feat: DNS-over-HTTPS (DoH) upstream forwarding (#14)

* feat: DNS-over-HTTPS upstream forwarding

Encrypt upstream queries via DoH — ISPs see HTTPS traffic on port 443,
not plaintext DNS on port 53. URL scheme determines transport:
https:// = DoH, bare IP = plain UDP. Falls back to Quad9 DoH when
system resolver cannot be detected.

- Upstream enum (Udp/Doh) with Display and PartialEq
- BytePacketBuffer::from_bytes constructor
- reqwest http2 feature for DoH server compatibility
- network_watch_loop guards against DoH→UDP silent downgrade
- 5 new tests (mock DoH server, HTTP errors, timeout)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* style: cargo fmt

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: add DoH to README — Why Numa, comparison table, roadmap

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/forward.rs

@@ -1,3 +1,4 @@
+use std::fmt;
 use std::net::SocketAddr;
 use std::time::Duration;
 
@@ -8,7 +9,46 @@ use crate::buffer::BytePacketBuffer;
 use crate::packet::DnsPacket;
 use crate::Result;
 
+#[derive(Clone)]
+pub enum Upstream {
+    Udp(SocketAddr),
+    Doh {
+        url: String,
+        client: reqwest::Client,
+    },
+}
+
+impl PartialEq for Upstream {
+    fn eq(&self, other: &Self) -> bool {
+        match (self, other) {
+            (Self::Udp(a), Self::Udp(b)) => a == b,
+            (Self::Doh { url: a, .. }, Self::Doh { url: b, .. }) => a == b,
+            _ => false,
+        }
+    }
+}
+
+impl fmt::Display for Upstream {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Upstream::Udp(addr) => write!(f, "{}", addr),
+            Upstream::Doh { url, .. } => f.write_str(url),
+        }
+    }
+}
+
 pub async fn forward_query(
+    query: &DnsPacket,
+    upstream: &Upstream,
+    timeout_duration: Duration,
+) -> Result<DnsPacket> {
+    match upstream {
+        Upstream::Udp(addr) => forward_udp(query, *addr, timeout_duration).await,
+        Upstream::Doh { url, client } => forward_doh(query, url, client, timeout_duration).await,
+    }
+}
+
+async fn forward_udp(
     query: &DnsPacket,
     upstream: SocketAddr,
     timeout_duration: Duration,
@@ -33,3 +73,174 @@ pub async fn forward_query(
 
     DnsPacket::from_buffer(&mut recv_buffer)
 }
+
+async fn forward_doh(
+    query: &DnsPacket,
+    url: &str,
+    client: &reqwest::Client,
+    timeout_duration: Duration,
+) -> Result<DnsPacket> {
+    let mut send_buffer = BytePacketBuffer::new();
+    query.write(&mut send_buffer)?;
+
+    let resp = timeout(
+        timeout_duration,
+        client
+            .post(url)
+            .header("content-type", "application/dns-message")
+            .header("accept", "application/dns-message")
+            .body(send_buffer.filled().to_vec())
+            .send(),
+    )
+    .await??
+    .error_for_status()?;
+
+    let bytes = resp.bytes().await?;
+    log::debug!("DoH response: {} bytes", bytes.len());
+
+    let mut recv_buffer = BytePacketBuffer::from_bytes(&bytes);
+    DnsPacket::from_buffer(&mut recv_buffer)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::future::IntoFuture;
+
+    use crate::header::ResultCode;
+    use crate::question::{DnsQuestion, QueryType};
+    use crate::record::DnsRecord;
+
+    #[test]
+    fn upstream_display_udp() {
+        let u = Upstream::Udp("9.9.9.9:53".parse().unwrap());
+        assert_eq!(u.to_string(), "9.9.9.9:53");
+    }
+
+    #[test]
+    fn upstream_display_doh() {
+        let u = Upstream::Doh {
+            url: "https://dns.quad9.net/dns-query".to_string(),
+            client: reqwest::Client::new(),
+        };
+        assert_eq!(u.to_string(), "https://dns.quad9.net/dns-query");
+    }
+
+    fn make_query() -> DnsPacket {
+        let mut q = DnsPacket::new();
+        q.header.id = 0xABCD;
+        q.header.recursion_desired = true;
+        q.questions
+            .push(DnsQuestion::new("example.com".to_string(), QueryType::A));
+        q
+    }
+
+    fn make_response(query: &DnsPacket) -> DnsPacket {
+        let mut resp = DnsPacket::response_from(query, ResultCode::NOERROR);
+        resp.answers.push(DnsRecord::A {
+            domain: "example.com".to_string(),
+            addr: "93.184.216.34".parse().unwrap(),
+            ttl: 300,
+        });
+        resp
+    }
+
+    fn to_wire(pkt: &DnsPacket) -> Vec<u8> {
+        let mut buf = BytePacketBuffer::new();
+        pkt.write(&mut buf).unwrap();
+        buf.filled().to_vec()
+    }
+
+    #[tokio::test]
+    async fn doh_mock_server_resolves() {
+        let query = make_query();
+        let response_bytes = to_wire(&make_response(&query));
+
+        let app = axum::Router::new().route(
+            "/dns-query",
+            axum::routing::post(move || {
+                let body = response_bytes.clone();
+                async move {
+                    (
+                        [(axum::http::header::CONTENT_TYPE, "application/dns-message")],
+                        body,
+                    )
+                }
+            }),
+        );
+
+        let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
+        let addr = listener.local_addr().unwrap();
+        tokio::spawn(axum::serve(listener, app).into_future());
+
+        let upstream = Upstream::Doh {
+            url: format!("http://{}/dns-query", addr),
+            client: reqwest::Client::new(),
+        };
+
+        let result = forward_query(&query, &upstream, Duration::from_secs(2))
+            .await
+            .expect("DoH forward should succeed");
+
+        assert_eq!(result.header.id, 0xABCD);
+        assert!(result.header.response);
+        assert_eq!(result.header.rescode, ResultCode::NOERROR);
+        assert_eq!(result.answers.len(), 1);
+        match &result.answers[0] {
+            DnsRecord::A { domain, addr, ttl } => {
+                assert_eq!(domain, "example.com");
+                assert_eq!(
+                    *addr,
+                    "93.184.216.34".parse::<std::net::Ipv4Addr>().unwrap()
+                );
+                assert_eq!(*ttl, 300);
+            }
+            other => panic!("expected A record, got {:?}", other),
+        }
+    }
+
+    #[tokio::test]
+    async fn doh_http_error_propagates() {
+        let app = axum::Router::new().route(
+            "/dns-query",
+            axum::routing::post(|| async {
+                (axum::http::StatusCode::INTERNAL_SERVER_ERROR, "bad")
+            }),
+        );
+
+        let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
+        let addr = listener.local_addr().unwrap();
+        tokio::spawn(axum::serve(listener, app).into_future());
+
+        let upstream = Upstream::Doh {
+            url: format!("http://{}/dns-query", addr),
+            client: reqwest::Client::new(),
+        };
+
+        let result = forward_query(&make_query(), &upstream, Duration::from_secs(2)).await;
+        assert!(result.is_err());
+    }
+
+    #[tokio::test]
+    async fn doh_timeout() {
+        let app = axum::Router::new().route(
+            "/dns-query",
+            axum::routing::post(|| async {
+                tokio::time::sleep(Duration::from_secs(10)).await;
+                "never"
+            }),
+        );
+
+        let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
+        let addr = listener.local_addr().unwrap();
+        tokio::spawn(axum::serve(listener, app).into_future());
+
+        let upstream = Upstream::Doh {
+            url: format!("http://{}/dns-query", addr),
+            client: reqwest::Client::new(),
+        };
+
+        let result = forward_query(&make_query(), &upstream, Duration::from_millis(100)).await;
+        assert!(result.is_err());
+    }
+}