letsencrypt
va: Handle IPv4 fallback for HTTP(S) redirects
When performing an HTTP-01 validation, the VA will try to fall back from IPv6 to IPv4 if the connection fails; and it will follow HTTP(S) redirects. However, the current structure of processHTTPValidation cannot handle a case where both are needed.
Reproducer: Publish IPv6 & IPv4 addresses in DNS for a test host. Misconfigure networking on the test host so that IPv6 connections will fail. Configure a self-signed certificate for HTTPS. Configure the Web server to redirect all HTTP traffic (including ACME challenges) to HTTPS. Attempt HTTP-01 validation (e.g. certbot certonly --webroot). It will fail: Boulder will retry to IPv4, and follow the redirect - but when trying to validate the redirect target, it will only try IPv6 and not attempt to retry to IPv4.
