[HECnet] DECnet for Linux Bug Fixes

[John Forecast]

Recently I have found, and fixed, a number of bugs in the DECnet for Linux kernel module. Some of
these bugs may be relevant for members of this mailing list who are using DECnet for Linux,
especially on a Raspberry Pi.

Major bugs fixed:

- Ethernet Listen Timer was not implemented

	In addition, the check for an address change for the designated router (DR) was missing
	although there was a check if the DR’s priority had changed.

	Note: this code changes the format of /proc/net/decnet_dev to include the listen timer value.
	          nml/nml2 needs to be rebuilt to understand the changed format.

- System panic when using the loopback device (lo)

	The DECnet code was missing a destructor routine which is used to avoid data copying.

- The neighbour (adjacent node) code could be broken by kernel changes

	The code made use of a now deprecated feature (zero length array at the end of a structure)
	in order to access data in a surrounding structure. This happened to work “by chance”
	until kernel 5.4.42 on 32-bit processors.

- Interrupt message flow control was broken

	The flow control was a mixture of using the SEND/DONTSEND status of the data
	subchannel and a message count. This seems to work between Linux systems but is broken
	when communicating with other systems - during the life of a logical link, the remote system
	could only send a single interrupt message while the Linux system could pretty much send
	as many interrupts as it wanted possibly overrunning the remote systems buffers.

- Optional data on received connect confirm message was corrupted

	The code was getting the optional data from the wrong offset in the message.

- Next hop cache problem

	30 seconds after a logical link was taken down, the next hop cache entry was deleted. As
	part of this deletion, the link was taken “down” which caused a neighbour entry to be
	created for the same node address but accessed via the loopback device. Sometimes this
	would cause the designated router to become accessible via the loopback device and
	subsequent connections would fail.

- Intra-ethernet bit ignored

	The intra-ethernet bit in the routing flags is ignored on inbound traffic. If there was a neighbour
	entry for the remote node at connection time, everything would work correctly. If there wad no
	entry, all outbound  traffic would be sent through the designated router for the duration of
	the logical link.

- Promiscuous mode alters DECnet behaviour

	If the ethernet NIC used for DECnet had promiscuous mode enabled (e.g. using tcpdump
	for traffic tracing), the DECnet code would start seeing endnode hello’s, populating
	neighbour structures and causing the problems described above for the intra-ethernet bit
	to go away.

New programs:

DECnet Test Send and DECnet Test Receiver (DTS/DTR)

	Test programs created via reverse engineering the protocol exchanges. Used to find a
	number of the bugs described above.

NML2

	New implementation of the Network Management Listener. Supports SUMMARY, STATUS
	and CHARACTERISTICS for NODES, CIRCUITS and AREAS. It does not support LINKS
	and OBJECTS which were in the old version but are system specific operations which
	were only visible from DECnet-VMS systems.

	The old version is still the default during installation. The new version can be installed by:

		cd dnprogs/nml2
		make
		sudo make install

	which will overwrite the installed executable and man page.

OS Support:

	As of 06/29/2020 the code has been tested with:

		Raspbian 2020-05-27 release with kernels 4.19.126 and 5.4.44 (32-bit only)

		Pi OS 2020-05-27 32-bit release with kernel 4.19.126 and 5.4.44

		pI OS 2020-05-27 (fully updated on 06/26/2020) with kernel 5.4.49


The source code and installation instructions are available at:

	<https://github.com/JohnForecast/RaspbianDECnet <https://github.com/JohnForecast/RaspbianDECnet>>

  John.

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