We have established a VPN in our office. How can we establish peer-to-peer wireless file transfers over that VPN?
The main advantage of peer-to-peer (ad hoc mode) wireless is avoiding the need for network infrastructure -- access points, switches, routers, gateways, firewalls. The main disadvantage is that you forego all of the functionality normally provided by that infrastructure -- in this case, your VPN gateway. There is no way to use your office VPN to secure file transfers conducted over peer-to-peer wireless connections.
However, depending on the VPN client, an end-to-end IPsec connection may work. Most remote access VPNs follow a client/server model: the VPN client connects to a VPN gateway at the edge of the network to be accessed. However, IPsec also supports gateway-to-gateway and host-to-host configurations. Tunnel mode IPsec creates site-to-site VPNs between gateways, while transport mode IPsec secures traffic between hosts. Check your VPN client's documentation to determine whether it supports transport mode. For example, if you use Windows XP/2000, see these instructions: part one, part two.
Alternatively, consider using a secure file transfer protocol, like SFTP. Links to several free SFTP clients can be found here. Or just enable WPA2-Personal security on the peer-to-peer wireless link to protect all messages sent over the air, including file transfer.
This was first published in November 2006