Number Routing
To route calls between platforms we propose utilising an ENUM like service, defined in RFC6116
ENUM performs DNS lookups for a number to resolve a Uniform Resource Identifier (URI) to route the call to. A URI might look something like sip:441632556789@example.com, which indicates a call should be sent via SIP to the number 441632556789, on the server that reached at example.com
ENUM also supports wildcards, allowing a single record to route an entire prefix, and delegation is also built in to allow a particular prefix, e.g a country code, to be redirected to another DNS server.
ENUM Overview
Name Authority Pointer (NAPTR) records are created on a DNS server, with the full number including country code reversed. e.g for the number 441632556789 you would create the a record for 9.8.7.6.5.5.2.3.6.1.4.4.e164.example.com that looks something like:
100 10 "u" "E2U+sip" "!^.*$!sip:441632556789@example.com!" .
The record has a specific syntax, described below:
| Field | Value | Explanation |
|---|---|---|
| Order | 100 |
Defines the processing order. Lower values have higher priority. |
| Preference | 10 |
Determines priority among records with the same ORDER. Lower values are preferred. |
| Flags | "u" |
Specifies the record type: "u" (URI), "s" (SRV lookup), "p" (non-terminal processing). |
| Service | "E2U+sip" |
Defines the ENUM service "E2U+sip" (ENUM to SIP mapping). |
| Regex | "!^.*$!sip:441632556789@example.com!" |
A regular expression for rewriting the ENUM query into a target URI. Rewrites any match (^.*$) to sip:441632556789@example.com. |
| Replacement | . |
Alternative domain (usually . for ENUM records using REGEX). No alternative domain; use the result of the Regex directly. |
You can test an example ENUM record for 441632556789 by running the following command. Note the URI doesn't route anywhere:
dig NAPTR 9.8.7.6.5.5.2.3.6.1.4.4.dnum.cutel.net
; <<>> DiG 9.16.1-Ubuntu <<>> NAPTR 9.8.7.6.5.5.2.3.6.1.4.4.dnum.cutel.net
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 53739
;; flags: qr rd ad; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; WARNING: recursion requested but not available
;; QUESTION SECTION:
;9.8.7.6.5.5.2.3.6.1.4.4.dnum.cutel.net. IN NAPTR
;; ANSWER SECTION:
9.8.7.6.5.5.2.3.6.1.4.4.dnum.cutel.net. 0 IN NAPTR 100 10 "u" "E2U+sip" "!^.*$!sip:441632556789@example.com!" .
;; Query time: 10 msec
;; SERVER: 172.31.48.1#53(172.31.48.1)
;; WHEN: Thu Jan 30 23:51:40 GMT 2025
;; MSG SIZE rcvd: 157
Secondary DNS servers
It's hoped that users will host secondary DNS servers to reduce load and increase resilience. And for fun!
Security
To secure the DNS records its envisioned we'll use DNSSEC, but this is a topic we don't currently know much about. If you do, please get in touch.