Skip to content

DNS Query Functionality

The code below provides an example of how to query a DNS record, specifically using the NAPTR regexp field specified in the NAPTR protocol. NAPTR fields are treated as a set , using Python to iterate it, and using the regexp field to directly access the data.

The BPC SML Domain

The BPC SML Domain is: bpcb2b.net

Example - Querying a DNS NAPTR Record

1. Import the necessary module

The example implements the dns.resolver Python module. 

import dns.resolver

2. Provide the hashed URN for look-up

For the example, the URN is already created, hashed, and the domain of "sc-b2b.us" is appended. (This was functional at the time of initial development.) 

hashed_value = "6c24uvqpxrfyweqimfxmsuym3bbjvoikuwmmidquz2a2zzyikdya.sc-b2b.us"

3. Do the DNS look-up

The look-up is returned from the resolve() method of dns.resolver. In this implementation, the method takes two parameters, the query string, and the look-up type. See Resolver Functions and the Default Resolver for the complete method signature.

dns.resolver.resolve() return type is dns.resolver.Answer

The dns.resolver.resolve() method return type is dns.resolver.Answer. See The dns.resolver.Resolver and dns.resolver.Answer Classes for details on the Answer response class.

4. The NAPTR DNS query response


4a. The response as Rdata

Rdata is typed data in one of the known DNS data types, i.e., an IPv4 address for a host, MX record, or NAPTR record. RRset is an Rdata object which supports the Python set API. For details on RRset see Rdataset, RRset, and Node Classes.

4b. The fields in the NAPTR response

The returned Rdata data set is in the format as specified by the NATPR protocol and contains the following fields which may be iterated as a set: 

Order Preference Flags Service Regexp Replacement

The set values are iterated using: 

for a in lookup.rrset:


4c. The "regexp" field of the NATPR response

The value of the "regexp" field contains the terminal address of the SMP in the format of a regular expression, which is how it would otherwise be used if doing a Session Initiation Protocol (SIP) lookup.
The value is directly accessed using: 

 for a in lookup.rrset:
    smp_uri = a.regexp


4d. The response as a string

Use the String decode() method to change the returned response from Binary to String format. 

smp_uri = smp_uri.decode()

The final response is the terminal SMP URL in the NAPTR regular expression substitution format.

Code

import dns.resolver

hashed_value = "6c24uvqpxrfyweqimfxmsuym3bbjvoikuwmmidquz2a2zzyikdya.sc-b2b.us"
lookup = dns.resolver.resolve(hashed_value,'NAPTR')
print(type(lookup))
for a in lookup.rrset:
    smp_uri = a.regexp
smp_uri = smp_uri.decode()
print(smp_uri)

Output

<class 'dns.resolver.Answer'>
!^.*$!https://my-smp-url.com/0123456789!