Captive DNS Server

Overview

This tutorial demonstrates how Mongoose Library can be used to implement a captive DNS portal. It is usually required for device configuration. For example, an un-configured device starts its own WiFi network, and on this network, any DNS name gets resolved to that device. This way, a user might not know device's IP address to get to the WiFi configuration page.

Full source code for this tutorial is at https://github.com/cesanta/mongoose/tree/master/tutorials/udp/captive-dns-server

Build and test

• Follow the Build Tools tutorial to setup your development environment.

• Start a terminal in the project directory; clone the Mongoose Library repo, and run the make all command:

git clone https://github.com/cesanta/mongoose
cd mongoose/tutorials/udp/captive-dns-portal
make all
cc mongoose.c main.c -W -Wall -Wextra -g -DMG_IO_SIZE=8192 -DMG_ENABLE_LINES -o example
./example 
f7292e1 3 net.c:182:mg_listen           1 0x4 udp://0.0.0.0:5533

Usually DNS servers use port 53. However that port is privileged - a program that opens that port must have root permissions. That is why our example uses port 5533 instead of port 53.

Now start another terminal and type a command that resolves any domain name, for example "example.com":

dig @localhost -p 5533 -4 example.com A
...
;; QUESTION SECTION:
;example.com.			IN	A
;; ANSWER SECTION:
example.com.		120	IN	A	1.2.3.4

Here we see that "example.com" has been resolved to "1.2.3.4" which is hardcoded by the server.

How it works

In the usual event manager initialisation code, we start a UDP listener:

In the event handler function, we catch MG_EV_READ, which is triggered every time we receive a DNS request:

There, we try to parse the received message. If it is a valid DNS request, we craft a response which has a hardcoded IP address of "1.2.3.4", send the response back, and clean up the c->recv IO buffer.