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nano /etc/sysconfig/proxy
Then add these lines
PROXY_ENABLED="yes" HTTP_PROXY="http://proxyhost:port" FTP_PROXY="http://proxy-host.com:port" HTTPS_PROXY="http://proxy-host:port"
If you are using a username and password for your proxy make the above lines looks like this
HTTP_PROXY="http://user:password@proxyhost:port"
After adding these file save it
and then exit
and reboot your machine
Yast and zypper are working now behind the proxy.
Njoy :)