When do you have to use quotes in the configuration?

Solution 1:

The exact answer is "never". You can either quote or \-escape some special characters like " " or ";" in strings (characters that would make the meaning of a statement ambiguous), so

add_header X-MyHeader "Test String;"; 

would work like

add_header X-MyHeader Test\ String\;;

In reality: Just use quotes :)

Edit: As some people love to nitpick: The not necessarily complete list of characters that can make a statement ambiguous is according to my understanding of the nginx config syntax:

<space> " ' { } ; $ \ 

and it might be necessary to escape $ and \ even in quoted strings to avoid variable expansion.

Unfortunately, I can't find a complete and authoritative list of such characters in the docs.

Solution 2:

Quotes are required for values which are containing space(s) and/or some other special characters, otherwise nginx will not recognize them. In your current example quotes make no difference, but anyway quoting values is a good practice/rule of thumb