Change Host header in nginx reverse proxy - nginx

Question

I am running nginx as reverse proxy for the site example.com to loadbalance a ruby application running in backend server. I have the following proxy_set_header field in nginx which will pass host headers to backend ruby. This is required by ruby app to identify the subdomain names.
location / {
proxy_pass http://rubyapp.com;
proxy_set_header Host $http_host;
}
Now I want to create an alias beta.example.com, but the host header passed to backend should still be www.example.com otherwise the ruby application will reject the requests. So I want something similar to below inside location directive.
if ($http_host = "beta.example.com") {
proxy_pass http://rubyapp.com;
proxy_set_header Host www.example.com;
}
What is the best way to do this?

Answer 1

You cannot use proxy_pass in if block, so I suggest to do something like this before setting proxy header:
set $my_host $http_host;
if ($http_host = "beta.example.com") {
set $my_host "www.example.com";
}
And now you can just use proxy_pass and proxy_set_header without if block:
location / {
proxy_pass http://rubyapp.com;
proxy_set_header Host $my_host;
}

Answer 2

map is better than set + if.
map $http_host $served_host {
default $http_host;
beta.example.com www.example.com;
}
server {
[...]
location / {
proxy_pass http://rubyapp.com;
proxy_set_header Host $served_host;
}
}

Answer 3

I was trying to solve the same situation, but with uwsgi_pass.
After some research, I figured out that, in this scenario, it's required to:
uwsgi_param HTTP_HOST $my_host;
Hope it helps someone else.

Answer 4

Just a small tip. Sometimes you may need to use X-Forwarded-Host instead of Host header. That was my case where Host header worked but only for standard HTTP port 80. If the app was exposed on non-standard port, then this port was lost when the app generated redirects. So finally what worked for me was:
proxy_set_header X-Forwarded-Host $http_host;