Configuring Linux to accept Packet connections

Linux Amateur Radio AX.25 HOWTO
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# 12. Configuring Linux to accept Packet connections

Linux is a powerful operating system and offers a great deal of flexibility in how it is configured. With this flexibility comes a cost in configuring it to do what you want. When configuring your Linux machine to accept incoming AX.25, NET/ROM or ROSE connections there are a number of questions you need to ask yourself. The most important of which is: "What do I want users to see when they connect?". People are developing neat little applications that may be used to provide services to callers, a simple example is the pms program included in the AX.25 utilities, a more complex example is the node program also included in the AX.25 utilities. Alternatively you might want to give users a login prompt so that they can make use of a shell account, or you might even have written your own program, such as a customized database or a game, that you want people to connect to. Whatever you choose, you must tell the AX.25 software about this so that it knows what software to run when it accepts an incoming AX.25 connection.

The ax25d program is similar to the inetd program commonly used to accept incoming TCP/IP connections on UNIX machines. It sits and listens for incoming connections, when it detects one it goes away and checks a configuration file to determine what program to run and connect to that connection. Since this the standard tool for accepting incoming AX.25, NET/ROM and ROSE connections I'll describe how to configure it.

## 12.1. Creating the `/etc/ax25/ax25d.conf` file

This file is the configuration file for the ax25d AX.25 daemon which handles incoming AX.25, NET/ROM and ROSE connections.

The file is a little cryptic looking at first, but you'll soon discover it is very simple in practice, with a small trap for you to be wary of.

The general format of the `ax25d.conf` file is as follows:

    # This is a comment and is ignored by the ax25d program.
    [port_name] || <port_name> || {port_name}
    <peer1>    window T1 T2 T3 idle N2 <mode> <uid> <cmd> <cmd-name> <arguments>
    <peer2>    window T1 T2 T3 idle N2 <mode> <uid> <cmd> <cmd-name> <arguments>
    parameters window T1 T2 T3 idle N2 <mode>
    <peer3>    window T1 T2 T3 idle N2 <mode> <uid> <cmd> <cmd-name> <arguments>
       ...
    default    window T1 T2 T3 idle N2 <mode> <uid> <cmd> <cmd-name> <arguments>

Where:

#

  at the start of a line marks a comment and is completely ignored by the ax25d program.

<port_name>

  is the name of the AX.25, NET/ROM or ROSE port as specified in the `/etc/ax25/axports`, `/etc/ax25/nrports` and `/etc/ax25/rsports` files. The name of the port is surrounded by the ``[]`' brackets if it is an AX.25 port, the ``<>`' brackets if it is a NET/ROM port, or the ``{}`' brackets if it is a ROSE port. There is an alternate form for this field, and that is use prefix the port name with ``callsign/ssid via`' to indicate that you wish accept calls to the callsign/ssid via this interface. The example should more clearly illustrate this.

<peer>

  is the callsign of the peer node that this particular configuration applies to. If you don't specify an SSID here then any SSID will match.

window

  is the AX.25 Window parameter (K) or MAXFRAME parameter for this configuration.

T1

  is the Frame retransmission (T1) timer in half second units.

T2

  is the amount of time the AX.25 software will wait for another incoming frame before preparing a response in 1 second units.

T3

  is the amount of time of inactivity before the AX.25 software will disconnect the session in 1 second units.

idle

  is the idle timer value in seconds.

N2

  is the number of consecutive retransmissions that will occur before the connection is closed.

<mode>

  provides a mechanism for determining certain types of general permissions. The modes are enabled or disabled by supplying a combination of characters, each representing a permission. The characters may be in either upper or lower case and must be in a single block with no spaces.

  u/U

    UTMP - currently unsupported.

  v/V

    Validate call - currently unsupported.

  q/Q

    Quiet - Don't log connection

  n/N

    check NET/ROM Neighbour - currently unsupported.

  d/D

    Disallow Digipeaters - Connections must be direct, not digipeated.

  l/L

    Lockout - Don't allow connection.

  */0

    marker - place marker, no mode set.

<uid>

  is the userid that the program to be run to support the connection should be run as.

<cmd>

  is the full pathname of the command to be run, with no arguments specified.

<cmd-name>

  is the text that should appear in a ps as the command name running (normally the same as <cmd> except without the directory path information.

<arguments>

  are the command line argument to be passed to the <:cmd> when it is run. You pass useful information into these arguments by use of the following tokens:

  %d

    Name of the port the connection was received on.

  %U

    AX.25 callsign of the connected party without the SSID, in uppercase.

  %u

    AX.25 callsign of the connected party without the SSID, in lowercase.

  %S

    AX.25 callsign of the connected party with the SSID, in uppercase.

  %s

    AX.25 callsign of the connected party with the SSID, in lowercase.

  %P

    AX.25 callsign of the remote node that the connection came in from without the SSID, in uppercase.

  %p

    AX.25 callsign of the remote node that the connection came in from without the SSID, in lowercase.

  %R

    AX.25 callsign of the remote node that the connection came in from with the SSID, in uppercase.

  %r

    AX.25 callsign of the remote node that the connection came in from with the SSID, in lowercase.

You need one section in the above format for each AX.25, NET/ROM or ROSE interface you want to accept incoming AX.25, NET/ROM or ROSE connections on.

There are two special lines in the paragraph, one starts with the string ``parameters`' and the other starts with the string ``default`' (yes there is a difference). These lines serve special functions.

The ``default`' lines purpose should be obvious, this line acts as a catch-all, so that any incoming connection on the <interface_call> interface that doesn't have a specific rule will match the ``default`' rule. If you don't have a ``default`' rule, then any connections not matching any specific rule will be disconnected immediately without notice.

The ``parameters`' line is a little more subtle, and here is the trap I mentioned earlier. In any of the fields for any definition for a peer you can use the `*' character to say `use the default value'. The ``parameters`' line is what sets those default values. The kernel software itself has some defaults which will be used if you don't specify any using the ``parameters`' entry. The trap is that the these defaults apply only to those rules below the ``parameters`' line, not to those above. You may have more than one ``parameters`' rule per interface definition, and in this way you may create groups of default configurations. It is important to note that the ``parameters`' rule does not allow you to set the ``uid`' or ``command`' fields.

## 12.2. A simple example `ax25d.conf` file

Okay, an illustrative example:

    # ax25d.conf for VK2KTJ - 02/03/97
    # This configuration uses the AX.25 port defined earlier.

    # <peer> Win T1  T2  T3  idl N2 <mode> <uid> <exec> <argv[0]>[<args....>]

    [VK2KTJ-0 via radio]
    parameters 1    10  *  *  *   *   *
    VK2XLZ     *     *  *  *  *   *   *    root  /usr/sbin/axspawn axspawn %u +
    VK2DAY     *     *  *  *  *   *   *    root  /usr/sbin/axspawn axspawn %u +
    NOCALL     *     *  *  *  *   *   L
    default    1    10  5 100 180 5   *    root  /usr/sbin/pms pms -a -o vk2ktj

    [VK2KTJ-1 via radio]
    default    *     *    *   *   *   0    root /usr/sbin/node node

    <netrom>
    parameters 1    10  *  *  *   *   *
    NOCALL     *     *  *  *  *   *   L
    default    *     *  *  *  *   *   0        root /usr/sbin/node node

    {VK2KTJ-0 via rose}
    parameters 1    10  *  *  *   *   *
    VK2XLZ     *     *  *  *  *   *   *    root  /usr/sbin/axspawn axspawn %u +
    VK2DAY     *     *  *  *  *   *   *    root  /usr/sbin/axspawn axspawn %u +
    NOCALL     *     *  *  *  *   *   L
    default    1    10  5 100 180 5   *    root  /usr/sbin/pms pms -a -o vk2ktj

    {VK2KTJ-1 via rose}
    default    *     *    *   *   *   0    root /usr/sbin/node node radio

This example says that anybody attempting to connect to the callsign ``VK2KTJ-0`' heard on the AX.25 port called ``radio`' will have the following rules applied:

Anyone whose callsign is set to `NOCALL' should be locked out, note the use of mode `L'.

The `parameters` line changes two parameters from the kernel defaults (Window and T1) and will run the /usr/sbin/axspawn program for them. Any copies of /usr/sbin/axspawn run this way will appear as axspawn in a ps listing for convenience. The next two lines provide definitions for two stations who will receive those permissions.

The last line in the paragraph is the `catch all' definition that everybody else will get (including VK2XLZ and VK2DAY using any other SSID other than -1). This definition sets all of the parameters implicitly and will cause the pms program to be run with a command line argument indicating that it is being run for an AX.25 connection, and that the owner callsign is `VK2KTJ`. (See the `Configuring the PMS' section below for more details).

The next configuration accepts calls to `VK2KTJ-1` via the `radio` port. It runs the node program for everybody that connects to it.

The next configuration is a NET/ROM configuration, note the use of the greater-then and less-than braces instead of the square brackets. These denote a NET/ROM configuration. This configuration is simpler, it simply says that anyone connecting to our NET/ROM port called ``netrom`' will have the node program run for them, unless they have a callsign of ``NOCALL`' in which case they will be locked out.

The last two configurations are for incoming ROSE connections. The first for people who have placed calls to ``vk2ktj-0`' and the second for ``VK2KTJ-1` at the our ROSE node address. These work precisely the same way. Not the use of the curly braces to distinguish the port as a ROSE port.

This example is a contrived one but I think it illustrates clearly the important features of the syntax of the configuration file. The configuration file is explained fully in the `ax25d.conf` man page. A more detailed example is included in the `ax25-utils` package that might be useful to you too.

## 12.3. Starting ax25d

When you have the two configuration files completed you start ax25d with the command:

    # /usr/sbin/ax25d

When this is run people should be able to make AX.25 connections to your Linux machine. Remember to put the `ax25d` command in your rc files so that it is started automatically when you reboot each time.

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