Domain name reverse resolution

    Domain name is a set of address translation system specially established for the convenience of memory. To access a server on the Internet, it must be realized by IP address. Domain name resolution is the process of re-translating a domain name into an IP address. This process is accomplished through the domain name resolution system DNS. 
Next, let's first understand what is IP reverse resolution. There are two areas in the DNS server we often use, namely "forward lookup area" and "reverse lookup area". The forward lookup area is what we usually call domain name resolution, and the reverse lookup area is what we mean here. Its function is to obtain the domain name pointed to by the IP address by querying the PTR record of the IP address. Of course, to obtain the domain name successfully, the PTR record of the IP address must be obtained. A PTR record is a type of mail exchange records. There are A records and PTR records in the mail exchange records. The A records resolve names to addresses, and the PTR records resolve addresses to names. Address refers to a client's IP address, and name refers to a client's fully qualified domain name. By querying the PTR records, the purpose of reverse checking is achieved. The Reverse DNS function ensures that the appropriate mail exchange records are in effect. This is one of the most common problems (especially in foreign mail systems). More email providers use reverse DNS lookups to confirm where the information is coming from. As the use of this method becomes more widespread, mail bounces may occur more often for domains that do not properly publish reverse DNS information. Therefore, when the forward domain name resolution is completed, you should also apply to your ISP for reverse address resolution, especially for foreign-related emails, to reduce the possibility of being bounced by foreign institutions. 
The reverse parsing verification is actually performed by the other server. If we do not perform reverse parsing, then the reverse parsing verification of the other server will fail, so that the other server will reject the message we send to because we are an unknown sender. Mail, which is the reason why we cannot send mail to the mail server without reverse analysis after excluding other reasons (such as being blacklisted by the other party, having no MX record, using a dynamic IP address, etc.). 
To do reverse resolution, you must first have a fixed public IP address and an available domain name. For example, if you have the domain name of abc.com, you can ask your domain name registrar to add a domain name of okmail.abc.com for you and point its A record to your SMTP server export public IP address. The ISP you belong to requires a reverse resolution to okmail.abc.com for your IP. Don't forget to change the HELO domain name of your SMTP server to okmail.abc.com after completion, so as to achieve the purpose. 
     
Successfully do IP reverse analysis (PTR record) In today's flood of spam, spam has brought great harm to our life, work and study. Due to the lack of an effective sending authentication mechanism between the SMTP servers, even if the spam identification and blocking technology is used, the effect is still average. Moreover, the spam identification and blocking technology is mainly used to identify the letter after receiving the letter according to certain conditions, which requires a lot of server resources. , If you can take certain measures before the letter reaches the server, it can greatly improve the efficiency of the server. Therefore, many mail servers such as sina.com, hotmail.com, yahoo.com.cn, etc. have adopted the spam identification and blocking technology + IP reverse parsing and verification technology to better block spam. 
       Let's first understand what is IP reverse resolution. In fact, friends who have worked as a DNS server will know that there are two areas in the DNS server, namely "forward lookup area" and "reverse lookup area". The reverse lookup area is the IP reverse resolution mentioned here. The function is to obtain the domain name pointed to by the IP address by querying the PTR record of the IP address. Of course, to obtain the domain name successfully, the PTR record of the IP address must be obtained. So how is IP reverse resolution applied to mail servers to block spam? Let's take a look at the following example: One day, Ah Q visited Company A, and he handed him a business card. The business card stated that he was from the "underworld killing club" and his phone number. Company A felt that it should treat Ah Q's I did a simple investigation on the origin, so I called the telecommunications bureau to which the phone number on Ah Q's business card belonged to check. If the telecommunications bureau told Company A that its phone number did not belong to the "underworld murder club", Company A would refuse Ah Q's visit. If its phone number does belong to the "Underworld Killing Club", Company A may accept Ah Q's visit or may further verify it, so it calls the registration organization to which the "Underworld Killing Club" belongs. If the answer confirms that the club does have this phone number, then Company A will accept Ah Q's visit, otherwise it will still refuse Ah Q's visit. In this example, Ah Q is like our mail server, Company A is the other party's mail server, and "Underworld Murder Club" is the HELO domain name (not the domain name after the email address @) used by our mail server to communicate with the other party's mail server. The phone number on the business card is the public IP address of our mail server exit. The process of Company A's investigation of Ah Q is equivalent to a reverse analysis and verification process. From this, it can be seen that the reverse parsing verification is actually performed by the other server. If we do not perform reverse parsing, then the reverse parsing verification of the other server will fail, so that the other server will reject us as an unknown sender. Receive emails from us, that is, after we exclude other reasons (such as being blacklisted by the other party, having no MX record, using a dynamic IP address, etc.), we cannot send messages to sina.com, The reason why homail.com sent the letter. 
       So how should we do reverse parsing smoothly? First of all, you must have a fixed public IP address and an available domain name (preferably not used by other services). For example, if you have a domain name of lunch-time.com, you can ask your domain registrar to add an okmail.lunch-time for you. com domain name, and point its A record to your SMTP server export public IP address, such as: 220.112.20.18, then please contact the ISP to which your fixed IP belongs to request reverse resolution for your IP to
okmail.lunch-time .com. Don't forget to change the HELO domain name of your SMTP server to okmail.lunch-time.com after completion, so that the purpose can be achieved. In addition: 
1. Letters sent to sina.com after reverse analysis may be transferred to the "unknown mail folder". At this time, please contact sina.com and ask to solve the problem for you. 
2. To check whether the reverse parsing is successful, use the following command: nslookup –qt=ptr yourIP, you can see the result of reverse parsing from the returned information. 
3. There must be a fixed public IP address and a real available domain name. At the same time, it should be noted that the A record of the reversely resolved domain name must point to this IP! 
4. For those with multiple fixed public IP addresses, you only need to do the reverse resolution of the main export IP. No matter how many SMTP servers you have, as long as you connect to the external network through this IP, change the HELO domain name of these SMTP servers. It can be converted into the domain name of IP reverse resolution. (It has nothing to do with the address domains on these SMTP servers.) 
      The domain names and IP addresses appearing here are all virtual information. If any inconvenience is caused to the real rights holders, please inform them, but I do not take any legal responsibility. How to do reverse domain name resolution (reverse DNS resolution) Suppose your server IP is 211.147.9.106 1) First, you need to know which DNS server is responsible for the reverse domain name resolution of this IP. You can use this page to query reverse domain name resolution information 

It probably means that you need to contact ns.cnc.ac.cn, and ns.cnc.ac.cn is responsible for the reverse domain name resolution of 211.147.9.106. 2) Then you can ask your IP provider, that is your hosting provider, if your business users have a fixed IP, you should find Telecom. Ask them to contact ns.cnc.ac.cn, add a reverse domain name resolution record to your IP, and generally reversely resolve this IP into the mx record corresponding to your mail server.