Some helpers to set dynamic dns on an OpenBSD firewall on namecheap.com. Usage is `dyndns-matches $interface $host $domain || namecheap-dyndns $host $domain` (with your namecheap dyndns password in /etc/namecheap-dyndns). Should be suitable for putting in cron.

dyndns-matches

#!/bin/sh
set -e

if=$1
host=$2
domain=$3

lastip="/tmp/dyndns-$host-$domain"
ttl=3600

myip=$( ifconfig $if | awk '/inet / { print $2 }' | head -1 )

# We can trust a longer TTL since we're going to be changing the IP 
[ -e "$lastip" ] \
  && [ $(( $( date +%s ) - $( stat -f %m "$lastip" ) )) -lt $ttl ] \
  && [ "$myip" = $(< "$lastip" ) ] \
  && exit

for s in $( host -t NS $domain | awk '/ name server / { print $4 }' ); do
	dnsip=$( host -r -t A $host.$domain $s |
	    awk '/ has address / { print $4 }' | head -1 )
	if [ "$myip" = "$dnsip" ]; then
		echo "$dnsip" > "$lastip"
		break
	else
		echo "$host.$domain has address $dnsip, should be $myip" >&2
		exit 1
	fi
done

namecheap-dyndns

#!/bin/sh
set -e

# https://www.namecheap.com/support/knowledgebase/article.aspx/29/11/how-do-i-use-a-browser-to-dynamically-update-the-hosts-ip/

if [ ! "$1" -o ! "$2" ] ; then
	echo "Usage: $0 host domain [password_file]" >&2
	exit 1
fi

url=https://dynamicdns.park-your-domain.com/update
url="$url?host=$1&domain=$2"

password_file=${3:-/etc/namecheap-dyndns}
if [ ! "$password_file" -o ! -s "$password_file" ]; then
	echo "Password file ($password_file) does not exist" >&2
	exit 2
fi

url="$url&password=$(< $password_file )"

# TIP: If you don't want to use any DNS client, you can just bookmark
# the URL (after inputting proper values there) and access it whenever
# you need to update your IP.
#url=$url&ip=[your_ip]"

status=$( ftp -VMN namecheap-dyndns -o- $url )

# While I would like to make this XML prettier, what a pain
if [ "$status" = "${status%<ErrCount>0</ErrCount>*}" ]; then
	echo $status >&2
	exit 255
fi