Comment 1 for bug 1559772

CentOS does not build the "ipddp" module / does not set "CONFIG_IPDDP" in the kernel config, so appears to not be affected by this issue. I would be happy if Ubuntu resolved this issue by likewise no longer shipping this module.

For anyone not familiar, some background: DDP was Apple's proprietary networking protocol, used by Apple and early Macintosh computers prior to the widespread adoption of TCP and Ethernet.

"IP-over-DDP" (what this particular module is used for) encapsulates IP packets inside DDP packets. It's useful if your network supports DDP traffic but does not support IP traffic -- for example, Apple's LocalTalk serial-port-based networking standard. So you can browse the Web on a Mac that predates Ethernet. (Apple stopped supporting LocalTalk with the initial release of Mac OS X, and stopped shipping computers with LocalTalk-compatible serial ports at about the same time.)

The kernel's "Documentation/networking/ipddp.txt" says the following regarding setting up the "ipddp" module:

"""
Decapsulation - You will need to download a software package called MacGate.
(...)
Encapsulation - You will need to download a software daemon called ipddpd.
"""

(If the module isn't configured with one of these two programs, it doesn't do anything useful.) MacGate was last shipped in Debian Potato; the sources from that release appear to want a 2.0 kernel, and do not compile on my machine. I can't find sources for ipddpd online.

The functionality provided by the "ipddp" module is now also mostly provided by the userspace program "macipgw":
https://github.com/zero2sixd/macipgw
This program builds and works correctly on a stock Ubuntu LTS 14.04 x86_64 machine with a modified Ubuntu 4.2 kernel that omits "CONFIG_IPDDP=m" from its kernel config. (Of course, it doesn't work with stock Ubuntu kernels because of this bug.)