Web crawlers are automated systems used by search engines, retrieval platforms, SEO tools, social networks, and AI systems to discover, access, evaluate, and revisit webpages across the internet.
For SEO, crawlers matter because they are often the first layer between your website and a searchable index. They help systems understand your site structure, discover new content, process updates, interpret links, and decide when a page should be revisited.
In simple terms: crawlers continuously explore the visible structure of the web in order to maintain searchable memory.
What Web Crawlers Do
A crawler visits URLs, downloads resources, follows links, reads directives, and sends information back to the system that operates it. That system may be a search engine, an SEO platform, a social media preview tool, a browser assistant, or an AI retrieval service.
Crawlers can be used to:
- Discover new webpages.
- Revisit known pages for updates.
- Follow internal and external links.
- Read sitemaps and feeds.
- Check HTTP status codes.
- Process canonical tags, robots directives, and structured data.
- Render pages that rely on JavaScript, depending on the crawler.
- Collect information for search indexes, link graphs, previews, reports, or retrieval systems.
Not every crawler has the same purpose. A search engine crawler is different from a social media preview crawler. An SEO tool crawler is different from a crawler used for AI training or live retrieval. Treating every bot as the same can lead to poor decisions.
How Crawling Works
Most web crawling follows a repeated sequence. The details vary by platform, but the basic pattern is stable.
1. URL Discovery
A crawler needs a place to start. It may discover URLs through:
- Links from other pages.
- Internal navigation.
- XML sitemaps.
- RSS or Atom feeds.
- Previously known URLs.
- Redirect chains.
- External links from other websites.
This is why internal linking matters. If an important page is isolated, not included in a sitemap, and rarely linked, crawlers may find it slowly or treat it as less central to the site.
2. Fetching
After discovering a URL, the crawler requests it from the server. The server responds with an HTTP status code, headers, and content.
Common status codes include:
- 200: The page loaded successfully.
- 301 or 308: The page has permanently moved.
- 302 or 307: The page has temporarily moved.
- 404: The page was not found.
- 410: The page is intentionally gone.
- 500-level errors: The server had a problem responding.
Search crawlers can usually tolerate some errors. Persistent errors on important URLs, however, can reduce discovery, delay updates, or create index quality issues.
3. Robots and Access Checks
Before or during crawling, many legitimate crawlers check robots.txt. This file tells crawlers which paths are allowed or disallowed for crawling.
Example:
User-agent: *
Disallow: /private/
Allow: /
Sitemap: https://www.example.com/sitemap.xmlrobots.txt controls crawling behavior for compliant bots. It does not securely protect private content. Sensitive content should be protected with authentication, server-side access control, or removed from public availability.
4. Parsing and Link Extraction
Once a crawler accesses a page, it may parse the HTML to identify:
- Links.
- Canonical tags.
- Meta robots tags.
- Headings and body content.
- Structured data.
- Images, scripts, and stylesheets.
- Language and hreflang signals.
This is where clean HTML, readable navigation, and consistent internal links help. A crawler does not “experience” a website the way a human does, but good structure often helps both.
5. Rendering
Some crawlers can render JavaScript. Others cannot, or do so only partially. Googlebot can render many JavaScript-based pages, but rendering may happen after the initial HTML crawl and may require additional processing.
For SEO-critical content, avoid relying entirely on client-side rendering unless you have tested how search crawlers see the page. Important text, links, canonical tags, and metadata should be available as reliably as possible.
6. Scheduling Future Crawls
Crawlers decide when to revisit URLs. Revisit frequency can be influenced by many signals, including:
- How often the page changes.
- How important the URL appears within the site.
- Server reliability.
- Internal and external link signals.
- Sitemap freshness hints.
- Past crawl behavior.
A homepage or frequently updated category page may be crawled more often than an old, rarely changed page buried deep in the site.
Crawling vs. Indexing vs. Ranking
These three concepts are related, but they are not the same.
- Crawling means a system discovered and accessed a URL.
- Indexing means the system stored and made the content eligible for retrieval.
- Ranking means the system selected and ordered results for a query or task.
A page can be crawled but not indexed. A page can be indexed but not rank prominently. A page can rank for one query and be irrelevant for another. Good SEO work keeps these layers separate so problems can be diagnosed accurately.
Important Crawlers SEOs Should Recognize
The crawler landscape changes over time. User-agent names, policies, and documentation may shift. Still, there are several crawler families that SEOs should understand because they appear often in logs, SEO tools, server rules, and platform documentation.
Search Engine Crawlers
| Crawler | Operated By | Why SEOs Should Know It |
|---|---|---|
| Googlebot | Google’s main web crawler. It discovers and revisits pages for Google Search. Googlebot has smartphone and desktop variants, with mobile-first indexing being the normal frame for most sites. | |
| Googlebot-Image | Used for image discovery and image search processing. Important for sites where images drive search visibility, such as ecommerce, publishing, recipes, visual portfolios, and local services. | |
| Googlebot-News | Relevant for eligible news publishers. Not every site needs to optimize for it, but publishers should understand how Google accesses fresh news content. | |
| Bingbot | Microsoft Bing | Bing’s primary crawler. Bing also powers or contributes to several search and discovery experiences, so Bingbot should not be ignored. |
| Applebot | Apple | Used by Apple for search-related and assistant-related experiences. It may affect visibility in Apple-controlled discovery environments. |
| DuckDuckBot | DuckDuckGo | Associated with DuckDuckGo crawling. DuckDuckGo also uses multiple sources, so DuckDuckBot is only one part of its discovery ecosystem. |
| YandexBot | Yandex | Relevant for sites targeting markets where Yandex is used. Less important for many U.S.-focused websites, but still common in server logs. |
| Baiduspider | Baidu | Relevant for sites targeting Chinese search visibility. Many sites outside that market choose to limit it depending on audience and server resources. |
Google Crawlers and Related User Agents
Google uses several crawler user agents for different purposes. SEOs should be careful not to block important Google crawlers accidentally.
- Googlebot Smartphone: The main Google crawler for mobile-first indexing.
- Googlebot Desktop: Still used, but mobile-first indexing means the smartphone crawler is usually the more important view.
- Googlebot-Image: Used for image search and image understanding.
- Googlebot-Video: Used for video discovery and processing.
- AdsBot-Google: Used in relation to Google Ads landing page quality and ad systems. It is not the same as organic Googlebot.
- GoogleOther: A Google crawler used for various non-primary search tasks. Its purpose is broader and may not map directly to traditional organic indexing.
If you are managing access rules, separate organic search crawlers from ads, testing, preview, and other Google user agents. A broad block can have unintended consequences.
SEO Tool Crawlers
SEO tools use crawlers to build link indexes, audit websites, monitor technical issues, and estimate competitive visibility. These crawlers do not directly determine search rankings, but they are often useful for diagnostics.
| Crawler | Associated Platform | Why SEOs Should Know It |
|---|---|---|
| AhrefsBot | Ahrefs | Used for backlink and web index data. Often visible in logs. Some sites allow it for third-party link analysis; others limit it to reduce server load. |
| SemrushBot | Semrush | Used for SEO visibility, backlink, and competitive research datasets. Can be useful, but may not be necessary for every site to allow. |
| Moz DotBot | Moz | Used for Moz link index and SEO metrics. Relevant if you rely on Moz data or want your site represented in that index. |
| Screaming Frog SEO Spider | Screaming Frog / user-operated | Usually run manually by site owners or SEOs. Useful for auditing internal URLs, metadata, status codes, canonicals, and technical structure. |
| Sitebulb Crawler | Sitebulb / user-operated | Another audit crawler used by SEOs to inspect site architecture, internal linking, indexability, and rendering issues. |
SEO tool crawlers are not automatically good or bad. The right choice depends on server capacity, reporting needs, privacy concerns, and whether you want third-party tools to include your pages in their datasets.
Social and Preview Crawlers
Social platforms crawl pages to generate link previews. These crawlers often request a page when someone shares a URL in a post, message, or profile.
| Crawler | Platform | Why It Matters |
|---|---|---|
| facebookexternalhit | Meta / Facebook | Fetches page information for Facebook previews. Open Graph tags can influence the title, description, and image shown. |
| Twitterbot | X / Twitter | Fetches page data for shared URL previews. Twitter Card tags can help control preview appearance. |
| LinkedInBot | Fetches preview information when URLs are shared on LinkedIn. | |
| Slackbot-LinkExpanding | Slack | Generates previews when links are shared in Slack workspaces. |
| Discordbot | Discord | Fetches page preview data when links are shared in Discord. |
These crawlers are not usually about rankings. They are about presentation, sharing, and user experience. If a shared link shows the wrong image or description, preview crawler access and metadata are good places to check.
AI and Retrieval-Related Crawlers
AI-related crawlers are increasingly important, but this area changes quickly. Some bots are used for training datasets, some for live retrieval, some for user-triggered browsing, and some for platform-specific search experiences.
Examples SEOs may see include:
- GPTBot: Associated with OpenAI crawling for model-related purposes, according to OpenAI’s public documentation.
- ChatGPT-User: Associated with user-triggered browsing or retrieval contexts in OpenAI systems.
- ClaudeBot: Associated with Anthropic crawling, depending on current public documentation and policy.
- PerplexityBot: Associated with Perplexity’s retrieval and answer systems.
- CCBot: Operated by Common Crawl, whose public web datasets may be used by many downstream systems.
- Google-Extended: Not a crawler in the same simple sense as Googlebot, but a user-agent token Google has documented for controlling certain uses of content in Gemini-related and Vertex AI-related systems.
Because policies differ, it is better to read each platform’s documentation than to assume all AI crawlers behave the same way. Some site owners allow them. Some block them. Some allow search crawlers while limiting training-related crawlers. The decision should match the site’s goals, rights, infrastructure, and content strategy.
How to Manage Crawler Access
Managing crawlers is not just about blocking bots. It is about making the right content accessible to the right systems while protecting private, duplicate, low-value, or resource-heavy areas.
Use robots.txt for Crawl Access
robots.txt is useful when you want to prevent compliant crawlers from crawling certain paths.
User-agent: *
Disallow: /wp-admin/
Disallow: /cart/
Disallow: /checkout/
Sitemap: https://www.example.com/sitemap_index.xmlCommon areas to evaluate include:
- Admin areas.
- Internal search result pages.
- Cart and checkout pages.
- Filtered URL combinations that create crawl traps.
- Staging environments.
- Duplicate parameter URLs.
Do not use robots.txt as a privacy tool. A disallowed URL can still be known, linked, or exposed elsewhere. Use authentication for truly private content.
Use Meta Robots for Indexing Directives
A page-level meta robots tag can tell compliant crawlers whether a page should be indexed or whether links should be followed.
<meta name="robots" content="noindex, follow">This is often used when a page may be crawled but should not appear in search results.
Use X-Robots-Tag for Files and Server-Level Control
The X-Robots-Tag HTTP header can apply indexing directives to non-HTML files, such as PDFs, or to groups of URLs at the server level.
X-Robots-Tag: noindexThis can be useful for documents, media files, and server-managed patterns where adding an HTML meta tag is not possible.
Use Canonical Tags for Duplicate or Similar Pages
Canonical tags help indicate the preferred version of a page when multiple URLs contain the same or very similar content.
<link rel="canonical" href="https://www.example.com/preferred-page/">Canonical tags are signals, not absolute commands. They work best when supported by consistent internal linking, redirects, sitemaps, and content structure.
Use Sitemaps to Support Discovery
XML sitemaps help crawlers discover important URLs. A sitemap does not guarantee indexing, but it can clarify which URLs you consider important and when they were last modified.
A good sitemap should generally include canonical, indexable URLs that return a successful status code.
Read: XML Sitemaps, HTML Sitemaps, and SEO
Crawler Signals in Log Files
Server logs show which user agents requested which URLs, when they visited, and how the server responded. For intermediate SEO work, log analysis can reveal what crawlers are actually doing rather than what a tool assumes they might do.
Useful questions include:
- Is Googlebot visiting important pages?
- Is Bingbot discovering new content?
- Are crawlers wasting time on parameter URLs, internal search pages, or old redirects?
- Are important pages returning 200 status codes?
- Are crawl requests hitting 500-level errors?
- Are blocked paths being requested repeatedly?
- Are AI or SEO tool crawlers consuming significant server resources?
When reviewing logs, remember that user-agent strings can be spoofed. A request claiming to be Googlebot is not always Googlebot.
Verifying Real Search Crawlers
For major search engines, verification usually involves DNS checks. Google, for example, documents a process using reverse DNS lookup and forward DNS confirmation. Bing and other platforms also provide guidance.
This matters because fake bots may use names like “Googlebot” to bypass rules. If server security decisions depend on crawler identity, verify the crawler rather than trusting the user-agent string alone.
Common Crawler Mistakes
Blocking Important Resources
If CSS, JavaScript, images, or API endpoints required for rendering are blocked, crawlers may see an incomplete version of the page. This can affect how content and layout are understood.
Confusing Noindex and Disallow
Disallow in robots.txt tells compliant crawlers not to crawl a URL. noindex tells compliant crawlers not to index a page. If a crawler is blocked from accessing a page, it may not see the noindex directive on that page.
Creating Crawl Traps
Crawl traps happen when a site generates many low-value URL variations. Examples include endless calendar pages, faceted navigation combinations, tracking parameters, and internal search result URLs.
Relying Only on XML Sitemaps
Sitemaps help discovery, but they do not replace internal links. Important pages should usually be reachable through meaningful site architecture.
Letting Staging Sites Be Crawled
Staging and development environments should not be publicly crawlable. Use password protection or server-level restrictions. A noindex tag is helpful, but it is not as strong as preventing access.
Blocking All Bots Without Reviewing Impact
Some server or firewall rules block bots broadly. This may reduce unwanted traffic, but it can also block search engines, social previews, ad review bots, monitoring tools, and legitimate accessibility or archiving systems.
Crawler Checklist for SEOs
Use this checklist when reviewing crawler behavior on a website.
- Confirm that important pages return 200 status codes.
- Check that canonical URLs are internally linked and included in sitemaps.
- Review
robots.txtfor accidental blocks. - Make sure important CSS, JavaScript, and image assets are crawlable when needed for rendering.
- Use
noindexwhere pages should be accessible but not indexed. - Use authentication for private or staging content.
- Review server logs for real crawler behavior.
- Verify major crawler identities before making security decisions based on user-agent strings.
- Watch for crawl traps caused by parameters, filters, calendars, or internal search pages.
- Separate search engine crawlers, SEO tool crawlers, social preview bots, and AI-related crawlers in your policy decisions.
FAQ
What is a web crawler?
A web crawler is an automated system that visits URLs, reads content, follows links, and sends information back to the platform that operates it. Search engines use crawlers to discover and refresh pages for their indexes.
Is crawling the same as indexing?
No. Crawling means a system accessed a URL. Indexing means the system stored the content and made it eligible for retrieval. A page can be crawled without being indexed.
Which crawler is most important for SEO?
For most websites, Googlebot is the most important crawler because Google Search is a major source of organic search visibility. Bingbot is also important, especially because Bing contributes to multiple search and discovery experiences.
Should I block SEO tool crawlers?
It depends. SEO tool crawlers do not directly control rankings, but they can help tools report on backlinks, visibility, and technical issues. Some site owners allow them; others limit them to reduce server load or data collection. The decision should match your goals and infrastructure.
Should I block AI crawlers?
There is no single answer. Some publishers allow AI-related crawlers for visibility in emerging retrieval systems. Others restrict them because of content rights, business strategy, or resource concerns. Review each crawler’s documentation and decide intentionally.
Can bots ignore robots.txt?
Yes. Legitimate crawlers usually respect robots.txt, but malicious or poorly behaved bots may ignore it. Do not use robots.txt to protect sensitive information.
How can I tell if Googlebot is real?
User-agent strings can be spoofed. To verify Googlebot, use Google’s documented DNS verification process, which involves reverse DNS lookup and forward DNS confirmation.
Why are crawlers visiting pages I do not want indexed?
Crawling and indexing are separate. A crawler may access a page to check directives, follow links, or revisit known URLs. If the page should not be indexed, use the correct indexing directive. If it should not be accessed at all, use access control or an appropriate crawl block.
Closing Thought
Crawlers are not just technical background noise. They are how search and retrieval systems maintain contact with the web. When a site has clear structure, stable signals, accessible content, and intentional crawler rules, it becomes easier for those systems to understand what exists, what matters, and what has changed.
Good crawler management is not about chasing every bot. It is about knowing which systems matter for your site, allowing the right access, limiting the wrong access, and keeping the structure readable over time.