Add to README.

README.md

@@ -82,7 +82,7 @@ login, and attempt to prompt you for your password.
 - [`matrix-commander-rs`](https://github.com/8go/matrix-commander-rs)
 - [Matrix](https://matrix.org/)
   - Your Matrix user
-  - A Matrix bot user (just a normal user with a noticeable username like `notification-bot`)
+  - A Matrix bot user (just a normal user with a noticeable username like `gpg-bot`)
   - A private Matrix channel between you and the bot (preferably encrypted).
   - A Matrix client app for mobile, e.g. [Element](https://element.io/)
 - A proxy server with HTTPS support (E.g. [`caddy`](https://caddyserver.com/))
@@ -92,6 +92,26 @@ For development contributors only:
 
 ## Installation instructions
 
+Configure the domain used for login links.  This should be a subdomain you
+control, e.g. `gpg.yourdomain.com`:
+
+```{bash}
+mkdir -p "$HOME/.config/web-pinentry"
+echo "gpg.yourdomain.com" >> "$HOME/.config/web-pinentry/domain"
+```
+
+Install and setup a reverse proxy with HTTPS support, e.g. `caddy`:
+
+- Install `caddy` / `nginx` / `apache` or any other reverse proxy with HTTPS support.
+- Configure the reverse proxy to forward requests from your chosen domain to
+  127.0.0.1:7563, where `web-pinentry` will be running its HTTP server.  For example, with `caddy`:
+
+```{caddyfile}
+gpg.yourdomain.com {
+  reverse_proxy 127.0.0.1:7563
+}
+```
+
 Building and installing the `web-pinentry` program:
 
 ```{bash}
@@ -140,7 +160,7 @@ Testing that you can receive messages from the bot:
 matrix-commander-rs --message 'Hello world!'
 ```
 
-The above should trigger a notification on your phone via the Matrix client app
+The above should trigger a message notification on your phone via the Matrix client app
 you have installed.
 
 You might notice the message come with a warning about the bot's client.

src/main.rs

@@ -1,5 +1,5 @@
 use std::env;
-use std::fs::File;
+use std::fs::{self, File};
 use std::io::{self, BufRead, BufReader, Read, Write};
 use std::net::{IpAddr, Ipv4Addr, SocketAddr, TcpListener, TcpStream};
 use std::process;
@@ -127,8 +127,23 @@ fn random_path_segment() -> io::Result<String> {
 		.collect())
 }
 
+fn advertised_domain() -> io::Result<String> {
+	let home = env::var("HOME").map_err(|error| io::Error::new(io::ErrorKind::NotFound, error))?;
+	let domain = fs::read_to_string(format!("{home}/.config/web-pinentry/domain"))?;
+	let domain = domain.trim();
+
+	if domain.is_empty() {
+		return Err(io::Error::new(
+			io::ErrorKind::InvalidData,
+			"domain file is empty",
+		));
+	}
+
+	Ok(domain.to_string())
+}
+
 fn advertised_url(route_path: &str) -> io::Result<String> {
-    Ok(format!("https://gpg.seanhealy.ie{route_path}"))
+	Ok(format!("https://{}{route_path}", advertised_domain()?))
 }
 
 fn send_url_message(url: &str) -> io::Result<()> {
@@ -148,7 +163,7 @@ fn send_url_message(url: &str) -> io::Result<()> {
 
 /**
  * Allow any connections from docker or 127.0.0.1.
- * Docker IPs looks like: 172.17.x.x or 172.18.x.x.
+ * Docker IPs looks like: 172.17.x.x, 172.18.x.x. etc.
  */
 fn is_allowed_client(address: SocketAddr) -> bool {
 	matches!(address.ip(), IpAddr::V4(ip) if ip == Ipv4Addr::LOCALHOST ||