404 redirect to another server/domain - haproxy

Question

I'm looking for a solution with redirects to another domain if the response from HTTP server was 404.
acl not_found status 404
acl found_ceph status 200
use_backend minio_s3 rsprep ^HTTP/1.1\ 404\ (.*)$ HTTP/1.1\ 302\ Found\nLocation:\ / if not_found
use_backend ceph if found_ceph
But still not working, this rule goes to minio_s3 backend.
Thank you for you advice.

Answer 1

When the response from this backend has status 404, first add a Location header that will send the browser to example.com with the original URI intact, then set the status code to 302 so the browser executes a redirect.
backend my-backend
mode http
server my-server 203.0.113.113:80 check inter 60000 rise 1 fall 2
http-response set-header Location http://example.com%[capture.req.uri] if { status eq 404 }
http-response set-status 302 if { status eq 404 }
Test:
$ curl -v http://example.org/pics/funny/cat.jpg
* Hostname was NOT found in DNS cache
* Trying 127.0.0.1...
* Connected to example.org (127.0.0.1) port 80 (#0)
> GET /pics/funny/cat.jpg HTTP/1.1
> User-Agent: curl/7.35.0
> Host: example.org
> Accept: */*
The actual back-end returns 404, but we don't see it. Instead...
< HTTP/1.1 302 Moved Temporarily
< Last-Modified: Thu, 04 Aug 2016 16:59:51 GMT
< Content-Type: text/html
< Content-Length: 332
< Date: Sat, 07 Oct 2017 00:03:22 GMT
< Location: http://example.com/pics/funny/cat.jpg
The response body from the back-end's 404 error page will still be sent to the browser, but -- as it turns out -- the browser will not display it, so no harm done. This requires HAProxy 1.6 or later.

Answer 2

#Michael's answer is rather good, but isno't working for me for two reasons:
Mainly because the %[capture.req.uri] tag resolves to empty (HA Proxy 1.7.9 Docker image)
Also due to the fact that the original assumptions are incomplete, due to the fact that the frontend section is missing...
So I struggled for a while, as you find all kinds of answers on the Internet, between those guys who swear the 404 logic should be put in the frontend, vs those who choose the backend, and any possible kind of tags...
This is my answer, which works for me.
My use case is that if an image is not found on the backend behind HA Proxy, then an S3 bucket is checked.
The entry point is: https://myhostname:8080/path/to/image.jpeg
defaults
mode http
global
log 127.0.0.1:514 local0 debug
frontend come_on_over_here
bind :8080
# The following two lines are here to save values while we have access to them. They won't be available in the backend section.
http-request set-var(txn.path) path
http-request set-var(txn.query) query
http-request replace-value Host localhost:8080 dev.local:80
default_backend onprems_or_s3_be
backend onprems_or_s3_be
log global
acl path_photos var(txn.path) -m beg /path/prefix/i/want/to/strip/off
acl p_ext_jpeg var(txn.path) -m end .jpeg
acl is404 status eq 404
http-response set-header Location https://mybucket.s3.eu-west-3.amazonaws.com"%[var(txn.path),regsub(^/path_prefix_i_want_to_strip_off/,/)]?%[var(txn.query)]" if path_photos p_ext_jpeg is404
http-response set-status 301 if is404
server onprems_server dev.local:80 check