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Getting it down on `paper`

MySQL DNS Caching

On December 12, 2008, SCCOOS metadata system was broken. The error was cryptic, but the message was clear: Connection to [IP] denied.


Using the MySQL command-line tool:

mysql -u admin -p [-h ] [-P ]
mysql> flush hosts;

That’s it!

Chain of Events

  • UCSD’s DNS was polluted, identifying as for the past few weeks.
  • Sandbar was restarted last week, freeing up both the OS and MySQL DNS caches, which meant all new connections will need to query a DNS server to identify whether it’s an allowed machine or not (do you see a security vulnerability?).
  • Both the OS and MySQL cached the hostname, but MySQL further rejected all connections from alfredo because MySQL thought that alfredo was ridge2000 instead.
  • The mysql database, user table identifies user sccoos can connect to sandbar from
  • The OS cache probably cleared, but since so few machines access sandbar, the MySQL cache hadn’t filled up yet.
  • Just last night (2008-12-22), the UCSD DNS purged the name from its list, so UCSD’s DNS is clean, but any caches may still be polluted.
  • Windows machines cycle their caches regularly, so the problem isn’t more wide-spread or commonly understood.
  • The long-lasting MySQL DNS cache is disturbing, but alternatives would cripple the imperative nature of a database.




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