RR Ips from their > secondary server, so > in theory the RR goal is often achieved....? Assuming they are independent and not configured to use the same forwarders, yes. You might be surprised how few DNS servers an organisation needs, though - Demon (my home ISP, and not a small one) has two, and could probably get away with one except for redundancy. I've not seen an ISP setup document yet that says to use primary and secondary DNS of ns47.isp.net and ns32.isp.net - they're almost all ns0 and ns1 or ns1 and ns2, indicating that there are probably very few in the organisation. > For example I just used DOS > nslookup to query my ISPs 2 main dns servers for > www.microsoft.com - they > each returned a different address, although repeatedly > querying each one > returns the same answer every time. If I go through a local > caching DNS on > my LAN, that returns a third address for MS - again, the same > one every time. Yup. So anyone using your ISP's DNS servers will get one of two IPs for www.microsoft.com at present, out of the however many they have. Lumpy load balancing in action :-). You likely haven't set up your own caching DNS to forward requests to your ISP's DNS servers; otherwise you'd have had one of the same answers. > Basically, it's "internal feature" of the DNS server > to decide how > it treats hostnames for which is has more than one IP. Indeed. > PS this has rekindled my interest so I just googled to > refresh my mind on > the basics, this seems a useful page that explains what we > are talking about above. > http://www.onjava.com/pub/a/onjava/2001/09/26/load.html Yes, that seems like a reaonable summary, although it doesn't really go into the caching effects we're discussing here. - Peter --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
