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I've been reading up on reverse proxying and am wondering when proxy_set_header Host $host is appropriate over proxy_set_header Host $proxy_host. I did some research and in this article it says that in most cases we set Host to $host. Then why does nginx default to $proxy_host? To help me understand more concretely, will the reverse proxy configuration here (bottom of article) still work if we use $proxy_host instead?

Thanks

Alex
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2 Answers2

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In general there is no need to explicitly do proxy_set_header Host $proxy_host because it's the default. If you need to call a server by something other than what is in the proxy_pass directive, then you will need to override via proxy_set_header something.

If you want to proxy the same host as was in your server_name directive, then you would have occasion to use proxy_set_header $host. This would commonly be the case if perhaps the actual application is hosted on another port or on some internal server.

server {
    listen 80;
    server_name site.example.com;
    
    location / {
       proxy_set_header Host $host;
       proxy_pass http://localhost:8080;
    }
}

If the name you are calling the upstream is not its actual DNS name, then you might have something like:

# 192.168.2.1 responds to site.example.com, but
# site.example.com doesn't actually resolve to 192.168.2.1
proxy_pass http://192.168.2.1;
proxy_set_header Host site.example.com;

Another case might be for "name-based" virtual hosting where there is a useful DNS name for the upstream, but you would like to call it by another name.

proxy_pass http://origin.example.com;
proxy_set_header Host site.example.com
YoYo
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Joshua DeWald
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    Does the order of `proxy_pass` and `proxy_set_header` matter? – iBug Jun 14 '19 at 17:05
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    @iBug Nope! the `upstream` module will read each of the settings applicable for its internal phases, and then apply the when it issues the actual `proxy_pass`. You'll get the exact same behavior if the `proxy_set_header` is *after* `proxy_pass` in the config. – Joshua DeWald Jun 16 '19 at 05:16
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    I am having a difficult time understanding the wording of the definitions of `$host`and `$proxy_host`. So is this wrong: A client sends a request to an nginx server **A** which proxy passes it to backend **B**. Then `$host` would be the url of **A**, while `$proxy_host` would be the url of **B**. So if I want *future* requests of the client to still be directed at **A**, I would want to use `$host`? Is that correct? – Felix B. May 01 '21 at 16:06
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$proxy_host is a tag about upstreams servers, so nginx do this one by default.

If you want to add other tag(header) about the host, use $host.

Ying-Shan Lin
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raheleh gilani
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