Runtime DNS resolution limits when using variables in proxy_pass

Answer to the Questions

1. Caching when valid is omitted

• NGINX uses the TTL returned by the DNS server for each name. If valid is not set, the record’s TTL becomes the cache lifetime.
• The cache is per‑worker and stored in memory. Subsequent requests that hit the same worker reuse the cached IP until the TTL expires.
• When the TTL expires, the next request that reaches the worker triggers a new lookup. While the lookup is in progress, the worker can continue serving the previously cached (stale) address; only the request that triggered the lookup is blocked until the resolution succeeds or fails.

2. Latency when resolver_timeout is hit

• The request is stalled for the full resolver_timeout duration (default 30 s) while waiting for a DNS reply.
• If the timeout expires, NGINX returns a 502 Bad Gateway to the client.
• Thus, a slow or unreachable DNS server directly adds that timeout period to the client’s round‑trip time.

Practical Steps for This Scenario

1. Verify the NGINX version to ensure the caching behaviour matches the documented implementation:

   nginx -v

2. Check the resolver configuration in nginx.conf or the relevant server block:

   resolver 10.0.0.53;            # without valid
   # or
   resolver 10.0.0.53 valid=30s;

3. To observe caching, enable DNS query logging on the resolver or capture traffic with tcpdump -i eth0 port 53 and watch that subsequent requests within the TTL do not trigger new queries.

4. To test latency impact, point resolver at an unreachable address and measure the response time with curl -w "%{time_total}\n" -o /dev/null -s http://example.com. The duration should match the configured resolver_timeout before a 502 is returned.

5. In production, consider lowering resolver_timeout to 2–5 s and using a local caching resolver to bound worst‑case latency.

Missing Diagnostic Detail

Could you confirm which NGINX version you are running? The caching and stale‑answer behaviour changed subtly in 1.21 and later, so knowing the exact release will help tailor the recommendation.