Nginx sub_filter for headers - nginx

Question

I want to use nginx proxy_pass before different applications which run on their own port.
so i.e. I have
server {
listen 443 ssl;
ssl on;
ssl_certificate /etc/ssl/certs/self-signed.crt;
ssl_certificate_key /etc/ssl/private/self-signed.key;
proxy_intercept_errors on;
location /app1/ {
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
proxy_set_header Host $http_host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_http_version 1.1;
proxy_redirect off;
proxy_pass http://localhost:1111;
}
location /app2/ {
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
proxy_set_header Host $http_host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_http_version 1.1;
proxy_redirect off;
proxy_pass http://localhost:2222;
}
}
However the problem is the applications itself have redirects in them which causes the /app1/ or /app2/ to disappear.
i.e. a 302 get /page1 will become https://example.com/page1 instead of https://example.com/app1/page1
So basically whenever someone is at a /app1/* page to always prefix /app1/.
and it would be really great if this doesn't happen when this is actually the full domain name so it's possible to load resources from a /app2/ url in a /app1/ page
Is there a way I can fix with Nginx config or can I only fix this in the application itself?
EDIT:
I found that Nginx has a function called sub_filter.
This replaced strings in the body of the response.
so:
app returns: 200 "hello world!"
//nginx
sub_filter "world" "moon";
then the browser will display "hello moon!"
However I also need to do this for 302 redirects.
Is there a "sub_filter equivalent" for headers?
EDIT2:
As Ivan suggested probably proxy redirect should do the trick, but it's not changing anything.
my nginx code is:
location /app1/ {
proxy_set_header Accept-Encoding ""; # no compression allowed or next won't work
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_http_version 1.1;
proxy_redirect off;
proxy_buffering off;
proxy_read_timeout 3600;
proxy_connect_timeout 3600;
fastcgi_read_timeout 3600s;
proxy_pass http://localhost:5000/;
proxy_redirect http://localhost:5000/ /app1/; #also tried full domain
}
As for the app I'm testing this with a simple Flask app like this:
#app.route('/')
def hello_world():
return redirect("http://localhost:5000/testing", code=302)
and the browser response headers are:
HTTP/1.1 302 FOUND
Server: nginx/1.14.0 (Ubuntu)
Date: Wed, 06 Nov 2019 15:56:24 GMT
Content-Type: text/html; charset=utf-8
Transfer-Encoding: chunked
Connection: keep-alive
Location: http://localhost:5000/testing

Answer 1

Use proxy_redirect directive for your case:
proxy_redirect / /app1/;
and
proxy_redirect / /app2/;
Remove any proxy_redirect off; directive because it cancels the effect of all proxy_redirect directives on the same level.