pip does not find packages

[Olive]

I am using virtualenv and pip (from archlinux). What I have done:
virtualenv was installed by my distribution. I have made a virtual environment and activate it, it has installed pip, so far so good.

Now I am trying to install package in the virtualenvironnement:

pip install Impacket
Downloading/unpacking Impacket
  Could not find any downloads that satisfy the requirement Impacket
No distributions at all found for Impacket

but Impacket is found by 
pip search Impacket
Impacket                  - Network protocols Constructors and Dissectors

exactly the same happens with pcapy. With PyGTK, the pip command just hang when trying to download it. What is going on? Maybe a misconfigured server? Is there anything that I can do?

Olive