A problem with git push via proxy

when I use:

$ git push origin master

 

I got that:
ssh: Could not resolve hostname github.com: Name or service not known
fatal: The remote end hung up unexpectedly

 

I have tried sth like this:

kalvin@kalvin-laptop:~/rails_projects/first_app$ git config --global http.proxy http://xxx.xxx.xxx.xxx:808

 

nothing happened..

Can Someone help me out~