What's the difference of $host and $http_host in Nginx - configuration

Question

In Nginx, what's the difference between variables $host and $http_host.

Answer 1

$host is a variable of the Core module.
$host
This variable is equal to line Host in the header of request or
name of the server processing the request if the Host header is not
available.
This variable may have a different value from $http_host in such
cases: 1) when the Host input header is absent or has an empty value,
$host equals to the value of server_name directive; 2)when the value
of Host contains port number, $host doesn't include that port number.
$host's value is always lowercase since 0.8.17.
$http_host is also a variable of the same module but you won't find it with that name because it is defined generically as $http_HEADER (ref).
$http_HEADER
The value of the HTTP request header HEADER when converted to lowercase and with 'dashes' converted to 'underscores', e.g. $http_user_agent, $http_referer...;
Summarizing:
$http_host equals always the HTTP_HOST request header.
$host equals $http_host, lowercase and without the port number (if present), except when HTTP_HOST is absent or is an empty value. In that case, $host equals the value of the server_name directive of the server which processed the request.

Answer 2

The accepted answer and its comments don't seem to be correct (anymore). The docs (http://nginx.org/en/docs/http/ngx_http_core_module.html#var_host) say that $host is
in this order of precedence: host name from the request line, or host name from the “Host” request header field, or the server name matching a request
So $http_host is always the value of the Host header field. They might differ if the host in the request line (if specified) differs from the Host header field. Or if the Host header is not set.
server_name matches only the Host header field (http://nginx.org/en/docs/http/request_processing.html), so that $host may differ from the matched server_name.

Answer 3

$http_host
$http_host always equals Host request header field
Host: example.org
$host
$host is in this order of precedence (from high to low):
Host name from the request line
GET http://example.org/test/ HTTP/1.1
Host request header field
The server_name (in Nginx config) matching a request, even if server_name is wildcard (Ex: server_name *.example.org;)
Host name from the request line
When open URL http://example.org/test/ ...
Most browser send the request like this
GET /test/ HTTP/1.1
Host: example.org
Most browser doesn't send the request like this (but this is valid request)
GET http://example.org/test/ HTTP/1.1
Validation
Nginx testing config
server {
listen 80;
server_name *.example.org;
location / {
default_type "text/plain";
return 200 "[host] = $host";
}
}
When all exist ...
$host = host name from the request line
curl http://127.0.0.1 -v \
--request-target http://request.example.org/test/ \
--path-as-is \
-H "Host: host.example.org"
This command will
Connect to 127.0.0.1
Send request path as GET http://request.example.org/test/ HTTP/1.1
Set Host header to Host: host.example.org
* Trying 127.0.0.1:80...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 80 (#0)
> GET http://request.example.org/test/ HTTP/1.1
> Host: host.example.org
> User-Agent: curl/7.68.0
> Accept: */*
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Server: nginx/1.23.1
< Date: Fri, 21 Oct 2022 02:00:56 GMT
< Content-Type: text/plain
< Content-Length: 28
< Connection: keep-alive
<
* Connection #0 to host 127.0.0.1 left intact
[host] = request.example.org
When only Host header exist ...
$host = Host header
curl http://127.0.0.1/test/ -v \
-H "Host: host.example.org"
* Trying 127.0.0.1:80...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 80 (#0)
> GET /test/ HTTP/1.1
> Host: host.example.org
> User-Agent: curl/7.68.0
> Accept: */*
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Server: nginx/1.23.1
< Date: Fri, 21 Oct 2022 02:01:37 GMT
< Content-Type: text/plain
< Content-Length: 25
< Connection: keep-alive
<
* Connection #0 to host 127.0.0.1 left intact
[host] = host.example.org
When none exist ...
$host = server_name (in Nginx config)
# HTTP 1.1 must have Host header, so use HTTP 1.0
curl http://127.0.0.1/test/ -v -H "Host:" -0
* Trying 127.0.0.1:80...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 80 (#0)
> GET /test/ HTTP/1.0
> User-Agent: curl/7.68.0
> Accept: */*
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Server: nginx/1.23.1
< Date: Fri, 21 Oct 2022 02:02:20 GMT
< Content-Type: text/plain
< Content-Length: 22
< Connection: close
<
* Closing connection 0
[host] = *.example.org
Ref: ngx_http_core_module, Nginx $host validation