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No one likes spam, and while most of it is simply a slight annoyance, there can be dangerous implications to not filtering your incoming mail. While simple spam can just be deleted, certain unwanted messages can include malicious code or links that take you to phishing sites. In order to improve your protection against such attacks, and to simply get rid of pesky spam messages, many turn to a Domain Name System Blacklists or DNSBL.
What is DNSBL?
Domain Name System Blacklists (DNSBL) are spam blocking lists that allow system moderators to block messages from specific systems that have a history of sending spam. As the name suggests, these lists are based on the Domain Name System (DNS) which converts numerical IP addresses into domain names. If the maintainer of the DNSBL receives spam from a specific domain name, that server would be blacklisted and all messages sent from it would either be flagged or rejected by any sites that use the said list.
There are three basic components that make up a DNSBL:
- A domain name to host it under
- A server to host the aforementioned domain
- A list of addresses that make up the blacklist
The History of DNSBLs
The first DNSBL was created by MAPS (i.e. Mail Abuse and Prevention System) in 1997. They called it the Real-time Blackhole List (RBL) and its original purpose was to block spam emails and educate ISPs and other websites on spam and its prevention. Nowadays, DNSBLs are rarely used for educational purposes, however, their primary purpose as a spam blocker and filter is being served to this day. In fact, nearly all email servers support at least one DNSBL in order to minimize the amount of junk mail their users receive.
How are blacklists created?
DNS blacklists (DNSBLs) are created to identify and block email from sources of spam or malicious activity. Here's an overview of how they are typically created:
- Pattern Recognition
By analyzing emails, some patterns such as repeated senders, IP addresses, domains, and content characteristics will be recognized which are commonly used by spammers.
- User Reports
Users and administrators report spam emails to the DNSBL maintainers.
- Criteria for Listing
DNSBL maintainers have some specific criteria to determine whether an IP or domain should be listed such as the volume of spam sent, the presence of malware, or the history of malicious behavior.
- Spam Traps
Special email addresses that are not used for legitimate communication are deployed. Any email received at these addresses is considered unsolicited and thus likely spam.
By following these steps and collecting the necessary information, DNS blacklist will create a blacklist to protect email systems and users from spam and other threats.
How does one get on a blacklist?
There are several anti-spam techniques and spam filters that have criteria to check whether an email is spam or not; therefore, if you step on one of their criteria, you will get blacklisted. Being added to a DNS blacklist involves exhibiting behaviors that are identified as sources of spam or malicious activity. Here are some common behaviors that businesses should avoid if they do not want to end up on a DNS blacklist:
- Sending Spam
You will probably end up on a DNS blacklist if you send large volumes of unsolicited emails to recipients or be reported by multiple recipients or email providers as a source of spam.
- Acting like a spammer
You will probably end up on a DNS blacklist if you show behaviors that are typical of spammers, such as sending emails with deceptive subject lines and spam words or using obfuscation techniques to hide the content or sender. If a receipt flags your email as spam, even when your email is legitimate, the email provider will consider you as a spammer.
- Compromised Systems
You will probably end up on a DNS blacklist if you have an IP address or domain associated with a server that has been compromised and used to send spam, malicious emails or operate an open relay or open proxy that allows anyone to send email through it, which can be exploited by spammers.
- Email lists
You will probably end up on a DNS blacklist if you acquire unverified email lists, since these lists usually contain some old and outdated emails. Maintain a good email list by regularly cleaning your mailing lists with the help of email verifying tools.
Modern DNSBLs
More than 20 years have passed since the introduction of RBL, and since then, dozens of different blacklists have sprung up and are available for public use. They all have their own criteria of what they do and do not consider spam, therefore all of them have their own lists. Because of this, DNSBLs vary vastly from one another. Some are way stricter than others, and some are much more lenient. There are DNS blacklists that list sites only for a set amount of time since the last received spam, while others are monitored and managed completely manually. In fact, some of the DNSBLs not only block the IP address of the website but even sometimes entire Internet Service Providers (ISPs) known to host spammers.
The variety of DNS blacklists available allows the user to choose the desired blacklists based on how well the DNSBL’s criteria for spam match with the user’s needs. Less lenient lists might allow some spam to be let through, but not block misidentified non-spam messages and vice versa.
Why Use a DNSBL?
Doing a DNSBL lookup on an email message during the SMTP connection is cheap in hardware cycles and system time. If the MTA already knows the incoming message is spam it can deny a spam message before having to take additional action; The DNS server may even have the results cached from previous attempts!
System costs:
- Passing it to a mail-scanner (medium cost);
- Using a Bayesian filter (medium)
- Running it through a virus scanner (medium to expensive)
- Doing SpamAssassin network tests that check blocklists, DCC, pyzor, razor, etc. (medium to expensive)
Mail rejected by a DNSBL during delivery is not silently discarded. A real-time DNSBL rejection creates a delivery status notification (DSN) to the sender identifying the cause of the rejection, allowing troubleshooting on the sender's end. Real-time rejection avoids the backscatter problem of some spam filters which accept delivery, close the connection, and then try to return the mail after it is determined to be spam.
Consequences of being added to a DNS blacklist
When your IP is blacklisted, your email will end up in the spam folder rather than the inbox. Being added to a DNS blacklist has several consequences which can be extremely harmful, especially for websites and email servers. Some of them are mentioned below:
- Reputation Damage
The most obvious consequence! If you are blacklisted, it will harm the reputation of your business or organization and it shows that your domain or IP address have been involved in malicious activities, such as sending spam or hosting malware.
- User Trust
If a user faces issues accessing a website or receiving spam emails, it can lead to losing trust in the organization. Consequently, customer loyalty and satisfaction will decrease over time.
- Search Engine Ranking
DNS blacklisting can indirectly affect search engine optimization (SEO). Search engines might consider a blacklisted domain less trustworthy, which could impact its ranking
- Email Delivery Issues
Emails sent from a blacklisted domain or IP address are likely to be blocked or marked as spam by major email providers. This can severely affect communication, especially for those relying on email marketing or customer support.
- Website Accessibility
A website with a blacklisted IP address may become inaccessible to users who rely on DNS servers that enforce these blacklists. This can lead to a loss of traffic and potential revenue and obstruct their online presence.
- Financial Costs
Losing traffic, potential revenue, customers trust, and many other opportunities, coupled with the resources needed to address the blacklisting, will undoubtedly result in substantial financial costs.
How to find out if you are on the blacklist?
To find out if your IP address or domain is on a DNS blacklist, you can follow these steps:
Using tools and services
You can use online blacklist-checking tools such as MXToolbox, DNSBL.info, and WhatIsMyIPAddress. In order to use these tools, you just need to enter your IP or domain on these website and they will show your status.
Sending a test email
To check your status, notice if your emails are being marked as spam or not delivered at all, this could be an indication that you are blacklisted.
Checking email metrics
You can also use email service provides. Many providers offer delivery reports that show if your emails are being blocked or not.
How to remove your IP from the list
If you have found yourself on a blacklist, do not worry. Here are some actions that can help maintain your domain and IP reputation, ensuring smooth email delivery and website access.
Identify what blacklists you're on
Firstly, you need to identify your blacklist. Tools like MXToolbox, DNSBL, or MultiRBL can help you to check which blacklists your IP is on.
Contact the administrators of blacklist services
If you are truly innocent and you have been blacklisted by mistake, contact the administrators and explain your issue and asked for their help.
Run security scan
You can also run comprehensive security scans to identify and resolve any issues.
To prevent it from happening again, you have to investigate why your IP was blacklisted in the first place. Common reasons include sending spam, malware, or other malicious activity. Remove malware, secure open relays, prevent further abuse, and ensure your network and email systems are secure. Finally, make sure you have proper SPF, DKIM, and DMARC records set up.
Conclusion
Having a DNSBL is recommended, especially if you receive lots of spam. It can even reduce your chances of being a target of phishing and malicious code injection. We hope that with the help of this article, you were able to better understand what DNSBL is and why you should use it. If you have any questions or suggestions, please leave them in the comment section below.
