[9fans] vpn without ipsec nor ssl

[geoff@collyer.net]

If your company's firewalls are awkward to get through because of
inadequate documentation, the complexity of ipsec, or windows-only VPN
clients, you can roll your own VPN (as long as the firewalls permit
arbitrary outgoing connections).  This may be obvious to some, but
perhaps not all.

Put up a Plan 9 CPU server inside the company and add these two
scripts to /rc/bin on your home file server:

; cat /bin/posttermnet 
#!/bin/rc
# posttermnet - post the terminal's /net
rfork e
svc=net.$user.office
if (test -e /srv/$svc) {
	echo $0: /srv/$svc already exists >[1=2]
	exit 'srv exists'
}
rfork s
exec srvfs $svc /mnt/term/net

; cat /bin/mntoffnet
#!/bin/rc
# mntoffnet - mount the office /net
rfork e
srv=/srv/net.$user.office
if (! test -e $srv) {
	if (test -e /env/cpu)
		import -a $cpu /srv
	if (! test -e $srv) {
		echo $0: no $srv here nor on $cpu >[1=2]
		exit 'no srv'
	}
}
exec mount -b $srv /net

At the office, connect to your local cpu server and run

	cpu -h your-home-cpu-server
	posttermnet

This creates an encrypted cpu connection to your home cpu server and
posts /mnt/term/net (the office cpu server's /net) as
/srv/net.$user.office.

Once home, you can then run

	mntoffnet

on a terminal or cpu server and get your office cpu server's /net on
top of your own /net.  You can then ssh, cpu or sam -r to machines at
the office.