On the firewalled/vpnd server;
1. find the package URL
apt-get –print-uris install dos2unix
2. wget over ssh from an internet bound server on the same network
ssh use@somehost ‘wget -O – http://mirrordirector.raspbian.org/raspbian/pool/main/d/dos2unix/dos2unix_6.0-1_armhf.deb’ >> dos2unix_6.0-1_armhf.deb
3. dpkg -i dos2unix_6.0-1_armhf.deb