Port disclosure via socket-dependent routing exception cache updates

Information disclosure in Linux IPv4 UDP allows remote attackers to infer a connected socket's ephemeral port via ICMP errors. The net/ipv4/udp.c error handler creates FIB nexthop exceptions only when the quoted UDP packet matches an existing socket, exposing port guesses through cache changes. Off-path exploitation requires ICMP Fragmentation Needed or Redirect messages to reach the host and observable cache effects; the disclosed consequence is port inference, not direct access to application data.