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.
A full source code for this tutorial is at https://github.com/cesanta/mongoose/tree/master/examples/captive-dns-server
Build and test
Assuming we're on a Mac or Linux workstation, start a new terminal and execute the following to build and start the websocket server:
$ git clone https://github.com/cesanta/mongoose $ cd mongoose/examples/captive-dns-portal $ make cc ../../mongoose.c -I../.. -W -Wall -DMG_ENABLE_LINES=1 -o example main.c ./example 2021-11-23 13:59:07 3 sock.c:502:mg_listen 1 accepting on udp://0.0.0.0:5533 (port 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 18.104.22.168
Here we see that "example.com" has been resolved to "22.214.171.124" 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 "126.96.36.199",
send the response back, and clean up the
c->recv IO buffer.