PMTUD exploration, start small then grow

udp/udp_freebsd.go

@@ -0,0 +1,25 @@
+//go:build freebsd && !e2e_testing
+// +build freebsd,!e2e_testing
+
+package udp
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+// EnablePathMTUDiscovery sets the don't-fragment bit on outbound packets.
+// FreeBSD exposes IP_DONTFRAG (v4) and IPV6_DONTFRAG (v6) in golang.org/x/sys/unix.
+// Unlike Linux, BSDs don't have an explicit "don't consume incoming ICMP
+// frag-needed" knob for unconnected UDP sockets; the kernel's PMTU cache will
+// be updated from ICMP, which is benign for our usage (the cache only affects
+// what EMSGSIZE gets surfaced for; the manager drives its own discovery via
+// authenticated probes).
+func (u *GenericConn) EnablePathMTUDiscovery() error {
+	v4 := u.isV4Socket()
+	return u.controlFD(func(fd uintptr) error {
+		if v4 {
+			return unix.SetsockoptInt(int(fd), unix.IPPROTO_IP, unix.IP_DONTFRAG, 1)
+		}
+		return unix.SetsockoptInt(int(fd), unix.IPPROTO_IPV6, unix.IPV6_DONTFRAG, 1)
+	})
+}