Sentry does cache DNS lookup results.
Positive Cache: 600 seconds (10 minutes)
Negative Cache: 10 seconds
This is the default behavior. You can also control the caching on a per remote policy basis. To be able to set the caching settings on a per policy basis, you have to enable the user preference "Enable DNS cache override" on the System>>Settings>>Preferences page.
You can clear the DNS cache with the CLI command "network utils dns-flush".
You can do a lookup with "network utils dns-lookup".
Lastly, you can add static host entries to bypass DNS with the CLI command "network static-host add".