No one ever said the Internet is squeaky clean. Malicious users and spammers always lurk in the dark corners of the web, ready to bother users and businesses. Thus, the IP blacklist created. While this might sound like a superhero trope, it’s pretty much about the hygiene of the Internet.
IP blacklisting is among the most used methods for avoiding and preventing spam and malware. Nobody likes spammy content, so that’s why blacklists are the go-to for anyone who wants to get rid of spam, malware, and more.
Today we will talk about what an IP blacklist is and how it works. We will also discuss the most common challenges blacklisting often has to face.
What is an IP blacklist?
In essence, it is a list of IP addresses that are known spam sources. You can use these to block IP addresses that you consider suspicious or malicious. Such blacklists are usually used by mail servers and some firewalls to decide whether to flag an email as junk, spam, or drop the traffic altogether.
IP blacklists are also known as DNSBL (Domain Name System-based Blackhole List) or RBL (Real-time Blackhole List). Organizations like Email Service Providers (ESP), Internet Service Providers (ISP), and Anti-spam agencies (ASA) use them to block messages from suspicious addresses.
Thus, they make sure messages sent from these addresses are flagged or rejected before arriving in the recipient’s inbox.
Popular examples of blacklists include:
When it comes to IP addresses only, IP-based blacklists include addresses of sending servers that are considered spammy, suspicious, or fraudulent. This is useful not only in stopping spam emails from being sent from those IPs, but also helps businesses and organizations filter malicious traffic according to their policies.
How does an IP blacklist work?
Depending on the blacklist, the process might be different. Suspicious addresses can be manually added to a blacklist, but there are also other methods of filling such blacklists with unwanted IPs.
For example, some mail servers use something called Feedback Complaint Loop. They collect data from users who keep clicking on the ‘This is Spam’ button whenever they get an unwanted email. This is reported to blacklist administrators, who will mark the IP address used to send those unwanted emails as suspicious or malicious.
Other blocklist operators use spam traps, also called honeypots. These are systems or ‘fake’ email addresses designed to attract spam, so they can immediately blacklist an IP address from which they receive spam.
On checking IP blacklists
There are many blacklists out there, and even more IP addresses (FYI, there are around 4,3 billion IPv4 addresses in use across the Internet). Sure, you can check if a certain IP is blacklisted, either manually in the blacklists of your choice or a blacklist compilator, like MultiRBL. This one checks whether your address is included in many blacklists.
But that’s not going to work – or at least it’s not going to work smoothly – if you want to check thousands of addresses. For that, a few blacklist operators, like Spamhaus or SORB, will let you download their list locally. You can combine that with a suitable script if you want to check not only a single IP address, but an entire block.
How are IPs removed from a blacklist?
Blacklists have different mechanisms and conditions for removing addresses. For some, you need to follow some technical steps, such as unrouting the IPs. For others, you might be required to provide relevant documentation that shows you took care of the problem and won’t make the same mistakes again.
There are cases in which IPs will get whitelisted after a certain period, as long as the address has a more or less clean record and/or you contacted the blacklist administrators and sorted out the problem.
What limitations do IP blacklists have to deal with?
As the Internet evolved more and more, so did the methods of avoiding blacklisting. So, even if they are a good way of preventing suspicious IPs from accessing networks or sending emails, blacklists are not invincible. Let’s have a look at what’s often considered the most prominent challenges they often have to face.
Changing IP addresses
A very straightforward way for spammers and other bad actors to dodge blacklists. They usually work with a range of IP addresses, so if one gets blacklisted, they move on to using another one.
Such rotations often make it difficult for authorities to track the attackers. Besides, businesses or other users will have a hard time fully keeping spammy or malicious content away.
In the case of network attacks like DDoS (Denial Distribution of Service), attackers can make use of IP spoofing to conceal their real IP. Such practice helps them avoid blacklisting and keeps them anonymous.
In some cases, IP spoofing can also enable attackers to trick security systems. They make compromised credentials appear legitimate.
This is one of the most prevalent threats on the Internet. Botnets are created by compromising and taking control over end-user devices and IoT (Internet of Things) devices.
Many attacks are done using large numbers of IP addresses. These might constantly change as devices leave or enter the botnet trap. This volatile nature of botnets gives blacklists such a hard time.
Inaccurate IP detection
There might be cases in which more users are working with the same IP address. When IPs are dynamically assigned, you cannot know who is currently using a particular address.
So, if you block an IP address due to an abusive user, you might prevent a legitimate user from accessing the network via that particular address.
The so-called “collateral damage”
Sometimes, blacklist administrators block traffic that shouldn’t be. There are instances where certain blacklists do this on purpose. This forces large IP block holders to act and prevent spam from reaching their customers.
The problem of clean history
Sometimes, IPs for sale or lease can have a history of being blacklisted, leading to a potentially damaged reputation. This is why businesses that want to get IP blocks should always work with trustworthy IP brokers. IPWAY works only with clients who value transparency and can provide clean, reputable, and customized IP addresses for companies that are looking to lease them.
The future of IP blacklists
Blacklists are used by many. Yet, they are not 100% effective, due to modern methods that allow attackers to avoid ending up on blacklists. Since modern problems need modern solutions, companies can use ‘reputation intelligence’.
In essence, these are services that offer data about users and cyber entities. This data can then be used to decide whether to block or allow activity on a particular IP address. Reputation intelligence bases itself on relevant information about user behavior. This helps in identifying threats according to the historical data of the IP ranges attackers might connect from.
All of this while reducing the need to individually check the potential risks of every network. Reputation intelligence can be called the next step in the evolution of IP blacklists. It helps with bolstering security and helping users identify threats such as:
- Malicious IP addresses;
- Anonymous proxies;
- TOR networks;
- Phishing URLs;
It’s quite clear that IP blacklists are here to stay, but they are not foolproof. Spam and malware will probably never disappear from the Internet, but they are kept in check both by IP blacklists and more modern solutions. Nobody likes getting spam in their mailbox. So, blacklists can help reduce the amount of junk mail by blocking the abusive IP address from which the mail is sent
At the end of the day, it’s important to check the reputation of the IP range you are planning on working with as well. This requires not only diligence from companies looking to get IP addresses but also transparency from IP sellers and brokers.