PERL's pack function is frustrating the heck out of me.

I need to build a UDP packet from the ethernet layer on up. I've found examples online for the IP and UDP layers, even so it seems like voodoo (I'm clearly missing something).

what should I be using for my template string???

for example: 0x0800 is the protocol number for an ip packet.

my $proto = 0x0800;
#my $template = 'hh' #this is wrong and I have no idea what would be right.
$packed_proto = pack($template, $proto);

I know how many bytes each part of my header should be (this is well documented...) and I know what values I want for each part of the header, but I can't figure out a sane method to convert between the two. Its clearly choosing the right template for the pack function.

(Or if any one can suggest a perl module that *works* and can help me pack the ethernet layer I would appreciate it.)

I had a lot of trouble with getting pack/unpack to do what I wanted, from memory if you are building a packet your template would need to be 'N' for network byte order.. hth

As it stands, $proto is an integer, not a string, so it can't be "h".

You probably want "n" - 16 bit short in network order.

I haven't used pack much, but you may find the tutorial helpful: http://perldoc.perl.org/perlpacktut.html.

Assuming you have a normal installation of Perl, you can also get this in a terminal via perldoc perlpacktut.

Have you looked at search.cpan.org eg http://search.cpan.org/search%3fmodule=NetPacket::UDP