> -----Original Message----- > From: centos-bounces at centos.org > [mailto:centos-bounces at centos.org] On Behalf Of Johnny Hughes > Sent: Monday, June 26, 2006 7:19 AM > To: CentOS ML > Subject: RE: [CentOS] Re: DNS Server > > On Sun, 2006-06-25 at 20:32 -0400, Thomas E Dukes wrote: > <snip> > > > > > So even if a service such as zoneedit, say they can do > reverse DNS, it > > won't work? > > > > I really don't understand how it can work in one direction > and not the > > reverse. If they can keep up with my IP address and match it to my > > domainanme, seems they could do the reverse. > > > > OK ... rather than you staying confused on this issue, I will > try to explain it in basic terms. > > DNS converts names to IPs (forward lookups) and IPs to names > (reverse lookups). > > A forward lookup is when you have a name (www.abcxyz.com) and > need a number. This this case, there is a domain owner and > that domain has it's own DNS Zone. The owner of that Zone > can put whatever IP addresses > (numbers) with names that they want in that zone. > > In the case of a forward lookup, there is no predefined zone > at all ... > you can have as many names as you want, and since people pay > for it (the name), it stands to reason that will keep it > updated properly. > > A reverse lookup is different. The standard for reverse > lookups break them down in "Class C" blocks (that is, the > first 3 groups of numbers are the network number, the last > group is the host number). If you have an ip address of: > > 192.87.99.234 > > The network number is 192.87.99.0, the subnet mask is > 255.255.255.0, the host number is 234, and the reverse lookup > domain is: > > 99.87.192.in-addr.arpa > > All 254 host addresses in that zone are normally assigned > from the owner of that zone from one machine. If someone > buys the whole class C network, they get to control the zone, > otherwise it is normally controlled by the ISP that owns all the IPs. > > It is possible, but not usually done, to break up the reverse > into smaller ranges. > > Tom Diehl has already mentioned RFC 2317: > > http://www.faqs.org/rfcs/rfc2317.html > > Using the techniques there, an ISP _CAN_ transfer control of > some reverse lookup domains. They will normally not do it > unless you have a fairly large network, however. > > I hope this helps you understand that forward zones are > designed to easily break them down into 1 or 2 names ... but > reverse zones are predefined and not designed for less than 1 > class C network blocks. Hello Johnny, I guess that makes sense. It seems it would create too much work for the ISP to handle the reverse lookup for a single IP. If they dole them out that way, they should either do it or delegate them. All this is to operate a mail server without bounces. Is this why it recommedned to use your ISP's mail server as smarthost? Does this mean I would be using the ISP's mail server for outgoing mail? Or is it just 'stamped' with the ISP's name to prevent bounces? Thanks, Eddie