Changing the Host passed to the backend - regex

Question

I have my frontend server running nginx. The backend is on another machine on the same VPN. This is its config:
server {
listen 80;
server_name *.vpn.domain.com;
location / {
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header Host $http_host;
proxy_set_header X-NginX-Proxy true;
proxy_pass http://10.8.25.102:8100/;
proxy_redirect http://10.8.25.102:8100/ http://$server_name/;
}
}
I would like to pass a different host to the backend... I'd like the backend to receive, for requests done tosubdomain.vpn.domain.com the host subdomain.local.domain.com
Is there any way to do this? I'm looking for a regexp substitution (or even a substring substitution) but I'm having surprisingly little success... I thought it would be a piece of cake. I think the solution would be in the lines of
server {
listen 80;
server_name *.vpn.domain.com;
set $my_host $http_host;
replace $my_host .vpn. .local.
location / {
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header Host $my_host;
proxy_set_header X-NginX-Proxy true;
proxy_pass http://10.8.25.102:8100/;
proxy_redirect http://10.8.25.102:8100/ http://$server_name/;
}
}
It's just that I haven't found yet the proper syntax for replace $my_host .vpn. .local. I don't really care about multiple substitutions... I won't have a.vpn.a.vpn.domain.com

Answer 1

I finally figured it out, I can do
if ($http_host ~ ^(.*)\.vpn\.(.*)$) {
set $my_host $1.local.$2;
}
And then, as there're CSRF validations in place, I also need to rewrite the Referer... so this is how it ended up looking
server {
listen 80;
server_name *.vpn.domain.com;
set $my_host $http_host;
if ($http_host ~ ^(.*)\.vpn\.(.*)$) {
set $my_host $1.local.$2;
}
set $referer $http_referer;
set $referer_host no;
if ($http_referer ~ ^(https?://)([^/]+)(/.*)$) {
set $referer_host $2;
set $rewritten_referer $1$my_host$3;
}
if ($referer_host = $http_host) {
set $referer $rewritten_referer;
}
location / {
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header Host $my_host;
proxy_set_header X-NginX-Proxy true;
proxy_set_header Referer $referer;
proxy_set_header IS_SECURE no;
proxy_pass http://10.8.25.102:8100/;
proxy_redirect https://$my_host/ https://$http_host/;
proxy_redirect http://$my_host/ http://$http_host/;
}
}