conn_waitpong: exempt loopback (per ipv4/ipv6) and per-class from the registration ping cookie

src/modules/conn_waitpong.rs

@@ -4,11 +4,16 @@
 //! ```text
 //! conn_waitpong = yes                       # require the pong before registering (default off)
 //! conn_waitpong_killonbadreply = yes        # disconnect on a wrong pong (default: keep waiting)
+//! conn_waitpong_exempt_localhost4 = yes     # skip the cookie for 127.0.0.0/8 (default off)
+//! conn_waitpong_exempt_localhost6 = yes     # skip the cookie for ::1 (default off)
+//! connectclass ... waitpongexempt=yes       # skip the cookie for a whole class
 //! ```
 //!
 //! The gate is the core `User.waitpong` field (checked in `try_register`); this
 //! module arms it at connect and clears it on the matching PONG.
 
+use std::net::IpAddr;
+
 use crate::server::Server;
 use crate::Uid;
 
@@ -19,10 +24,29 @@ fn cookie() -> String {
     b.iter().map(|x| format!("{x:02x}")).collect()
 }
 
+/// Whether this client is exempt from the cookie: a per-class opt-out, or a
+/// loopback address whose family the config trusts. Both default off, so the
+/// challenge still applies everywhere unless explicitly relaxed.
+fn exempt(s: &Server, uid: Uid) -> bool {
+    if crate::modules::connclass::waitpong_exempt(s, uid) {
+        return true;
+    }
+    let Some(ip) = s.users.get(&uid).map(|u| u.addr.ip()) else {
+        return false;
+    };
+    match ip {
+        IpAddr::V4(a) if a.is_loopback() => s.conf_bool("conn_waitpong_exempt_localhost4", false),
+        IpAddr::V6(a) if a.is_loopback() => s.conf_bool("conn_waitpong_exempt_localhost6", false),
+        // a loopback client on an IPv6 listener can arrive v4-mapped (::ffff:127.0.0.1)
+        IpAddr::V6(a) => a.to_ipv4_mapped().is_some_and(|m| m.is_loopback()) && s.conf_bool("conn_waitpong_exempt_localhost4", false),
+        _ => false,
+    }
+}
+
 /// At connect: if enabled, stash a cookie on the user and PING it. `try_register`
-/// will not complete while `User.waitpong` is set.
+/// will not complete while `User.waitpong` is set. Exempt sources skip it.
 pub fn arm(s: &mut Server, uid: Uid) {
-    if !s.conf_bool("conn_waitpong", false) {
+    if !s.conf_bool("conn_waitpong", false) || exempt(s, uid) {
         return;
     }
     let c = cookie();

src/modules/connclass.rs

@@ -52,6 +52,7 @@ pub struct ConnClass {
     pub requireident: bool,          // refuse if the ident lookup fails
     pub resolvehostnames: bool,      // resolve rDNS for this class (default yes)
     pub maxconnwarn: bool,           // snotice opers when a limit refuses a client
+    pub waitpongexempt: bool,        // skip the conn_waitpong cookie for this class
 }
 
 /// Split a `key=value` value on commas into non-empty pieces.
@@ -91,6 +92,7 @@ fn apply(c: &mut ConnClass, k: &str, v: &str) {
         "requireident" => c.requireident = v.eq_ignore_ascii_case("yes"),
         "resolvehostnames" => c.resolvehostnames = !v.eq_ignore_ascii_case("no"),
         "maxconnwarn" => c.maxconnwarn = v.eq_ignore_ascii_case("yes"),
+        "waitpongexempt" => c.waitpongexempt = v.eq_ignore_ascii_case("yes"),
         _ => {}
     }
 }
@@ -512,6 +514,10 @@ pub fn flood_over(s: &Server, uid: Uid) -> Option<(Option<usize>, Option<u64>, b
 pub fn resolve_hostnames(s: &Server, uid: Uid) -> bool {
     class_of(s, uid).map(|c| c.resolvehostnames).unwrap_or(true)
 }
+/// Whether this client's class opts out of the conn_waitpong cookie.
+pub fn waitpong_exempt(s: &Server, uid: Uid) -> bool {
+    class_of(s, uid).map(|c| c.waitpongexempt).unwrap_or(false)
+}
 /// `(useident, requireident)` for this client's class.
 pub fn ident_policy(s: &Server, uid: Uid) -> (bool, bool) {
     class_of(s, uid)