rDNS Look up: What It Is and How to Use It Published: 17 Aug, 2023
Reverse DNS look up (rDNS) is a process of resolving an IP address to a hostname, which is the opposite of the normal DNS lookup that resolves a hostname to an IP address. rDNS Look up is useful for various purposes, such as email security, network troubleshooting, and geolocation. In this article, you will learn what rDNS is, how it works, how to perform a rDNS lookup, how to set up a rDNS record, and how to optimize rDNS for email delivery.
How to Perform a RDNS Look up
A forward DNS lookup is the process of finding the IP address of a hostname, such as www.example.com. A reverseDNS lookup is the process of finding the hostname of an IP address, such as 192.0.2.1. To perform a rDNS look up, you need to use a special domain name called in-addr.arpa, which is used for mapping IP addresses to hostnames in reverse order. For example, to find the hostname of 192.0.2.1, you need to query the domain name 1.2.0.192.in-addr.arpa.
There are different ways to perform a rDNS look up, depending on the tool you use. You can use command-line utilities, such as nslookup, dig, or host, which are available on most operating systems. Alternatively, you can use online services, such as IPinfo.io, IPLocation, or DNS Propagation Checker, which provide a graphical interface and additional information.
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rDNS look up Examples
Here are some examples of rDNS look up results using different tools:
Using nslookup
$ nslookup 192.0.2.1
Server: 8.8.8.8
Address: 8.8.8.8#53
Non-authoritative answer:
1.2.0.192.in-addr.arpa name = www.example.com.
Using dig
$ dig -x 192.0.2.1
; <<>> DiG 9.16.1-Ubuntu <<>> -x 192.0.2.1
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 12345
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;1.2.0.192.in-addr.arpa. IN PTR
;; ANSWER SECTION:
1.2.0.192.in-addr.arpa. 86400 IN PTR www.example.com.
;; Query time: 23 msec
;; SERVER: 8.8.8.8#53(8.8.8.8)
;; WHEN: Tue Aug 15 11:25:34 UTC 2023
;; MSG SIZE rcvd: 77
Using host
$ host 192.0.2.1
1.2.0.192.in-addr.arpa domain name pointer www.example.com.
Using IPinfo.io
To interpret the results of a rDNS lookup, you need to look at the PTR record, which is the type of DNS record that maps an IP address to a hostname for rDNS purposes. The PTR record contains the IP address in reverse order followed by in-addr.arpa as the domain name, and the hostname as the value. For example, the PTR record for 192.0.2.1 is:
1.2.0.192.in-addr.arpa IN PTR www.example.com.
The rDNS lookup results provide information about the owner and location of the IP address, such as the domain name, the organization, the country, and the city. For example, from the IPinfo.io screenshot above, you can see that the IP address 192.0.2.1 belongs to Example Inc., which is located in Los Angeles, United States.
How to Set Up a Reverse DNS Record
To set up a rDNS record for an IP address, you need to have two things: the ownership or control of the IP address, and the ownership or control of the hostname. The ownership or control of the IP address means that you have either purchased or leased the IP address from an Internet Service Provider (ISP) or a hosting provider. The ownership or control of the hostname means that you have registered the domain name and configured the DNS zone for the hostname.
The steps and requirements to set up a rDNS record for an IP address may vary depending on the provider and the scenario, but the general process is as follows:
- Contact the IP owner or ISP and request them to delegate the rDNS authority for your IP address to your DNS server. This means that they will create a PTR record for your IP address in their DNS zone and point it to your DNS server, which will handle the rDNS queries for your IP address. For example, if your IP address is 192.0.2.1 and your DNS server is ns1.example.com, they will create a PTR record like this:
1.2.0.192.in-addr.arpa IN NS ns1.example.com.
- Choose a hostname for your IP address and make sure it is valid and consistent with your forward DNS record. A valid hostname is one that follows the DNS naming conventions and does not contain any invalid characters or spaces. A consistent hostname is one that matches the forward DNS record for the same IP address, which means that the rDNS lookup and the forward DNS lookup return the same hostname. For example, if your IP address is 192.0.2.1 and your hostname is www.example.com, you should have a forward DNS record like this:
www.example.com IN A 192.0.2.1
- Configure the DNS zone for your hostname and create a PTR record for your IP address in your DNS server. This means that you will add a new entry in your DNS zone file for your hostname and specify the IP address in reverse order followed by in-addr.arpa as the domain name, and the hostname as the value. For example, if your IP address is 192.0.2.1 and your hostname is www.example.com, you will create a PTR record like this:
1.2.0.192.in-addr.arpa IN PTR www.example.com.
Using BIND
$ORIGIN 2.0.192.in-addr.arpa.
$TTL 86400
@ IN SOA ns1.example.com. hostmaster.example.com. (
2023081501 ; serial
3600 ; refresh
1800 ; retry
604800 ; expire
86400 ) ; minimum
IN NS ns1.example.com.
IN NS ns2.example.com.
1 IN PTR www.example.com.
To verify and troubleshoot the PTR record, you can use rDNS lookup tools to check if the rDNS lookup returns the correct hostname for your IP address. You can also use online tools, such as DNS Checker, WhatsMyDNS, or DNS Propagation Checker, to check if the PTR record has propagated to different DNS servers around the world.
How to Optimize Reverse DNS for Email Delivery
rDNS affects email delivery because many email servers use rDNS as a way to verify the identity and reputation of the sender. rDNS helps prevent email spoofing, phishing, and spamming by checking if the IP address of the sender matches the hostname of the sender. If the rDNS lookup fails or returns an invalid or inconsistent hostname, the email may be rejected, bounced, or marked as spam by the recipient’s email server.
To optimize rDNS for email delivery, you should follow these best practices and tips:
- Use consistent and valid hostnames for your IP addresses and make sure they match your forward DNS records. This will ensure that the rDNS lookup and the forward DNS lookup return the same hostname for your IP address, which will increase your email credibility and deliverability.
- Avoid using dynamic or generic hostnames for your IP addresses, such as those generated by ISPs or hosting providers. These hostnames may indicate that you are using a shared or residential IP address, which may have a low reputation or be blacklisted by email servers. Instead, use a dedicated or static IP address with a custom hostname that reflects your domain name or brand name.
rDNS Look up FAQs
Here are some frequently asked questions about rDNS based on the people also ask section for the main keyword:
What is reverse DNS used for?
Reverse DNS is used for various purposes, such as:
<!--[if !supportLists]-->· <!--[endif]-->Email security: rDNS helps prevent email spoofing, phishing, and spamming by verifying the identity and reputation of the sender. Many email servers use rDNS to check if the IP address of the sender matches the hostname of the sender. If the rDNS lookup fails or returns an invalid or inconsistent hostname, the email may be rejected, bounced, or marked as spam by the recipient’s email server.
<!--[if !supportLists]-->· <!--[endif]-->Network troubleshooting: rDNS helps diagnose network issues and errors by providing information about the owner and location of the IP address. For example, rDNS can help identify the source of a malicious attack, trace the route of a packet, or locate a server outage.
<!--[if !supportLists]-->· <!--[endif]-->Geolocation: rDNS helps determine the approximate geographic location of an IP address by using the hostname as a clue. For example, rDNS can help estimate the country, region, or city of an IP address based on the domain name suffix or prefix.
How do I find my reverse DNS name?
To find your reverse DNS name, you need to perform a rDNS lookup for your IP address using a tool such as nslookup, dig, host, or an online service such as IPinfo.io, IPLocation, or DNS Propagation Checker. The rDNS lookup will return the hostname associated with your IP address, which is your reverse DNS name. For example, if your IP address is 192.0.2.1 and your hostname is www.example.com, your reverse DNS name is www.example.com.
How do I change my reverse DNS?
To change your reverse DNS, you need to set up a PTR record for your IP address in your DNS server. A PTR record is the type of DNS record that maps an IP address to a hostname for rDNS purposes. To set up a PTR record, you need to have two things: the ownership or control of the IP address, and the ownership or control of the hostname. You also need to contact the IP owner or ISP and request them to delegate the rDNS authority for your IP address to your DNS server. Then, you need to choose a hostname for your IP address and make sure it is valid and consistent with your forward DNS record. Finally, you need to configure the DNS zone for your hostname and create a PTR record for your IP address in your DNS server.
How long does reverse DNS take to propagate?
Reverse DNS propagation is the process of updating and spreading the PTR record for an IP address across different DNS servers around the world. Reverse DNS propagation may take from a few minutes to several hours or days, depending on various factors such as the TTL (time to live) value of the PTR record, the caching behavior of different DNS servers, and the network latency and congestion.
Conclusion
In this article, you have learned what rDNS is, how it works, how to perform a rDNS lookup, how to set up a rDNS record, and how to optimize rDNS for email delivery. You have also learned some frequently asked questions about rDNS and their answers.
rDNS is a useful and important feature of the DNS system that allows you to resolve an IP address to a hostname in reverse order. rDNS can help you with various tasks such as email security, network troubleshooting, and geolocation. To use rDNS effectively, you need to understand how it works and how to set up and maintain your PTR records.
We hope you enjoyed this article and found it informative and helpful.