wait-for-dns

#!/bin/bash

# wait-for-dns - a script which waits for the availability of a DNS record.
#
# This script is useful in cases where it is necessary to ensure a certain DNS record is resolvable
# without "contaminating" DNS resolvers with cached NXDOMAIN responses.
#
# This script accepts a DNS zone and a record. It then figures out the authoritative nameservers
# for for the zone and queries each of them for the existence of the provided record. If the record
# exists on all of the nameservers, the script exits successfully. If the record doesn't exist on
# any of the nameservers by the time the maximum attempts counter is reached, the script exits with
# an error.

#!/bin/bash

set -eou pipefail

if [[ $# -ne 3 ]]; then
    echo "Usage: $0 <zone> <record> <max_attempts>"
    exit 1
fi

zone=$1
record=$2
max_attempts=$3

echo "Figuring out the nameservers for $zone"
nameservers=""
counter=0
while [[ $counter -lt $max_attempts ]]; do
    out=$(dig +short +timeout=2 $zone ns)
    ret=$?
    if [[ $ret -eq 0 && "$out" != "" ]]; then
        nameservers=$out
        break
    fi
    echo "Not available yet"
    sleep 1
    counter=$[$counter+1]
done

[[ "$nameservers" != "" ]] || (echo "Could not resolve nameservers for $zone"; exit 1)

for ns in $nameservers; do
    echo "Polling $ns for $record.$zone..."
    counter=0
    ok=false
    while [[ $counter -lt $max_attempts ]] ; do
        out=$(dig +short +timeout=2 @$ns $record.$zone a)
        ret=$?
        if [[ $ret -eq 0 && "$out" != "" ]]; then
            echo "Looks good!"
            ok=true
            break
        fi
        echo "Not available yet"
        sleep 1
        counter=$[$counter+1]
    done
    if ! $ok; then
        echo "$record.$zone didn't become available within the allowed time"
        exit 1
    fi
done

echo "$record.$zone is available on all nameservers!"
exit 0