proxy_pass and weird behaviour

[Michael Grimm]

Maxim Dounin <mdounin at mdounin.ru> wrote:
> On Sat, Mar 11, 2017 at 09:07:54AM +0100, Michael Grimm wrote:

> [...]
> 
>> Well, that is working, somehow, except: If the LE server
>> addresses Host A, the challenge file is going to be retrieved
>> instantaneously. If the LE server addresses Host B, only every
>> *other* request is being served instantaneously:
>> 
>> 	1. access: immediately download
>> 	2. access: 60 s wait, then download
>> 	3. access: immediately download
>> 	4. access: 60 s wait, then download
>> 	etc.
>> 
>> 
>> Hmm, default proxy_connect_timeout is 60s, I know. But why every
>> other connect?
> 
> You are using "proxy_pass http://local-at-host-A.lan;" in your
> configuration.  What are the IP addresses it resolves to?
> 
> The behaviour observed suggests that the name resolves to 2
> different addresses, so nginx uses round-robin to balance between
> these addresses, and only one of these addresses is reacheable.

Bingo! I had had two issues in that regard: My local resolver returned 
one IPv4 and on IPv6 address for local-at-host-A.lan, and in my server 
block I had had an include statement with listen statements for IPv4 and 
IPv6 addresses. (Those were left-overs I didn't bear in mind when 
removing IPv6 functionality for that given nginx server.)

Now, everything is working as expected. Thank you very much for pointing 
me to the right direction!

With kind regards,
Michael