CRON to resolve and update the IP addresses used throughout the Gasmask *.hst config files. Tested only on OSX 10.8.3 with Gasmask 0.7.

fetchip-gasmask-hosts.sh

#!/bin/bash

# Job for automatically resolving and updating GasMask host file entries.
# 
# Spencer Creasey (github.com/spence)
#
# Gasmask: http://www.clockwise.ee/gasmask/
# 
# To use, on the line before your host entry, add:
# 
#   # fetchip <resolvable hostname>
# 
# Example:
# 
#   If host entry contains: 
# 
#   # fetchip dev.site.com
#   127.0.0.1 site.com
# 
#   And "dev.site.com" resoves to 10.10.10.10, running this will update the host file to:
#   
#   # fetchip dev.site.com (updated 1970-01-01T00:00:00)
#   10.10.10.10 site.com
# 
#   Running again will update the datetime. 
# 
# Features:
#
# - Supports multiple unique entires of "# fetchip" per file. 
# - Ignores entries without "# fetchip".
#
# To install:
#  
#  1. Place file in CRON folder (e.g., ~/.cron/fetchip-gasmask.hosts.sh)
#  2. Add execute permissions
#     > chmod +x ~/.cron/fetchip-gasmask.hosts.sh
#  3. Update crontab
#     > crontab -e
#     */5 * * * * /Users/myusername/.cron/fetchip-gasmask-hosts.sh

date=$(date +%Y-%m-%dT%H:%M:%S)
for file in /Users/$USER/Library/Gas\ Mask/Local/*.hst /etc/hosts; do 
  hosts=$(sed -E -n 's/^# *fetchip +([^ ]+)( +\(updated [-T0-9 :]+\))?/\1/p' "$file") || continue
	for host in $hosts; do
		ip=$(dig +short $host | grep -e '^\d' | head -n 1)
		escaped_host=$(echo "$host" | sed 's/[^-A-Za-z0-9_]/\\&/g')
		if [[ -n $ip ]]; then
			sed -E -i.bak "/^# *fetchip +$escaped_host/ { s/^(# *fetchip +[^ ]+)( +\(updated [-T0-9 :]+\))?/\1 (updated $date)/; n ; s/[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3} +(.*)/$ip \1/; }" "$file"
			rm "$file.bak"
		fi
	done
done