Googlebot Can't Access Your Site - DNS Errors
DNS Hacking Help - 1 November 2015
The same applies to unnecessarily allocating new resources to your DNS server (purchasing more bandwidth) due to your site been targeted by scrappers, spammers bots and hackers.
Below is what Google has to say when it comes to Googlebot been unable to access your site.
Above Google seem to have covered the basics of Googlebot and DNS issues, but neglected to mention hacking, bots, spamming and scrapping (the bandwidth thieves) which in my opinion relates to most DNS issues, such as Googlebot been unable to access your site/pages.
Unknown robot (identified by 'bot' followed by a space or one of the following characters _+:,.;/\-)
Unknown robots, bots spoofing IP addresses and hackers are usually the culprits of DNS errors. They hack or try login via other user accounts to spam, the majority of time they target forums or websites that allow commenting.
The above Russian bot network targeted this exact site beginning 2015 when I created a PHBB Forum. The forum was removed 9 months back, but it seems the Russian bot network is still trolling the old forum links. The image below displays the old forum index page that enabled log-in, which no longer exists.
The bot hit the 404 PHBB Forum not found page every second (thousands of hits) until my site no longer responded. Hence, the warning from Google:
Googlebot can't access your site. Over the last 24 hours, Googlebot encountered 9 errors while attempting to retrieve DNS information for your site. The overall error rate for DNS queries for your site is 37.5%
Russian Bots Spoofing IP addresses - DNS Errors
The spoofed IP addresses associated with the DNS errors tracked back to Russia - 22.214.171.124/24, 126.96.36.199, 188.8.131.52, 184.108.40.206, 220.127.116.11, 18.104.22.168, 22.214.171.124, 126.96.36.199, etc.
As well as the following IP addresses - 188.8.131.52, 184.108.40.206, 220.127.116.11, 18.104.22.168, 22.214.171.124, 126.96.36.199, 188.8.131.52, only to mention a few of the spoofed Russian IP addresses.
Any site that ranks moderately high in Google that has a forum has been under various forms of spam attack. So they can post nonsensical text and links to get you to buy pills or whatever the latest scam is. Posting links on respected sites also gives there sites more legitimacy in search results ranking (PageRank, PR)
Those same Russian IP addresses are also associated with other hacked sites (below image)
Once the site has been hacked the hacker will redirect user agents (Google, Bing, etc) to porn or pharmaceutical sites or they create new folders and files then spam the crap out of the website. In this instance the Russian bot-net spams nonsensical text and links to pharmaceutical drugs (Below image) while spoofing its IP address.
Hence, Google flagged the site as been hacked.
DNS errors from Google need to be taken seriously. If this happens to you on multiple occasions its a possibility your site is in the process of been hacked, spammed, SQL attacked or your email header been injected.