Unable to connect to security.ubuntu.com

When I do `apt-get update` I got many error messages. One of them is "W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/precise-security/main/binary-i386/Packages Unable to connect to security.ubuntu.com:http: [IP: 91.189.92.151 80]". I think the problem is proxy settings, but the `set | grep proxy` gave me the correct http_proxy and no_proxy. Before doing that, I've also set org.gnome.system.proxy.socks host, org.gnome.system.proxy.socks port, org.gnome.system.proxy ignore-hosts, and org.gnome.system.proxy mode 'manual' with `gsettings set`.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: apt 0.8.16~exp12ubuntu10
ProcVersionSignature: Ubuntu 3.2.0-24.39-generic 3.2.16
Uname: Linux 3.2.0-24-generic i686
ApportVersion: 2.0.1-0ubuntu8
Architecture: i386
Date: Sat Jun 23 15:10:03 2012
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release i386 (20110427.1)
SourcePackage: apt
UpgradeStatus: Upgraded to precise on 2012-05-05 (48 days ago)