Question: no exception or message if graylog2 server unreachable

[adorechen]

I use logback-gelf send log to graylog2, all the things are good but I know nothing if the graylog2 server unreachable. can you give some suggestion?

and how to debug if I send a message, log out in my client console, but graylog2 server get nothing.
for example:
I change the configuration as follows: (want to display java naming conversion in graylog2)
<additionalField>applicationId:_applicationId</additionalField>

thanks a lot.

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me.moocar:logback-gelf:0.12
Graylog v2.4.3+2c41897