After a while I tried to upgrade my Ubuntu server 11.04 with IspConfig installed. It doesn't work, but it have already worked. I pinged security.ubuntu.com and this is the result: PING security.ubuntu.com (126.96.36.199) 56(84) bytes of data. After ctrl+c I got: --- security.ubuntu.com ping statistics --- 63 packets transmitted, 0 received, 100% packet loss, time 62008ms I've tried to modify /etc/resolv.conf with add OpenDNS's nameservers. My server is behind router.