In-depth understanding of the matching logic of Server and Location in Nginx

Server matching logic

When Nginx decides which server block to execute a request, it focuses on the listen and server_name fields in the server block.

The listen directive

The listen field defines the IP and port that the server responds to. If the listen field is not explicitly configured, the default listener is 0.0.0.0:80 (root) or 0.0.0.0:8080 (non-root)

listen can be configured as:

1. A combination of an IP and a port
2. A single IP, listening on port 80 by default
3. A single port, listening on all IP interfaces by default
4. A Unix socket path

The last one is usually only used to transfer requests between different servers.

The rules for selecting the server to use are as follows:

1. Nginx first converts all "incomplete" listen instructions, such as those without a listen field are converted to listen 0.0.0.0:80, listen 1.1.1.1 is converted to listen 1.1.1.1:80, etc.
2. Nginx creates a list of server blocks that best match the request based on the requested IP and port, giving priority to server blocks that specify a specific IP, and then selecting server blocks such as listen 0.0.0.0. In either case, the port must be an exact match.
3. If there is only one best match, then the matching server block will be used to respond to the request, otherwise the server_name directive of each server block will be evaluated.

Once again, the server_name directive is evaluated only if the listen directive cannot find a best match.

For example, let's assume that the example.com domain is pointing to 192.168.0.1, and that nginx on 192.168.0.1 has only the following two server blocks:

# server block 1server {
  listen 192.168.0.1;
  server_name other.com
  ...
}

# server block 2server {
  listen 80;
  server_name example.com
  ...
}

Server_name Directive

If the best match cannot be found according to the listen directive, the server_name directive will be parsed. Nginx will check the "Host" header in the request. This value contains the domain name or IP address that the client is actually trying to request. Nginx will match the server_name directive based on this value. The matching rules are as follows:

1. Nginx will try to find a server block that exactly matches the server_name and Host values. If multiple exact matches are found, the first matching server block will be used.
2. If no exact matching server block is found, nginx tries to find a server block with a server_name starting with *. If multiple server blocks are found, the longest matching server block is selected.
3. If no server block beginning with is found, a server block ending with is searched. Again, if there are multiple matches, the longest match is chosen.
4. If no server block matching * is found, a server block defining server_name using a regular expression (starting with ~) is searched for. If multiple matches are found, the first match is used.
5. If no server block matching the regular expression is found, nginx will select a default server block matching the listen field. Each IP and port combination can be configured with one and only one default default_server block. If there is none, the first server in the available list will be selected (the selection is random at this time, and the order is not fixed)

Here is an example:

(1) Accurate server_name matching, for example:

server {
   listen 80;
   server_name www.domain.com;
   ...
}

(2) A string starting with the * wildcard character:

server {
   listen 80;
   server_name *.domain.com;
   ...
}

(3) A string ending with the * wildcard character:

server {
   listen 80;
   server_name www.*;
   ...
}

(4) Matching regular expressions:

server {
   listen 80;
   server_name ~^(?.+)\.domain\.com$;
   ...
}

(5) If none of the above matches, the default_server is used. If no default_server is specified, the first available server is selected. We can specify that when there is no matching host value, an error is returned to the client. This can be used to prevent others from redirecting junk traffic to your website.

server {
  listen 80 default_server;
  server_name _; return 444;
}

Let nginx disconnect from the browser by returning 444, a non-standard error code of nginx

Location matching logic

Location Syntax Analysis

location optional_modifier location_match {
  ...
}

The available modifiers are as follows

Decision rules

1. Nginx first checks for prefix-based location matches (i.e. matches that do not contain regular expressions)

2. If a location block using the = modifier completely matches the requested URL, the location is immediately used to respond to the request.

3. If no location block match with the = modifier is found, the inexact prefix will continue to be calculated, the longest matching prefix will be found according to the given URI, and then the following processing will be performed:

(1) If the longest matching location has the ^~ modifier, nginx immediately uses that location to respond to the request

(2) If the longest matching location does not have the ^~ modifier, nginx will temporarily store the match and then continue with the subsequent match

4. After determining and storing the longest matching prefix location block, nginx continues to check the regular expression matching location (case sensitive/case insensitive). If there is a regular expression that meets the requirements, the first regular expression location that matches the requested URI will be selected to respond to the request.

5. If no regular expression location matching the requested URI is found, the longest prefix location previously stored is used to respond to the request.

Replenish

Normally, once a location is chosen to respond to a request, the request will be processed within that location, regardless of other locations. However, some instructions in the location will trigger a new location match, such as:

(1) try_files

(2) rewrite

(3) error_page

For more information about configuring default_server for https, refer to Properly setting up a “default” nginx server for https

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