High speed whois lookups
There is always a temptation to perform domain lookups at high speeds using multiple threads. If you do this you will soon start seeing outputs similar to the following
Welcome to the NSI Registrar Whois Server.
Your IP address is contained within a list of IP addresses that may have failed to abide by Network Solutions' Whois policy.
If you feel this is in error, please contact us at x-xxx-xxx-xxxx or (xxx)-xxx-xxxx or at email@example.com.
Remember that whois servers are being used and abused. Whois server administrators have responded with limits on how many lookups may be performed per minute, per day, etc.
Whois Lookup Policies
Here are some examples of the limits set by different domain registrars. Please note that these are just a few examples. Almost all registrars have similar limits in place.
Tucows (whois.opensrs.net) rejects multiple simultaneous connections from the same IP. You will have to be satisfied with one connection at a time.
BulkRegister (whois.bulkregiter.net) will immediately block the IP temporarily if they notice a large number of connections being made. This is done by their software automatically.
Network Solutions (whois.networksolutions.com) will block the IP temporarily if you do more than 1000 lookups in a day.
Go Daddy (whois.godaddy.com) will simply start hiding all the relevant information from the returned whois text if your IP gets in their list.
...most other registrars also have similar restrictions.
Both Watch My Domains and Domain Name Analyzer Pro allow you to configure the number of simultaneous connections and a connection interval. This will ensure that you don't abuse the whois servers and get banned. The ideal setting in software like Watch My Domains is 1 connection and a lookup interval of 12 seconds or more (5 connections per minute).
If your internet connection has a permanent IP, you should be very careful while using public resources like whois servers. Make sure that your IP doesn't find its way into various registrar ban lists. If you ever find your IP banned give them a call and explain your situation. Find out what got you banned and assure them that you will follow their whois policy usage guidelines.