The IETF's IPSEC working group is specifying a security protocol for use at the IP layer called IPSP that should provide the encrypted link portion of what you want. A implementation of an earlier experimental standard called swIPe is widely available. The idea behind these things is to encrypt all your IP packets so the higher layers can assume security to some extent... .pm Stephen D. Williams says:
Has anyone produced software (preferably Unix based) that provides a public key authenticated, DES/3DES encrypted datastream over WAN firewalls/bastions?
A typical use would be to connect transaction feeds between two data centers in a secure, automated way over Internet.
I can't see anything conceptually difficult with it, and it seems the PGP library could be used to create simple Unix daemons.
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