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We set SO_REUSEADDR on sockets used for host port mappings by docker-proxy - which means it's possible to bind the same port on a specific address as well as 0.0.0.0/::. For TCP sockets, an error is raised when listen() is called on both sockets - and the port allocator will be called again to avoid the clash (if the port was allocated from a range, otherwise the container will just fail to start). But, for UDP sockets, there's no listen() - so take more care to avoid the clash in the portallocator. The port allocator keeps a set of allocated ports for each of the host IP addresses it's seen, including 0.0.0.0/::. So, if a mapping to 0.0.0.0/:: is requested, find a port that's free in the range for each of the known IP addresses (but still only mark it as allocated against 0.0.0.0/::). And, if a port is requested for specific host addresses, make sure it's also free in the corresponding 0.0.0.0/:: set (but only mark it as allocated against the specific addresses - because the same port can be allocated against a different specific address). Signed-off-by: Rob Murray <rob.murray@docker.com>