sasl external: request tls client cert, plumb sha256 certfp through to user, relay to services; advertise sasl=PLAIN,EXTERNAL on tls

echoircd.conf.example

@@ -24,6 +24,11 @@ bind_server = 0.0.0.0:7000
 # link = <name> <ip> <port> <password> [autoconnect]   (the password is a shared secret)
 # link = peer.example.net 203.0.113.5 7000 CHANGE_THIS_LINK_SECRET autoconnect
 
+# services: the linked server that handles SASL (client AUTHENTICATE is relayed to
+# it). Leave unset to disable SASL. SASL EXTERNAL additionally needs the client on
+# TLS with a client certificate (its fingerprint is sent to services).
+# sasl_server = services.example.net
+
 # IRC operators — oper = <name> <password>
 oper = admin CHANGE_THIS_PASSWORD
 

src/coremods/core_user.rs

@@ -67,13 +67,18 @@ impl Command for Cap {
         match params[0].to_ascii_uppercase().as_str() {
             "LS" => {
                 let cap302 = params.get(1).map(|v| v == "302").unwrap_or(false);
+                let secure = s.users.get(&uid).map(|u| u.secure).unwrap_or(false);
                 if let Some(u) = s.users.get_mut(&uid) {
                     u.cap = true; // hold registration until CAP END
                     u.cap_302 |= cap302;
                 }
                 s.send(
                     uid,
-                    format!(":{} CAP {who} LS :{}", s.name, Caps::ls_line(cap302)),
+                    format!(
+                        ":{} CAP {who} LS :{}",
+                        s.name,
+                        Caps::ls_line(cap302, secure)
+                    ),
                 );
             }
             "REQ" => {
@@ -177,6 +182,28 @@ impl Command for Authenticate {
                     }
                     s.send(uid, "AUTHENTICATE +".to_string());
                     CmdResult::Ok
+                } else if arg.eq_ignore_ascii_case("EXTERNAL") {
+                    // CertFP: only works on TLS with a client cert; the fingerprint
+                    // goes to services, which map it to an account.
+                    let certfp = s.users.get(&uid).and_then(|u| u.certfp.clone());
+                    match certfp {
+                        Some(fp) if have_services => {
+                            if let Some(u) = s.users.get_mut(&uid) {
+                                u.sasl_mech = Some("EXTERNAL".to_string());
+                            }
+                            s.sasl_relay(uid, &format!("S EXTERNAL {fp}"));
+                            s.send(uid, "AUTHENTICATE +".to_string());
+                            CmdResult::Ok
+                        }
+                        _ => {
+                            s.numeric(
+                                uid,
+                                ERR_SASLFAIL,
+                                ":SASL EXTERNAL requires a client certificate",
+                            );
+                            CmdResult::Fail
+                        }
+                    }
                 } else {
                     s.numeric(uid, RPL_SASLMECHS, "PLAIN :are available SASL mechanisms");
                     s.numeric(uid, ERR_SASLFAIL, ":Unsupported SASL mechanism");

src/ircd.rs

@@ -24,8 +24,9 @@ pub enum Event {
         out: OutSink,
         sock: Option<TcpStream>,
         secure: bool,
-        link: bool,     // a server-to-server connection, not a client
+        certfp: Option<String>, // TLS client-cert fingerprint (clients only)
-        outbound: bool, // (link) we dialed them
+        link: bool,             // a server-to-server connection, not a client
+        outbound: bool,         // (link) we dialed them
     },
     Line {
         uid: Uid,
@@ -70,13 +71,14 @@ impl Ircd {
                     out,
                     sock,
                     secure,
+                    certfp,
                     link,
                     outbound,
                 } => {
                     if link {
                         self.server.add_link(uid, addr, out, sock, outbound);
                     } else {
-                        self.server.add_conn(uid, addr, out, sock, secure);
+                        self.server.add_conn(uid, addr, out, sock, secure, certfp);
                     }
                 }
                 Event::Line { uid, line } => {

src/server.rs

@@ -182,6 +182,7 @@ impl Server {
         out: OutSink,
         sock: Option<TcpStream>,
         secure: bool,
+        certfp: Option<String>,
     ) {
         let uuid = self.next_uuid();
         self.uuid_local.insert(uuid.clone(), uid);
@@ -198,6 +199,7 @@ impl Server {
                 cloak: String::new(),
                 vhost: None,
                 secure,
+                certfp,
                 account: None,
                 signon: now(),
                 addr,
@@ -666,6 +668,7 @@ mod tests {
                 cloak: String::new(),
                 vhost: None,
                 secure: false,
+                certfp: None,
                 account: None,
                 signon: 0,
                 addr: "127.0.0.1:1".parse().unwrap(),

src/socketengine.rs

@@ -184,6 +184,7 @@ pub fn run_reactor(mut listener: MioListener, core: Sender<Event>, counter: Arc<
                                     out,
                                     sock: None,
                                     secure: false,
+                                    certfp: None,
                                     link: false,
                                     outbound: false,
                                 })
@@ -380,6 +381,7 @@ pub fn accept_loop(
                         out: OutSink::Thread(out_tx),
                         sock: Some(shutdown),
                         secure: false,
+                        certfp: None,
                         link,
                         outbound: false,
                     })
@@ -427,6 +429,7 @@ pub fn connect_link(addr: &str, core: Sender<Event>, counter: Arc<AtomicU64>) {
             out: OutSink::Thread(out_tx),
             sock: Some(shutdown),
             secure: false,
+            certfp: None,
             link: true,
             outbound: true,
         })
@@ -501,6 +504,7 @@ fn tls_conn(
             return; // handshake failed
         }
     };
+    let certfp = conn.peer_cert_fp();
     let (out_tx, out_rx) = mpsc::channel::<String>();
     if core
         .send(Event::Connect {
@@ -509,6 +513,7 @@ fn tls_conn(
             out: OutSink::Thread(out_tx),
             sock: Some(shutdown),
             secure: true,
+            certfp,
             link,
             outbound: false,
         })

src/tls.rs

@@ -12,7 +12,8 @@ use std::io::{self, Read, Write};
 use std::net::{Shutdown, TcpStream};
 use std::time::Duration;
 
-use openssl::ssl::{SslAcceptor, SslFiletype, SslMethod, SslStream};
+use openssl::hash::MessageDigest;
+use openssl::ssl::{SslAcceptor, SslFiletype, SslMethod, SslStream, SslVerifyMode};
 
 /// A live TLS connection: read/write plaintext, tune the read timeout (the
 /// socket engine polls with one to interleave reads and queued writes), and shut
@@ -23,6 +24,9 @@ pub trait TlsConn: Send {
     fn flush(&mut self) -> io::Result<()>;
     fn set_read_timeout(&self, dur: Option<Duration>) -> io::Result<()>;
     fn shutdown(&self);
+    /// SHA-256 fingerprint (lowercase hex) of the peer's certificate, if it sent
+    /// one. Drives SASL EXTERNAL / CertFP.
+    fn peer_cert_fp(&self) -> Option<String>;
 }
 
 /// A TLS backend: performs the server-side handshake on an accepted socket.
@@ -47,6 +51,10 @@ impl OpensslBackend {
         b.set_private_key_file(key, SslFiletype::PEM).map_err(err)?;
         b.set_certificate_chain_file(cert).map_err(err)?;
         b.check_private_key().map_err(err)?;
+        // Request (but don't require) a client cert so SASL EXTERNAL / CertFP can
+        // read its fingerprint. We never validate the chain — services match the
+        // fingerprint to an account — so the callback always accepts.
+        b.set_verify_callback(SslVerifyMode::PEER, |_valid, _ctx| true);
         Ok(OpensslBackend {
             acceptor: b.build(),
         })
@@ -78,4 +86,9 @@ impl TlsConn for OpensslConn {
     fn shutdown(&self) {
         let _ = self.0.get_ref().shutdown(Shutdown::Both);
     }
+    fn peer_cert_fp(&self) -> Option<String> {
+        let cert = self.0.ssl().peer_certificate()?;
+        let digest = cert.digest(MessageDigest::sha256()).ok()?;
+        Some(digest.iter().map(|b| format!("{b:02x}")).collect())
+    }
 }

src/users.rs

@@ -127,12 +127,17 @@ impl Caps {
     }
 
     /// The `CAP LS` token list; `sasl` carries its mechanisms for 302 clients.
-    pub fn ls_line(cap302: bool) -> String {
+    /// EXTERNAL is only offered on TLS connections (it needs a client cert).
+    pub fn ls_line(cap302: bool, secure: bool) -> String {
         SUPPORTED_CAPS
             .iter()
             .map(|c| {
                 if *c == "sasl" && cap302 {
-                    "sasl=PLAIN".to_string()
+                    if secure {
+                        "sasl=PLAIN,EXTERNAL".to_string()
+                    } else {
+                        "sasl=PLAIN".to_string()
+                    }
                 } else {
                     (*c).to_string()
                 }
@@ -209,6 +214,7 @@ pub struct User {
     pub cloak: String, // masked host shown under +x ("" until computed)
     pub vhost: Option<String>, // displayed-host override (CHGHOST/SETHOST vhost)
     pub secure: bool,  // connected over TLS (drives WHOIS 671 / sslinfo)
+    pub certfp: Option<String>, // TLS client-cert fingerprint (SASL EXTERNAL / CertFP)
     pub account: Option<String>, // logged-in account name (set by services)
     pub signon: u64,   // unix secs at registration (WHOIS 317)
     pub addr: SocketAddr,
@@ -465,8 +471,13 @@ mod tests {
         assert!(!c.set("bogus-cap", true)); // unknown cap rejected
         assert!(c.has("server-time") && c.has("multi-prefix") && !c.has("sasl"));
         assert_eq!(c.enabled(), "server-time multi-prefix"); // SUPPORTED order
-        assert!(Caps::ls_line(true).contains("sasl=PLAIN")); // 302 shows mechs
+        assert!(Caps::ls_line(true, false).contains("sasl=PLAIN")); // 302 shows mechs
-        assert!(Caps::ls_line(false).contains("sasl") && !Caps::ls_line(false).contains("sasl="));
+        assert!(!Caps::ls_line(true, false).contains("EXTERNAL")); // plaintext: no EXTERNAL
+        assert!(Caps::ls_line(true, true).contains("sasl=PLAIN,EXTERNAL")); // TLS offers it
+        assert!(
+            Caps::ls_line(false, false).contains("sasl")
+                && !Caps::ls_line(false, false).contains("sasl=")
+        );
         c.set("server-time", false);
         assert!(!c.has("server-time"));
     }