RMI does not support _ underscore in host names.

Resolving The Problem

Applications using rmi to connect to host machines do not work if the application engine host name contains an underscore. The config console application wasn¿t able to connect the router on such systems since the rmi connection was trying to connect to a machine with an underscore and rmi connections do not work well with underscore in host names.

RMI does not support _ in host names. DOD RFC 952/1035 specifies that a host name is alpha numeric characters and dashes. Underscore is not a supported character in host name aliases. The workaround is fairly simple, the server name can have additional alias names that do not include an underscore that can be added to the host table or DNS.

Applications that use domain names (e.g., mail, TELNET).
<domain> ::= <subdomain> | " "
<subdomain> ::= <label> | <subdomain> "." <label>
<label> ::= <letter> [ [ <ldh-str> ] <let-dig> ]
<ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>
<let-dig-hyp> ::= <let-dig> | "-"
<let-dig> ::= <letter> | <digit>
<letter> ::= any one of the 52 alphabetic characters A through Z in
upper case and a through z in lower case
<digit> ::= any one of the ten digits 0 through 9
Note that while upper and lower case letters are allowed in domain
names, no significance is attached to the case. That is, two names with the same spelling but different case are to be treated as if identical.

The labels must follow the rules for ARPANET host names. They must start with a letter, end with a letter or digit, and have as interior
characters only letters, digits, and hyphen. There are also some
restrictions on the length. Labels must be 63 characters or less.
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To reproduce: Use an application engine with an underscore in the DNS entry for its name. Open config console and try to connect to the router. The connection is not established and an error saying, cannot connect to the router is displayed.

Workaround: When starting a router to a server that has an underscore in the name, you need to specify the IP address rather than the name, or the router will not start.t.