Web Crawler List: Good Bots, Bad Bots & How to Block Them

Your website gets thousands of visitors every day. Many of them are not people. Some are Googlebot, indexing your pages for search results. Some are Ahrefs, mapping your backlinks. Some are Facebook, reading your Open Graph tags to build link previews when someone shares your URL.

And some are bots designed to click your ads, scrape your product pricing, and stuff your contact forms with fake leads.

Web crawlers are not inherently malicious. The internet depends on good bots. But telling them apart from bad ones, and actually stopping the harmful ones from reaching your site, is harder than it looks.

This guide gives you a complete reference list of the most common web crawlers and bots, explains how bot lists work, and covers exactly why a list alone is not enough to protect your advertising budget and analytics data.

What Is a Web Crawler?

A web crawler is an automated program that systematically browses the internet, visiting pages, reading content, and following links to index, analyze, or copy website data.

The terms "web crawler" and "bot" are often used interchangeably, with a slight distinction worth knowing. A crawler typically moves across the web by following links, discovering new pages as it goes. A bot is any automated program that interacts with websites, which includes crawlers but also programs that target specific pages without navigating the broader web: click bots, form submission bots, and inventory bots, for example.

For practical purposes, both terms describe the same category: automated programs that interact with websites without a real human behind the browser.

Not all crawlers are the same. Some are essential infrastructure for the modern web. Others are designed to exploit the websites they visit. Understanding this distinction is the starting point for managing them intelligently.

What Are the Most Common Good Bots and Crawlers?

Good bots include search engine crawlers, SEO tools, social media link previewers, and AI training crawlers. They are generally identified by official user-agent strings and can be verified against published IP ranges.

Blocking good bots has consequences. Blocking Googlebot removes your site from Google search results. Blocking social media crawlers breaks link previews when your content is shared. The goal is not to stop all crawlers. It is to allow the ones that benefit you and block the ones that do not.

Search Engine Crawlers

Search engines send crawlers to discover, read, and index web pages for their search results. These are the most important good bots to allow, and the most important to verify, because bad bots frequently impersonate them.

Google operates multiple crawlers beyond the main Googlebot. Google-Extended specifically handles AI training data collection and can be blocked in robots.txt without affecting your search rankings. Website owners concerned about their content being used for AI model training can target this crawler specifically.

SEO Tool Crawlers

SEO platforms operate their own crawlers to build keyword databases, backlink indexes, and site audit tools. These are generally safe to allow. Some website owners choose to block them strategically to prevent competitors from easily analyzing their site structure.

Social Media Crawlers

Social platforms send crawlers to read the Open Graph and Twitter Card metadata on your pages. This is how they generate rich link previews when someone shares your URL on their platform.

Blocking social media crawlers prevents rich previews from generating when your content is shared on those platforms, which reduces click-through rates from social shares.

AI Training Crawlers

A growing category of crawlers has emerged as AI companies collect training data from the web. Unlike search crawlers, these bots do not drive traffic back to your site. Whether to allow or block them is a business decision about content use rather than a technical one.

All of these can be blocked in robots.txt without affecting search indexing.

Monitoring and Performance Crawlers

These bots check your site's uptime, page speed, and technical health. They originate from monitoring and development tools.

What Are the Most Common Bad Bots?

Bad bots include click fraud bots, web scrapers, credential stuffers, spam bots, and vulnerability scanners. Unlike good bots, they operate without permission and cause direct, measurable harm to the sites they target.

No public list covers all bad bots. New bot infrastructure launches constantly, and professional bot operations specifically choose fresh IP addresses and user-agents that appear on no existing blocklist. The categories below describe the types, not a fixed inventory.

Click Fraud Bots

Click fraud bots visit pages with advertising, click on ads, and generate fraudulent charges against ad budgets. Simple versions run basic scripts. Sophisticated ones use antidetect browsers specifically designed to mimic real visitor behavior across every dimension of a browsing session.

The cost goes further than the click itself. Bots that click ads enter retargeting audiences, meaning you pay again to show them follow-up ads. They send signals to Smart Bidding algorithms that train on the wrong data. They contribute to conversion events that make unprofitable campaigns appear to be working. Our click fraud guide covers how this compounding damage builds over time.

Scrapers and Content Thieves

Web scrapers automatically copy content, product data, pricing, and proprietary information from websites. For publishers, scraped content appearing on other domains dilutes SEO authority and cannibalizes search rankings. For retailers, scraped pricing allows competitors to undercut you automatically in real time.

Many bot farms operate scraping as a paid service, providing competitive intelligence to businesses that pay for it.

Credential Stuffing Bots

Credential stuffing bots take lists of usernames and passwords leaked from data breaches and systematically test them against login forms. Because many people reuse passwords across multiple services, a breach at one site enables account takeover attempts across many others.

This is one of the scenarios where a CAPTCHA at the login page provides genuine value, adding per-attempt friction that makes mass automated testing impractical. The broader distinction between CAPTCHAs and bot detection is covered in our CAPTCHA vs bot detection guide.

Spam Bots

Spam bots submit fake entries to contact forms, registration pages, and comment sections. The goals vary: publishing promotional links, flooding CRMs with junk leads, or overwhelming support pipelines with fake inquiries. For businesses where sales teams work inbound leads, contaminated pipelines waste significant time on contacts that will never respond.

How Do Bot Lists Work?

Bot lists identify crawlers by user-agent string, IP address, or ASN range. Websites check incoming requests against these lists to decide whether to allow, block, or challenge each visitor.

Two mechanisms form the foundation of most list-based bot management.

User-Agent Matching

Every web request includes a User-Agent header: a string that identifies the software sending the request. Googlebot sends "Googlebot/2.1 (+http://www.google.com/bot.html)". AhrefsBot sends "AhrefsBot/7.0". Your robots.txt file uses these strings to instruct crawlers which pages to visit:

Web application firewalls and CDN services use the same mechanism to block specific user-agents at the network edge. The critical limitation: user-agent strings are trivially spoofable. Any program can send any string it wants. Bad bots routinely claim to be Googlebot to avoid user-agent-based filters.

IP Address and ASN Blocklists

The second mechanism checks the IP address making the request against databases of known bot infrastructure: data center IP ranges, commercial proxy services, and previously identified malicious addresses.

Google publishes its official Googlebot IP ranges for verification. Data center ASN (Autonomous System Number) ranges for major cloud providers (AWS, Google Cloud, Azure) can be blocked as a group, since most legitimate web users do not browse from data center IPs.

IP-based blocking has the same fundamental limitation as user-agent matching: the identifiers rotate and can be spoofed. Residential proxy networks route bot traffic through real home internet connections from real internet service providers, making requests appear to originate from legitimate residential addresses that no blocklist covers.

What robots.txt Actually Does

robots.txt is a text file that tells crawlers which pages they can and cannot access. It is the standard protocol for managing good bot behavior: directing Googlebot away from staging environments, preventing SEO tools from crawling admin pages.

robots.txt is a courtesy signal, not a security control. Legitimate crawlers respect it. Malicious bots ignore it completely. It has no ability to enforce compliance.

Why Aren't Bot Lists Enough to Protect Your Website?

Bot lists are static and reactive. IP addresses rotate constantly, user-agents are spoofable, and residential proxies make bot traffic appear to come from legitimate home connections. Lists can only catch bots already catalogued.

This is the protection gap that leaves most websites exposed when they rely on lists alone.

The Rotation Problem

IP addresses used by bot operations are not fixed. Professional bot networks rotate across thousands of addresses continuously to avoid triggering blocklists. By the time a specific IP address gets added to a list, the bot operation has already moved to fresh infrastructure.

Residential proxy networks compound this problem significantly. Services that route traffic through real home internet connections provide addresses that no IP blocklist covers, because blocking residential IP ranges would also block millions of real people.

The Spoofing Problem

User-agent strings are plain text. Any program can declare itself to be anything. A bot configured to impersonate Googlebot sends "Googlebot/2.1" and passes through most user-agent-based filters without any obstruction.

The correct defense against Googlebot impersonation is reverse DNS verification: checking whether the IP address resolves to a legitimate Google hostname (googlebot.com or google.com). Most basic blocklist implementations do not perform this check.

The Unknown Bot Problem

Every bot list is a historical record. It only identifies bots that have already been observed, analyzed, and added. New bot infrastructure, new evasion tools, and new operators launching campaigns use fresh addresses and user-agents that no list has ever seen.

The invalid traffic that costs advertisers billions annually comes predominantly from bots operating with clean infrastructure specifically chosen to avoid every existing list. Static lists do not adapt to new threats. Bot operators specifically take advantage of this.

The Scope Problem

Bot lists only protect the entry points where you implement them. They do nothing about what happens in the first milliseconds of a page load, before any list check can run.

When a bot visits your landing page, your tag management container fires immediately. Your Google Ads click is recorded. Your Meta pixel sends a PageView. Your retargeting audience gains a new member. All of this happens before any list-based rule has had a chance to block the request.

A bot can be on every blocklist that exists, and if your pixels fire before the block, the ad fraud damage is already done.

Want to see how much bot traffic is getting through to your site right now? Try our free traffic analyzer. No signup required.

How Do You Use Crawler Lists Effectively?

Use robots.txt to manage good bot access, CDN-level firewall rules to block known bad infrastructure, and reverse DNS verification to confirm claimed identities of legitimate crawlers.

Even with their limitations, bot lists are a practical foundation for crawler management. Here is how to use them well.

robots.txt for Good Bot Management

robots.txt is the right tool for controlling legitimate crawler behavior. Block AI training crawlers if you do not want your content used for model training, without affecting search indexing:

Keep your robots.txt updated as new major crawlers emerge. AI training bots in particular are a fast-growing and frequently updated category.

CDN and WAF Rules for Known Bad Infrastructure

Cloudflare's Bot Fight Mode and similar products use a combination of user-agent matching, IP reputation databases, and behavioral analysis to block known malicious traffic at the network edge. This reduces server load and provides a baseline protection layer.

Web application firewalls can apply block rules based on ASN ranges associated with known bot infrastructure and user-agent patterns of common attack tools. This is the appropriate layer for blocking vulnerability scanners and known scraping tools.

Verify Claimed Crawlers with Reverse DNS

Any crawler claiming to be Googlebot can be verified. Perform a reverse DNS lookup on the IP address. If it resolves to a hostname ending in googlebot.com or google.com, the claim is legitimate. If it resolves to anything else or fails to resolve, the bot is impersonating Googlebot.

This verification matters because Googlebot impersonation is common among scrapers and malicious crawlers that want to be automatically allowed through filters.

When Do You Need More Than a Bot List?

When bots are draining your advertising budget, corrupting your analytics data, or reaching your site through residential proxies that bypass every list you have, real-time detection is what is actually needed.

Bot lists are the right tool for crawler management and known infrastructure blocking. They are the wrong tool for protecting paid advertising campaigns against the sophisticated bots that target ad budgets specifically.

The bots causing the most damage to advertisers operate specifically to evade detection. They use residential proxies not covered by any IP list. They rotate user-agents continuously and never match a known bad bot signature. They pass every list check cleanly while still clicking your ads, firing your pixels, and contaminating your retargeting audiences.

The limitation is structural: list-based protection is reactive. It identifies threats that already exist in the catalogue. The bots actively targeting advertising budgets treat this as an engineering constraint to design around, not an obstacle.

Real-time bot detection addresses this differently. Rather than checking visitors against a list of known bad actors, it analyzes every session's full behavioral and technical profile, looking for the contradictions that reveal automated behavior regardless of whether the bot appears on any list. A bot using a clean residential IP and a legitimate-looking user-agent still produces technical and behavioral signals that are inconsistent with a real human browsing session.

For businesses where bots are actively wasting advertising budget, a layered approach that combines blocklists with real-time behavioral detection provides comprehensive coverage. Our ad fraud prevention guide covers how these layers work together. For evaluating specific tools that go beyond list matching, see our bot detection software comparison. And for the manual investigation steps to assess your current bot exposure using GA4, the how to detect bot traffic guide walks through the process.

Hyperguard identifies bots based on behavioral and technical signals, not blocklists. It works against bots that no list has ever seen. Setup takes under 5 minutes. See how it works or get started today.

Frequently Asked Questions

What is a web crawler?

A web crawler is an automated program that systematically browses the internet by visiting web pages, reading their content, and following links to discover more pages. Search engines use crawlers to build their indexes, SEO tools use them to track backlinks and rankings, and social platforms use them to generate link previews. Not all crawlers are beneficial: bad bots use the same mechanism to click ads, scrape content, and submit fake leads.

How do I see which crawlers are visiting my website?

Server logs record every request your site receives, including the user-agent string and IP address of each visitor. Filtering log entries by user-agent string reveals which crawlers visit most frequently and how often. Google Search Console shows Googlebot's specific crawl activity. For a broader investigation of bot traffic using GA4, our guide on how to detect bot traffic walks through the step-by-step process.

What is the difference between a good bot and a bad bot?

Good bots perform legitimate functions that benefit website owners or the web broadly: indexing content for search, generating link previews, monitoring uptime, and analyzing performance. They identify themselves with accurate user-agent strings and generally respect robots.txt instructions. Bad bots operate without permission and cause harm: clicking ads fraudulently, scraping proprietary content, testing stolen passwords, or submitting spam. The practical difference matters because blocking good bots causes collateral damage (losing search rankings, broken previews), while failing to block bad bots allows ongoing harm.

Does robots.txt actually stop bots?

robots.txt is a courtesy protocol, not a security control. Legitimate crawlers respect robots.txt instructions. Malicious bots ignore it completely. robots.txt is the right tool for managing good bot access: telling Googlebot to avoid staging environments, directing SEO tools away from internal pages. It provides no protection against bots that do not comply with the standard.

How do I block bad bots without blocking Googlebot?

Verify Googlebot's identity using reverse DNS lookup before applying any block rules. If the requesting IP resolves to a hostname ending in googlebot.com or google.com, it is legitimate and should be allowed. If it does not resolve correctly, block it regardless of what user-agent string it claims. For CDN-level rules, target specific bad bot user-agent patterns rather than broad network blocks that could include search engine infrastructure.

Why do bots keep getting through my blocklist?

Bot operations specifically rotate IP addresses to stay ahead of blocklists. When an IP gets added to a list, the operation moves to fresh infrastructure. Residential proxy networks make this worse by routing bot traffic through real home internet connections that no IP blocklist covers. User-agent strings are also trivially spoofable. Every static list has a built-in lag between when a new bot appears and when it gets catalogued, and professional bot operations exploit that lag deliberately.

What is the IAB/ABC International Spiders and Bots List?

The IAB/ABC International Spiders and Bots List is an industry standard catalogue of known bot user-agents maintained by the Interactive Advertising Bureau. GA4 and most major analytics platforms use this list to filter known crawlers from your analytics data. However, it only identifies bots already catalogued. Sophisticated bots designed to evade detection never appear on this list, which is why GA4's built-in bot filtering misses the ones causing the most damage to ad campaigns.

Can I block all bots and just allow real visitors?

Technically possible, but not practical without consequences. Blocking Googlebot removes your site from Google search. Blocking social media crawlers breaks link previews. Blocking monitoring bots removes your uptime visibility. A targeted approach works better: allow verified good bots, block known bad infrastructure, and use real-time behavioral detection to catch the sophisticated bots that bypass every list. Our guide to bot detection software covers the tools that handle this detection layer.