Add Dynamic DNS Script (#9)

* Initial commit for Dynamic DNS code. Not in a runnable state but as far as I can get for now.

* Bring shell script for Dynamic DNS to working order. Dry runs are going well. Have not tested against a production environment yet.

* Add instructions for No-IP in case it needs installed later..

* Variablize the full path to the keyfile. Enforce strict permissions on the file.

* Final updates after successful testing.

* Update instructions, add examples.

* Fix usage.

* Remove unnecessary asterisk.

Config/DynamicDNS/update_dns.sh

@@ -0,0 +1,149 @@
+#!/bin/bash
+# 2023-05-18 Hyperling
+# Keep afraid.org dynamic DNS synced on ISP connections without static IPs.
+
+## Setup ##
+
+DIR="`dirname $0`"
+PROG=`basename $0`
+if [[ $DIR == "."* ]]; then
+	DIR="`pwd`"
+fi
+
+DOMAIN="sync.afraid.org"
+PROTOCOL="http"
+
+KEYFILE_NAME="private.key"
+KEYFILE="$DIR/$KEYFILE_NAME"
+
+## Functions ##
+
+function usage {
+	# Accepts 1 parameter: The exit code to use.
+	exit_status=$1
+	echo "Usage: $PROG [-4] [-6] [-d | -t] [-v] [-h]" 1>&2
+	cat <<- EOF
+	   Program reads the local $KEYFILE_NAME and syncs with the Dynamic DNS provider.
+
+	   Current DNS providers are mentioned in the README, but initally only
+	      afraid.org is being supported since that is the maintainer's primary
+	      usage. More may be added eventually or you're welcome to contribute.
+
+	   Parameters:
+	      -4 : Update IPV4.
+	      -6 : Update IPV6.
+	         * If neither 4 or 6 are provided, 4 is assumed as Yes.
+	      -d : Perform a dry run, echoing the commands rather than doing the update.
+	      -t : Test run. Alias for the dry run option above.
+	      -v : Enable extra output, helpful for debugging.
+	      -h : Print this list of parameters.
+	EOF
+	exit $exit_status
+}
+
+function check {
+	# Accepts parameter of status and whether the program should quit.
+	status=$1
+	quit=$2
+	if [[ $status != 0 ]]; then
+		echo "ERROR: Did not receive a successful return message, got $status." 1>&2
+		if [[ $quit == "Y" ]]; then
+			exit $status
+		fi
+	fi
+	echo "Status $status is acceptable."
+}
+
+## Validations ##
+
+# Ensure the account key is present and has contents.
+if [[ ! -s $KEYFILE ]]; then
+	echo "ERROR: Key file '$KEYFILE' is empty or does not exist." 1>&2
+	echo -e "Please see '$DIR/README.md' for instructions.\n"
+	usage
+fi
+
+## Parameters ##
+
+while getopts ":46dtvh" opt; do
+	case $opt in
+		4)
+			v4="Y"
+			;;
+		6)
+			v6="Y"
+			;;
+		d | t)
+			dry_run="Y"
+			;;
+		v)
+			set -x
+			;;
+		h)
+			usage 0
+			;;
+		*)
+			echo "ERROR: Parameter $opt not recognized."
+			usage 1
+			;;
+	esac
+done
+
+# Cannot set both 4 and 6, otherwise only the 2nd takes effect.
+if [[ -n $v4 && -n $v6 ]]; then
+	echo "ERROR: Cannot set both v4 and v6, please only choose one." 1>&2
+	usage 1
+fi
+
+# If neither parameter was passed, assume 4 is wanted.
+if [[ -z $v4 && -z $v6 ]]; then
+	v4="Y"
+fi
+
+## Main ##
+
+# Use echo instead of cURL if doing a dry/test run.
+command="curl -w HTTP%{http_code}\n"
+if [[ $dry_run == "Y" ]]; then
+	command="echo $command"
+fi
+
+# Ensure permissions are strict.
+chmod -c 600 $KEYFILE
+
+# Get the user's key
+if [[ -n $KEYFILE ]]; then
+	key=`cat $KEYFILE`
+else
+	echo "ERROR: Cannot find '$KEYFILE'. Please set up your account key." 1>&2
+	usage 1
+fi
+
+# Remove any padding like newlines or trailing spaces
+key=`echo $key`
+
+# Ensure we got a value
+if [[ -z $key ]]; then
+	echo "ERROR: Key contents were not read, is '$KEYFILE' set up proerly?." 1>&2
+	usage 1
+fi
+
+# Try to ensure the key is not going to cause a malformed link somehow.
+if [[ $key == *" "* ]]; then
+	echo "WARNING: Space character found in key. Is that correct? Converting to %20." 1>&2
+	key=${key// /%20}
+fi
+
+uri="$DOMAIN/u/$key/"
+
+# Connect with the provider.
+if [[ $v4 == "Y" ]]; then
+	$command $PROTOCOL://$uri
+	check $? Y
+fi
+if [[ $v6 == "Y" ]]; then
+	$command $PROTOCOL://v6.$uri
+	check $? Y
+fi
+
+exit 0