feat(odoh): ship ODoH client + self-hosted relay (RFC 9230)

Client (mode = "odoh"): URL-query target routing per RFC 9230 §5,
/.well-known/odohconfigs TTL cache with 60s backoff on failure, HPKE
seal/open via odoh-rs, strict-mode default that SERVFAILs on relay
failure instead of silently downgrading. Host-equality config
validation rejects same-operator relay/target pairs.

Relay (`numa relay [PORT]`): axum server with /relay + /health.
SSRF-hardened hostname validator (RFC 1035 ASCII + dot + dash),
4 KiB body cap at the axum layer, 5s full-transaction timeout, and
static 502 on target failure (reqwest internals logged, not leaked).
Aggregate counters only — no per-request logs.

Observability: new `UpstreamTransport { Udp, Doh, Dot, Odoh }`
orthogonal to `QueryPath`, so /stats can tally wire protocols
symmetrically. Recursive mode records `Some(Udp)` for honest
"bytes egressing in cleartext" accounting.

Tests: Suite 8 exercises the client end-to-end via Frank Denis's
public relay + Cloudflare target; Suite 9 exercises `numa relay`
forwarding + guards against Cloudflare as the real far end. Full
probe script at tests/probe-odoh-ecosystem.sh verifies the entire
public ODoH ecosystem (4 targets + 1 relay per DNSCrypt's curated
list — confirms deploying Numa's relay doubles global supply).

src/config.rs

@@ -134,6 +134,7 @@ pub enum UpstreamMode {
     #[default]
     Forward,
     Recursive,
+    Odoh,
 }
 
 impl UpstreamMode {
@@ -142,6 +143,7 @@ impl UpstreamMode {
             UpstreamMode::Auto => "auto",
             UpstreamMode::Forward => "forward",
             UpstreamMode::Recursive => "recursive",
+            UpstreamMode::Odoh => "odoh",
         }
     }
 }
@@ -154,7 +156,7 @@ pub struct UpstreamConfig {
     pub address: Vec<String>,
     #[serde(default = "default_upstream_port")]
     pub port: u16,
-    #[serde(default)]
+    #[serde(default, deserialize_with = "string_or_vec")]
     pub fallback: Vec<String>,
     #[serde(default = "default_timeout_ms")]
     pub timeout_ms: u64,
@@ -166,6 +168,20 @@ pub struct UpstreamConfig {
     pub prime_tlds: Vec<String>,
     #[serde(default = "default_srtt")]
     pub srtt: bool,
+
+    /// Only used when `mode = "odoh"`. Full https:// URL of the relay
+    /// endpoint (including path, e.g. `https://odoh-relay.numa.rs/relay`).
+    #[serde(default)]
+    pub relay: Option<String>,
+    /// Only used when `mode = "odoh"`. Full https:// URL of the target
+    /// resolver (`https://odoh.cloudflare-dns.com/dns-query`).
+    #[serde(default)]
+    pub target: Option<String>,
+    /// Only used when `mode = "odoh"`. When true (the default), relay failure
+    /// returns SERVFAIL instead of downgrading to the `fallback` upstream —
+    /// a user who configured ODoH rarely wants a silent non-oblivious path.
+    #[serde(default)]
+    pub strict: Option<bool>,
 }
 
 impl Default for UpstreamConfig {
@@ -180,10 +196,75 @@ impl Default for UpstreamConfig {
             root_hints: default_root_hints(),
             prime_tlds: default_prime_tlds(),
             srtt: default_srtt(),
+            relay: None,
+            target: None,
+            strict: None,
         }
     }
 }
 
+/// Parsed ODoH config fields. `mode = "odoh"` requires both URLs to be
+/// present, to parse as `https://`, and to resolve to distinct hosts.
+#[derive(Debug)]
+pub struct OdohUpstream {
+    pub relay_url: String,
+    pub target_host: String,
+    pub target_path: String,
+    pub strict: bool,
+}
+
+impl UpstreamConfig {
+    /// Validate and extract ODoH-specific fields. Called during `load_config`
+    /// so misconfigured ODoH fails fast at startup, the same care we take
+    /// with the DNSSEC strict boot check.
+    pub fn odoh_upstream(&self) -> Result<OdohUpstream> {
+        let relay = self
+            .relay
+            .as_deref()
+            .ok_or("mode = \"odoh\" requires upstream.relay")?;
+        let target = self
+            .target
+            .as_deref()
+            .ok_or("mode = \"odoh\" requires upstream.target")?;
+
+        let relay_url = reqwest::Url::parse(relay)
+            .map_err(|e| format!("upstream.relay invalid URL '{}': {}", relay, e))?;
+        let target_url = reqwest::Url::parse(target)
+            .map_err(|e| format!("upstream.target invalid URL '{}': {}", target, e))?;
+
+        if relay_url.scheme() != "https" || target_url.scheme() != "https" {
+            return Err("upstream.relay and upstream.target must both use https://".into());
+        }
+        if relay_url.host_str().is_none() || target_url.host_str().is_none() {
+            return Err("upstream.relay and upstream.target must include a host".into());
+        }
+        if relay_url.host_str() == target_url.host_str() {
+            return Err(format!(
+                "upstream.relay and upstream.target resolve to the same host ({}); the privacy property requires distinct operators",
+                relay_url.host_str().unwrap_or("?")
+            )
+            .into());
+        }
+
+        let target_host = target_url
+            .host_str()
+            .ok_or("upstream.target has no host")?
+            .to_string();
+        let target_path = if target_url.path().is_empty() {
+            "/".to_string()
+        } else {
+            target_url.path().to_string()
+        };
+
+        Ok(OdohUpstream {
+            relay_url: relay.to_string(),
+            target_host,
+            target_path,
+            strict: self.strict.unwrap_or(true),
+        })
+    }
+}
+
 fn string_or_vec<'de, D>(deserializer: D) -> std::result::Result<Vec<String>, D::Error>
 where
     D: serde::Deserializer<'de>,
@@ -643,12 +724,22 @@ mod tests {
     }
 
     #[test]
-    fn fallback_parses() {
+    fn fallback_array_parses() {
         let config: Config =
             toml::from_str("[upstream]\nfallback = [\"8.8.8.8\", \"1.1.1.1\"]").unwrap();
         assert_eq!(config.upstream.fallback, vec!["8.8.8.8", "1.1.1.1"]);
     }
 
+    #[test]
+    fn fallback_string_parses_as_singleton_vec() {
+        let config: Config =
+            toml::from_str("[upstream]\nfallback = \"tls://1.1.1.1#cloudflare-dns.com\"").unwrap();
+        assert_eq!(
+            config.upstream.fallback,
+            vec!["tls://1.1.1.1#cloudflare-dns.com"]
+        );
+    }
+
     #[test]
     fn empty_address_gives_empty_vec() {
         let config: Config = toml::from_str("").unwrap();
@@ -656,6 +747,88 @@ mod tests {
         assert!(config.upstream.fallback.is_empty());
     }
 
+    // ── [upstream] mode = "odoh" ────────────────────────────────────────
+
+    #[test]
+    fn odoh_config_parses_and_validates() {
+        let toml = r#"
+[upstream]
+mode = "odoh"
+relay = "https://odoh-relay.numa.rs/relay"
+target = "https://odoh.cloudflare-dns.com/dns-query"
+"#;
+        let config: Config = toml::from_str(toml).unwrap();
+        assert!(matches!(config.upstream.mode, UpstreamMode::Odoh));
+        let odoh = config.upstream.odoh_upstream().unwrap();
+        assert_eq!(odoh.relay_url, "https://odoh-relay.numa.rs/relay");
+        assert_eq!(odoh.target_host, "odoh.cloudflare-dns.com");
+        assert_eq!(odoh.target_path, "/dns-query");
+        assert!(odoh.strict, "strict defaults to true under mode=odoh");
+    }
+
+    #[test]
+    fn odoh_strict_false_is_honoured() {
+        let toml = r#"
+[upstream]
+mode = "odoh"
+relay = "https://odoh-relay.numa.rs/relay"
+target = "https://odoh.cloudflare-dns.com/dns-query"
+strict = false
+"#;
+        let config: Config = toml::from_str(toml).unwrap();
+        assert!(!config.upstream.odoh_upstream().unwrap().strict);
+    }
+
+    #[test]
+    fn odoh_rejects_same_host_relay_and_target() {
+        let toml = r#"
+[upstream]
+mode = "odoh"
+relay = "https://odoh.example.com/relay"
+target = "https://odoh.example.com/dns-query"
+"#;
+        let config: Config = toml::from_str(toml).unwrap();
+        let err = config.upstream.odoh_upstream().unwrap_err().to_string();
+        assert!(err.contains("same host"), "got: {err}");
+    }
+
+    #[test]
+    fn odoh_rejects_non_https() {
+        let toml = r#"
+[upstream]
+mode = "odoh"
+relay = "http://odoh-relay.numa.rs/relay"
+target = "https://odoh.cloudflare-dns.com/dns-query"
+"#;
+        let config: Config = toml::from_str(toml).unwrap();
+        let err = config.upstream.odoh_upstream().unwrap_err().to_string();
+        assert!(err.contains("https"), "got: {err}");
+    }
+
+    #[test]
+    fn odoh_missing_relay_rejected() {
+        let toml = r#"
+[upstream]
+mode = "odoh"
+target = "https://odoh.cloudflare-dns.com/dns-query"
+"#;
+        let config: Config = toml::from_str(toml).unwrap();
+        let err = config.upstream.odoh_upstream().unwrap_err().to_string();
+        assert!(err.contains("upstream.relay"), "got: {err}");
+    }
+
+    #[test]
+    fn odoh_missing_target_rejected() {
+        let toml = r#"
+[upstream]
+mode = "odoh"
+relay = "https://odoh-relay.numa.rs/relay"
+"#;
+        let config: Config = toml::from_str(toml).unwrap();
+        let err = config.upstream.odoh_upstream().unwrap_err().to_string();
+        assert!(err.contains("upstream.target"), "got: {err}");
+    }
+
     // ── issue #82: [[forwarding]] config section ────────────────────────
 
     #[test]