dnsbl on connect (m_dnsbl-style, ipv4+ipv6, own module), cache + parallelize resolver, group connect notices before cap

echoircd.conf.example

@@ -39,6 +39,17 @@ resolve_hosts = on
 # and reports "Found your hostname". Only matters when resolve_hosts = on.
 use_resolved_host = on
 
+# DNS blocklist (DNSBL) checks on connect, like InspIRCd's m_dnsbl. Repeat `dnsbl`
+# for multiple zones. On a listing, `dnsbl_action` decides what happens:
+#   mark  = just show the "*** ... LISTED" notice, let them in (default, safe)
+#   kill  = disconnect them (no persistent ban)
+#   kline / gline / zline = add a 1-day ban and disconnect
+# (leave commented to disable DNSBL entirely)
+# dnsbl = dnsbl.dronebl.org
+# dnsbl = rbl.efnetrbl.org
+# dnsbl_action = mark
+# dnsbl_reason = Your host is listed in a DNS blocklist
+
 # antimixedutf8 — block spam that mixes look-alike scripts within words.
 # action = block | kill | gline | kline | zline ; target = both | channel | private
 antimixedutf8 = off

src/config.rs

@@ -71,6 +71,9 @@ pub struct Config {
     pub amu: AntiMixedCfg,             // antimixedutf8 module config
     pub resolve_hosts: bool,           // reverse-DNS clients on connect (default on)
     pub use_resolved_host: bool,       // put the resolved hostname in the hostmask (default on)
+    pub dnsbl_zones: Vec<String>,      // DNS blocklist zones to check on connect
+    pub dnsbl_action: String,          // mark | kline | gline | zline (on a hit)
+    pub dnsbl_reason: String,          // ban reason for a DNSBL hit
 }
 
 impl Default for Config {
@@ -94,6 +97,9 @@ impl Default for Config {
             amu: AntiMixedCfg::default(),
             resolve_hosts: true,
             use_resolved_host: true,
+            dnsbl_zones: Vec::new(),
+            dnsbl_action: "mark".to_string(),
+            dnsbl_reason: "Your host is listed in a DNS blocklist".to_string(),
         }
     }
 }
@@ -219,6 +225,13 @@ impl Config {
                         "off" | "false" | "no" | "0"
                     )
                 }
+                "dnsbl" | "dnsbl_zone" => {
+                    if !v.is_empty() {
+                        c.dnsbl_zones.push(v.to_string());
+                    }
+                }
+                "dnsbl_action" => c.dnsbl_action = v.to_ascii_lowercase(),
+                "dnsbl_reason" => c.dnsbl_reason = v.to_string(),
                 _ => {}
             }
         }

src/coremods/core_rehash.rs

@@ -63,6 +63,9 @@ impl Command for Rehash {
                 s.amu = fresh.amu;
                 s.resolve_hosts = fresh.resolve_hosts;
                 s.use_resolved_host = fresh.use_resolved_host;
+                s.dnsbl_zones = fresh.dnsbl_zones;
+                s.dnsbl_action = fresh.dnsbl_action;
+                s.dnsbl_reason = fresh.dnsbl_reason;
                 s.announce("Server configuration reloaded.");
                 s.numeric(uid, RPL_REHASHING, &format!("{path} :Rehashing"));
             }

src/ircd.rs

@@ -34,10 +34,12 @@ pub enum Event {
     Disconnect {
         uid: Uid,
     },
-    /// A client's reverse-DNS lookup finished (`None` = no confirmed hostname).
+    /// A client's connect-time DNS work finished: the reverse-DNS hostname
+    /// (`None` = none confirmed) and the DNSBL outcome.
     ResolvedHost {
         uid: Uid,
         host: Option<String>,
+        dnsbl: crate::modules::dnsbl::Outcome,
     },
     /// Background timer tick — drives ping/idle timeouts.
     Tick,
@@ -93,8 +95,16 @@ impl Ircd {
                         self.quit_user(uid, "Connection closed");
                     }
                 }
-                Event::ResolvedHost { uid, host } => {
-                    self.server.on_resolved(uid, host);
+                Event::ResolvedHost { uid, host, dnsbl } => {
+                    self.server.on_resolved(uid, host, dnsbl);
+                    // now that the notice block has printed, replay the handshake
+                    // lines we held while resolving
+                    for line in self.server.take_deferred(uid) {
+                        if !self.server.users.contains_key(&uid) {
+                            break; // a replayed QUIT/ban already dropped them
+                        }
+                        self.on_line(uid, &line);
+                    }
                     self.try_register(uid); // DNS may have been the last thing we waited on
                 }
                 Event::Tick => self.on_tick(),
@@ -122,6 +132,12 @@ impl Ircd {
             .map(|u| u.registered)
             .unwrap_or(false);
 
+        // Hold the handshake while the connect-time DNS/DNSBL lookups run, so the
+        // "*** ..." notices print as one block; replayed in Event::ResolvedHost.
+        if !registered && self.server.defer_if_resolving(uid, line) {
+            return;
+        }
+
         // module pre-command gate
         for m in &mut self.modules {
             if m.on_pre_command(&mut self.server, uid, cmd, &msg.params) == ModResult::Deny {

src/modules/dnsbl.rs

@@ -0,0 +1,110 @@
+//! DNSBL — DNS blocklist checks on connect, InspIRCd `m_dnsbl` style. On connect
+//! the resolver thread reverses the client's IP under each configured blocklist
+//! zone and A-looks it up (see [`crate::resolver`]); a listing triggers the
+//! configured action. Works for IPv4 **and** IPv6 (v4 reversed octets or v6
+//! reversed nibbles under the zone) — a v4-only blocklist simply NXDOMAINs a v6
+//! query, which reads as "not listed".
+//!
+//! Actions (`dnsbl_action`): `mark` just shows the notice and lets them in
+//! (default, safe), `kill` disconnects, `kline`/`gline`/`zline` add a 1-day ban
+//! and disconnect. This isn't a hook `Module` — it's driven from the connection
+//! lifecycle (`Server::add_conn` → `on_resolved`) — but it lives here as its own
+//! self-contained unit.
+
+use std::net::{IpAddr, Ipv4Addr};
+use std::time::Duration;
+
+use crate::resolver;
+use crate::server::Server;
+use crate::xline::XKind;
+use crate::Uid;
+
+/// Ban length applied by the `*line` actions on a hit.
+const DNSBL_BAN: u64 = 86_400; // 1 day
+
+/// Outcome of a DNSBL check for one connecting client.
+pub enum Outcome {
+    /// No blocklists configured — the check didn't run.
+    Skipped,
+    /// Checked against every zone; the address is not listed.
+    Clean,
+    /// Listed: `zone` returned `reply` (`127.0.0.x`, last octet = reason code).
+    Hit { zone: String, reply: Ipv4Addr },
+}
+
+/// Check `ip` against every blocklist `zone`; the first listing wins. Runs off the
+/// core thread (called from the resolver worker), so it may block on DNS.
+pub fn check(ip: IpAddr, zones: &[String], timeout: Duration) -> Outcome {
+    if zones.is_empty() {
+        return Outcome::Skipped;
+    }
+    for zone in zones {
+        let z = zone.trim().trim_end_matches('.');
+        let qname = format!("{}.{z}", resolver::reverse_labels(ip));
+        if let Some(reply) = resolver::a_lookup(&qname, timeout) {
+            return Outcome::Hit {
+                zone: zone.clone(),
+                reply,
+            };
+        }
+    }
+    Outcome::Clean
+}
+
+/// Emit the DNSBL notices for `outcome` and, on a hit, take the configured action.
+/// Called from `Server::on_resolved` on the core thread.
+pub fn report(s: &mut Server, uid: Uid, outcome: Outcome) {
+    match outcome {
+        Outcome::Skipped => {}
+        Outcome::Clean => {
+            s.notice_star(uid, "Checking for DNSBL");
+            s.notice_star(uid, "Checking for DNSBL done, no hit.");
+        }
+        Outcome::Hit { zone, reply } => {
+            s.notice_star(uid, "Checking for DNSBL");
+            s.notice_star(
+                uid,
+                &format!("Checking for DNSBL done — LISTED on {zone} ({reply})."),
+            );
+            act(s, uid, &zone, reply);
+        }
+    }
+}
+
+/// Act on a hit per `dnsbl_action`: `mark` just informs; the `*line` actions add a
+/// temporary ban and close; `kill` closes without a persistent ban.
+fn act(s: &mut Server, uid: Uid, zone: &str, reply: Ipv4Addr) {
+    let (mask, ip) = match s.users.get(&uid) {
+        Some(u) => (u.prefix(), u.addr.ip()),
+        None => return,
+    };
+    let action = s.dnsbl_action.clone();
+    s.snotice(&format!(
+        "DNSBL: {mask} is listed on {zone} ({reply}); action={action}"
+    ));
+    let reason = format!("{} (listed on {zone})", s.dnsbl_reason);
+    let ipstr = ip.to_string();
+    match action.as_str() {
+        "kline" => s.add_xline(
+            XKind::Kline,
+            &format!("*@{ipstr}"),
+            DNSBL_BAN,
+            "dnsbl",
+            &reason,
+        ),
+        "gline" => s.add_xline(
+            XKind::Gline,
+            &format!("*@{ipstr}"),
+            DNSBL_BAN,
+            "dnsbl",
+            &reason,
+        ),
+        "zline" => s.add_xline(XKind::Zline, &ipstr, DNSBL_BAN, "dnsbl", &reason),
+        "kill" | "reject" => {}
+        _ => return, // "mark" or unknown: notify only, don't disconnect
+    }
+    // pre-registration users aren't caught by add_xline's enforce sweep, so close
+    // this connection explicitly (the ERROR flushes before the socket).
+    s.send(uid, format!("ERROR :Closing link: ({reason})"));
+    s.remove_user(uid, &reason);
+}

src/modules/mod.rs

@@ -1,8 +1,11 @@
 //! Optional, pluggable modules — echoIRCd's answer to InspIRCd's `src/modules/`.
-//! Each hooks lifecycle events via the [`crate::module::Module`] trait.
+//! Most hook lifecycle events via the [`crate::module::Module`] trait; [`dnsbl`]
+//! is the exception — it's driven straight from the connection lifecycle rather
+//! than the hook bus, but lives here as its own self-contained unit.
 
 pub mod antimixedutf8;
 pub mod cloak;
+pub mod dnsbl;
 pub mod flood;
 pub mod snoop;
 

src/resolver.rs

@@ -1,14 +1,21 @@
-//! Reverse-DNS host resolution — echoIRCd's answer to InspIRCd's async resolver,
-//! done from scratch with std UDP (no DNS crate, no `unsafe`). Given a client IP
-//! it looks up the PTR record and **forward-confirms** it (the name must resolve
-//! back to the same IP, so a client can't fake a hostname — same anti-spoofing
-//! InspIRCd does). Best-effort: any failure returns `None` and the caller keeps
-//! the IP. It runs off the core thread, so it never blocks the daemon, and it's
-//! bounded in time (the UDP read timeout) and in concurrency (`try_acquire`).
+//! DNS lookups — echoIRCd's answer to InspIRCd's async resolver + `m_dnsbl`, done
+//! from scratch with std UDP (no DNS crate, no `unsafe`). Two things:
+//!
+//!   * **reverse-DNS**: PTR-resolve a client IP and **forward-confirm** it (the name
+//!     must resolve back to the same IP, so a client can't fake a hostname — the
+//!     anti-spoofing InspIRCd does);
+//!   * **DNSBL**: reverse the client's v4 octets under a blocklist zone and A-lookup
+//!     it (`m_dnsbl` style), reporting the listing reply.
+//!
+//! Best-effort: any failure returns "not found / clean" and the caller keeps the
+//! IP. Runs off the core thread (never blocks the daemon), bounded in time (the UDP
+//! read timeout) and in concurrency (`try_acquire`).
 
-use std::net::{IpAddr, ToSocketAddrs, UdpSocket};
+use std::collections::HashMap;
+use std::net::{IpAddr, Ipv4Addr, ToSocketAddrs, UdpSocket};
 use std::sync::atomic::{AtomicUsize, Ordering};
-use std::time::Duration;
+use std::sync::{Mutex, OnceLock};
+use std::time::{Duration, Instant};
 
 /// How long to wait for the DNS server before giving up.
 pub const DNS_TIMEOUT: Duration = Duration::from_millis(2500);
@@ -17,6 +24,40 @@ pub const DNS_TIMEOUT: Duration = Duration::from_millis(2500);
 const MAX_ACTIVE: usize = 512;
 static ACTIVE: AtomicUsize = AtomicUsize::new(0);
 
+// --- caches -------------------------------------------------------------------
+// Memoize lookups so reconnects and clients sharing an IP (NAT/CGNAT, bouncers)
+// skip the network entirely. These Mutexes are internal to the resolver worker
+// threads — the single-threaded core never touches them, so the "no locks in the
+// core" rule still holds. Entries store their expiry `Instant`; a bounded map
+// (purge-expired, then clear on a pathological unique-IP flood) caps memory.
+const CACHE_CAP: usize = 65_536;
+const NS_TTL: Duration = Duration::from_secs(30); // re-read resolv.conf at most this often
+const RDNS_TTL_HIT: Duration = Duration::from_secs(600); // resolved hostname
+const RDNS_TTL_MISS: Duration = Duration::from_secs(60); // no/unconfirmed PTR
+const A_TTL_HIT: Duration = Duration::from_secs(300); // A record present (e.g. DNSBL listing)
+const A_TTL_MISS: Duration = Duration::from_secs(120); // NXDOMAIN / no A (e.g. not listed)
+
+/// A lazily-initialised, expiry-tagged lookup cache keyed by `K` holding `V`.
+type Cache<K, V> = OnceLock<Mutex<HashMap<K, (V, Instant)>>>;
+
+static NS_CACHE: Mutex<Option<(String, Instant)>> = Mutex::new(None);
+static RDNS_CACHE: Cache<IpAddr, Option<String>> = OnceLock::new();
+static A_CACHE: Cache<String, Option<Ipv4Addr>> = OnceLock::new();
+
+/// Keep a cache map bounded: once it hits the cap, drop expired entries, and if
+/// it's *still* full (a flood of distinct fresh IPs), clear it — degrading to
+/// no-cache rather than growing without bound.
+fn evict_if_full<K: Eq + std::hash::Hash, V>(map: &mut HashMap<K, (V, Instant)>) {
+    if map.len() >= CACHE_CAP {
+        let now = Instant::now();
+        map.retain(|_, (_, exp)| *exp > now);
+        if map.len() >= CACHE_CAP {
+            map.clear();
+        }
+    }
+}
+
+const QTYPE_A: u16 = 1;
 const QTYPE_PTR: u16 = 12;
 const QCLASS_IN: u16 = 1;
 
@@ -37,8 +78,32 @@ pub fn release() {
 }
 
 /// Reverse-resolve `ip` and forward-confirm. `Some(host)` only if a PTR exists
-/// and that host resolves back to `ip`.
+/// and that host resolves back to `ip`. Cached by IP so reconnects and clients
+/// behind the same NAT resolve instantly.
 pub fn reverse_confirmed(ip: IpAddr, timeout: Duration) -> Option<String> {
+    let cache = RDNS_CACHE.get_or_init(|| Mutex::new(HashMap::new()));
+    if let Ok(g) = cache.lock() {
+        if let Some((val, exp)) = g.get(&ip) {
+            if Instant::now() < *exp {
+                return val.clone();
+            }
+        }
+    }
+    let val = resolve_reverse(ip, timeout);
+    let ttl = if val.is_some() {
+        RDNS_TTL_HIT
+    } else {
+        RDNS_TTL_MISS
+    };
+    if let Ok(mut g) = cache.lock() {
+        evict_if_full(&mut g);
+        g.insert(ip, (val.clone(), Instant::now() + ttl));
+    }
+    val
+}
+
+/// The actual reverse lookup + forward-confirm (uncached; see `reverse_confirmed`).
+fn resolve_reverse(ip: IpAddr, timeout: Duration) -> Option<String> {
     let ns = nameserver();
     let ptr = ptr_lookup(&ns, &reverse_name(ip), timeout)?;
     // forward-confirm: the resolved name must map back to this IP
@@ -49,26 +114,54 @@ pub fn reverse_confirmed(ip: IpAddr, timeout: Duration) -> Option<String> {
     (ok && !ptr.is_empty()).then_some(ptr)
 }
 
-/// The `in-addr.arpa` / `ip6.arpa` reverse name for `ip`.
-fn reverse_name(ip: IpAddr) -> String {
+/// The reversed digit/nibble labels for `ip`, without any suffix — `1.2.3.4` →
+/// `4.3.2.1`, `2001:db8::1` → the 32 reversed hex nibbles. PTR appends
+/// `.in-addr.arpa` / `.ip6.arpa`; DNSBL appends the blocklist zone.
+pub fn reverse_labels(ip: IpAddr) -> String {
     match ip {
         IpAddr::V4(a) => {
             let o = a.octets();
-            format!("{}.{}.{}.{}.in-addr.arpa", o[3], o[2], o[1], o[0])
+            format!("{}.{}.{}.{}", o[3], o[2], o[1], o[0])
         }
         IpAddr::V6(a) => {
-            let mut s = String::with_capacity(72);
+            let mut s = String::with_capacity(64);
             for octet in a.octets().iter().rev() {
                 s.push_str(&format!("{:x}.{:x}.", octet & 0xf, octet >> 4));
             }
-            s.push_str("ip6.arpa");
+            s.pop(); // drop the trailing '.'
             s
         }
     }
 }
 
-/// First `nameserver` in /etc/resolv.conf, else a sensible fallback.
+/// The `in-addr.arpa` / `ip6.arpa` reverse name for `ip` (for PTR lookups).
+fn reverse_name(ip: IpAddr) -> String {
+    let suffix = if ip.is_ipv4() {
+        "in-addr.arpa"
+    } else {
+        "ip6.arpa"
+    };
+    format!("{}.{suffix}", reverse_labels(ip))
+}
+
+/// First `nameserver` in /etc/resolv.conf, else a sensible fallback — cached for
+/// `NS_TTL` so we don't stat+read the file on every single DNS query.
 fn nameserver() -> String {
+    if let Ok(mut g) = NS_CACHE.lock() {
+        if let Some((ns, at)) = g.as_ref() {
+            if at.elapsed() < NS_TTL {
+                return ns.clone();
+            }
+        }
+        let ns = read_nameserver();
+        *g = Some((ns.clone(), Instant::now()));
+        return ns;
+    }
+    read_nameserver()
+}
+
+/// Read the first `nameserver` from /etc/resolv.conf (uncached; see `nameserver`).
+fn read_nameserver() -> String {
     if let Ok(text) = std::fs::read_to_string("/etc/resolv.conf") {
         for line in text.lines() {
             let line = line.trim();
@@ -83,27 +176,101 @@ fn nameserver() -> String {
     "1.1.1.1:53".to_string()
 }
 
-/// Send a PTR query for `qname` to `ns` and return the first PTR answer name.
-fn ptr_lookup(ns: &str, qname: &str, timeout: Duration) -> Option<String> {
+/// Build a `qtype` query for `qname`, send it to `ns`, and return the raw reply
+/// (with the transaction id validated).
+fn send_query(ns: &str, qname: &str, qtype: u16, id: u16, timeout: Duration) -> Option<Vec<u8>> {
     let sock = UdpSocket::bind("0.0.0.0:0")
         .or_else(|_| UdpSocket::bind("[::]:0"))
         .ok()?;
     sock.set_read_timeout(Some(timeout)).ok()?;
-
-    let id: u16 = 0x4543; // fixed query id ("EC"); we match it on the reply
     let mut query = Vec::with_capacity(qname.len() + 18);
     query.extend_from_slice(&id.to_be_bytes());
     query.extend_from_slice(&[0x01, 0x00]); // flags: RD=1
     query.extend_from_slice(&[0, 1]); // QDCOUNT=1
     query.extend_from_slice(&[0, 0, 0, 0, 0, 0]); // AN/NS/AR = 0
     encode_name(&mut query, qname);
-    query.extend_from_slice(&QTYPE_PTR.to_be_bytes());
+    query.extend_from_slice(&qtype.to_be_bytes());
     query.extend_from_slice(&QCLASS_IN.to_be_bytes());
-
     sock.send_to(&query, ns).ok()?;
     let mut buf = [0u8; 1500];
     let n = sock.recv(&mut buf).ok()?;
-    parse_ptr_reply(&buf[..n], id)
+    if n < 12 || u16::from_be_bytes([buf[0], buf[1]]) != id {
+        return None;
+    }
+    Some(buf[..n].to_vec())
+}
+
+/// Send a PTR query for `qname` to `ns` and return the first PTR answer name.
+fn ptr_lookup(ns: &str, qname: &str, timeout: Duration) -> Option<String> {
+    let reply = send_query(ns, qname, QTYPE_PTR, 0x4543, timeout)?;
+    parse_ptr_reply(&reply, 0x4543)
+}
+
+/// Resolve `qname`'s first A record. Generic — the DNSBL module builds a
+/// `<reversed-ip>.<zone>` name and calls this to test a listing. Cached by qname
+/// so repeat DNSBL checks for the same IP+zone don't re-hit the network.
+pub fn a_lookup(qname: &str, timeout: Duration) -> Option<Ipv4Addr> {
+    let cache = A_CACHE.get_or_init(|| Mutex::new(HashMap::new()));
+    if let Ok(g) = cache.lock() {
+        if let Some((val, exp)) = g.get(qname) {
+            if Instant::now() < *exp {
+                return *val;
+            }
+        }
+    }
+    let val = send_query(&nameserver(), qname, QTYPE_A, 0x4544, timeout)
+        .and_then(|reply| parse_a_reply(&reply, 0x4544));
+    let ttl = if val.is_some() { A_TTL_HIT } else { A_TTL_MISS };
+    if let Ok(mut g) = cache.lock() {
+        evict_if_full(&mut g);
+        g.insert(qname.to_string(), (val, Instant::now() + ttl));
+    }
+    val
+}
+
+/// Parse a DNS reply for the first A record (4-byte address). Bounds-checked.
+fn parse_a_reply(msg: &[u8], want_id: u16) -> Option<Ipv4Addr> {
+    if msg.len() < 12 || u16::from_be_bytes([msg[0], msg[1]]) != want_id {
+        return None;
+    }
+    if msg[3] & 0x0f != 0 {
+        return None; // rcode != NOERROR (e.g. NXDOMAIN = not listed)
+    }
+    let qd = u16::from_be_bytes([msg[4], msg[5]]);
+    let an = u16::from_be_bytes([msg[6], msg[7]]);
+    if an == 0 {
+        return None;
+    }
+    let mut pos = 12;
+    for _ in 0..qd {
+        pos = skip_name(msg, pos)?;
+        pos = pos.checked_add(4)?;
+        if pos > msg.len() {
+            return None;
+        }
+    }
+    for _ in 0..an {
+        pos = skip_name(msg, pos)?;
+        if pos + 10 > msg.len() {
+            return None;
+        }
+        let rtype = u16::from_be_bytes([msg[pos], msg[pos + 1]]);
+        let rdlen = u16::from_be_bytes([msg[pos + 8], msg[pos + 9]]) as usize;
+        let rdata = pos + 10;
+        if rdata + rdlen > msg.len() {
+            return None;
+        }
+        if rtype == QTYPE_A && rdlen == 4 {
+            return Some(Ipv4Addr::new(
+                msg[rdata],
+                msg[rdata + 1],
+                msg[rdata + 2],
+                msg[rdata + 3],
+            ));
+        }
+        pos = rdata + rdlen;
+    }
+    None
 }
 
 /// Encode a dotted name into wire format (length-prefixed labels + root 0).

src/server.rs

@@ -17,6 +17,7 @@ use crate::extensible::Extensible;
 use crate::ircd::Event;
 use crate::link::{Link, RemoteServer, RemoteUser};
 use crate::module::Hook;
+use crate::modules::dnsbl;
 use crate::resolver;
 use crate::socketengine::OutSink;
 use crate::users::{Caps, User, UserFlags};
@@ -99,6 +100,9 @@ pub struct Server {
     pub amu: crate::config::AntiMixedCfg,          // antimixedutf8 module config
     pub resolve_hosts: bool,                       // reverse-DNS clients on connect
     pub use_resolved_host: bool,                   // apply the resolved name to the hostmask
+    pub dnsbl_zones: Vec<String>,                  // DNS blocklist zones checked on connect
+    pub dnsbl_action: String,                      // mark | kline | gline | zline
+    pub dnsbl_reason: String,                      // ban reason on a DNSBL hit
     pub event_tx: Sender<Event>,                   // self-inject events (DNS results)
 }
 
@@ -135,6 +139,9 @@ impl Server {
             amu: cfg.amu,
             resolve_hosts: cfg.resolve_hosts,
             use_resolved_host: cfg.use_resolved_host,
+            dnsbl_zones: cfg.dnsbl_zones,
+            dnsbl_action: cfg.dnsbl_action,
+            dnsbl_reason: cfg.dnsbl_reason,
             event_tx,
         }
     }
@@ -194,6 +201,7 @@ impl Server {
                 addr,
                 registered: false,
                 dns_pending: false,
+                deferred: Vec::new(),
                 cap: false,
                 cap_302: false,
                 caps: Caps::default(),
@@ -218,19 +226,38 @@ impl Server {
         // is real (see `resolver`) — its result arrives later as an Event.
         self.notice_star(uid, "Checking Ident");
         self.notice_star(uid, "No Ident response");
-        self.notice_star(uid, "Looking up your hostname...");
-        if self.resolve_hosts && resolver::try_acquire() {
+        let do_rdns = self.resolve_hosts;
+        let zones = self.dnsbl_zones.clone(); // DNSBL runs if any zones are configured
+        if do_rdns {
+            self.notice_star(uid, "Looking up your hostname...");
+        }
+        if (do_rdns || !zones.is_empty()) && resolver::try_acquire() {
             if let Some(u) = self.users.get_mut(&uid) {
-                u.dns_pending = true; // hold registration until the lookup returns
+                u.dns_pending = true; // hold registration until the lookups return
             }
             let tx = self.event_tx.clone();
             thread::spawn(move || {
-                let host = resolver::reverse_confirmed(ip, resolver::DNS_TIMEOUT);
+                // rDNS and DNSBL are independent (DNSBL only needs the IP), so run
+                // them concurrently — the client waits on max(rdns, dnsbl), not the
+                // sum. Only spin up the extra thread when both are actually needed.
+                let (host, dnsbl) = if do_rdns && !zones.is_empty() {
+                    let job =
+                        thread::spawn(move || dnsbl::check(ip, &zones, resolver::DNS_TIMEOUT));
+                    let host = resolver::reverse_confirmed(ip, resolver::DNS_TIMEOUT);
+                    (host, job.join().unwrap_or(dnsbl::Outcome::Skipped))
+                } else if do_rdns {
+                    (
+                        resolver::reverse_confirmed(ip, resolver::DNS_TIMEOUT),
+                        dnsbl::Outcome::Skipped,
+                    )
+                } else {
+                    (None, dnsbl::check(ip, &zones, resolver::DNS_TIMEOUT))
+                };
                 resolver::release();
-                let _ = tx.send(Event::ResolvedHost { uid, host });
+                let _ = tx.send(Event::ResolvedHost { uid, host, dnsbl });
             });
-        } else {
-            // resolution off (or too many in flight): keep the IP as the host
+        } else if do_rdns {
+            // wanted rDNS but couldn't start (too many in flight): keep the IP
             self.notice_star(
                 uid,
                 "Couldn't look up your hostname; using your IP address instead",
@@ -239,20 +266,47 @@ impl Server {
     }
 
     /// A pre-registration `:server NOTICE * :*** <msg>` line.
-    fn notice_star(&self, uid: Uid, msg: &str) {
+    pub(crate) fn notice_star(&self, uid: Uid, msg: &str) {
         self.send(uid, format!(":{} NOTICE * :*** {msg}", self.name));
     }
 
+    /// While a client's connect-time DNS/DNSBL lookups are still running, hold its
+    /// handshake lines instead of processing them, so the "*** ..." notices print
+    /// as one contiguous block rather than interleaving with the CAP/NICK replies.
+    /// Returns true if `line` was buffered. Bounded — past the cap we let lines
+    /// through (degrading to interleaved output rather than dropping input).
+    pub fn defer_if_resolving(&mut self, uid: Uid, line: &str) -> bool {
+        const MAX_DEFERRED: usize = 32;
+        match self.users.get_mut(&uid) {
+            Some(u) if u.dns_pending && u.deferred.len() < MAX_DEFERRED => {
+                u.deferred.push(line.to_string());
+                true
+            }
+            _ => false,
+        }
+    }
+
+    /// Take and clear the handshake lines held while `uid`'s lookups ran, to replay
+    /// once the notice block has printed.
+    pub fn take_deferred(&mut self, uid: Uid) -> Vec<String> {
+        self.users
+            .get_mut(&uid)
+            .map(|u| std::mem::take(&mut u.deferred))
+            .unwrap_or_default()
+    }
+
     /// A client's reverse-DNS lookup finished. Set the resolved host (so WHOIS,
     /// bans and cloaking use the hostname, not the IP), tell the client, and clear
     /// the flag that was holding their registration.
-    pub fn on_resolved(&mut self, uid: Uid, host: Option<String>) {
+    pub fn on_resolved(&mut self, uid: Uid, host: Option<String>, outcome: dnsbl::Outcome) {
+        // hostname result (only announced if we actually attempted the lookup)
         match &host {
             Some(h) => self.notice_star(uid, &format!("Found your hostname ({h})")),
-            None => self.notice_star(
+            None if self.resolve_hosts => self.notice_star(
                 uid,
                 "Couldn't look up your hostname; using your IP address instead",
             ),
+            None => {}
         }
         let apply = self.use_resolved_host;
         if let Some(u) = self.users.get_mut(&uid) {
@@ -263,6 +317,12 @@ impl Server {
                     u.host = h;
                 }
             }
+        }
+        // DNSBL notices + action (InspIRCd m_dnsbl style) — see `modules::dnsbl`.
+        // May close the connection if the zone is listed and the action bans.
+        dnsbl::report(self, uid, outcome);
+        // release the registration hold (no-op if a DNSBL ban already removed them)
+        if let Some(u) = self.users.get_mut(&uid) {
             u.dns_pending = false;
         }
     }
@@ -561,6 +621,7 @@ mod tests {
                 addr: "127.0.0.1:1".parse().unwrap(),
                 registered: true,
                 dns_pending: false,
+                deferred: Vec::new(),
                 cap: false,
                 cap_302: false,
                 caps: Caps::default(),
@@ -592,13 +653,21 @@ mod tests {
     fn resolved_host_applied_only_when_configured() {
         let mut s = srv(); // use_resolved_host = true (default)
         let _a = add_user(&mut s, 1, "ann"); // host starts "localhost"
-        s.on_resolved(1, Some("host.example.net".to_string()));
+        s.on_resolved(
+            1,
+            Some("host.example.net".to_string()),
+            dnsbl::Outcome::Skipped,
+        );
         assert_eq!(s.users[&1].host, "host.example.net");
         assert!(!s.users[&1].dns_pending);
 
         s.use_resolved_host = false; // resolve + report, but keep the IP in the mask
         let _b = add_user(&mut s, 2, "bob");
-        s.on_resolved(2, Some("host.example.net".to_string()));
+        s.on_resolved(
+            2,
+            Some("host.example.net".to_string()),
+            dnsbl::Outcome::Skipped,
+        );
         assert_eq!(s.users[&2].host, "localhost");
         assert!(!s.users[&2].dns_pending); // registration still un-held either way
     }

src/users.rs

@@ -205,16 +205,17 @@ pub struct User {
     pub signon: u64,   // unix secs at registration (WHOIS 317)
     pub addr: SocketAddr,
     pub registered: bool,
-    pub dns_pending: bool, // holding registration for a reverse-DNS lookup
-    pub cap: bool,         // CAP negotiation in progress (holds registration)
-    pub cap_302: bool,     // client sent CAP LS 302 (cap-notify aware)
-    pub caps: Caps,        // enabled IRCv3 capabilities
+    pub dns_pending: bool,     // holding registration for a reverse-DNS lookup
+    pub deferred: Vec<String>, // handshake lines held while dns_pending (replayed after)
+    pub cap: bool,             // CAP negotiation in progress (holds registration)
+    pub cap_302: bool,         // client sent CAP LS 302 (cap-notify aware)
+    pub caps: Caps,            // enabled IRCv3 capabilities
     pub sasl_mech: Option<String>, // SASL mechanism chosen, mid-handshake
     pub channels: HashSet<String>, // lowercased channel keys
-    pub watch: Vec<String>, // WATCH list — lowercased nicks
-    pub monitor: Vec<String>, // MONITOR list — lowercased nicks
-    pub silence: Vec<String>, // SILENCE masks — nick!user@host globs
-    pub accept: Vec<String>, // ACCEPT list — lowercased nicks (callerid +g)
+    pub watch: Vec<String>,    // WATCH list — lowercased nicks
+    pub monitor: Vec<String>,  // MONITOR list — lowercased nicks
+    pub silence: Vec<String>,  // SILENCE masks — nick!user@host globs
+    pub accept: Vec<String>,   // ACCEPT list — lowercased nicks (callerid +g)
     pub quitting: Option<String>, // set by QUIT; drained by the core
     pub flags: UserFlags,
     pub last_active: u64, // unix secs of the last line we received