Recently I changed ISPs.  This generally isn’t a big deal. It didn’t cause any difference in speed or  connection reliability, however my old ISP had, regardless of me not ordering one, given me a static IP address.  I’d taken advantage of this and assigned the IP to a DNS record to give me easy to remember access to my files when I’m away from home.  Anyway when I changed ISPs I discovered my new one changed my IP address every few days, which is an unfortunate thing to discover when you’re not at home.

I took a look at my router and found it only supported DynDns, which I then found out that, apart from to some DLink users, didn’t provide provide a free version of their service to the general public, and I don’t have a DLink router.

The next option was a free alternative to DynDns, there were several, and they all required running some kind of linux demon / cron script, which I would then have to alter my existing A record DNS entry to a CNAME to point to the new dynamic DNS record.

Before setting this up I thought I’d look into my existing DNS provider just incase there was any way of updating DNS records on the fly without having to maintain two separate DNS providers. I found that CloudFlare had an API, which I could update records with. The example provided was a Curl example, which made me wonder, surely I could just write a bash script to update the record with via a cron job. I didn’t want to have to install any major scripting language on my minimal Linux distro, excluding any php, perl etc. solution I’d found that worked with CloudFlare.

I checked CloudFlare’s API rate limiting and found it certainly wouldn’t be limiting for what I was trying to do (it’s approximately 1200 requests every 5 mins if you’re wondering). I then found MyExternalIP that could provide my external IP address with a rate limit that was easy to deal with of roughly 1/s, after which it was just a case of writing the bash script.

I worked a script out using Curl requests to get the information I needed including the specific id of the DNS record I was trying to update and then used sed to get the information I needed out each requests response. I made it compatible with the CloudFlareAPI call I was making that way any parameter passed to the call could be passed by a command line argument to the script itself.

The resultant script I added to my crontab to run every half hour.  Everything worked perfectly so I thought  I’d create a gist at github so others could take advantage of this:

#!/bin/sh
#
# CloudFlare Dynamic DNS
#
# Updates CloudFlare records with the current public IP address
#
# Takes the same basic arguments as A/CNAME updates in the CloudFlare v4 API
# https://www.cloudflare.com/docs/client-api.html#s5.2
#
# Use with cron jobs etc.
#
# e.g.
#
# manually run:
# cloudflare_dyn_dns.sh -key 404613183ab3971a2118ae5bf03d63e032f9e -email test@example.com -zone example.com -name extra
#
# cronjob entry to run every 5 minutes:
# */5 * * * * /path/to/cloudflare_dyn_dns.sh -key 404613183ab3971a2118ae5bf03d63e032f9e -email test@example.com -zone example.com -name extra >> /path/to/cloudflare_dyn_dns.log
#
# will both set the type A DNS record for extra.example.com to the current public IP address for user test@example.com with the provided API key

key=
email=
zone=
zone_id=
type=A
rec_id=
name=
content=
ttl=1
proxied=false

while [ "$1" != "" ]; do
    case $1 in
        -key )     shift
                   key=$1
                   ;;
        -email )   shift
                   email=$1
                   ;;
        -zone )    shift
                   zone=$1
                   ;;
        -zone_id ) shift
                   zone_id=$1
                   ;;
        -type )    shift
                   type=$1
                   ;;
        -rec_id )  shift
                   rec_id=$1
                   ;;
        -name )    shift
                   name=$1
                   ;;
        -content ) shift
                   content=$1
                   ;;
        -ttl )     shift
                   ttl=$1
                   ;;
        -proxied ) shift
                   proxied=$1
                   ;;
        * )        echo "unknown parameter $1"
                   exit 1
    esac
    shift
done

if [ "$content" = "" ]
then
    content=`curl -s http://myexternalip.com/raw`
    if [ "$content" = "" ]
    then
        date
        echo "No IP address to set record value with."
        exit 1
    fi
fi

if [ "$name" = "" ]
then
    echo "You must provide the name of the record you wish to change."
    exit 1
fi

if [ "$zone" = "" ]
then
    echo "You must provide the domain you wish to change."
    exit 1
fi

if [ "$name" = "$zone" ]
then
    hostname="$name"
else
    hostname="$name.$zone"
fi

command -v host > /dev/null 2>&1
if [ "$?" = "1" ]
then
    command -v nslookup > /dev/null 2>&1
    if [ "$?" = "1" ]
    then
        echo "Cannot find a way to check existing $type record for $hostname"
        exit 1
    fi
    existing_content=`nslookup -type=$type $hostname | awk -F 'Address: ' 'NR==6 { print $2 }'`
else
    existing_content=`host -t $type $hostname | sed -E 's/.+?\s+([^\s]+)$/\1/'`
fi

if [ "$content" = "$existing_content" ]
then
    echo "Existing record value $existing_content is the same as provided content $content. Exiting."
    exit
fi

if [ "$key" = "" ]
then
    echo "You must provide your user API token."
    exit 1
fi

if [ "$email" = "" ]
then
    echo "You must provide your user email."
    exit 1
fi

# Get the zone id for the entry we're trying to change if it's not provided
if [ "$zone_id" = "" ]
then
    zone_response_json=`curl -X GET "https://api.cloudflare.com/client/v4/zones?name=$zone" -H "X-Auth-Email: $email" -H "X-Auth-Key: $key" -H "Content-Type: application/json"`
    zone_id=`echo $zone_response_json | sed -E "s/.+\"result\":\[\{\"id\":\"([a-f0-9]+)\"[^\}]+$zone.+/\1/g"`
    if [ "$zone_id" = "" ]
    then
        echo "Cloudflare DNS Zone id could not be found, please make sure it exists"
        exit 1
    fi
fi

# Get the record id for the entry we're trying to change if it's not provided
if [ "$rec_id" = "" ]
then
    rec_response_json=`curl -X GET "https://api.cloudflare.com/client/v4/zones/$zone_id/dns_records?name=$hostname" -H "X-Auth-Email: $email" -H "X-Auth-Key: $key" -H "Content-Type: application/json"`
    rec_id=`echo $rec_response_json | sed -E "s/.+\"result\":\[\{\"id\":\"([a-f0-9]+)\"[^\}]+\"type\":\"$type\"[^\}]+$hostname.+/\1/g"`
    if [ "$rec_id" = "" ]
    then
        echo "Cloudflare DNS Record id could not be found, please make sure it exists"
        exit 1
    fi
fi

# Update the DNS record
update_response=`curl -X PUT "https://api.cloudflare.com/client/v4/zones/$zone_id/dns_records/$rec_id" -H "X-Auth-Email: $email" -H "X-Auth-Key: $key" -H "Content-Type: application/json" --data "{\"id\":\"$rec_id\",\"type\":\"$type\",\"name\":\"$hostname\",\"content\":\"$content\",\"ttl\":$ttl,\"proxied\":$proxied}"`

success_val=`echo $update_response | sed -E "s/.+\"success\":(true|false).+/\1/g"`
if [ "$success_val" = "true" ]
then
    echo "Record Updated."
else
    echo "Record update failed."
    exit 1
fi