As noted while debugging <https://issues.guix.gnu.org/59784>, turning off networking while ‘guix substitute’ is downloading (be it with “ifconfig X down” or by physically unplugging the cable) leads it to be stuck in ‘read’ or ‘recvfrom’ for a long time.
Most likely this can be addressed by setting SO_RCVTIMEO and SO_SNDTIMEO to a reasonable value on the socket. Ludo’.
