PMTUD exploration, start small then grow

udp/udp_openbsd.go

@@ -0,0 +1,23 @@
+//go:build openbsd && !e2e_testing
+// +build openbsd,!e2e_testing
+
+package udp
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+// EnablePathMTUDiscovery sets the don't-fragment bit on outbound packets.
+// OpenBSD exposes IPV6_DONTFRAG via golang.org/x/sys/unix but the kernel does
+// not provide a socket-level knob for setting DF on v4 UDP. The only IP-layer
+// constant exposed is IP_DF, which is the wire header flag, not a sockopt.
+// quic-go skips OpenBSD for the same reason. So v4 sockets stay at nebula's
+// historical behavior (kernel may fragment); v6 gets DF.
+func (u *GenericConn) EnablePathMTUDiscovery() error {
+	if u.isV4Socket() {
+		return nil
+	}
+	return u.controlFD(func(fd uintptr) error {
+		return unix.SetsockoptInt(int(fd), unix.IPPROTO_IPV6, unix.IPV6_DONTFRAG, 1)
+	})
+}