DNS ENUM
Overview
DNS ENUM (Electronic Numbering, E.164 Number Mapping) is a critical protocol in modern telecommunications, particularly in Voice over LTE (VoLTE) and Voice over New Radio (VoNR) networks. It provides a standardized method for mapping telephone numbers to various communication services and endpoints using the Domain Name System (DNS) infrastructure.
DNS ENUM is a protocol that translates telephone numbers into Internet-based Uniform Resource Identifiers (URIs) using the Domain Name System (DNS). It adheres to the standards specified in RFC 6116.
ENUM Query Structure
DNS ENUM queries utilize a hierarchical domain name structure. ENUM uses NAPTR (Naming Authority Pointer) DNS records for service resolution. Each NAPTR record contains:
Order and preference values
Service identification
Regular expressions for rewriting
Replacement domain
Example:
For the E.164 number +1-800-555-1234
:
Reverse the number:
4.3.2.1.5.5.5.0.0.8.1
Append the ENUM domain:
e164.arpa
Full query:
4.3.2.1.5.5.5.0.0.8.1.e164.arpa
The DNS server responds with a URI, such as sip:18005551234@domain.com
.
IN NAPTR 100 10 "u" "E2U+sip" "!^.*$!sip:18005551234@domain.com!" .
Service Parameters
Supported services in VoLTE and VoNR ENUM include:
E2U+sip
: SIP-based communicationE2U+mailto
: Email contactE2U+h323
: H.323 communicationE2U+voice:tel
: Traditional telephonyE2U+im
: Instant messaging
Role of DNS ENUM in VoLTE and VoNR
In VoLTE, ENUM enables number resolution for voice calls by bridging the gap between traditional telephone numbers and IP-based addressing. Here's a breakdown of its function and the network elements involved:
How ENUM Works in IMS Network during a voice call ;
Originating UE (User Equipment): A user on a VoLTE phone dials a number. The phone sends a SIP INVITE message to the network.
ENUM Query: The P/I/S-CSCF initiates an ENUM query to find the destination's SIP URI. This query involves:
Reversing the dialed number (e.g., +1-555-123-4567 becomes 7.6.5.4.3.2.1.5.5.5.1).
Appending ".e164.arpa" to create a domain name (e.g., 7.6.5.4.3.2.1.5.5.5.1.e164.arpa).
Querying DNS servers for NAPTR records associated with this domain.
DNS Resolution: The DNS system resolves the ENUM query, potentially traversing through root servers, country code top-level domains (ccTLDs), and authoritative name servers.
NAPTR Records: The DNS servers return NAPTR (Naming Authority Pointer) records. These records contain information about available services and how to access them, including the all-important SIP URI.
SIP Routing: The P/I/S-CSCF uses the SIP URI from the NAPTR record to route the call towards the destination network.
Error Codes ;
'0' : 'No error',
'1' : 'Format error',
'2' : 'Server failure',
'3' : 'No such name',
'4' : 'Not implemented',
'5' : 'Refused',
'6' : 'Name exists',
'7' : 'RRset exists',
'8' : 'RRset does not exist',
'9' : 'Not authoritative',
'10': 'Name out of zone',
Troubleshooting in AGILITY
DNS ENUM sessions can be found under drop down menu :