Custom 404 in Nginx - nginx

Question

I'm trying to specify a custom 404 page and preserve the URL, however using the below gives me the error nginx: [emerg] "proxy_pass" cannot have URI part in location given by regular expression, or inside named location, or inside "if" statement, or inside "limit_except" block in...
location / {
error_page 404 #404;
}
location #404 {
proxy_pass https://example.com/404.html;
}
Does anyone know a solution to this?

Answer 1

I have tested your config on nginx 1.6.2. The error message complains about the URL in proxy_pass having path specified. Specifying only https://server would be OK, but doesn't solve your issue.
Named locations (like #fallback) have been introduced also to avoid having to use non-existing locations (which could later become existing and cause issues). But we can also use a location which should never exist on the server itself, for example:
location / { error_page 404 /xx-404-xx; }
location /xx-404-xx { proxy_pass https://example.com/404.html; }
Redirecting to relative URL causes only internal nginx redirect, which does not change the URL as browser sees it.
EDIT: Here is the test result:
kenny:/etc/nginx/sites-enabled:# wget -S server.com/abc/abc
HTTP request sent, awaiting response...
HTTP/1.1 404 Not Found
Server: nginx/1.6.2
Date: Thu, 16 Jul 2015 07:01:38 GMT
Content-Type: text/html
Content-Length: 5
Connection: keep-alive
Last-Modified: Thu, 16 Jul 2015 07:00:59 GMT
ETag: "5037762-5-51af8a241434b"
Accept-Ranges: bytes
2015-07-16 09:01:38 ERROR 404: Not Found.
From apache access log:
1.2.3.4 - - [16/Jul/2015:09:01:38 +0200] "GET /test.html HTTP/1.0" 200 5 "-" "Wget/1.13.4 (linux-gnu)"
In nginx I have proxy_pass to a html file on Apache webserver with just "test\n" in it. As you can see, nginx fetched that, added headers from Apache (Last-Mod, ETag) and also Content-Length: 5, so it received the html file from Apache, but wget doesn't save the content of 404 errors. Also many browsers don't display 404 errors by default if they are smaller than 1 kB (they show their own error page instead). So either make your 404 page bigger or you can configure nginx to serve it as normal html with "200" result code (error_page 404 =200 /xx).
I have no idea why you are receiving external redirects with the same config. Try it with wget to see which headers exactly nginx sent. Also mixing http and https for proxy should be no issue. Try to remove everything from your config and test only this error page, maybe some other directive is causing this (like other location is used instead).