The Domain Name System
The Domain Name System (DNS) is a hierarchical naming system for computers, services, or any resource participating in the Internet. It associates various information with domain names assigned to such participants. Most importantly, it translates domain names meaningful to humans into the numerical (binary) identifiers associated with networking equipment for the purpose of locating and addressing these devices world-wide. An often used analogy to explain the Domain Name System is that it serves as the "phone book" for the Internet by translating human-friendly computer hostnames into IP addresses.
The Domain Name System makes it possible to assign domain names to groups of Internet users in a meaningful way, independent of each user's physical location. Because of this, World-Wide Web (WWW) hyperlinks and Internet contact information can remain consistent and constant even if the current Internet routing arrangements change or the participant uses a mobile device. Internet domain names are easier to remember than IP addresses such as 208.77.188.166 (IPv4) or 2001:db8:1f70::999:de8:7648:6e8 (IPv6). People take advantage of this when they recite meaningful URLs and e-mail addresses without having to know how the machine will actually locate them.
Whether you realize it or not, your computer might at this very moment be changing its Internet address. Some computers have what are called changing IP addresses. This isn't necessarily a problem, unless your website doesn't change along with that IP address.
In order to avoid this problem, you need Dynamic DNS. This stands for Dynamic Domain Name System, and it is the solution to a vexing problem that many people are not aware of. Using Dynamic DNS, you can maintain a website or a web server and not worry that others may suddenly be unable to access it.
When you connect to the Internet, your Internet Service Provider assigns a temporary IP address that lasts until you disconnect. If, during your Internet session, you happen to update your website, the IP address of your website will change. The resulting situation is a problem for computers that aren't equipped to track your website when its IP address changes.
Since your website is probably some sort of http configuration of letters rather than numbers, you likely won't notice that anything has changed. Dynamic DNS takes care of that by changing your website's IP address correspondingly, to keep up with your connectivity changes. In a sense, the numbers change but the letters never do. This eliminates the need for the person who wants to view your website to type in the exact numbers of the IP address. Your name address will do.
Dynamic DNS is available for Web users large and small. Personal users have just one or a handful of uses for Dynamic DNS. Large companies have multiple needs and require larger amounts of equipment and/or software. Some of the largest companies using Dynamic DNS are Internet Service Providers themselves.
DNS reverse (PTR) records
The simplest definition of PTR records is resolving IP addresses into hostnames. Certainly you appreciate this if you are for example sending e-mails. Having outgoing e-mail rejected due to domain name system configuration issues is a common problem that Internet e-mail server administrators face.
For maximum delivery, you should make sure that the public IP address used by your outgoing e-mail server to connect to remote e-mail servers has a DNS PTR record that resolves it to a hostname. The PTR record is a pointer record in the in.addr-arpa infrastructure domain that maps the IP address to a hostname.
In order to create or change this PTR record, you will typically need to contact the owner of the IP address that you have been assigned. Most e-mail server administrators do not own their own IP addresses, but instead lease IP address space from their Internet service providers. If you don’t know who to contact about your IP address’s PTR record, you can probably get started in your sleuthing by contacting your ISP.
But not every IP address has a corresponding PTR record. In fact, if you took a random sampling of addresses your firewall blocked because they were up to no good, you'd probably find most have no PTR record - a dig -x gets you no information. That's also apt to be true for mail spammers, or their PTR doesn't match up: if you do a dig -x on their IP you get a result, but if you look up that result you might not get the same IP you started with.
That's why PTR records have become important. Originally, PTR records were just intended as a convenience, and perhaps as a way to be neat and complete. There still are no requirements that you have a PTR record or that it be accurate, but because of the abuse of the internet by spammers, certain conventions have grown up. For example, you may not be able to send email to some sites if you don't have a valid PTR record, or if your pointer is "generic":
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