A found tunnel carries its MTU to the mesh

The host parses MTU from the found [Interface] and reports it, so the
mesh's interface can come up with the same MTU when it takes the tunnel
over. A path tuned to 1380 regresses to the 1420 default otherwise —
invisible to ping, fatal to TLS handshakes and transfers over that path
(novox/hq: the mesh had no MTU concept). Zero when the config named
none, and the mesh writes no MTU line then.

cmd/mesh-host/main.go

@@ -910,7 +910,7 @@ func rekeyOnto(mine identity.Identity, found tunnel.Found) (identity.Identity, l
 // carried is a found tunnel as it is presented to the mesh: everything but its private key.
 func carried(t tunnel.Found) *link.Tunnel {
 	out := &link.Tunnel{Interface: t.Interface, Unit: t.Unit, Config: t.Config, Port: t.Port,
-		Address: t.Address, Range: t.Range, PublicKey: t.PublicKey}
+		Address: t.Address, Range: t.Range, MTU: t.MTU, PublicKey: t.PublicKey}
 	for _, p := range t.Peers {
 		out.Peers = append(out.Peers, link.TunnelPeer{PublicKey: p.PublicKey, Address: p.Address})
 	}