MV.COM policy: Reverse DNS

You have probably received a pointer to this web page because you attempted to access a service at MV Communications and were denied because of a reverse DNS issue; i.e. your IP address does not have a valid reverse DNS configured. Not all of our services require a strict reverse DNS configuration, but some may. If you're participating in that kind of service on the Internet, you really ought to have this DNS configuration correct. If you're responsible for the configuration, please fix it- otherwise get your provider to fix it for you.

Succinct explanation

1. There was no reverse DNS configured for your IP address, and thus no host name could be found;
2. A host name was found, but when our server looked up that host name, the result did not match your IP address.

Long-winded explanation

Normally you don't deal with IP addresses-- you deal with names (called "host names"). Names are again a convenience for humans. When you access a host name, e.g. www.mv.com, the program that you are using translates that name to its corresponding IP address (a number), and communication ensues. This translation is done via the Domain Name System (DNS), the translation being referred to as a "DNS Lookup."

Sometimes it's necessary to find out what host name is associated with an IP address. There is a provision in the DNS for this kind of lookup, and since it's the opposite of the usual name-to-address translation, it's referred to as a "reverse DNS lookup."

All IP addresses that are used on the Internet should have a proper configuration for the reverse DNS lookup. Furthermore, the result of this lookup should match the forward lookup. That is, when you look up the name associated with an IP address, you ought to be able to then look up the IP address associated with that name and get the IP address you started with. Any other setup (including no name at all, or a name that doesn't match) is fishy and often indicates some kind of hanky-panky.

Example:

• The IP address 199.125.85.1 has a reverse DNS of ns1.mv.net
• When you do a DNS lookup for the host name ns1.mv.net, you get the IP address 199.125.85.1 .

Notes

The IP address that led you here may be the one associated with your computer, or it maybe another one. You may be accessing a server at MV directly from your own computer, in which case the client IP address is the one assigned to your computer. Or some other computer may be making this access on your behalf, such as when you send email through your ISP's mail server to a user at MV (then that mail server connects to MV's mail server on your behalf, to deliver your mail), or such as when you access a web site at MV via a proxy server (then that proxy server makes the request on your behalf).

Whatever the IP address, it should be properly configured. If you've been assigned a single IP address from your ISP, they should have made sure it's got a valid reverse lookup that maps to a valid forward lookup. If they haven't gotten that right, let them know! Their misconfiguration will cause problems for you and for others. If you've been assigned a subnet from your ISP, you probably need to let the ISP know how to configure the reverse DNS for that subnet (as they probably do not know how you intend to name those IP addresses). Of course, if you are responsible for the DNS for your IP addresses directly, it's up to you to get it right yourself!