Configuring HTTPS servers

HTTPS server optimization
SSL certificate chains
A single HTTP/HTTPS server
Name-based HTTPS servers
     An SSL certificate with several names
     Server Name Indication
Compatibility

To configure an HTTPS server, the ssl parameter must be enabled on listening sockets in the server block, and the locations of the server certificate and private key files should be specified:

server {
    listen              443 ssl;
    server_name         www.example.com;
    ssl_certificate     www.example.com.crt;
    ssl_certificate_key www.example.com.key;
    ssl_protocols       SSLv3 TLSv1 TLSv1.1 TLSv1.2;
    ssl_ciphers         HIGH:!aNULL:!MD5;
    ...
}

The server certificate is a public entity. It is sent to every client that connects to the server. The private key is a secure entity and should be stored in a file with restricted access, however, it must be readable by nginx’s master process. The private key may alternately be stored in the same file as the certificate:

    ssl_certificate     www.example.com.cert;
    ssl_certificate_key www.example.com.cert;

in which case the file access rights should also be restricted. Although the certificate and the key are stored in one file, only the certificate is sent to a client.

The directives ssl_protocols and ssl_ciphers can be used to limit connections to include only the strong versions and ciphers of SSL/TLS. Since version 1.0.5, nginx uses “ssl_protocols SSLv3 TLSv1” and “ssl_ciphers HIGH:!aNULL:!MD5” by default, so configuring them explicitly only makes sense for the earlier nginx versions. Since versions 1.1.13 and 1.0.12, nginx uses “ssl_protocols SSLv3 TLSv1 TLSv1.1 TLSv1.2” by default.

CBC-mode ciphers might be vulnerable to a number of attacks and to the BEAST attack in particular (see CVE-2011-3389). Configuration of ciphers can be adjusted to prefer RC4-SHA as the following:

    ssl_ciphers RC4:HIGH:!aNULL:!MD5;
    ssl_prefer_server_ciphers on;

HTTPS server optimization

SSL operations consume extra CPU resources. On multi-processor systems several worker processes should be run, no less than the number of available CPU cores. The most CPU-intensive operation is the SSL handshake. There are two ways to minimize the number of these operations per client: the first is by enabling keepalive connections to send several requests via one connection and the second is to reuse SSL session parameters to avoid SSL handshakes for parallel and subsequent connections. The sessions are stored in an SSL session cache shared between workers and configured by the ssl_session_cache directive. One megabyte of the cache contains about 4000 sessions. The default cache timeout is 5 minutes. It can be increased by using the ssl_session_timeout directive. Here is a sample configuration optimized for a multi-core system with 10 megabyte shared session cache:

worker_processes auto;

http {
    ssl_session_cache   shared:SSL:10m;
    ssl_session_timeout 10m;

    server {
        listen              443 ssl;
        server_name         www.example.com;
        keepalive_timeout   70;

        ssl_certificate     www.example.com.crt;
        ssl_certificate_key www.example.com.key;
        ssl_protocols       SSLv3 TLSv1 TLSv1.1 TLSv1.2;
        ssl_ciphers         HIGH:!aNULL:!MD5;
        ...

SSL certificate chains

Some browsers may complain about a certificate signed by a well-known certificate authority, while other browsers may accept the certificate without issues. This occurs because the issuing authority has signed the server certificate using an intermediate certificate that is not present in the certificate base of well-known trusted certificate authorities which is distributed with a particular browser. In this case the authority provides a bundle of chained certificates which should be concatenated to the signed server certificate. The server certificate must appear before the chained certificates in the combined file:

$ cat www.example.com.crt bundle.crt > www.example.com.chained.crt

The resulting file should be used in the ssl_certificate directive:

server {
    listen              443 ssl;
    server_name         www.example.com;
    ssl_certificate     www.example.com.chained.crt;
    ssl_certificate_key www.example.com.key;
    ...
}

If the server certificate and the bundle have been concatenated in the wrong order, nginx will fail to start and will display the error message:

SSL_CTX_use_PrivateKey_file(" ... /www.example.com.key") failed
   (SSL: error:0B080074:x509 certificate routines:
    X509_check_private_key:key values mismatch)

because nginx has tried to use the private key with the bundle’s first certificate instead of the server certificate.

Browsers usually store intermediate certificates which they receive and which are signed by trusted authorities, so actively used browsers may already have the required intermediate certificates and may not complain about a certificate sent without a chained bundle. To ensure the server sends the complete certificate chain, the openssl command-line utility may be used, for example:

$ openssl s_client -connect www.godaddy.com:443
...
Certificate chain
 0 s:/C=US/ST=Arizona/L=Scottsdale/1.3.6.1.4.1.311.60.2.1.3=US
     /1.3.6.1.4.1.311.60.2.1.2=AZ/O=GoDaddy.com, Inc
     /OU=MIS Department/CN=www.GoDaddy.com
     /serialNumber=0796928-7/2.5.4.15=V1.0, Clause 5.(b)
   i:/C=US/ST=Arizona/L=Scottsdale/O=GoDaddy.com, Inc.
     /OU=http://certificates.godaddy.com/repository
     /CN=Go Daddy Secure Certification Authority
     /serialNumber=07969287
 1 s:/C=US/ST=Arizona/L=Scottsdale/O=GoDaddy.com, Inc.
     /OU=http://certificates.godaddy.com/repository
     /CN=Go Daddy Secure Certification Authority
     /serialNumber=07969287
   i:/C=US/O=The Go Daddy Group, Inc.
     /OU=Go Daddy Class 2 Certification Authority
 2 s:/C=US/O=The Go Daddy Group, Inc.
     /OU=Go Daddy Class 2 Certification Authority
   i:/L=ValiCert Validation Network/O=ValiCert, Inc.
     /OU=ValiCert Class 2 Policy Validation Authority
     /CN=http://www.valicert.com//emailAddress=info@valicert.com
...

In this example the subject (“s”) of the www.GoDaddy.com server certificate #0 is signed by an issuer (“i”) which itself is the subject of the certificate #1, which is signed by an issuer which itself is the subject of the certificate #2, which signed by the well-known issuer ValiCert, Inc. whose certificate is stored in the browsers’ built-in certificate base (that lay in the house that Jack built).

If a certificate bundle has not been added, only the server certificate #0 will be shown.

A single HTTP/HTTPS server

It is possible to configure a single server that handles both HTTP and HTTPS requests:

server {
    listen              80;
    listen              443 ssl;
    server_name         www.example.com;
    ssl_certificate     www.example.com.crt;
    ssl_certificate_key www.example.com.key;
    ...
}

Prior to 0.7.14 SSL could not be enabled selectively for individual listening sockets, as shown above. SSL could only be enabled for the entire server using the ssl directive, making it impossible to set up a single HTTP/HTTPS server. The ssl parameter of the listen directive was added to solve this issue. The use of the ssl directive in modern versions is thus discouraged.

Name-based HTTPS servers

A common issue arises when configuring two or more HTTPS servers listening on a single IP address:

server {
    listen          443 ssl;
    server_name     www.example.com;
    ssl_certificate www.example.com.crt;
    ...
}

server {
    listen          443 ssl;
    server_name     www.example.org;
    ssl_certificate www.example.org.crt;
    ...
}

With this configuration a browser receives the default server’s certificate, i.e. www.example.com regardless of the requested server name. This is caused by SSL protocol behaviour. The SSL connection is established before the browser sends an HTTP request and nginx does not know the name of the requested server. Therefore, it may only offer the default server’s certificate.

The oldest and most robust method to resolve the issue is to assign a separate IP address for every HTTPS server:

server {
    listen          192.168.1.1:443 ssl;
    server_name     www.example.com;
    ssl_certificate www.example.com.crt;
    ...
}

server {
    listen          192.168.1.2:443 ssl;
    server_name     www.example.org;
    ssl_certificate www.example.org.crt;
    ...
}

An SSL certificate with several names

There are other ways that allow sharing a single IP address between several HTTPS servers. However, all of them have their drawbacks. One way is to use a certificate with several names in the SubjectAltName certificate field, for example, www.example.com and www.example.org. However, the SubjectAltName field length is limited.

Another way is to use a certificate with a wildcard name, for example, *.example.org. A wildcard certificate secures all subdomains of the specified domain, but only on one level. This certificate matches www.example.org, but does not match example.org and www.sub.example.org. These two methods can also be combined. A certificate may contain exact and wildcard names in the SubjectAltName field, for example, example.org and *.example.org.

It is better to place a certificate file with several names and its private key file at the http level of configuration to inherit their single memory copy in all servers:

ssl_certificate     common.crt;
ssl_certificate_key common.key;

server {
    listen          443 ssl;
    server_name     www.example.com;
    ...
}

server {
    listen          443 ssl;
    server_name     www.example.org;
    ...
}

Server Name Indication

A more generic solution for running several HTTPS servers on a single IP address is TLS Server Name Indication extension (SNI, RFC 6066), which allows a browser to pass a requested server name during the SSL handshake and, therefore, the server will know which certificate it should use for the connection. However, SNI has limited browser support. Currently it is supported starting with the following browsers versions:

Only domain names can be passed in SNI, however some browsers may erroneously pass an IP address of the server as its name if a request includes literal IP address. One should not rely on this.

In order to use SNI in nginx, it must be supported in both the OpenSSL library with which the nginx binary has been built as well as the library to which it is being dynamically linked at run time. OpenSSL supports SNI since 0.9.8f version if it was built with config option “--enable-tlsext”. Since OpenSSL 0.9.8j this option is enabled by default. If nginx was built with SNI support, then nginx will show this when run with the “-V” switch:

$ nginx -V
...
TLS SNI support enabled
...

However, if the SNI-enabled nginx is linked dynamically to an OpenSSL library without SNI support, nginx displays the warning:

nginx was built with SNI support, however, now it is linked
dynamically to an OpenSSL library which has no tlsext support,
therefore SNI is not available

Compatibility

written by Igor Sysoev
edited by Brian Mercer