DNS Lookups
DNS or Domain Name System is a hierarchical naming system for services, computers, or any resource connected to a private network or the internet. It helps associate and gather information by assigning domain names to the participants. One of its main jobs is to translate domain names into a binary identifier string so it can be used to address and locate devices all over the world. In some ways the DNS system is like a phonebook that maps names of people and their phone numbers, except the DNS maps the hostnames to IP addresses.
When you type www.google.com into your web browser, the browser contacts a DNS resolver which returns the IP address of the server you are requesting. Many people are using DNS lookups to get more info, but it comes at a cost. It usually takes 20-120 milliseconds for the DNS service to find the hostname IP address, but during that time the browser cannot do anything until the hostname has been found.
In order to get better performance, you can cache your DNS lookup. DNS caching occurs on a special server that is maintained by the user’s local network or ISP, but you could also cache on your individual computer. Many web browsers have their own cache system that is separate from the operating system cache. If a browser keeps its own DNS cache, it will help web pages to load much faster than normal.
Unique DNS lookups provide you with more information, but they take longer to load so you should cache them if you can. Internet Explorer caches Domain Name System for 30 minutes, although it can be changed to longer by accessing the browser settings. Firefox only caches the DNS lookups for one minute, but it can be changed to one hour by switching to Fasterfox.
A DNS lookup is a program that tries to find an IP address using an Internet domain name, while a reverse DNS lookup is using an IP address to search for a domain name. The reverse DNS lookup method is able to identify if the incoming email server has a valid host name and legitimate properties. Reverse DNS lookups are very time consuming so they are rarely used. The receiving server will perform a lookup on the incoming connection’s IP address to check if it is associated with a valid domain name.
Many servers are using DNS lookups to eliminate spam from email servers that connect through dial-up, cable, and ADSL connections. The IP addresses of these connections are not usually registered to a DNS which means that they do not have a registered host name and static IP like mail.domain.com.
A sender’s address lookup is an alternative to DNS lookups. When the ISP checks if an incoming message is accepted, they can perform a DNS check on the email address of the sender. For example, you have an email address of email@domain.com , then the ISP perform a lookup on domain.com. If there are no records found, the message will be rejected.
When sending mass emails, most spammers use non-configured host servers to disguise the spam’s source. Using a DNS query will not recover a matching IP address and host name but it will provide an indication that the email is spam.
It is not always a good solution to use a DNS lookup. Many legitimate email servers have not registered their name with DNS, or are incorrectly configured, so the query may not provide a matching host name. In addition, this anti-spamming method runs queries which can consume valuable network resources. Improperly configured servers and network delays can also prevent legitimate emails from getting past the filter.
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