Fix Network: Listeners bound to the unspecified IPv6 address ([::]) fall back to IPv4 when socket creation fails with EPROTONOSUPPORT

CHANGELOG.md

@@ -13,6 +13,7 @@ If you are upgrading from v0.16.x, replace the binary (or run `docker pull`). If
 ## Fixed
 - LDAP: Active Directory servers that answer an unauthenticated bind (a non-empty DN with a zero-length password) with success no longer authenticate accounts without a password.
 - IMAP: Every command in a pipelined `STATUS` or `FETCH` batch receives its tagged completion, instead of the first failing command dropping the responses for all commands queued behind it.
+- Network: Listeners bound to the unspecified IPv6 address (`[::]`) fall back to IPv4 when socket creation fails with `EPROTONOSUPPORT`.
 
 ## [0.16.20] - 2026-08-30
 

crates/common/src/config/server/listener.rs

@@ -121,7 +121,9 @@ impl Listeners {
             } {
                 Ok(socket) => socket,
                 Err(err)
-                    if is_eafnosupport(&err) && addr.is_ipv6() && addr.ip().is_unspecified() =>
+                    if is_ipv6_unsupported(&err)
+                        && addr.is_ipv6()
+                        && addr.ip().is_unspecified() =>
                 {
                     let v4_addr =
                         StdSocketAddr::new(IpAddr::V4(Ipv4Addr::UNSPECIFIED), addr.port());
@@ -327,15 +329,15 @@ impl Listeners {
     }
 }
 
-fn is_eafnosupport(err: &std::io::Error) -> bool {
+fn is_ipv6_unsupported(err: &std::io::Error) -> bool {
     let code = err.raw_os_error();
     #[cfg(unix)]
     {
-        code == Some(libc::EAFNOSUPPORT)
+        matches!(code, Some(libc::EAFNOSUPPORT) | Some(libc::EPROTONOSUPPORT))
     }
     #[cfg(windows)]
     {
-        code == Some(10047)
+        matches!(code, Some(10047) | Some(10043))
     }
     #[cfg(not(any(unix, windows)))]
     {