nginx proxy_pass DNS cache causes a common issue when proxying S3

We use nginx to proxy to an Amazon S3 bucket to serve static content to customers. Yesterday, I noticed a high failure rate through the proxy but I couldn’t figure out why. After some debugging, I finally discovered that the IP address nginx was hitting was different from the one that DNS was returning. It turns out that nginx resolves hostnames only once on load, meaning whatever IP address it got on load would stick around until reload.