In a December 6 blog post on Google Webmaster Central blog, Pierre Far, Webmaster Trends Analyst shared some tips to help both hosting providers and webmasters recognize, diagnose, and fix hosting-related common issues affecting their sites.
Per Far's blog post:
Blocking of Googlebot crawling. This is a very common issue usually due to a misconfiguration in a firewall or DoS protection system and sometimes due to the content management system the site runs. Because, however, Googlebot often performs more requests than a human user, these protection systems may decide to block Googlebot and prevent it from crawling your website.
Google offer following tools to, who want more control over Googlebot's crawling, and to improve crawling efficiency:
- Detailed help about how you control Googlebot's crawling using the robots exclusion protocol and configuring URL parameters.
- worried about rogue bots using Googlebot user-agent, verify whether a crawler is actually Googlebot.
- If Googlebot crawls your site too frequently, you can change Googlebot's crawl rate.
For more information, refer this crawling and indexing FAQ.
Availability issues. Check out report in Webmaster Tools about the URL unreachable errors or crawl errors, that Google reports when a websites being unavailable when Googlebot (and users) attempt to access the site. This includes DNS issues, overloaded servers leading to timeouts and refused connections, misconfigured content distribution networks (CDNs) etc.
Invalid SSL certificates. For SSL certificates to be valid for your website, they need to match the name of the site. Common problems include expired SSL certificates and servers misconfigured such that all websites on that server use the same certificate. The fix for these problems is to make sure to use SSL certificates that are valid for all your website's domains and subdomains your users will interact with.
Wildcard DNS. Websites can be configured to respond to all subdomain requests. For example, the website at example.com can be configured to respond to requests to foo.example.com, made-up-name.example.com and all other subdomains.
To minimize problems in wildcard DNS setups, either configure your website to not use them, or configure your server to not respond successfully to non-existent hostnames, either by refusing the connection or by returning an HTTP 404 header.
Misconfigured virtual hosting. The symptom of this problem is that multiple hosts and/or domain names hosted on the same server always return the contents of only one site. To diagnose the issue, you need to check that the server responds correctly to the Host HTTP header.
Content duplication through hosting-specific URLs. Many hosts helpfully offer URLs for your website for testing/development purposes. For example, if you're hosting the website http://a.com/ on the hosting provider example.com, the host may offer access to your site through a URL like http://a.example.com/ or http://example.com/~a/. Our recommendation is to have these hosting-specific URLs not publicly accessible (by password protecting them); and even if these URLs are accessible, our algorithms usually pick the URL webmasters intend. If our algorithms select the hosting-specific URLs, you can influence our algorithms to pick your preferred URLs by implementing canonicalization techniques correctly.
Soft error pages. Some hosting providers show error pages using an HTTP 200 status code (meaning "Success") instead of an HTTP error status code. For example, a "Page not found" error page could return HTTP 200 instead of 404, making it a soft 404 page; or a "Website temporarily unavailable" message might return a 200 instead of correctly returning a 503 HTTP status code. This may cause ranking or cross-domain URL selection issues.
It's easy to check the status code returned: simply check the HTTP headers the server returns using any one of a number of tools, such as Fetch as Googlebot. If an error page is returning HTTP 200, change the configuration to return the correct HTTP error status code. Also, keep an eye out for soft 404 reports in Webmaster Tools, on the Crawl errors page in the Diagnostics section.
Content modification and frames. To check whether a web host is changing your content in unexpected ways, simply check the source code of the page as served by the host and compare it to the code you uploaded. Note that some server-side code modifications may be very useful. For example, a server using Google's mod_pagespeed Apache module or other tools may be returning your code minified for page speed optimization.