Enable virtual DNS by default for the CLI.

This commit enables virtual DNS by default for the CLI.

Pros for this decision:
  - It works out of the box in most cases.
  - Users do not need to install third-party software.
  - Users do not need to update their /etc/resolv.conf.

Cons:
  - Not every server might support this.
  - Hijacking port 53 without asking is somewhat intrusive.

Also, make the --dns argument an enum type. In the future, native
support for DNS over TCP/TLS might be added. In that case we could e.g.
supply tcp://1.1.1.1 to the --dns argument without breaking the CLI.

src/main.rs

@@ -12,13 +12,25 @@ struct Args {
     #[arg(short, long, value_name = "name", default_value = "tun0")]
     tun: String,
 
-    /// The proxy URL in the form proto://[username[:password]@]host:port
+    /// Proxy URL in the form proto://[username[:password]@]host:port
     #[arg(short, long, value_parser = Proxy::from_url, value_name = "URL")]
     proxy: Proxy,
 
-    /// Enable virtual DNS feature
-    #[arg(short = 'd', long = "dns")]
-    virtual_dns: bool,
+    /// DNS handling
+    #[arg(
+        short,
+        long,
+        value_name = "method",
+        value_enum,
+        default_value = "virtual"
+    )]
+    dns: ArgDns,
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, clap::ValueEnum)]
+enum ArgDns {
+    Virtual,
+    None,
 }
 
 fn main() {
@@ -31,7 +43,7 @@ fn main() {
     log::info!("Proxy {proxy_type} server: {addr}");
 
     let mut options = Options::new();
-    if args.virtual_dns {
+    if args.dns == ArgDns::Virtual {
         options = options.with_virtual_dns();
     }
 

src/tun2proxy.rs

@@ -501,7 +501,8 @@ impl<'a> TunToProxy<'a> {
                 // The connection handler builds up the connection or encapsulates the data.
                 // Therefore, we now expect it to write data to the server.
                 self.write_to_server(&resolved_conn);
-            } else if resolved_conn.proto == IpProtocol::Udp.into() {
+            } else if resolved_conn.proto == IpProtocol::Udp.into() && resolved_conn.dst.port == 53
+            {
                 if let Some(virtual_dns) = &mut self.options.virtdns {
                     let payload = &frame[_payload_offset.._payload_offset + _payload_size];
                     if let Some(response) = virtual_dns.receive_query(payload) {

src/virtdns.rs

@@ -34,7 +34,7 @@ pub struct VirtualDns {
 impl Default for VirtualDns {
     fn default() -> Self {
         let start_addr = Ipv4Addr::from_str("198.18.0.0").unwrap();
-        let cidr = Ipv4Cidr::new(start_addr.into(), 28);
+        let cidr = Ipv4Cidr::new(start_addr.into(), 15);
 
         Self {
             next_addr: start_addr.into(),