Here is a DDNS update script for a unix environment that should be 
useful for your users.
It allows a user to update the ddns of a router from a server behind an 
ethernet firewall. It allows the user to modify several different 
domains at once, and to put the access passwords into a secret file for 
security purposes.

The name of the script is DDNS_UPDATE.pl

#! /usr/bin/perl -w

###############################################################
#
# Copyright (C) 2003 Philip Lawrence <www.amazingcs.com>
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# version 1.0 ddns_update.pl for systems with an ethernet router
# (WGR614 specifically, but it is easily changed to work with others)
#
# This allows a system to have an ethernet router, with port
# forwarding, and a server hidden behind the router firewall.
#
# this version uses curl to get the router status info, then uses that
# ip to update one or more domains on the server.
#
# The servers hidden behind the firewall can use dynamic or static
# ip addresses assigned by the router, as long as the router can
# route to them with port forwarding.
#
# The router status command, domain names, userids, and passwords
# are hidden in a secret file (which is checked at runtime).
#
# A log file is appended to after completion of the update command
#
###############################################################

use Date::Format;

#$verbose = 'false';
$verbose = 'true';

$ruid = '';
$rpwd = '';
$cuid = '';
$cpwd = '';
$rurl = '';
@domain = ();

$CFGDIR = '/etc/changeip';
$LOGFILE = '/var/log/cip.log';
$DNSSERVER = 'www.changeip.com:443';

sub get_pwd
{
# this file MUST be owned by root with mode 600
my $sfile = "$CFGDIR/changeip.secret";

my $statstr = `stat -c "%U %a" $sfile`;
my $dstr = '';

($statstr ne 'root 600') or
die "$sfile MUST be set to mode 600 and root";

# the format of the file is:
# url to the router status (http://<router ip address>/s_status.htm)
# colon separated list of domain names (domain.com:www.domain.com)
# router user id
# router password
# changeip account user id
# changeip account password

open(IN, "<$sfile") or die "unable to open $sfile";
my @lines = <IN>;
close(IN);

($rurl) = ($lines[0] =~ /\s*([\w:.\/]*)\s*/);
($dstr) = ($lines[1] =~ /\s*([\w:.]*)\s*/);
@domain = split(/:/,$dstr);
($ruid) = ($lines[2] =~ /\s*(\w*)\s*/);
($rpwd) = ($lines[3] =~ /\s*(\w*)\s*/);
($cuid) = ($lines[4] =~ /\s*(\w*)\s*/);
($cpwd) = ($lines[5] =~ /\s*(\w*)\s*/);

if ($verbose eq 'true') {
print "rurl - $rurl\n";
print "dstr - $dstr\n";
print "ruid - $ruid\n";
print "rpwd - $rpwd\n";
print "cuid - $cuid\n";
print "cpwd - $cpwd\n";
}
}

sub get_router_ip
{
my $line = '';
my @lines;
my $cnt = 0;

@lines = `/usr/bin/curl -s --user $ruid:$rpwd $rurl | grep -A 1 
"IP Address"`;

while ($#lines < 1 && $cnt < 3) {
$cnt++;
sleep(1);
}
if ($#lines < 1) {
print "unable to get the router ip\n";
exit 1;
}

if ($verbose eq 'true') {
print "router line: $lines[1]\n";
}

$_ = $lines[1];
($line) = ($lines[1] =~ /[\s\w]*>([\w.]*)<\w*/);

if ($verbose eq 'true') {
print "router_ip: $line\n";
}

return $line;
}

sub get_ns_ip
{
my $str = '';
my $dnsstr = `host $_[0]`;

($str) = ($dnsstr =~ /(\S+)\n?$/);

if ($verbose eq 'true') {
print "domain: $_[0] ns_ip: $str\n";
}

return $str;
}

sub update_ns
{
my $successstr = 'Successful Update!';
my $ip = $_[0];
my $dname = $_[1];

my $getstring = "GET 
/update.asp?u=$cuid&p=$cpwd&cmd=update&hostname=$dname&ip=$ip";
my $cmd = qq~echo "$getstring" | openssl s_client -quiet 
-connect $DNSSERVER 2>&1~;
# run the update command

if ($verbose eq 'true') {
print "updating dns server with command:\n";
print "$cmd\n";
}

my @output = `$cmd | grep "<ResultText>" | grep "$successstr"`;

if ($verbose eq 'true') {
print "response from cmd:\n";
print "$output[0]\n";
}

open(LOGFIL, ">>$LOGFILE");

print LOGFIL time2str("%D %H:%M", time()) . "\n";

if (index($output[0],$successstr) >= 0) {
print LOGFIL "$successstr for domain ip: $router_ip\n";
}
else {
print LOGFIL "update failed for domain ip: $router_ip\n";
}

close(LOGFIL);
}

get_pwd();

$router_ip = get_router_ip();

foreach my $tname (@domain)
{
$ns_ip = get_ns_ip($tname);

# only modify the ddns ip address if it has changed
if ($router_ip ne $ns_ip) {
update_ns($router_ip,$tname);
}
}