DnsServer - give the devices on your network friendly names

This makes your ESP32 a tiny DNS server, so other devices can say printer.lan instead of remembering 192.168.1.50. It is handy on a network with no internet (a lab bench, a robot, an off-grid site), where the usual name lookups do not work. It is written for a beginner.

What is DNS, in one breath?

Computers talk to each other using numbers (IP addresses like 192.168.1.50). People prefer names (printer.lan). DNS is the phone book that turns a name into a number. Normally your router or your internet provider runs it for you - but on an isolated network there is no phone book, so nothing can find anything by name. This example gives your network its own little phone book.

What you will need

• An ESP32 board and your WiFi details.
• Another device to test from (a laptop or phone on the same network).

Set it up

1. Open DnsServer.ino and set your SSID and PASSWORD.

2. Edit the name records - the phone-book entries - to match your network:

   pc_dns_server_add("printer.lan", 192, 168, 1, 50);   // name, then the four numbers of the IP
   pc_dns_server_add("nas.lan",     192, 168, 1, 60);

   (esp32.lan is added for you and points at the board itself.)

3. Upload and open the Serial Monitor at 115200. It prints the board's IP and DNS server on UDP/53.

Try it

You do not have to change any settings to test a single lookup - just ask this server directly. From another computer on the network (replace <board-ip> with the IP from the Serial Monitor):

nslookup printer.lan <board-ip>      # Windows / Linux / macOS
# or
dig @<board-ip> printer.lan          # Linux / macOS

You should get back 192.168.1.50 (or whatever you set). An unknown name returns "NXDOMAIN", which just means "not in the phone book".

To make every lookup on a device go through the ESP32, set that device's DNS server to the board's IP (in its WiFi/network settings). Then names like esp32.lan work everywhere on that device. Most people only do this on a dedicated offline network.

Troubleshooting

• The lookup times out. DNS uses UDP port 53. The most common cause on a home network is AP/client isolation - the router stops WiFi devices from talking to each other. Turn it off, or test from a device on a wired port.
• A name comes back NXDOMAIN. It is not in your table, or the spelling differs (matching is case-insensitive, but printer.lan and printer.local are different names).
• Only A records. This server answers IPv4 name lookups (the common case). It politely returns no answer for other record types (IPv6/AAAA, MX, etc.).

Build and run (PlatformIO)

The feature lives in the library, so the flag must reach the whole build:

pio ci examples/L7-Application/DnsServer \
  --board esp32dev --lib "." \
  --project-option="build_flags=-DPC_ENABLE_DNS_SERVER=1"

(The Arduino IDE reads the flag from build_opt.h beside the sketch automatically.)

How it works (for the curious)

pc_dns_server_add(name, a, b, c, d) stores a name -> IPv4 record in a small fixed table. pc_dns_server_begin() binds UDP/53 through the library's transport UDP service; each query is handed to the pure pc_dns_server_build_response(), which parses the question, looks the name up (case-insensitively), and - for an A/IN query that hits - appends one answer record (using DNS name compression: a 2-byte pointer back to the question). Misses return NXDOMAIN. Everything is fixed-buffer and heap-free, and the wire format is unit-tested on a PC (see test/test_dns_server). It is a self-contained authoritative resolver, separate from the captive-portal DNS used during WiFi provisioning (which points every name at the setup page).