Captive DNS Server Example (Mongoose More Than an Embedded Web Server)

This week we continue our series of examples for Mongoose Embedded Web Server with an example of a DNS server.

In this case, it is a DNS server intended to be used to build a captive portal, i.e. where the user needs to log in before using public WiFi. Such a server responds to all queries with its own IP address and an HTTP server running alongside serves the portal page to the user.

Building a DNS server with Mongoose is easy: mg_bind to a udp:// port, call mg_set_protocol_dns on the connection and handle incoming MG_DNS_MESSAGEevents.

Our captive_dns_server example puts this all together:

$ ./captive_dns_server
Listening on 'udp://:5533'
ev_handler           Q type 1 name example.org

---

$ dig @localhost -p 5533 example.org

; <<>> DiG 9.9.5-11ubuntu1.3-Ubuntu <<>> @localhost -p 5533 example.org
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 44936
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 1, AUTHORITHY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;example.org.                   IN      A

;; ANSWER SECTION:
example.org.            10      IN      A       127.0.0.1

;; Query time: 0 msec
;; SERVER: 127.0.0.1#5533(127.0.0.1)
;; WHEN: Mon Apr 18 19:56:51 IST 2016
;; MSG SIZE  rcvd: 56

Follow the Example

To follow the example download Mongoose Embedded Webserver. It's open source under GPLv2, so free of charge for non-commercial use.