Linux AX.25 Socket programming

Linux AX.25 Socket programming

[Jonathan Naylor]

I have seen some messages on Linux-Hams about AX.25 socket programming 
that seem to miss the potential simplicity of it. By this I mean making 
use of the existing infrastructure within the ax25-utils/ax25-lib.

AX.25 (and NET/ROM and ROSE) server development can by careful design of 
the server be made extremely easy. The program ax25d is exactly 
analogous to inetd, in that it can launch programs when an incoming 
connection is received. This is controlled by the ax25d.conf file which 
also allows for different servers to be started depending on the 
incoming interface and/or incoming callsign. The started program only 
has to read and write to stdin and stdout respectively, ax25d is also 
capable of passing command line arguments to the started program based 
on a set of substitution parameters and literal text given in its 
configuration file.

This mode of server program assumes that one program will handle one 
connection. This makes (usually) for much simpler programs that don't 
have to internally multi-task, but then places more complexity into any 
needed inter-process communication that may be needed between different 
instances of the same server. An example I can think of is for a DX 
Cluster implementation, whereby the IPC would be via a database accessed 
from all of the instances. Each server could either be multi-threaded 
with one thread to listen for incoming commands from the user and the 
other thread sending incoming spot data to the user, or alternatively a 
polling structure using select(2) with a timeout.

Servers that can be started by ax25d do not need to be in C or C++, but 
can be in Perl, Java, Ruby, COBOL or anything else that can read and 
write to stdin/stdout.

As a side note, a server designed to be used from ax25d, can just as 
easily be started by inetd and therefore be made available to users of 
an IP network also. The only caveat would be that the end of line 
convention between AX.25 and the Internet are different, I am not sure 
if the ax25-lib has support for this, but the alternative would be to 
pass an indication of the required EOL character via the command line, 
possibly as an incoming connection type e.g. ax25, netrom, rose, inet.

As I said at the beginning this only applies to server programming, for 
a client you have to get your hands dirty, and indeed for any program 
that needs to monitor traffic like listen.

Jonathan  G4KLX

Re: Linux AX.25 Socket programming

[Kelly Black]

On Wed, 2003-01-01 at 12:59, Jonathan Naylor wrote:
> AX.25 (and NET/ROM and ROSE) server development can by careful design of 
> the server be made extremely easy. The program ax25d is exactly 
> analogous to inetd, in that it can launch programs when an incoming 
> connection is received. This is controlled by the ax25d.conf file which 
> also allows for different servers to be started depending on the 
> incoming interface and/or incoming callsign. The started program only 
> has to read and write to stdin and stdout respectively, ax25d is also 
> capable of passing command line arguments to the started program based 
> on a set of substitution parameters and literal text given in its 
> configuration file.

Thanks G4KLX!

Excellent!!!  Does anybody know where I can find an example of a script
that is doing just this read from stdin and write to stdout.

Sounds like a great way to prototype a server quickly.  Neat way to
implement a quick service outside of the regular ones to get emergency
services via a simple scripted telnet client as well!

Kelly
KB0GBJ

Re: Linux AX.25 Socket programming

[Andrew Bates]

> Excellent!!!  Does anybody know where I can find an example of a script
> that is doing just this read from stdin and write to stdout.
>

Here is a very simply perl script that reads and writes to the stdin/stdout:

#!/usr/bin/perl
# -- beginning of script

# display our message to the stdout
print "Testing ax25d\n";
print "Please enter your callsign: ";

# read input from the command line, until
# a carriage return is received
$call = (<>);

# remove the trailing carriage return
chomp $call;

# display callsign to the stdout
print "Hello ".$call."!\n";

exit;
# -- end of script
hope that helps, more information about perl can be found at www.perl.com...
other scripting languages (like korn shell) have howto's at the linux
documentation project (www.tldp.org).  Any language that can communicate with
*nix's STDIN/STDOUT (C/C++, Java, ksh, sh, bash, perl, etc.) can be used for
AX25d processes.
Andrew

Re: Linux AX.25 Socket programming

[Kelly Black]

On Wed, 2003-01-01 at 17:00, Andrew Bates wrote:
> > Excellent!!!  Does anybody know where I can find an example of a script
> > that is doing just this read from stdin and write to stdout.
> >
> 
> Here is a very simply perl script that reads and writes to the stdin/stdout:
> 
SNIP---

O.K. Thanks for the tip.  Can you get the callsign or other environment
info from the environment ( $ENV ?) as well like a CGI script does.

I will probably still write the program in C though as I was impressed
at how well I was able to view a curses based application via TCP/IP
over AX.25, and was floored by its responsiveness.

One of my reasons for re-writing the existing application is to make
user training simple by providing a simple interface option instead of
just the terse command line option that exists now.

Kelly
KB0GBJ

P.S. The application I am trying to re-create will be a sub-set of
ARES-DATA.  I will tailor it for tracking runners in a marathon race.

Re: Linux AX.25 Socket programming

[Curt Mills, WE7U]

On 1 Jan 2003, Kelly Black wrote:

> O.K. Thanks for the tip.  Can you get the callsign or other environment
> info from the environment ( $ENV ?) as well like a CGI script does.

Yes.  Very easily:

    $display=$ENV{"DISPLAY"};


> I will probably still write the program in C though as I was impressed
> at how well I was able to view a curses based application via TCP/IP
> over AX.25, and was floored by its responsiveness.

Remember though that Perl is a compiled language as well.  It just
happens to compile it each time you invoke it.  It's very fast.

Perls strongpoint is in ripping apart text and reconstructing it in
various ways.  It does most everything else as well.


> One of my reasons for re-writing the existing application is to make
> user training simple by providing a simple interface option instead of
> just the terse command line option that exists now.

There's also PerlTk.  Not a bad way to get a program with a GUI up
and running in a hurry.

-- 
Curt Mills, WE7U                    hacker_NO_SPAM_@tc.fluke.com
Senior Methods Engineer/SysAdmin
"Lotto:    A tax on people who are bad at math!"
"Windows:  Microsoft's tax on computer illiterates!" -- WE7U
"The world DOES revolve around me:  I picked the coordinate system!"

Re: Linux AX.25 Socket programming

[Tomi Manninen]

On Wed, 1 Jan 2003, Jonathan Naylor wrote:

> Servers that can be started by ax25d do not need to be in C or C++, but 
> can be in Perl, Java, Ruby, COBOL or anything else that can read and 
> write to stdin/stdout.

One word of caution though: AX.25 connections are so called sequential
packet connections which means that packet boundaries have meaning in the
protocol. To make a long story short, this means that you are yourself
responsible for packetizing the data in correct size chunks.

When you read from stdin, you have to pass the read() system call a buffer
that is larger than any incoming packet might be. Otherwise data will be
lost. See for example axconv where read() is always called with a buffer
of 4096 bytes.

When you write to stdout, you need to make sure you don't write() a buffer
longer than the MTU (or PACLEN). Also, you probably want to avoid a
situation where you write() several short buffers in a row as they will be 
sent separately. In axconv I have used the stdio functions for writing 
(and buffering) and used setvbuf() to set the effective paclen.

The above holds for C, and I'm not completely sure how you can accomplish 
the same in eg. Perl. There was a thread about Perl and ax25d recently on 
this list but I have already forgotten what the correct procedure was...

-- 
Tomi Manninen           Internet:  oh2bns@sral.fi
OH2BNS                  AX.25:     oh2bns@oh2rbi.fin.eu
KP20ME04                Amprnet:   oh2bns@oh2rbi.ampr.org

Re: Linux AX.25 Socket programming

[Kelly Black]

OK, just to be silly I set up a simple call to fortune in my ax25d.conf
to see what would happen, and had great fun watching the fortune flash
up on the screen and then have the call program exit.  This is great
fun! (Although I should get on with writing the program now)...

73's,
Kelly
KB0GBJ

Re: Linux AX.25 Socket programming

[Wilbert Knol]

On Thu, 2 Jan 2003, Tomi Manninen wrote:

> the same in eg. Perl. There was a thread about Perl and ax25d recently on 
> this list but I have already forgotten what the correct procedure was...

The trick was, to 'unbuffer' the Perl interpreter with:

$|=1;
# or any non-zero value

My Perl Bible says:

 English name: $OUTPUT_AUTOFLUSH

 If set to any non-zero value, forces a flush after every write or 
 print on the currently selected output device


I tried out the recently posted Perl script (having 'unbuffered it 
first...)

It looks as though AX25  scrambles the '\n', because the script keeps 
waiting for input (much 
like a Windows shutdown :-)

When I set $/ to any character, the script then gets past the
$call=(<>);
statement once that character appears on stdin.

How does ax25d present end-of-line?


Wilbert, ZL2BSJ

Re: Linux AX.25 Socket programming

[Tomi Manninen OH2BNS]

On Fri, 3 Jan 2003, Wilbert Knol wrote:

> On Thu, 2 Jan 2003, Tomi Manninen wrote:
> 
> > the same in eg. Perl. There was a thread about Perl and ax25d recently on 
> > this list but I have already forgotten what the correct procedure was...
> 
> The trick was, to 'unbuffer' the Perl interpreter with:

Ok, but how about input? Didn't it need something special too?

> How does ax25d present end-of-line?

The standard end-of-line in AX.25 related world is CR (yes, a really
bright idea indeed...). Ax25d doesn't touch that at all so any program
lauched from ax25s needs to do the conversions itself.

-- 
Tomi Manninen           Internet:  oh2bns@sral.fi
OH2BNS                  AX.25:     oh2bns@oh2rbi.fin.eu
KP20ME04                Amprnet:   oh2bns@oh2rbi.ampr.org

Re: Linux AX.25 Socket programming

[Wilbert Knol]

> The standard end-of-line in AX.25 related world is CR

Thanks. In that case, in order to make it work with ax25d, Andrew 
Bates's Perl script needs to start off like this:

   #!/usr/bin/perl
   # -- beginning of script
   # Un-buffer STDIN:
   $|=1;
   # Use AX25 EOL:
   # (Check the '$/' paragraph in 'man perlvar')
   $/="\r";

I found that the AX25 socket gets closed as soon as the script runs 
out. The remote end gets a UI disconnect, hot on the heels of the last 
Info frame. The remote user never gets to see the result of the script. 

A graceful closedown by the remote user would fix that:

   print "Hit ENTER to quit:";
   $call = (<>);

   exit;
   # end of script

So, it looks like it _really_ is as easy as advertised by G4KLX! I am 
now scripting a search-my-log gizmo :-)


Wilbert, ZL2BSJ

Re: Linux AX.25 Socket programming

[Kelly Black]

On Fri, 2003-01-03 at 14:47, Wilbert Knol wrote:
snip----
> So, it looks like it _really_ is as easy as advertised by G4KLX! I am 
> now scripting a search-my-log gizmo :-)
> 
> 
> Wilbert, ZL2BSJ
> 

Thanks! Now I kno why the fortune program would not send more than one
fortune per connect and the call program exited after the first fortune
when I requested multiples.

This ax25d trick can be a great boon to the local AX25 users!  I needed
something besides showing converse nodes to the local club at a future
meeting...  I think it will spark interest in a few more people in the
club to do other projects as well!

Kelly
KB0GBJ

Re: Linux AX.25 Socket programming

[M Taylor]

On Sat, Jan 04, 2003 at 09:47:05AM +1300, Wilbert Knol wrote:
> 
> 
> > The standard end-of-line in AX.25 related world is CR
> 
>    # Un-buffer STDIN:
>    $|=1;
>    # Use AX25 EOL:
>    # (Check the '$/' paragraph in 'man perlvar')
>    $/="\r";

For portable Perl, 
	$/ = \015;	# CR
because \r and \n can vary across different Perl platforms (e.g. MacOS as
I recall).

Re: Linux AX.25 Socket programming

[Wilbert Knol]

On Fri, 3 Jan 2003, M Taylor wrote:

> For portable Perl, 
> 	$/ = \015;	# CR

Try:
        $/ = "\015";

Perl wants a string. If you give it an integer number, it gets 
interpreted as a buffer size....


Wilbert, ZL2BSJ

Re: Linux AX.25 Socket programming

[Wilbert Knol]

On a similar note: I am now trying to get a Bash script, started by 
the ax25d daemon,  to accept input from a user.

The Bash command I am trying to press into service is the 'read'
built-in. The function keeps waiting for a line-feed that never comes.
I have tried things like:

 read -d \x0d input_string
 read -d \015 input_string
 read -d \r   input_string

etc etc, without any success.

How can I read user input with the Bash shell?


Wilbert, ZL2BSJ