text-relay

This is an overly simplistic text-relay server written on behalf of a friend as a challenge.

The Goal

This is meant to help handle the following situation:

Someone uses a cellphone to start their business. At some point, they decide to
port their number over to use a VoIP solution. Their old cell number is now
their main work line. This number may still receive text messages (for things
like 2fa, etc) from folks that haven't gotten the new personal number yet.

Rules

A text cannot be sent from a number that isn't controlled by the provider (no spoofing!)
A postfix should be added with a reference to the original number
The text should be sent to a new arbitrary number
Texts from numbers not listed in relays.csv will be ignored

Flow

The original number receives a text to some VoIP stack
The stack will issue an HTTP POST to a specified url with the text details
The url is handled by the text-relay service
The payload is re-written using the relays.csv data file
The service sends the new payload to a text messaging service, forwarding the text

Data

The actual relays are stored in relays.csv, colocated with the binary. There is minimal validation on the actual numbers. They must be in whatever format the text messaging service expects.

The first column is the original destination (and thus, the new originator). The second column is the target to receive the message.

Building

This application is written in Go. You'll need a relatively recent installation of Go. It must at least support modules. Versions other than 1.19.1 have not been tested.

To test locally:

go build

The resulting binary should be named text-relay and can run locally.

To build for running on a linux server:

GOOS=linux GOARCH=amd64 go build -o text-relay-linux

Substitute whatever GOARCH or GOOS matches your target system.

Running

The application expects the relays.csv to be colocated with the binary. It will fatally error should this not be true.

To run and configure, use the output below as a guide.

$ PORT=9001 TOKEN=<API TOKEN> ACCOUNT_ID=<ACCOUNT ID> ./text-relay
{"level":"debug","time":1674512034,"message":"starting text-relay on port 9001"}

Set the TOKEN and ACCOUNT_ID as required to communicate with the text messaging service. Set the PORT that the service should be exposed on. Once started, it will start logging to standard out in JSON.

ryanfaerman / text-relay