[HECnet] DECnet for Linux Bug Fixes

Mon Jun 29 18:44:11 PDT 2020

Hi John,

This is amazing news. I have been unable to use DECnet under Linux for a very long time as the implementation was broken.

The last time I saw DECnet for Linux working was back in the Ubuntu 14 timeframe. Nothing modern worked at all with the end result usually being the entire VM locking as soon as you went to initiate any form of DECnet connection (this required a power cycle or reset to recover).

I did some research and it looked like no-one was maintaining the code. I saw a few attempts from a few people to resurge this, but I could never get their code trees to compile.

I noticed in the readme that you’ve previously used this with Debian Buster 10 which puts this in a similar space to Ubuntu 18.04 Bionic, but your comments said “Due to kernel changes, the above no longer work.”

So should I be able to use your kernel module code under Ubuntu 18.04 (running kernel 4.15.0 or similar) or has something recently broken?

Cheers, Wiz!!

From: owner-hecnet at Update.UU.SE [mailto:owner-hecnet at Update.UU.SE] On Behalf Of John Forecast
Sent: Tuesday, 30 June 2020 10:45 AM
To: hecnet at update.uu.se
Subject: [HECnet] DECnet for Linux Bug Fixes

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>

  John.

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