New whatismydns feature (#29)

README.md

@@ -9,11 +9,12 @@
 - [What is my IP address](#what-is-my-ip-address)
   - [Features](#features)
   - [Endpoints](#endpoints)
+  - [DNS discovery](#dns-discovery)
   - [Build](#build)
   - [Usage](#usage)
   - [Examples](#examples)
     - [Run a default TCP server](#run-a-default-tcp-server)
-    - [Run a TLS (HTTP/2) server only](#run-a-tls-http2-server-only)
+    - [Run a TLS (HTTP/2) and enable What is my DNS](#run-a-tls-http2-and-enable-what-is-my-dns)
     - [Run an HTTP/3 server](#run-an-http3-server)
     - [Run a default TCP server with a custom template and trust a pair of custom headers set by an upstream proxy](#run-a-default-tcp-server-with-a-custom-template-and-trust-a-pair-of-custom-headers-set-by-an-upstream-proxy)
   - [Download](#download)
@@ -21,14 +22,17 @@
     - [Run a container locally using test databases](#run-a-container-locally-using-test-databases)
     - [From Docker Hub](#from-docker-hub)
 
+> [!NOTE]  
+> Since version 2.3.0, the application includes an optional client [DNS discovery](#dns-discovery)
+
 Just another "what is my IP address" service, including geolocation, TCP open port checking, and headers information. Written in go with high performance in mind,
 it uses [gin](https://github.com/gin-gonic/gin) which uses [httprouter](https://github.com/julienschmidt/httprouter) a lightweight high performance HTTP multiplexer.
 
-Take a look at [ifconfig.es](https://ifconfig.es) a live site using `whatismyip`
+Take a look at [ifconfig.es](https://ifconfig.es) a live site using `whatismyip` and the `DNS discovery` enabled.
 
 Get your public IP easily from the command line:
 
-```bash
+```text
 curl ifconfig.es
 127.0.0.1
 
@@ -36,10 +40,18 @@ curl -6 ifconfig.es
 ::1
 ```
 
+Get the IP of your DNS provider:
+
+```text
+curl -L dns.ifconfig.es
+2a04:e4c0:47::67 (Spain / OPENDNS)
+```
+
 ## Features
 
 - TLS and HTTP/2.
 - Experimental HTTP/3 support. HTTP/3 requires a TLS server running (`-tls-bind`), as HTTP/3 starts as a TLS connection that then gets upgraded to UDP. The UDP port is the same as the one used for the TLS server.
+- Beta DNS discovery: A best-effort approach to discovering the DNS server that is resolving the client's requests.
 - Can run behind a proxy by trusting a custom header (usually `X-Real-IP`) to figure out the source IP address. It also supports a custom header to resolve the client port, if the proxy can only add a header for the IP (for example a fixed header from CDNs) the client port is shown as unknown.
 - IPv4 and IPv6.
 - Geolocation info including ASN. This feature is possible thanks to [maxmind](https://dev.maxmind.com/geoip/geolite2-free-geolocation-data?lang=en) GeoLite2 databases. In order to use these databases, a license key is needed. Please visit Maxmind site for further instructions and get a free license.
@@ -69,6 +81,38 @@ curl -6 ifconfig.es
 - https://ifconfig.es/headers
   - https://ifconfig.es/<header_name>
 - https://ifconfig.es/scan/tcp/<port_number>
+- https://dns.ifconfig.es
+
+## DNS discovery
+
+The DNS discovery works by forcing the client to make a request to `<uuid>.dns.ifconfig.es` this DNS request is handled by a microdns server
+included in the `whatismyip` binary. In order to run the discovery server, a configuration file in the following form has to be created:
+
+```yaml
+---
+domain: dns.example.com
+redirect_port: ":8000"
+resource_records:
+  - "1800 IN SOA xns.example.com. hostmaster.example.com. 1 10000 2400 604800 1800"
+  - "3600 IN NS xns.example.com."
+ipv4:
+  - "127.0.0.2"
+ipv6:
+  - "aaa:aaa:aaa:aaaa::1"
+```
+
+The DNS authority for example.com has delegated the subdomain zone `dns.example.com` to the server running the `whatismyip` service.
+
+The client can request the URL `dns.example.com` by following the redirection `curl -L dns.example.com`.
+
+To avoid the redirection, you can provide a valid URL, for example, for the real [ifconfig.es](https://ifconfig.es):
+
+```bash
+curl $(uuidgen).dns.ifconfig.es
+
+curl $(cat /proc/sys/kernel/random/uuid).dns.ifconfig.es
+```
+
 
 ## Build
 
@@ -114,11 +158,12 @@ Usage of whatismyip:
 ./whatismyip -geoip2-city ./test/GeoIP2-City-Test.mmdb -geoip2-asn ./test/GeoLite2-ASN-Test.mmdb
 ```
 
-### Run a TLS (HTTP/2) server only
+### Run a TLS (HTTP/2) and enable What is my DNS
 
 ```bash
 ./whatismyip -geoip2-city ./test/GeoIP2-City-Test.mmdb -geoip2-asn ./test/GeoLite2-ASN-Test.mmdb \
-             -bind "" -tls-bind :8081 -tls-crt ./test/server.pem -tls-key ./test/server.key
+             -bind "" -tls-bind :8081 -tls-crt ./test/server.pem -tls-key ./test/server.key \
+             -resolver ./test/resolver.yml
 ```
 
 ### Run an HTTP/3 server
@@ -137,7 +182,7 @@ Usage of whatismyip:
 
 ## Download
 
-Download the latest version from https://github.com/dcarrillo/whatismyip/releases
+Download the latest version from [github](https://github.com/dcarrillo/whatismyip/releases)
 
 ## Docker