Setting a NAPTR record

In this article we explain how to set a NAPTR record in the DNS settings of your domain name.

A NAPTR record is a DNS record that is often used for internet telephony applications, for example in the mapping of servers and user addresses in the Session Initiation Protocol (SIP). This is a protocol used for multimedia communication such as Voice over IP (VoIP). NAPTR records are commonly used in combination with SRV records.

NAPTR stands for 'Name Authority Pointer' and specifies a rule that may use a 'regular expression' (or 'regex') to rewrite a 'string' (sequence of characters) to a domain name or a socalled 'Uniform Resource Identifier'. This is a way to make devices, such as fax machines and VoIP phones, reachable on the internet. Of course this only works with specific protocols.

Where do I add a NAPTR record?

You can add all your DNS records easily and free of charge via your control panel. Go to 'Domain' and select the domain in the left column for which you want to set the NAPTR record (don't check the box).

Now scroll to 'Advanced Domain Settings', followed by 'DNS'. If you do not see this yet, first click the switch behind 'TransIP settings' so the TransIP settings are switched off. The DNS records of your domain name will become visible, after which you can change them at will. After saving your own DNS records, make sure to leave the 'TransIP settings' switched off.

How do I set a NAPTR record?

A NAPTR record is made up of 6 different parts. Make sure to stick to the correct order when setting a NAPTR record. The correct order is as follows:

1. Order (also called 'priority'. This indicates which NAPTR record should be used first).
2. Preference (also called 'weight'. This indicates which NAPTR record should be used first if multiple NAPTR records have the same 'order').
3. Flags (These are important for the correct rewriting and interpretation of the other parts of the NAPTR record.)
4. Service (This specifies which services are available and, if necessary, which particular protocol should be used to connect with a service).
5. Regular expression (also called 'regexp'. This is used when the value "U" is set in the 'flags' option).
6. Replacement (This is a (sub)domain that is used when the value "S" or "A" is set in the 'flags' option).

The images below show 2 examples of NAPTR records.

The different parts of the record are explained in more detail below.

Name

You set a NAPTR record by starting with the 'Name'. Here you specify for which part of the domain the DNS record should work. A NAPTR record can be set for both the root domain and for subdomains.

• If you want to set a NAPTR record for the root domain, enter a @ in the 'Name' field.
• If you want to set a NAPTR record for a subdomain, only enter the subdomain in the 'Name' field. Our DNS software will automatically add the root domain name in the background.

Below are two examples of NAPTR records for the subdomain ex.domainname.com.

TTL

The 'TTL' of a DNS record determines how long the record can remain in the cache. We recommend keeping the TTL low, for example at 1 hour.

Type

Because you wish to set a NAPTR record, choose 'NAPTR' under 'Type'.

Value

In the 'Value' field of the NAPTR record you consecutively add the order, preference, flags, service, regular expression and replacement, seperated by spaces.Suppose you would like to set the following information:

    Value: "S" "sip+D2U" _sip._udp.example.com.
    Order: 100
    Preference: 10

In this example you would need to enter all the parts above in the correct order in the 'Value' field. Since there is no input for the 'regular expression' part, you will need to enter """ for that part. The end result looks like this:

In the above example you see the information in the correct order, seperated by spaces. These are the order (10), the preference (100), the flag ("S"), the service ("sip+D2U"), an empty string because there is no regular expression ("") and the replacement (_sip._udp.example.com.).

Below we will explain what the different elements of a NAPTR record mean and how to enter them in the NAPTR record.

Order

The order, also known as priority, indicates the order in which the NAPTR record needs to be processed when there are multiple NAPTR records. The lower the number, the higher the priority of the record. You can use a number between 0 and 65535 for the 'order'. As a rule, values such as 0, 10 or 100 are often used for convenience.

If a matching NAPTR record is found by a visitor's system, it will not look at NAPTR records with a higher number as 'order'. This is a difference with the 'preference' of the record, where it may still look at other NAPTR records with higher numbers as 'preference'.

Preference

The preference, also known as weight, also indicates an order in which the NAPTR record needs to be processed when there are multiple NAPTR records. The lower the number, the higher the priority of the record. You can use a number between 0 and 65535 for the 'preference'. As a rule, values such as 0, 10 or 100 are often used for convenience.

The difference between the 'order' and the 'preference' of a record is that a visitor's system (depending on their configuration) will look for NAPTR records with a higher number as 'preference' after finding a matching NAPTR record. It will however not look for NAPTR records with a higher number as 'order' after finding a matching NAPTR record.

Flags

The flags are specific characters to control aspects of the rewriting and interpretation of the other values in the NAPTR record. A 'flag' will always be a character between A and Z or between 0 and 9. There are currently 4 possibilities you can enter as 'flag':

• The letter "S". This indicates the NAPTR record is the last record and the next step is determined by the 'flag'. "S" means that after processing this NAPTR record the visitor's system should look for an SRV record for the domain entered as 'replacement'.
• The letter "A". This indicates the NAPTR record is the last record and the next step is determined by the 'flag'. "A" means that after processing this NAPTR record the visitor's system should look for an A record or an AAAA record for the domain entered as 'replacement'.
• The letter "U". This indicates the NAPTR record is the last record and the next step is determined by the 'flag'. "U" means that after processing this NAPTR record the visitor's system should not look for another DNS record but instead needs to look at the output of the 'regular expression'.
• The letter "P". This indicates that the visitor's system needs to continue according to the rules of the protocol that is entered in the 'service' part of the record. Since these rules differ per protocol we will not go into this indepth since that would be outside the scope for this article.

Service

The service specifies which services can be used for the rest of the NAPTR record, for example for the outcome of the 'regular expression'. You can use the 'service' to set a specific protocol that needs to be used. If you set the letter "P" as the 'flag' you are obligated to also set a protocol in the 'service'. The format of the 'service' part generally looks like this:"protocol+rs". The "rs" part stands for "Resolution Service".

The above format means that first it specifies the protocol, for example sip, then the plus sign and then the value that you are using as the resolution service. Normally the application for which you are setting the NAPTR record will tell you what exactly to enter for the 'service' part.

Regular expression

The 'regular expression' (also known as 'regex') specifies the substitution expression that is applied to rewrite the original (sub)domain to construct the next domain name that the visitor's system should look up. More information about the rules for this substitution expression can be found in the official RFC of the NAPTR record type.

Replacement

The replacement is the next (sub)domain that a visitor's system should look up, depending on what is set as 'flag'. It is important that this is a fully qualified domain name.

End result

Below you see two examples of complete NAPTR records. The first example shows a situation where only the 'replacement' is used and no 'regular expression'. The second example shows a situation where only the 'regular expression' is used and no 'replacement'.

With the steps in this article you can add NAPTR records. Most commonly the application that you set the record for will tell you whether you need one or more NAPTR records as well as the correct values for the order, preference, flag, service, regular expression and replacement.

More information about NAPTR records and their syntax can be found in RFC 2915 as described by the IETF.

In this article we explained how to configure NAPTR records in your control panel. For a general explanation about DNS records and entering them, see the article ‘DNS and nameservers'.

For more information about the use of NAPTR records, we advise you to read the RFC about this.

If you have any questions regarding this article, please contact our support team. You can reach them using the 'Contact us' button below or via the 'Contact' button inside your control panel.