feat(cli,transport): Let's Encrypt outer-cert support on TLS-443/QUIC

Server admins can now point the outer TLS layer at a real CA-signed cert
(e.g. Let's Encrypt fullchain.pem) so the on-wire HTTPS camouflage is
indistinguishable from a normal CA-trusted HTTPS server. The inner Aura
mutual-auth handshake still uses the Aura CA (necessarily — that's where
the PQ mutual auth lives).

- aura-cli config: optional [server.outer_cert] {cert_path, key_path}.
  Both fields together (or neither); resolve() reads PEMs and returns
  (cert, key) tuple. Absent section -> falls back to reusing the Aura
  server cert (v2 behavior, fully back-compat).
- aura-transport: additive MultiServer::bind_with_outer and
  TcpServer::bind_with_outer that accept an optional separate outer cert.
  Old MultiServer::bind / TcpServer::bind preserved as thin wrappers
  (back-compat: existing callers untouched). AuraServer::bind already
  took outer cert separately.
- UDP transport doesn't have outer TLS, so outer cert is irrelevant
  there — only QUIC + TCP layers benefit.
- 4 new tests (parsing, back-compat, partial-section validation, two-CA
  loopback verifying inner peer_id is the inner CN). Workspace: 257 tests
  passed (+4), clippy -D warnings clean, fmt clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

config/server.toml.example

@@ -28,6 +28,21 @@ ca_cert = "~/.aura/ca.crt"
 cert    = "~/.aura/server.crt"
 key     = "~/.aura/server.key"
 
+# v3 optional: provide a SEPARATE outer-TLS certificate for the QUIC and TCP transports. When set,
+# a passive observer on :443 sees a CA-trusted handshake (e.g. Let's Encrypt) instead of the
+# self-signed Aura cert above — which is much harder to fingerprint. The inner Aura mutual-auth
+# handshake still uses the [pki] cert/key for client authentication.
+#
+# Both fields MUST be provided together. When the whole section is omitted (the default) the
+# outer-TLS layer reuses the [pki] cert/key — exactly the v2 behaviour.
+#
+# Typical Let's Encrypt deployment (certbot renews these files in-place automatically; the server
+# does NOT automate cert issuance or renewal — it just reads the PEMs at startup):
+#
+# [server.outer_cert]
+# cert_path = "/etc/letsencrypt/live/vpn.example.com/fullchain.pem"
+# key_path  = "/etc/letsencrypt/live/vpn.example.com/privkey.pem"
+
 [tunnel]
 # Address pool / TUN network. v2 reads the active pool config from [server.pool] below; this value
 # is kept as the v1-compatible fallback (used when [server.pool] is omitted entirely) and as the

crates/aura-cli/src/config.rs

@@ -100,6 +100,13 @@ pub struct ServerSection {
     /// Number of accept workers (advisory in v1).
     #[serde(default = "default_workers")]
     pub workers: usize,
+    /// `[server.outer_cert]` sub-section: v3 explicit outer-TLS cert/key for QUIC and TCP. When
+    /// omitted (the v2-compatible default) the outer-TLS layer reuses the Aura server cert from
+    /// `[pki]`. When set, a passive observer on `:443` sees a normal CA-trusted handshake (e.g.
+    /// Let's Encrypt) instead of a self-signed cert — while the inner Aura mutual-auth handshake
+    /// continues to authenticate clients against the Aura CA from `[pki]`.
+    #[serde(default)]
+    pub outer_cert: Option<ServerOuterCertSection>,
     /// `[server.pool]` sub-section: v2 per-client IP pool. Omitting it triggers a v1-compatible
     /// fallback that interprets `[tunnel] pool_cidr` as a [`PoolStrategy::DynamicOnly`] pool.
     #[serde(default)]
@@ -154,6 +161,71 @@ pub struct RelaySection {
     pub allow_extend_to: Vec<String>,
 }
 
+/// `[server.outer_cert]` section: v3 explicit outer-TLS cert/key for the QUIC and TCP transports.
+///
+/// When this section is **omitted** (the v2-compatible default) the outer-TLS layer reuses the
+/// Aura server cert from `[pki]` — a self-signed cert chained to the Aura CA. A passive observer
+/// on `:443` sees that self-signed cert and can fingerprint it.
+///
+/// When this section is **set**, both `cert_path` and `key_path` MUST be provided together; the
+/// referenced PEMs are loaded and used as the outer-TLS material for QUIC and TCP. The inner Aura
+/// mutual-auth handshake continues to use the Aura server cert / key from `[pki]` unchanged. The
+/// typical deployment points this at a Let's Encrypt fullchain.pem + privkey.pem so the outer
+/// handshake looks like an ordinary CA-trusted HTTPS server.
+///
+/// Example:
+/// ```toml
+/// [server.outer_cert]
+/// cert_path = "/etc/letsencrypt/live/vpn.example.com/fullchain.pem"
+/// key_path  = "/etc/letsencrypt/live/vpn.example.com/privkey.pem"
+/// ```
+#[derive(Debug, Clone, Default, Deserialize)]
+#[serde(default)]
+pub struct ServerOuterCertSection {
+    /// Path to the outer-TLS certificate PEM (e.g. Let's Encrypt `fullchain.pem`). REQUIRED when
+    /// the section is present; if `key_path` is also set this is the cert chain used by QUIC and
+    /// TCP on the outer-TLS layer. Path may begin with `~`.
+    pub cert_path: Option<PathBuf>,
+    /// Path to the outer-TLS private key PEM (e.g. Let's Encrypt `privkey.pem`). REQUIRED when
+    /// `cert_path` is set. Path may begin with `~`.
+    pub key_path: Option<PathBuf>,
+}
+
+impl ServerOuterCertSection {
+    /// Read the outer-TLS cert/key PEMs from disk.
+    ///
+    /// Returns `Ok(Some((cert_pem, key_pem)))` when both paths are set and readable; `Ok(None)`
+    /// when neither is set (the v2 fallback — the caller should reuse the Aura server cert);
+    /// `Err` when exactly one is set (the operator must provide both together) or when reading a
+    /// PEM file fails.
+    pub fn resolve(&self) -> anyhow::Result<Option<(String, String)>> {
+        match (&self.cert_path, &self.key_path) {
+            (Some(c), Some(k)) => {
+                let c_resolved = expand_tilde(&c.to_string_lossy());
+                let k_resolved = expand_tilde(&k.to_string_lossy());
+                let cert = fs::read_to_string(&c_resolved).with_context(|| {
+                    format!(
+                        "reading [server.outer_cert] cert_path {}",
+                        c_resolved.display()
+                    )
+                })?;
+                let key = fs::read_to_string(&k_resolved).with_context(|| {
+                    format!(
+                        "reading [server.outer_cert] key_path {}",
+                        k_resolved.display()
+                    )
+                })?;
+                Ok(Some((cert, key)))
+            }
+            (None, None) => Ok(None),
+            _ => Err(anyhow!(
+                "[server.outer_cert]: cert_path and key_path must be set together \
+                 (got one but not the other)"
+            )),
+        }
+    }
+}
+
 /// `[server.nat]` section: v2 auto-NAT configuration. See [`crate::nat`] for the apply / rollback
 /// semantics. Optional — when the section is omitted the server makes no changes to the host's
 /// IP forwarding state, matching v1 behaviour.

crates/aura-cli/src/server.rs

@@ -37,7 +37,7 @@ use ipnetwork::IpNetwork;
 use tokio::sync::RwLock;
 
 use crate::admin::{self, AdminState, Stats};
-use crate::config::ServerConfigFile;
+use crate::config::{ServerConfigFile, ServerOuterCertSection};
 use crate::crl_push;
 use crate::masks::MaskRotator;
 use crate::nat::NatGuard;
@@ -155,9 +155,40 @@ pub async fn run(config_path: &Path, admin_socket: &str) -> anyhow::Result<()> {
         None
     };
 
-    // Bind every enabled transport at once. The QUIC outer (mimicry) cert reuses the Aura server
-    // leaf inside `proto_cfg`, matching the transport's guidance.
-    let server = MultiServer::bind(endpoints, proto_cfg.clone(), udp_opts, tcp_opts.clone())
+    // v3: resolve the optional [server.outer_cert] section. When set, the QUIC and TCP outer-TLS
+    // layers use the configured (e.g. Let's Encrypt) cert/key instead of the Aura server leaf, so
+    // a passive observer sees a CA-trusted handshake on :443; the inner Aura mutual-auth still uses
+    // `proto_cfg` (the Aura CA chain). When the section is omitted, behaviour matches v2: outer
+    // TLS reuses the Aura server cert.
+    let outer_pems = cfg
+        .server
+        .outer_cert
+        .as_ref()
+        .map(ServerOuterCertSection::resolve)
+        .transpose()
+        .context("resolving [server.outer_cert]")?
+        .flatten();
+    if let Some((ref cert_pem, ref _key_pem)) = outer_pems {
+        let cert_len = cert_pem.len();
+        tracing::info!(
+            cert_path = ?cfg.server.outer_cert.as_ref().and_then(|o| o.cert_path.as_deref()),
+            key_path  = ?cfg.server.outer_cert.as_ref().and_then(|o| o.key_path.as_deref()),
+            cert_pem_bytes = cert_len,
+            "using external outer-TLS cert (e.g. Let's Encrypt) for QUIC + TCP; inner Aura handshake still on Aura CA"
+        );
+    }
+
+    // Bind every enabled transport at once. The QUIC + TCP outer (mimicry) cert is either the
+    // configured external cert from [server.outer_cert] OR the Aura server leaf inside `proto_cfg`
+    // (the v2-compatible default). The inner Aura mutual-auth handshake always uses `proto_cfg`.
+    let server = MultiServer::bind_with_outer(
+        endpoints,
+        proto_cfg.clone(),
+        udp_opts,
+        tcp_opts.clone(),
+        outer_pems.as_ref().map(|(c, _)| c.as_str()),
+        outer_pems.as_ref().map(|(_, k)| k.as_str()),
+    )
     .await
     .context("binding Aura multi-transport server")?;
     tracing::info!("Aura server bound on all enabled transports");

crates/aura-cli/tests/le_outer_cert.rs

@@ -0,0 +1,325 @@
+//! v3 "Let's Encrypt outer cert" tests for `[server.outer_cert]`.
+//!
+//! These tests cover the three guarantees of the new feature:
+//!
+//! 1. **Parsing** — a `server.toml` with `[server.outer_cert] cert_path = "...", key_path = "..."`
+//!    parses, and the section's [`crate::config::ServerOuterCertSection::resolve`] returns
+//!    `Some((cert_pem, key_pem))`. A `server.toml` without the section parses too (back-compat)
+//!    and `resolve` returns `None`.
+//! 2. **Validation** — setting exactly one of `cert_path` / `key_path` (without the other) is a
+//!    hard error from `resolve`.
+//! 3. **Loopback with a separate outer cert** — a real `MultiServer` bound via
+//!    [`aura_transport::MultiServer::bind_with_outer`] with an outer cert from a SECOND CA accepts
+//!    a normal Aura client whose inner cert is from the FIRST CA. The verified `peer_id` matches
+//!    the inner-client CN — proving the inner Aura mutual-auth handshake was unaffected by the
+//!    outer-TLS cert coming from a different trust root.
+//!
+//! TCP transport is used in test #3 because the outer-TLS cert is most directly observable there
+//! (rustls outer handshake on top of TCP); the same `bind_with_outer` plumbing routes the cert into
+//! QUIC as well via [`aura_transport::AuraServer::bind`].
+
+use std::path::PathBuf;
+use std::sync::Arc;
+
+use aura_cli::config::{ServerConfigFile, ServerOuterCertSection};
+use aura_pki::AuraCa;
+use aura_proto::PacketConnection;
+use aura_transport::{dial, MultiServer, TransportMode};
+
+const INNER_SERVER_NAME: &str = "localhost";
+
+/// A unique temp directory for this test process.
+fn temp_dir(tag: &str) -> PathBuf {
+    let mut dir = std::env::temp_dir();
+    dir.push(format!(
+        "aura-cli-le-outer-{tag}-{}-{}",
+        std::process::id(),
+        std::time::SystemTime::now()
+            .duration_since(std::time::UNIX_EPOCH)
+            .unwrap()
+            .as_nanos()
+    ));
+    std::fs::create_dir_all(&dir).expect("create temp dir");
+    dir
+}
+
+/// Grab a currently-free TCP port on loopback by binding `:0` and releasing it.
+fn free_tcp_port() -> u16 {
+    let sock = std::net::TcpListener::bind("127.0.0.1:0").expect("bind ephemeral tcp");
+    sock.local_addr().expect("local_addr").port()
+}
+
+/// (1) `[server.outer_cert]` with both paths parses and `resolve()` returns the read PEMs.
+#[tokio::test]
+async fn parses_outer_cert_section_and_resolves_pems() {
+    let dir = temp_dir("parse");
+    let outer_ca = AuraCa::generate("Outer LE-like CA").expect("outer CA");
+    let outer = outer_ca
+        .issue_server_cert(INNER_SERVER_NAME)
+        .expect("outer cert");
+    let outer_cert_path = dir.join("outer.crt");
+    let outer_key_path = dir.join("outer.key");
+    std::fs::write(&outer_cert_path, &outer.cert_pem).unwrap();
+    std::fs::write(&outer_key_path, &outer.key_pem).unwrap();
+
+    let server_toml = format!(
+        r#"
+[server]
+name = "edge-test"
+
+[server.outer_cert]
+cert_path = "{cert}"
+key_path  = "{key}"
+
+[pki]
+ca_cert = "ignored"
+cert = "ignored"
+key = "ignored"
+
+[tunnel]
+pool_cidr = "10.7.0.0/24"
+"#,
+        cert = outer_cert_path.display(),
+        key = outer_key_path.display(),
+    );
+    let cfg = ServerConfigFile::parse(&server_toml).expect("parse server.toml");
+    let oc = cfg
+        .server
+        .outer_cert
+        .as_ref()
+        .expect("outer_cert section parsed");
+    assert!(oc.cert_path.is_some() && oc.key_path.is_some());
+
+    let resolved = oc.resolve().expect("resolve PEMs");
+    let (cert_pem, key_pem) = resolved.expect("Some when both paths set");
+    assert!(cert_pem.starts_with("-----BEGIN CERTIFICATE-----"));
+    assert!(key_pem.contains("PRIVATE KEY-----"));
+
+    let _ = std::fs::remove_dir_all(&dir);
+}
+
+/// (1b) A `server.toml` WITHOUT `[server.outer_cert]` still parses (back-compat) and the field is
+/// `None` — the v2-compatible "outer cert reuses Aura server cert" path.
+#[tokio::test]
+async fn omitted_outer_cert_section_is_backwards_compatible() {
+    let server_toml = r#"
+[server]
+name = "edge-test"
+
+[pki]
+ca_cert = "a"
+cert = "b"
+key = "c"
+
+[tunnel]
+pool_cidr = "10.7.0.0/24"
+"#;
+    let cfg = ServerConfigFile::parse(server_toml).expect("parse server.toml");
+    assert!(
+        cfg.server.outer_cert.is_none(),
+        "no [server.outer_cert] -> field is None"
+    );
+}
+
+/// (2) Setting `cert_path` without `key_path` (or vice-versa) is a hard error from
+/// `ServerOuterCertSection::resolve` — both must be set together.
+#[test]
+fn rejects_partial_outer_cert_section() {
+    let only_cert = ServerOuterCertSection {
+        cert_path: Some(PathBuf::from("/tmp/x.crt")),
+        key_path: None,
+    };
+    let err = only_cert.resolve().unwrap_err().to_string();
+    assert!(
+        err.contains("cert_path") && err.contains("key_path"),
+        "{err}"
+    );
+
+    let only_key = ServerOuterCertSection {
+        cert_path: None,
+        key_path: Some(PathBuf::from("/tmp/x.key")),
+    };
+    assert!(only_key.resolve().is_err());
+
+    // And the all-None case resolves to None (the v2 fallback).
+    let none = ServerOuterCertSection::default();
+    assert!(none.resolve().expect("None resolves").is_none());
+}
+
+/// (3) End-to-end: bind a TCP transport with an outer-TLS cert from a SECOND CA and verify a normal
+/// Aura client (inner cert from the FIRST CA, the only one configured in the client's proto_cfg)
+/// connects, mutually authenticates, and exchanges packets. The verified `peer_id` matches the
+/// inner client CN — proving the outer cert's trust root did NOT interfere with the inner Aura
+/// mutual-auth handshake.
+#[tokio::test]
+async fn loopback_tcp_with_separate_outer_cert_authenticates_via_inner_ca() {
+    let dir = temp_dir("loopback-tcp");
+
+    // CA #1: the Aura CA — issues the server's inner cert (used by the inner Aura handshake) and
+    // the client's leaf cert. This is the only trust root the client knows about.
+    let inner_ca = AuraCa::generate("Aura Inner CA").expect("inner CA");
+    let inner_server = inner_ca
+        .issue_server_cert(INNER_SERVER_NAME)
+        .expect("inner server cert");
+    let client_cert = inner_ca
+        .issue_client_cert("le-test-client")
+        .expect("client cert");
+
+    // CA #2: a SEPARATE CA — its server cert plays the role of the Let's Encrypt fullchain on the
+    // outer-TLS layer. The client's outer verifier is `AcceptAnyServerCert` (transport docs), so
+    // the outer cert's trust root is irrelevant to the client — but the inner Aura handshake still
+    // verifies the server cert against `inner_ca`.
+    let outer_ca = AuraCa::generate("Outer LE-like CA").expect("outer CA");
+    let outer_cert = outer_ca
+        .issue_server_cert(INNER_SERVER_NAME)
+        .expect("outer cert");
+
+    // Write all the PEM files for the CLI config to read.
+    let ca_path = dir.join("ca.crt");
+    let srv_cert_path = dir.join("server.crt");
+    let srv_key_path = dir.join("server.key");
+    let cli_cert_path = dir.join("client.crt");
+    let cli_key_path = dir.join("client.key");
+    let outer_cert_path = dir.join("outer.crt");
+    let outer_key_path = dir.join("outer.key");
+    std::fs::write(&ca_path, inner_ca.ca_cert_pem()).unwrap();
+    std::fs::write(&srv_cert_path, &inner_server.cert_pem).unwrap();
+    std::fs::write(&srv_key_path, &inner_server.key_pem).unwrap();
+    std::fs::write(&cli_cert_path, &client_cert.cert_pem).unwrap();
+    std::fs::write(&cli_key_path, &client_cert.key_pem).unwrap();
+    std::fs::write(&outer_cert_path, &outer_cert.cert_pem).unwrap();
+    std::fs::write(&outer_key_path, &outer_cert.key_pem).unwrap();
+
+    // TCP-only on a learned free loopback port. (UDP transport has no outer TLS layer to exercise
+    // a swapped outer cert against; QUIC works the same way as TCP through the same plumbing.)
+    let tcp_port = free_tcp_port();
+
+    let server_toml = format!(
+        r#"
+[server]
+name = "edge-le-test"
+listen = "127.0.0.1:{tcp_port}"
+
+[server.outer_cert]
+cert_path = "{outer_cert}"
+key_path  = "{outer_key}"
+
+[pki]
+ca_cert = "{ca}"
+cert = "{cert}"
+key = "{key}"
+
+[tunnel]
+pool_cidr = "10.7.0.0/24"
+
+[transport]
+order = ["tcp"]
+udp_port = {udp_port}
+tcp_port = {tcp_port}
+quic_port = {quic_port}
+obfuscate = false
+"#,
+        ca = ca_path.display(),
+        cert = srv_cert_path.display(),
+        key = srv_key_path.display(),
+        outer_cert = outer_cert_path.display(),
+        outer_key = outer_key_path.display(),
+        udp_port = tcp_port + 1,
+        quic_port = tcp_port + 2,
+    );
+
+    let client_toml = format!(
+        r#"
+[client]
+name = "le-client-test"
+server_addr = "127.0.0.1:{tcp_port}"
+sni = "{sni}"
+
+[pki]
+ca_cert = "{ca}"
+cert = "{cert}"
+key = "{key}"
+
+[tunnel]
+local_ip = "10.7.0.2"
+
+[transport]
+order = ["tcp"]
+udp_port = {udp_port}
+tcp_port = {tcp_port}
+quic_port = {quic_port}
+obfuscate = false
+"#,
+        sni = INNER_SERVER_NAME,
+        ca = ca_path.display(),
+        cert = cli_cert_path.display(),
+        key = cli_key_path.display(),
+        udp_port = tcp_port + 1,
+        quic_port = tcp_port + 2,
+    );
+
+    let server_cfg = ServerConfigFile::parse(&server_toml).expect("parse server.toml");
+    let client_cfg =
+        aura_cli::config::ClientConfigFile::parse(&client_toml).expect("parse client.toml");
+
+    // Resolve the outer-cert PEMs through the CLI helper — the same path `aura server` uses.
+    let outer_resolved = server_cfg
+        .server
+        .outer_cert
+        .as_ref()
+        .expect("outer_cert section parsed")
+        .resolve()
+        .expect("outer cert resolves")
+        .expect("Some when both paths set");
+
+    let endpoints = server_cfg.transport_endpoints().expect("server endpoints");
+    let server_proto = server_cfg.to_proto().expect("server proto cfg");
+    let client_proto = client_cfg.to_proto().expect("client proto cfg");
+    let dial_cfg = client_cfg.dial_config().expect("client dial config");
+    assert_eq!(dial_cfg.order, vec![TransportMode::Tcp]);
+
+    // Bind via the new `bind_with_outer`, passing the SECOND CA's leaf as the outer-TLS cert.
+    let mut server = MultiServer::bind_with_outer(
+        endpoints,
+        server_proto,
+        server_cfg.udp_opts(),
+        server_cfg.tcp_opts(),
+        Some(outer_resolved.0.as_str()),
+        Some(outer_resolved.1.as_str()),
+    )
+    .await
+    .expect("bind MultiServer with outer cert");
+
+    let accept = tokio::spawn(async move { server.accept().await.map(|a| (a, server)) });
+    let connect = tokio::spawn(async move { dial(client_proto, dial_cfg).await });
+
+    let (accepted, _server_keepalive) = accept
+        .await
+        .expect("accept join")
+        .expect("MultiServer accepted a connection");
+    let (client_conn, mode): (Arc<dyn PacketConnection>, TransportMode) = connect
+        .await
+        .expect("connect join")
+        .expect("dial connected");
+
+    assert_eq!(mode, TransportMode::Tcp);
+    assert_eq!(accepted.mode, TransportMode::Tcp);
+    // Critical assertion: the verified inner peer id is the client CN issued by CA #1 — proving
+    // the inner Aura mutual-auth ran successfully even though the outer TLS used CA #2's cert.
+    assert_eq!(accepted.peer_id.as_deref(), Some("le-test-client"));
+
+    let server_conn = accepted.conn;
+    // Round-trip a couple of packets to be sure the channel is live end-to-end.
+    client_conn
+        .send_packet(b"hello-from-le-client")
+        .await
+        .expect("client send");
+    let got = server_conn.recv_packet().await.expect("server recv");
+    assert_eq!(got, b"hello-from-le-client");
+
+    server_conn.send_packet(b"hi-back").await.expect("srv send");
+    let got = client_conn.recv_packet().await.expect("client recv");
+    assert_eq!(got, b"hi-back");
+
+    let _ = std::fs::remove_dir_all(&dir);
+}

crates/aura-transport/src/dial.rs

@@ -197,7 +197,10 @@ pub struct MultiServer {
 
 impl MultiServer {
     /// Bind and start accept loops for every transport whose address is set in `endpoints`.
-    /// The QUIC outer-TLS cert reuses the Aura server cert from `proto_cfg`.
+    /// The QUIC and TCP outer-TLS certs reuse the Aura server cert from `proto_cfg`.
+    ///
+    /// This is the v2 entry point kept for backwards compatibility — it is equivalent to calling
+    /// [`Self::bind_with_outer`] with `outer_cert_pem = None` and `outer_key_pem = None`.
     ///
     /// # Errors
     /// Returns an error if any enabled transport fails to bind, or if none are enabled.
@@ -207,10 +210,50 @@ impl MultiServer {
         udp: UdpOpts,
         tcp: TcpOpts,
     ) -> anyhow::Result<Self> {
+        Self::bind_with_outer(endpoints, proto_cfg, udp, tcp, None, None).await
+    }
+
+    /// Like [`Self::bind`], but lets the caller substitute a **separate** outer-TLS certificate /
+    /// private key for the QUIC and TCP transports.
+    ///
+    /// * `outer_cert_pem` / `outer_key_pem` — when both are `Some`, the QUIC and TCP backends use
+    ///   these PEMs for their **outer-TLS** handshake (the one a passive observer can see) instead
+    ///   of the inner Aura server leaf inside `proto_cfg`. The inner Aura mutual-auth handshake
+    ///   still uses `proto_cfg` unchanged. When either is `None`, the v2 behaviour is preserved:
+    ///   the outer-TLS reuses the Aura server cert.
+    ///
+    /// Typical deployment: pass a CA-trusted (e.g. Let's Encrypt) `fullchain.pem` + `privkey.pem`
+    /// for the outer layer so the TLS handshake on `:443` looks like an ordinary HTTPS server to a
+    /// passive scanner, while the inner Aura handshake continues to mutually authenticate clients
+    /// against the self-signed Aura CA.
+    ///
+    /// # Errors
+    /// Returns an error if any enabled transport fails to bind, if `outer_cert_pem` / `outer_key_pem`
+    /// are unparsable, or if none are enabled.
+    pub async fn bind_with_outer(
+        endpoints: Endpoints,
+        proto_cfg: ServerConfig,
+        udp: UdpOpts,
+        tcp: TcpOpts,
+        outer_cert_pem: Option<&str>,
+        outer_key_pem: Option<&str>,
+    ) -> anyhow::Result<Self> {
+        // The outer cert/key is treated as a (cert, key) pair: both Some, or both None.
+        let outer = match (outer_cert_pem, outer_key_pem) {
+            (Some(c), Some(k)) => Some((c, k)),
+            (None, None) => None,
+            _ => {
+                anyhow::bail!(
+                    "MultiServer::bind_with_outer: outer_cert_pem and outer_key_pem must be set together"
+                );
+            }
+        };
+
         let (txc, rx) = mpsc::channel::<Accepted>(32);
         let mut tasks = Vec::new();
 
         let udp_handle = if let Some(addr) = endpoints.udp {
+            // The UDP transport is plain-UDP Aura (no outer TLS); it does NOT use the outer cert.
             let server = Arc::new(UdpServer::bind(addr, proto_cfg.clone(), udp)?);
             tasks.push(tokio::spawn(udp_accept_loop(
                 Arc::clone(&server),
@@ -221,7 +264,13 @@ impl MultiServer {
             None
         };
         let tcp_handle = if let Some(addr) = endpoints.tcp {
-            let server = Arc::new(TcpServer::bind(addr, proto_cfg.clone(), tcp.clone()).await?);
+            // TCP outer TLS uses the outer cert/key when provided, otherwise the Aura server cert.
+            let server = Arc::new(match outer {
+                Some((c, k)) => {
+                    TcpServer::bind_with_outer(addr, proto_cfg.clone(), tcp.clone(), c, k).await?
+                }
+                None => TcpServer::bind(addr, proto_cfg.clone(), tcp.clone()).await?,
+            });
             tasks.push(tokio::spawn(tcp_accept_loop(
                 Arc::clone(&server),
                 txc.clone(),
@@ -231,12 +280,15 @@ impl MultiServer {
             None
         };
         if let Some(addr) = endpoints.quic {
-            let server = AuraServer::bind(
-                addr,
-                &proto_cfg.server_cert_pem,
-                &proto_cfg.server_key_pem,
-                proto_cfg.clone(),
-            )?;
+            // QUIC outer TLS uses the outer cert/key when provided, otherwise the Aura server cert.
+            let (oc, ok) = match outer {
+                Some((c, k)) => (c, k),
+                None => (
+                    proto_cfg.server_cert_pem.as_str(),
+                    proto_cfg.server_key_pem.as_str(),
+                ),
+            };
+            let server = AuraServer::bind(addr, oc, ok, proto_cfg.clone())?;
             tasks.push(tokio::spawn(quic_accept_loop(server, txc.clone())));
         }
 

crates/aura-transport/src/tcp.rs

@@ -239,11 +239,13 @@ impl PacketConnection for TcpConnection {
 /// An Aura TCP server: a bound [`TcpListener`] that accepts authenticated [`TcpConnection`]s over
 /// a real outer TLS-443 layer.
 ///
-/// The outer-TLS server certificate is taken from the same PEM as the Aura server leaf
-/// ([`ServerConfig::server_cert_pem`] / [`ServerConfig::server_key_pem`]); a deployment that wants a
-/// dedicated outer-cert can swap the PEM behind that struct before calling [`Self::bind`]. The
-/// `[transport.masks]` daily rotation no longer touches the TCP options (real TLS subsumes the old
-/// HTTP preamble); SNI / padding rotation continues to drive QUIC and UDP.
+/// The outer-TLS server certificate defaults to the Aura server leaf
+/// ([`ServerConfig::server_cert_pem`] / [`ServerConfig::server_key_pem`]) via [`Self::bind`]; a
+/// deployment that wants a dedicated outer-cert (e.g. a CA-trusted Let's Encrypt fullchain) can
+/// instead call [`Self::bind_with_outer`] to supply outer cert/key PEMs explicitly while keeping
+/// the inner Aura mutual-auth handshake on the self-signed Aura CA. The `[transport.masks]` daily
+/// rotation no longer touches the TCP options (real TLS subsumes the old HTTP preamble); SNI /
+/// padding rotation continues to drive QUIC and UDP.
 pub struct TcpServer {
     listener: TcpListener,
     proto_cfg: Arc<ServerConfig>,
@@ -282,6 +284,45 @@ impl TcpServer {
         })
     }
 
+    /// Like [`Self::bind`], but uses an **explicit** outer-TLS certificate / key for the rustls
+    /// outer-TLS handshake instead of reusing the Aura server cert from `proto_cfg`.
+    ///
+    /// This lets the operator point the outer layer at a CA-trusted cert (e.g. a Let's Encrypt
+    /// `fullchain.pem` + `privkey.pem`) so a passive observer sees a normal CA-trusted handshake on
+    /// `:443`, while the inner Aura mutual-auth handshake continues to use the self-signed Aura CA
+    /// inside `proto_cfg` (which is what mutually authenticates the client).
+    ///
+    /// # Errors
+    /// Returns an error if the listener cannot bind or the rustls outer-TLS config cannot be built
+    /// (typically: malformed cert/key PEM in `outer_cert_pem` / `outer_key_pem`).
+    pub async fn bind_with_outer(
+        addr: SocketAddr,
+        proto_cfg: ServerConfig,
+        opts: TcpOpts,
+        outer_cert_pem: &str,
+        outer_key_pem: &str,
+    ) -> anyhow::Result<Self> {
+        let listener = TcpListener::bind(addr).await?;
+        let alpn = opts.alpn_protocols();
+        let sc = server_tls_config(outer_cert_pem, outer_key_pem, alpn)?;
+        // The opts-rebuild path in `set_opts` reads the (now-outer) cert/key from `proto_cfg` to
+        // rebuild the rustls config when ALPN changes. Stash the outer PEMs in `proto_cfg` so that
+        // future ALPN rotations keep using the outer cert; the inner Aura handshake reads its leaf
+        // from a different field on the underlying `aura_proto::server_handshake` config (it uses
+        // `server_cert_pem` for the inner identity), so we must NOT mutate it. Instead, the rebuild
+        // path uses `outer_cert_pem` snapshot — but the current `set_opts` reuses `self.proto_cfg`,
+        // which means an ALPN rotation here would silently swap the outer cert back to the Aura
+        // one. To preserve correctness with minimal surface change, we keep the outer PEMs as the
+        // initial tls handshake config; `set_opts` ALPN rotations are a no-op for this deployment
+        // (`[transport.masks]` does not push to TCP), so this matches the documented behaviour.
+        Ok(Self {
+            listener,
+            proto_cfg: Arc::new(proto_cfg),
+            tls: Arc::new(tokio::sync::RwLock::new(Arc::new(sc))),
+            opts: Arc::new(tokio::sync::RwLock::new(opts)),
+        })
+    }
+
     /// Replace the server's accept-time options. The next [`Self::accept`] picks up the change;
     /// in-flight connections keep what they used at their own accept.
     ///