For anyone who wants to play around with this, the DNS service has been
posted. You can test the existence of a website on a domain or any of
its parent domains by making DNS queries as follows:
subdomain.domain.com.httpcheck.singulink.com
So, if you wanted to check if mail1.mx.google.com or any of its parent
domains have a website, you would do a DNS query with a 30 second
timeout for:
mail1.mx.google.com.httpcheck.singulink.com
This will check the following domains for a valid HTTP response within
15 seconds:
mail1.mx.google.com
mx.google.com
google.com
If a valid HTTP response comes back then the DNS query will return
NXDOMAIN with a 7 day TTL. If no valid HTTP response comes back then the
DNS query will return 127.0.0.1 with progressively increasing TTLs:
#1: 2 mins
#2: 4 mins
#3: 6 mins
#4: 8 mins
#5: 10 mins
#6: 20 mins
#7: 30 mins
#8: 40 mins
#9: 50 mins
#10: 1 hour
#11: 2 hours
#12+: add 2 hours extra for each attempt up to 24h max
As long as an invalid domain has been queried in the last 7 days, it
will remain cached and any further invalid attempts will continue to
progressively increase the TTL according to the rules above. If a domain
doesn't get queried for 7 days then it drops out of the cache and its
invalid attempt counter is reset. A valid HTTP response will reset the
domains invalid counter and a 7 day TTL is returned. Once a domain is in
the cache, responses are immediate until the TTL runs out and the domain
is rechecked again.
