Domain name/IP anti-resolution

Source: Internet
Author: User
Tags net domain nslookup

Today, with the proliferation of spam, spam has brought great harm to our life, work, and study. Due to the lack of an effective sending authentication mechanism between SMTP servers, even if spam recognition blocking technology is adopted, in addition, the spam recognition blocking technology mainly identifies emails based on certain conditions after receiving them, which requires a lot of server resources. If you can take some measures before the emails arrive at the server, this greatly improves the server efficiency. Therefore, many email servers, such as,, and, have adopted the spam identification blocking technology + reverse IP resolution verification technology to better block spam.

Let's take a look at what reverse IP resolution is. In fact, a friend who has worked on a DNS server will surely know that there are two regions in the DNS server, namely "forward lookup area" and "reverse lookup area". The reverse lookup area is the IP reverse resolution mentioned here, it is used to query the PTR record of the IP address to obtain the domain name pointed to by the IP address. Of course, to obtain the domain name successfully, you must have the PTR record of the IP address.
So how is reverse IP resolution applied to the mail server to block Spam? Let's take a look at the following example:

One day, a q visited Company A and handed him A business card containing information such as his "hacker murder club" and phone number, company A thought it was necessary to make A simple investigation into the origins of Q, so he called the Telecommunications Bureau of the phone number on Q's business card to check the situation, if the Telecommunications Bureau tells Company A that its phone number does not belong to the "black road killing club", Company A will reject the visit by Q. If the phone number does belong to the "black road killing club ", company A may accept A visit from Q and may further investigate the situation. Therefore, the company called the registrar of the "black road murder club". If the answer was received to confirm that the club had the phone number, company A will accept A visit from Q; otherwise, the visit will be rejected.

In this example, a q is like our mail server, and company A is the mail server of the other party, the "black road murder club" is the HELO domain name used by the email server to communicate with the other's email server (not the domain name after the email address ), the phone number on the business card is the public IP address at the exit of our email server. Company A's investigation of Q is equivalent to A reverse resolution verification process. From this we can see that reverse resolution verification is actually performed by the other server. If we do not do reverse resolution, the reverse resolution verification of the other server will fail, in this way, the recipient's server rejects the email we sent because we are an unknown sender, this is why we cannot send emails to or without reverse resolution after we exclude other reasons (such as being blacklisted by the other party, having no MX records, and using dynamic IP addresses.

So how can we smoothly perform reverse resolution? First, you must have a fixed public IP address and available domain name (preferably not used by other services). For example, if you have a domain name, you can ask your domain name registrar to add an domain name for you and direct its A record to the public IP address at the exit of your SMTP server, for example:, then, contact your ISP (China Telecom or China Netcom) to request reverse resolution of your IP address to After that, do not forget to change the HELO domain name of your SMTP server to


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Nslookup-qt = ptr yourIP

For example, my domain name is
My mail server address is
Email user name format is
Set the HELO of the mail system to on the mail server.
The email server IP address is
Find the local telecom to do the IP anti-resolution to

In Windows, run cmd and then in the window that opens, enter nslookup-qt = ptr

The returned information is as follows:
C: \ Documents ents and Settings \ user> nslookup-qt = ptr

This indicates that the anti-resolution of this IP address is successful.

The returned information is as follows:
C: \ Documents ents and Settings \ user> nslookup-qt = ptr
* ** Can't find server name for address Non-existent domain
Server: UnKnown

This indicates that the anti-resolution of this IP address is not performed.

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