[Hadoop Wiki] Update of "UnsetHostnameOrPort" by SteveLoughran

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The "UnsetHostnameOrPort" page has been changed by SteveLoughran:
https://wiki.apache.org/hadoop/UnsetHostnameOrPort

Comment:
Create wiki entry on the all zeros address, for matching JIRA

New page:
= Unset Hostname Or Port =

It is an error for a Hadoop client to try to connect to a service (including a web server/web service) with a network address of all zeros "0.0.0.0", or a network port of "0".

Why not? Because it's meaningless on a client.

The address "0.0.0.0" means, on a server, "start your server on all network interfaces you have". On a client, ''it tells you nothing about where the host is''. The client cannot talk to a service at 0.0.0.0 because there is no information as to where the service is running.

Similarly, a port of "0" tells a server application to "find a free port". The server can find a free port, but the client cannot know what it is.

Usually problems with 0.0.0.0 addresses and 0 ports surface when a client application has been given the -site configuration of a service, the one the service uses to start up, which has a 0.0.0.0 address. You cannot use the same configuration file for the client and the server in this situation. The client needs to know a real hostname or IP address of the server which is hosting the service. 

Fix your client configuration and try again.


This is not a Hadoop problem, it is an application configuration issue. As it is your cluster, [[YourNetworkYourProblem|only you can find out and track down the problem.]]. Please do not file bug reports related to your problem, as they will be closed as [[http://wiki.apache.org/hadoop/InvalidJiraIssues|Invalid]]