Support - Zerigo DNS

Reverse DNS FAQ

How do I add a reverse domain?

We support both IPv4 and IPv6 reverse domains. When you create a zone with the proper domain name, our system will automatically detect that it is a reverse domain and allow you to add PTR host records.

IPv4 reverse domains look like this: 3.2.1.in-addr.arpa

An IPv6 reverse domain looks more like this: 0.4.a.6.0.e.0.f.1.0.7.4.0.1.0.0.2.ip6.arpa

True to their name, reverse domains list each segment of the IP address in reverse order. In the above example, 3.2.1.in-addr.arpa is the reverse domain for an IPv4 address 1.2.3.x.

Note that reverse DNS is configured on a per-subnet basis, not per IP. Attempting to add a single reverse DNS record to a normal, forward DNS domain will not work.

How do I configure a reverse domain for an IPv4 subnet?

First, ask your ISP or hosting provider to delegate your IPv4 subnet to Zerigo's nameservers.

If your subnet is a /25 or smaller, there are at least 3 different ways that your ISP might choose to name your reverse DNS domain/zone. It's easiest to just ask them what your reverse domain will be.

Once you know the reverse domain/zone name, simply add that as a new domain to your Zerigo DNS account.

After the domain is added, then add PTR Host Records for each specific IP. The Hostname field will be the final segment of the IPv4 address.

Let's take two examples. First, a IP in a full /24 subnet: 198.51.100.7. In this case, the reverse domain would be "100.51.198.in-addr.arpa". The PTR host record's hostname would simply be "7".

Next, an IP in a /27 subnet: 203.0.113.166. At noted above, there are several ways to name the reverse domain. One of the more common ways is this: "160/27.113.0.203.in-addr.arpa". Your ISP will tell you which naming method they have selected though.

The PTR host records will look the same as a full subnet. In this example that would be "166".

How do I configure a reverse domain for an IPv6 subnet?

First, ask your ISP or hosting provider to delegate your IPv6 subnet to Zerigo's nameservers.

IPv6 subnets are usually quite large—often at a /64, /56, or /48 boundary. Because IPv6 reverse domains are segmented on each hexadecimal digit, the subnet boundary for reverse DNS will always be a multiple of 4.

You will need to know the exact reverse domain name your ISP is delegating. If you don't know what that is, ask your ISP.

Once you have the reverse domain/zone name, simply add that as a new domain to your Zerigo DNS account.

After the domain is added, then add a PTR record for each IP. Since the total address space is 128 bits and the subnet is usually only 64, 56, or 48 of those bits, the remaining portion is quite large. That entire remaining portion should be added to the Hostname field of the PTR record.

For example, the IPv6 address 2607:fc88:1001::800 looks like this when mapped to a reverse DNS name: 0.0.8.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.0.0.1.8.8.c.f.7.0.6.2.ip6.arpa.

If subnetted as a /64, the reverse domain would be "0.0.0.0.1.0.0.1.8.8.c.f.7.0.6.2.ip6.arpa". The Hostname portion of the actual PTR record would be everything remaining (on the left side): "0.0.8.0.0.0.0.0.0.0.0.0.0.0.0.0".

What if my ISP won't delegate my reverse DNS addresses?

The right to manage reverse DNS entries is always packaged with the official assignment of the IP block to your ISP or hosting provider. If they are unable or unwilling to delegate your sub-block of IPs for your own management, there's nothing you can do.

Some ISP's are unable to delegate reverse DNS, but will configure it for you if you open a ticket. While this may not be as easy long-term as managing it yourself, it may be much easier than changing providers.