>>> I've been attempting to get a PPP connection between two boxes
>>> using a null modem cable (with the 4-1&6 1&6-4 etc).
>> Unless this is a mockup of a dialup connection, my experience
>> suggests that you would be better off using SLIP instead of PPP.
> Ultimately this is supposed to be a dial-in server.

Ah, then yes, you'll want to get PPP working.

> Besides [its] inability to handle native IPv6,

This is trivial to fix.  I fixed it back in 2002 and even wrote up the
fix; apparently nobody cared enough to do anything with it.
(ftp.rodents.montreal.qc.ca:/mouse/docs/ipv6-slip in case anyone cares.)

> SLIP's 296 octet MTU even precludes IPv6 tunneling (unless it
> fragments or something automatically).

Even my antique 1.4T systems accept "ifconfig sl0 mtu 1500"....

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