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My OS is 12.04 and I am running it on my laptop. Whenever I install or remove a software from the software centre I receive the error below. Can anyone show me how to fix it?

Location: [following]
--2012-11-26 16:02:34--
Connecting to (||:3128... connected.
Proxy request sent, awaiting response... 200 OK
Length: 5307 (5.2K) [text/html]
Saving to: `./jdk-7u3-linux-x64.tar.gz'

0K .....                                                 100% 1.87M=0.003s

2012-11-26 16:02:35 (1.87 MB/s) - `./jdk-7u3-linux-x64.tar.gz' saved [5307/5307]

Download done.
sha256sum mismatch jdk-7u3-linux-x64.tar.gz
Oracle JDK 7 is NOT installed.
dpkg: error processing oracle-java7-installer (--configure):
 subprocess installed post-installation script returned error exit status 1
Setting up icedtea-netx-common (1.2-2ubuntu1.3) ...
Setting up icedtea-netx (1.2-2ubuntu1.3) ...
update-alternatives: using /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/itweb-settings to provide /usr/bin/itweb-settings (itweb-settings) in auto mode.
Errors were encountered while processing:
Error in function: 
SystemError: E:Sub-process /usr/bin/dpkg returned an error code (1)
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Seems like you are sitting behind a proxy and didn't configure it right. You can see that a connection is build to and then you get some information on what is downloaded:

Length: 5307 (5.2K) [text/html]

A package file would mostly be larger than 5.2K and it's type is not text or html. This most likely means that your proxy produces some kind of error page that is downloaded instead of the package file, thus the checksum is wrong.

You have to get your proxy settings right. You can find them at System settings...->Network->Network proxy.

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