Documentation / networking / decnet.txt

Based on kernel version 5.7.10. Page generated on 2020-07-23 22:17 EST.

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                    Linux DECnet Networking Layer Information

1) Other documentation....

   o Project Home Pages                      	    - Kernel info                - Userland tools   - Status page

2) Configuring the kernel

Be sure to turn on the following options:

    CONFIG_DECNET (obviously)
    CONFIG_PROC_FS (to see what's going on)
    CONFIG_SYSCTL (for easy configuration)

if you want to try out router support (not properly debugged yet)
you'll need the following options as well...

    CONFIG_DECNET_ROUTER (to be able to add/delete routes)
    CONFIG_NETFILTER (will be required for the DECnet routing daemon)

Don't turn on SIOCGIFCONF support for DECnet unless you are really sure
that you need it, in general you won't and it can cause ifconfig to

Run time configuration has changed slightly from the 2.4 system. If you
want to configure an endnode, then the simplified procedure is as follows:

 o Set the MAC address on your ethernet card before starting _any_ other
   network protocols.

As soon as your network card is brought into the UP state, DECnet should
start working. If you need something more complicated or are unsure how
to set the MAC address, see the next section. Also all configurations which
worked with 2.4 will work under 2.5 with no change.

3) Command line options

You can set a DECnet address on the kernel command line for compatibility
with the 2.4 configuration procedure, but in general it's not needed any more.
If you do st a DECnet address on the command line, it has only one purpose
which is that its added to the addresses on the loopback device.

With 2.4 kernels, DECnet would only recognise addresses as local if they
were added to the loopback device. In 2.5, any local interface address
can be used to loop back to the local machine. Of course this does not
prevent you adding further addresses to the loopback device if you
want to.

N.B. Since the address list of an interface determines the addresses for
which "hello" messages are sent, if you don't set an address on the loopback
interface then you won't see any entries in /proc/net/neigh for the local
host until such time as you start a connection. This doesn't affect the
operation of the local communications in any other way though.

The kernel command line takes options looking like the following:


the two numbers are the node address 1,2 = 1.2 For 2.2.xx kernels
and early 2.3.xx kernels, you must use a comma when specifying the
DECnet address like this. For more recent 2.3.xx kernels, you may
use almost any character except space, although a `.` would be the most
obvious choice :-)

There used to be a third number specifying the node type. This option
has gone away in favour of a per interface node type. This is now set
using /proc/sys/net/decnet/conf/<dev>/forwarding. This file can be
set with a single digit, 0=EndNode, 1=L1 Router and  2=L2 Router.

There are also equivalent options for modules. The node address can
also be set through the /proc/sys/net/decnet/ files, as can other system

Currently the only supported devices are ethernet and ip_gre. The
ethernet address of your ethernet card has to be set according to the DECnet
address of the node in order for it to be autoconfigured (and then appear in
/proc/net/decnet_dev). There is a utility available at the above
FTP sites called dn2ethaddr which can compute the correct ethernet
address to use. The address can be set by ifconfig either before or
at the time the device is brought up. If you are using RedHat you can
add the line:


or something similar, to /etc/sysconfig/network-scripts/ifcfg-eth0 or
wherever your network card's configuration lives. Setting the MAC address
of your ethernet card to an address starting with "hi-ord" will cause a
DECnet address which matches to be added to the interface (which you can
verify with iproute2).

The default device for routing can be set through the /proc filesystem
by setting /proc/sys/net/decnet/default_device to the
device you want DECnet to route packets out of when no specific route
is available. Usually this will be eth0, for example:

    echo -n "eth0" >/proc/sys/net/decnet/default_device

If you don't set the default device, then it will default to the first
ethernet card which has been autoconfigured as described above. You can
confirm that by looking in the default_device file of course.

There is a list of what the other files under /proc/sys/net/decnet/ do
on the kernel patch web site (shown above).

4) Run time kernel configuration

This is either done through the sysctl/proc interface (see the kernel web
pages for details on what the various options do) or through the iproute2
package in the same way as IPv4/6 configuration is performed.

Documentation for iproute2 is included with the package, although there is
as yet no specific section on DECnet, most of the features apply to both
IP and DECnet, albeit with DECnet addresses instead of IP addresses and
a reduced functionality.

If you want to configure a DECnet router you'll need the iproute2 package
since its the _only_ way to add and delete routes currently. Eventually
there will be a routing daemon to send and receive routing messages for
each interface and update the kernel routing tables accordingly. The
routing daemon will use netfilter to listen to routing packets, and
rtnetlink to update the kernels routing tables. 

The DECnet raw socket layer has been removed since it was there purely
for use by the routing daemon which will now use netfilter (a much cleaner
and more generic solution) instead.

5) How can I tell if its working ?

Here is a quick guide of what to look for in order to know if your DECnet
kernel subsystem is working.

   - Is the node address set (see /proc/sys/net/decnet/node_address)
   - Is the node of the correct type 
                             (see /proc/sys/net/decnet/conf/<dev>/forwarding)
   - Is the Ethernet MAC address of each Ethernet card set to match
     the DECnet address. If in doubt use the dn2ethaddr utility available
     at the ftp archive.
   - If the previous two steps are satisfied, and the Ethernet card is up,
     you should find that it is listed in /proc/net/decnet_dev and also
     that it appears as a directory in /proc/sys/net/decnet/conf/. The
     loopback device (lo) should also appear and is required to communicate
     within a node.
   - If you have any DECnet routers on your network, they should appear
     in /proc/net/decnet_neigh, otherwise this file will only contain the
     entry for the node itself (if it doesn't check to see if lo is up).
   - If you want to send to any node which is not listed in the
     /proc/net/decnet_neigh file, you'll need to set the default device
     to point to an Ethernet card with connection to a router. This is
     again done with the /proc/sys/net/decnet/default_device file.
   - Try starting a simple server and client, like the dnping/dnmirror
     over the loopback interface. With luck they should communicate.
     For this step and those after, you'll need the DECnet library
     which can be obtained from the above ftp sites as well as the
     actual utilities themselves.
   - If this seems to work, then try talking to a node on your local
     network, and see if you can obtain the same results.
   - At this point you are on your own... :-)

6) How to send a bug report

If you've found a bug and want to report it, then there are several things
you can do to help me work out exactly what it is that is wrong. Useful
information (_most_ of which _is_ _essential_) includes:

 - What kernel version are you running ?
 - What version of the patch are you running ?
 - How far though the above set of tests can you get ?
 - What is in the /proc/decnet* files and /proc/sys/net/decnet/* files ?
 - Which services are you running ?
 - Which client caused the problem ?
 - How much data was being transferred ?
 - Was the network congested ?
 - How can the problem be reproduced ?
 - Can you use tcpdump to get a trace ? (N.B. Most (all?) versions of 
   tcpdump don't understand how to dump DECnet properly, so including
   the hex listing of the packet contents is _essential_, usually the -x flag.
   You may also need to increase the length grabbed with the -s flag. The
   -e flag also provides very useful information (ethernet MAC addresses))


A quick FAQ on ethernet MAC addresses to explain how Linux and DECnet
interact and how to get the best performance from your hardware. 

Ethernet cards are designed to normally only pass received network frames 
to a host computer when they are addressed to it, or to the broadcast address.

Linux has an interface which allows the setting of extra addresses for
an ethernet card to listen to. If the ethernet card supports it, the
filtering operation will be done in hardware, if not the extra unwanted packets
received will be discarded by the host computer. In the latter case,
significant processor time and bus bandwidth can be used up on a busy
network (see the NAPI documentation for a longer explanation of these

DECnet makes use of this interface to allow running DECnet on an ethernet 
card which has already been configured using TCP/IP (presumably using the 
built in MAC address of the card, as usual) and/or to allow multiple DECnet
addresses on each physical interface. If you do this, be aware that if your
ethernet card doesn't support perfect hashing in its MAC address filter
then your computer will be doing more work than required. Some cards
will simply set themselves into promiscuous mode in order to receive
packets from the DECnet specified addresses. So if you have one of these
cards its better to set the MAC address of the card as described above
to gain the best efficiency. Better still is to use a card which supports
NAPI as well.

8) Mailing list

If you are keen to get involved in development, or want to ask questions
about configuration, or even just report bugs, then there is a mailing
list that you can join, details are at:

9) Legal Info

The Linux DECnet project team have placed their code under the GPL. The
software is provided "as is" and without warranty express or implied.
DECnet is a trademark of Compaq. This software is not a product of
Compaq. We acknowledge the help of people at Compaq in providing extra
documentation above and beyond what was previously publicly available.

Steve Whitehouse <>