wiki:KnownSVNIssues

Known SVN issues

svn doesn't honor $http_proxy

If you are behind a proxy, and

svn co http://svn.cc.gatech.edu/fastlab/mlpack/trunk/

gives

svn: OPTIONS of 'http://svn.cc.gatech.edu/fastlab/mlpack/trunk': could not connect to server (http://svn.cc.gatech.edu)

then you may need to update ~/.subversion/servers:

[global]
# http-proxy-exceptions = *.exception.com, www.internal-site.org
http-proxy-host = <proxy_url>                                      
http-proxy-port = <proxy_port>                                      
# http-proxy-username = defaultusername                                
# http-proxy-password = defaultpassword

Fill in that section with your proxy information, and this should fix your problem.