ISP Check

Compare the IP address reserved for the cable modem against the DNS entry registered for the same. If there is a mis-match alert the sysadmin to update the DNS record.

Public repo on gitHub

https://github.com/stevegroom/ispcheck

https://github.com/stevegroom/ispcheck.git
git@github.com:stevegroom/ispcheck.git

Flow

@startuml

start
floating note right: ~/development/ispcheck/isMyDnsOk.sh

fork

:process lookup current IP address;
:process parse address with jq;

fork again

:process lookup reverse DNS address;
:process parse address with jq;

end fork

if (addresses match?) then (yes)
  :process message = success;
else (no)
  :process message = failure;
endif

:process Send message;

stop

@enduml

DNS Check

https://dns.google.com/resolve?name=groom.ch&type=A

returns

{
"Status": 0,
"TC": false,
"RD": true,
"RA": true,
"AD": false,
"CD": false,
"Question":[ {"name": "groom.ch.","type": 1}],
"Answer":[ {"name": "groom.ch.","type": 1,"TTL": 299,"data": "178.198.102.49"}],
"Comment": "Response from 216.40.47.26."}

Filter the results with the jq command:

curl -s https://dns.google.com/resolve?name=groom.ch | tee | jq -r .Answer[0].data
178.198.102.49

Internet address check

https://api.ipify.org?format=json

returns

{"ip":"178.198.102.49"}

curl -s https://api.ipify.org?format=json | tee | jq -r .ip
178.198.102.49

Compare the two

if [ `curl -s https://api.ipify.org?format=json | tee | jq -r .ip` ==  `curl -s https://dns.google.com/resolve?name=groom.ch | tee | jq -r .Answer[0].data` ] ;then echo "OK" ; fi

The macos script

isMyDnsOk.sh

#!/bin/bash

# Mac OS version
# Checks the dns entry for the cable modem against the current IP address
# and sends an pop-up notification.
# Steve Groom
# 06.04.2019

dnsRec=`curl -s https://dns.google.com/resolve?name=groom.ch | tee | jq -r .Answer[0].data`
ipAddr=`curl -s https://api.ipify.org?format=json | tee | jq -r .ip`

if
[ $ipAddr == $dnsRec ]
then
  message="DNS record is correct: $dnsRec"
else
  message="DNS record needs updating to: $ipAddr"
fi

osascript -e "display notification \"$message\" with title \"DNS Checker\" "

Scheduling

Add this line:

0 20 * * * /Users/steve/development/docker/ispcheck/isMyDnsOk.sh

to your crontab:

$ crontab -e
crontab: no crontab for steve - using an empty one
crontab: installing new crontab

Next steps

Create version that can be called from Jenkins (or Jenkins in a docker container).

Steve Groom
2019-04-06