Server names are defined using the server_name directive and determine which server block is used for a given request. See also “How nginx processes a request”. They may be defined using exact names, wildcard names, or regular expressions:

server {
    listen       80;
    server_name  nginx.org  www.nginx.org;
    ...
}

server {
    listen       80;
    server_name  *.nginx.org;
    ...
}

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

server {
    listen       80;
    server_name  ~^(?<user>.+)\.nginx\.net$;
    ...
}

The names are tested in the following order:

1. exact names;
2. wildcard names starting with an asterisk: *.nginx.org;
3. wildcard names ending with an asterisk: mail.*;
4. and regular expressions in the order listed in the configuration file.

The first match stops the search.

A wildcard name may contain an asterisk only on the name's start or end, and only on a dot border. The names “www.*.nginx.org” and “w*.nginx.org” are invalid. However, these names can be specified using regular expressions, for example, “~^www\..+\.nginx\.org$” and “~^w.*\.nginx\.org$”. An asterisk can match several name parts. The name “*.nginx.org” matches not only www.nginx.org but www.sub.nginx.org as well.

A special wildcard in the form “.nginx.org” can be used to match both the exact name “nginx.org” and the wildcard name “*.nginx.org”.

The regular expressions used by nginx are compatible with those used by the Perl programming language (PCRE). To use a regular expression, the server name must start with the tilde character:

server_name  ~^www\d+\.nginx\.net$;

otherwise it will be treated as an exact name, or if the expression contains an asterisk, as a wildcard name (and most likely as an invalid one). Do not forget to set “^” and “$” anchors. They are not required syntactically, but logically. Also note that domain name dots should be escaped with a backslash. A regular expression containing the characters “{” and “}” should be quoted:

server_name  "~^(?<name>\w\d{1,3}+)\.nginx\.net$";

otherwise nginx will fail to start and display the error message:

directive "server_name" is not terminated by ";" in ...

A named regular expression capture can be used later as a variable:

server {
    server_name   ~^(www\.)?(?<domain>.+)$;

    location / {
        root   /sites/$domain;
    }
}

The PCRE library supports named captures using the following syntax:

?<name>	Perl 5.10 compatible syntax, supported since PCRE-7.0
?'name'	Perl 5.10 compatible syntax, supported since PCRE-7.0
?P<name>	Python compatible syntax, supported since PCRE-4.0

pcre_compile() failed: unrecognized character after (?< in ...

this means that the PCRE library is old and you should try the syntax “?P<name>”. The captures can also be used in digital form:

server {
    server_name   ~^(www\.)?(.+)$;

    location / {
        root   /sites/$2;
    }
}

However, such usage should be limited to simple cases (like the above), since the digital references can easily be overwritten.

If you want to process requests without a “Host” header line in a server block which is not the default, you should specify an empty name:

server {
    listen       80;
    server_name  nginx.org  www.nginx.org  "";
    ...
}

If no server_name is defined in a server block, then nginx uses the empty name as the server name.

nginx versions up to 0.8.48 used the hostname as the server name in this case.

If someone makes a request using an IP address instead of a server name, the request’s “Host” header line will contain the IP address and you can handle the request using the IP address as the server name:

server {
    listen       80;
    server_name  nginx.org
                 www.nginx.org
                 ""
                 192.168.1.1
                 ;
    ...
}

In catch-all server examples you may see the strange name “_”:

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

There is nothing special about this name, it is just one of a myriad of invalid domain names which never intersect with any real name. You may also use something like “--”, “!@#”, and so on.

nginx versions up to 0.6.25 supported the special name “*” which was erroneously interpreted to be a catch-all name. It never functioned as a catch-all or wildcard server name. Instead, it supplied the functionality that is now provided by the server_name_in_redirect directive. The special name “*” is now deprecated and the server_name_in_redirect directive should be used. Note that there is no way to specify the catch-all name or the default server using the server_name directive. This is a property of the listen directive and not of the server_name directive. See also “How nginx processes a request”. You can define servers listening on ports *:80 and *:8080, and direct that one will be the default server for port *:8080, while the other will be the default for port *:80:

server {
    listen       80;
    listen       8080  default_server;
    server_name  nginx.net;
    ...
}

server {
    listen       80  default_server;
    listen       8080;
    server_name  nginx.org;
    ...
}

Exact names and wildcard names are stored in hashes. The hashes are bound to the listen ports and every listen port may have up to three hashes: an exact names hash, a wildcard names hash starting with an asterisk, and a wildcard names hash ending with an asterisk. The sizes of the hashes are optimized at the configuration phase so that a name can be found with the fewest CPU cache misses. The exact names hash is searched first. If a name is not found using the exact name hash, then the wildcard names hash starting with an asterisk is searched. If the name is not found there, the wildcard names hash ending with an asterisk is searched. Searching wildcard names hashes is slower than searching exact name hash because names are searched by domain parts. Note that the special wildcard form “.nginx.org” is stored in a wildcard names hash and not in an exact names hash. Regular expressions are tested sequentially and therefore are the slowest method and are non-scalable.

For these reasons, it is better to use exact names where possible. For example, if the most frequently requested names of a server are nginx.org and www.nginx.org, it is more efficient to define them explicitly:

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

than to use the simplified form:

server {
    listen       80;
    server_name  .nginx.org;
    ...
}

If you have defined a large number of server names, or defined unusually long server names, you may need to tune the server_names_hash_max_size and server_names_hash_bucket_size directives at the http level. The default value of the server_names_hash_bucket_size may be equal to 32, or 64, or another value, depending on your CPU cache line size. If the default value is 32 and you define “too.long.server.name.nginx.org” as a server name, then nginx will fail to start and display the error message:

could not build the server_names_hash,
you should increase server_names_hash_bucket_size: 32

In this case, you should set the directive value to the next power of 2:

http {
    server_names_hash_bucket_size  64;
    ...

If you have defined a large number of server names, you will get another error message:

could not build the server_names_hash,
you should increase either server_names_hash_max_size: 512
or server_names_hash_bucket_size: 32

You should first try to set server_names_hash_max_size to a number close to the number of server names. Only if this does not help, or if nginx’s start time is unacceptably long, should you try to increase server_names_hash_bucket_size.

If a server is the only server for a listen port, then nginx will not test server names at all (and will not build the hashes for the listen port). However, there is one exception. If a server_name is a regular expression with captures, then nginx has to execute the expression to get the captures.

• A default server name value is an empty name “” since 0.8.48.
• Named regular expression server name captures have been supported since 0.8.25.
• Regular expression server name captures have been supported since 0.7.40.
• An empty server name “” has been supported since 0.7.12.
• A wildcard server name or regular expression has been supported for use as the first server name since 0.6.25.
• Regular expression server names have been supported since 0.6.7.
• Wildcard form nginx.* has been supported since 0.6.0.
• The special form .nginx.org has been supported since 0.3.18.
• Wildcard form *.nginx.org has been supported since 0.1.13.