Couldn't Connect to External Sites

My WordPress blog is unable to connect to external sites.
The below quote text can be seen when i am trying to install a new plugin directly from wordpress panel.

An unexpected error occurred. Something may be wrong with www.remarpro.com or this server’s configuration. If you continue to have problems, please try the support forums.

This one shows under the WordPress News of Dashboard

RSS Error: WP HTTP Error: Couldn’t resolve host ‘www.remarpro.com’

I tried reinstalling my server but that doesn’t work.And i even i tried that plugins thingy by disabling all,I was disappointed with too.
I am able to ping other sites from my SSH and I checked resolv.conf – it looks fine too.
Please help me to solve this problem.

Thank You