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Many shared hosting companies enable open_basedir by default. This is done for security purposes, since it limits the files that can be opened by PHP to a specific user.
An explanation of open_basedir can be found in the PHP manual:
When a script tries to open a file [...], the location of the file is checked. When the file is outside the specified directory-tree, PHP will refuse to open it.
This error message is received when trying to access Magento Connect. This is typically not something you can fix unless yourself, unless you’re on a VPS or dedicated server with root access.
For the majority of people out there, you’ll need to ask your host to disable open_basedir on your account.
By the way, the error message you’ll receive will have the following somewhere in it:
open_basedir restriction in effect
This restriction can be a good or bad thing depending on how you look at it.
We also use Suhosin to further protect clients, which also applies to those that do have the open_basedir restriction removed.
You might have seen something about Suhosin in the .htaccess file for Magento:
php_flag suhosin.session.cryptua off