Doug McK asked:
I’ve been trying to debug an issue on our servers and Im confused by this response from netcat. Can anyone explain why Im getting these 2 contradictory messages when checking to see if a port is working? Is it failing to connect via TCP and then succeeding with some other method (*)?
ubuntu@1-2-3-4:/var/log$ nc -vz localhost 7777 nc: connect to localhost port 7777 (tcp) failed: Connection refused Connection to localhost 7777 port [tcp/*] succeeded!
nc -vz4 localhost 7777 Connection to localhost 7777 port [tcp/*] succeeded! nc -vz6 localhost 7777 nc: connect to localhost port 7777 (tcp) failed: Connection refused
This is happening because your daemon is only listening on IPv4.
IPv6 is the default protocol, so if a given hostname has both IPv4 and IPv6 addresses, the IPv6 address is always tried first.
In your case,
localhost has the IPv4 address
127.0.0.1 and the IPv6 address
::1. But your daemon is only listening on
nc tries to connect to
localhost it first connects to
::1, finds nothing is listening, and returns
Connection refused. It then tries to connect to
127.0.0.1 and finds your daemon.
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