First: Whoa, that's pretty cool :) It's been a while since I saw someone do something clever with PPP.
Second: Really not clear on why. You already have IP based communication between the two hosts, it doesn't give you much in the way of privacy (probably not anything, really)...?
Now, the brass tacks:
The IP addresses specified in the arguments to pppd seem wrong for two reasons:
They appear to be the same as the IP addresses of the two machines on different (non-PPP) interfaces. I can't think of a situation where that is correct. In my opinion this is the cause of your problem. You're trying to ping an IP address that exists on the other end of two interfaces, and that itself is intended to have two interfaces with that IP address.
You're specifying the IP addresses on both the client and the server. That's technically okay, but since over-specifying here can cause a failure to negotiate, I'd remove the IP addresses from the client and instead set noipdefault there.
I would suggest picking an IP address pair for PPP negotiation that are NOT on a valid public network (you've done that), and that are NOT associated with a network that is already accessible to your two computers. E.g. 10.1.1.1:10.1.1.2
This looks like the following when done over the loopback interface:
Server:
$ sudo pppd 10.1.1.1:10.1.1.2 nodetach pty "nc -l 3333" Using interface ppp0 Connect: ppp0 <--> /dev/pts/4 Deflate (15) compression enabled local IP address 10.1.1.1 remote IP address 10.1.1.2 Client:
jnisbet2@decimate:~$ sudo pppd noipdefault nodetach pty "nc 127.0.0.1 3333" Using interface ppp1 Connect: ppp1 <--> /dev/pts/6 Deflate (15) compression enabled local IP address 10.1.1.2 remote IP address 10.1.1.1 The only practical effect of this process is to give you a different IP address for communicating with the remote host, and this is only really visible from inside the two endpoint hosts, so I'm not sure why you're interested in this project, but shrug. Oh, and there is a timeout for the server; if there is no connection within a minute or two, the server quits:
LCP: timeout sending Config-Requests Connection terminated. Modem hangup Child process nc -l 3333 (pid ####) terminated with signal 15