To listen to any IPv4 address: server_bind_addr = "0.0.0.0"
Or to listen to any address including IPv6 (if supported): server_bind_addr = "any"
A too low number here will mean the hub will not accept connections fast enough when users are reconnecting really fast. The hub should under normal circumstances be able to empty the listen backlog several times per second.
Windows users: file_acl = "c:\uhub\users.conf"