connect() on a guest datagram socket checked the address family before
doing anything, so an AF_UNSPEC (null) address failed with EAFNOSUPPORT
and left the socket connected: getpeername() still reported the old
peer, send() without an address still went there, and datagrams from
any other sender were dropped by the host.
FreeBSD 11.4 disconnects a connected datagram socket first and only then
rejects the address. sys/kern/uipc_socket.c, soconnectat():
* If protocol is connection-based, can only connect once.
* Otherwise, if connected, try to disconnect first. This allows
* user to disconnect by connecting to, e.g., a null address.
*/
if (so->so_state & (SS_ISCONNECTED|SS_ISCONNECTING) &&
((so->so_proto->pr_flags & PR_CONNREQUIRED) ||
(error = sodisconnect(so)))) {
udp_disconnect() (sys/netinet/udp_usrreq.c) clears the peer and resets
only the local address, keeping the local port:
in_pcbdisconnect(inp);
inp->inp_laddr.s_addr = INADDR_ANY;
after which in_pcbconnect_setup() (sys/netinet/in_pcb.c) returns
EAFNOSUPPORT for the full-size address, as main already does. connect(2)
documents it: "Datagram sockets may dissolve the association by
connecting to an invalid address, such as a null address."
A connected internet datagram socket given an address whose family byte
is 0 is now disconnected on the host first (AF_UNSPEC on Linux, a zeroed
address on Windows) and the call then fails as before. Linux also drops
an ephemeral local port on that disconnect, so the port is bound again
on the wildcard address, which is what FreeBSD leaves. A host
failure during the disconnect is returned as the call's error. Stream
sockets keep the previous path.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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