First off let me say that I am very new to Linux (Ubuntu). I am trying to load a package (dsniff) using the synaptic package manager onto my ubuntu machine. I have added a repository to the list of depositories. Now listed as repositories are:

CD Ubuntu 5.04 Hoary
Ubuntu 5.04 Hoary (archive.ubuntu.com/ubuntu/)

My problem is when I try to reload the packages I get an error "407 Proxy Authentication Req'd". I have added the correct proxy info into the preference settings under synaptic manager. I have also set the proxy settings under System->Administration->Network Proxy and included my proxy authentication credentials.

I am am able to surf the web through the proxy, but the first time I go to an external website I am prompted for a user name and password, then it works. Synaptic never prompts me, so I get the error above.

Anyone have any ideas?
Thanks
ccm030

Im not quite sure about synaptic.
But you could use a terminal instead, and as root:
sudo -s (you get prompt to input you password)
apt-get install dsniff

I just did it and it worked well for me.
I am also using Kubuntu, but maybe this can help you
here is my list:

deb cdrom:[Kubuntu 5.04 _Hoary Hedgehog_ - Release i386 (20050407)]/ hoary main restricted


deb http://us.archive.ubuntu.com/ubuntu hoary main restricted
deb-src http://us.archive.ubuntu.com/ubuntu hoary main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://us.archive.ubuntu.com/ubuntu hoary-updates main restricted
deb-src http://us.archive.ubuntu.com/ubuntu hoary-updates main restricted

## Uncomment the following two lines to add software from the 'universe'
## repository.
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## universe WILL NOT receive any review or updates from the Ubuntu security
## team.
deb http://us.archive.ubuntu.com/ubuntu hoary universe
deb-src http://us.archive.ubuntu.com/ubuntu hoary universe

deb http://security.ubuntu.com/ubuntu hoary-security main restricted
deb-src http://security.ubuntu.com/ubuntu hoary-security main restricted

deb http://security.ubuntu.com/ubuntu hoary-security universe
deb-src http://security.ubuntu.com/ubuntu hoary-security universe

deb http://archive.ubuntu.com/ubuntu hoary multiverse
deb-src http://archive.ubuntu.com/ubuntu hoary multiverse

## Backports

deb http://ubuntu-backports.mirrormax.net/ hoary-backports main universe multiverse restricted
deb http://ubuntu-backports.mirrormax.net/ hoary-extras main universe multiverse restricted

## Update KDE to 3.4.2

deb http://kubuntu.org/hoary-kde342/ hoary-updates main
deb-src http://kubuntu.org/hoary-kde342/ hoary-updates main

Or maybe if you get admin privileges you could set direct access to Internet
Hope it helps