mod_gnutls is a module for the Apache web server that provides HTTPS (HTTP over Transport Layer Security (TLS) or the older Secure Sockets Layer (SSL)) using the GnuTLS library. More information about the module can be found at the project's website.

Compilation & Installation

mod_gnutls uses the ./configure && make && make install mechanism common to many Open Source programs. Most of the dirty work is handled by either ./configure or Apache's apxs utility. If you have built Apache modules before, there shouldn't be any surprises for you.

The interesting options you can pass to configure are:

--with-apxs=PATH

This option is used to specify the location of the apxs utility that was installed as part of apache. Specify the location of the binary, not the directory it is located in.

--with-apu-config=PATH

Path to APR Utility Library config tool (apu-1-config)

--help

Provides a list of all available configure options.

It is recommended to run make check before installation. If your system doesn't have a loopback device with IPv6 and IPv4 support or localhost does not resolve to at least one of [::1] and 127.0.0.1, you may have to set the TEST_HOST or TEST_IP environment variables when running ./configure to make the test suite work correctly.

Integration

To activate mod_gnutls just add the following line to your httpd.conf and restart Apache:

LoadModule gnutls_module modules/mod_gnutls.so

Configuration Directives

GnuTLSEnable

Enable GnuTLS for this virtual host

GnuTLSEnable [on|off]

Default: off
Context: virtual host

This directive enables SSL/TLS Encryption for a Virtual Host.

GnuTLSCache

Configure SSL Session Cache

GnuTLSCache [dbm|gdbm|memcache|none] [PATH|SERVERLIST|-]

Default: GnuTLSCache none
Context: server config

This directive configures the SSL Session Cache for mod_gnutls. This could be shared between machines of different architectures.

dbm (Requires Berkeley DBM)

Uses the default Berkeley DB backend of APR DBM to cache SSL Sessions results. The argument is a relative or absolute path to be used as the DBM Cache file. This is compatible with most operating systems, but needs the Apache Runtime to be compiled with Berkeley DBM support.

gdbm

Uses the GDBM backend of APR DBM to cache SSL Sessions results.

The argument is a relative or absolute path to be used as the DBM Cache file. This is the recommended option.

memcache

Uses a memcached server to cache the SSL Session.

The argument is a space separated list of servers. If no port number is supplied, the default of 11211 is used. This can be used to share a session cache between all servers in a cluster.

none

Turns off all caching of SSL Sessions.

This can significantly reduce the performance of mod_gnutls since even followup connections by a client must renegotiate parameters instead of reusing old ones. This is the default, since it requires no configuration.

GnuTLSCacheTimeout

Timeout for SSL Session Cache expiration

GnuTLSCacheTimeout SECONDS

Default: GnuTLSCacheTimeout 300
Context: server config

Sets the timeout for SSL Session Cache entries expiration. This directive is valid even if Session Tickets are used, and indicates the expiration time of the ticket in seconds.

GnuTLSSessionTickets

Enable Session Tickets for the server

GnuTLSSessionTickets [on|off]

Default: off
Context: server config, virtual host

To avoid storing data for TLS session resumption it is allowed to provide client with a ticket, to use on return. Use for servers with limited storage, and don't combine with GnuTLSCache. For a pool of servers this option is not recommended since the tickets are unique for the issuing server only.

GnuTLSCertificateFile

Set to the PEM Encoded Server Certificate

GnuTLSCertificateFile FILEPATH

Default: none
Context: server config, virtual host

Takes an absolute or relative path to a PEM-encoded X.509 certificate to use as this Server's End Entity (EE) certificate. If you need to supply certificates for intermediate Certificate Authorities (iCAs), they should be listed in sequence in the file, from EE to the iCA closest to the root CA. Optionally, you can also include the root CA's certificate as the last certificate in the list.

Since version 0.7 this can be a PKCS #11 URL.

GnuTLSKeyFile

Set to the PEM Encoded Server Private Key

GnuTLSKeyFile FILEPATH

Default: none
Context: server config, virtual host

Takes an absolute or relative path to the Server Private Key. Set GnuTLSPIN if the key file is encrypted.

Since version 0.7 this can be a PKCS #11 URL.

Security Warning:
This private key must be protected. It is read while Apache is still running as root, and does not need to be readable by the nobody or apache user.

GnuTLSPGPCertificateFile

Set to a base64 Encoded Server OpenPGP Certificate

GnuTLSPGPCertificateFile FILEPATH

Default: none
Context: server config, virtual host

Takes an absolute or relative path to a base64 Encoded OpenPGP Certificate to use as this Server's Certificate.

GnuTLSPGPKeyFile

Set to the Server OpenPGP Secret Key

GnuTLSPGPKeyFile FILEPATH

Default: none
Context: server config, virtual host

Takes an absolute or relative path to the Server Private Key. This key cannot currently be password protected.

Security Warning:
This private key must be protected. It is read while Apache is still running as root, and does not need to be readable by the nobody or apache user.

GnuTLSClientVerify

Enable Client Certificate Verification
GnuTLSClientVerify [ignore|request|require]

Default: ignore
Context: server config, virtual host, directory, .htaccess

This directive controls the use of SSL Client Certificate Authentication. If used in the .htaccess context, it can force TLS re-negotiation.

ignore

mod_gnutls will ignore the contents of any SSL Client Certificates sent. It will not request that the client sends a certificate.

request

The client certificate will be requested, but not required. The Certificate will be validated if sent. The output of the validation status will be stored in the SSL_CLIENT_VERIFY environment variable and can be SUCCESS, FAILED or NONE.

require

A Client certificate will be required. Any requests without a valid client certificate will be denied. The SSL_CLIENT_VERIFY environment variable will only be set to SUCCESS.

GnuTLSClientCAFile

Set to the PEM Encoded Certificate Authority Certificate

GnuTLSClientCAFile FILEPATH

Default: none Context: server config, virtual host

Takes an absolute or relative path to a PEM Encoded Certificate to use as a Certificate Authority with Client Certificate Authentication. This file may contain a list of trusted authorities.

GnuTLSPGPKeyringFile

Set to a base64 Encoded key ring

GnuTLSPGPKeyringFile FILEPATH

Default: none
Context: server config, virtual host

Takes an absolute or relative path to a base64 Encoded Certificate list (key ring) to use as a means of verification of Client Certificates. This file should contain a list of trusted signers.

GnuTLSDHFile

Set to the PKCS #3 encoded Diffie Hellman parameters

GnuTLSDHFile FILEPATH

Default: none
Context: server config, virtual host

Takes an absolute or relative path to a PKCS #3 encoded DH parameters.Those are used when the DHE key exchange method is enabled. You can generate this file using certtool --generate-dh-params --bits 2048. If not set mod_gnutls will use the included parameters.

GnuTLSSRPPasswdFile

Set to the SRP password file for SRP ciphersuites

GnuTLSSRPPasswdFile FILEPATH

Default: none
Context: server config, virtual host

Takes an absolute or relative path to an SRP password file. This is the same format as used in libsrp. You can generate such file using the command srptool --passwd /etc/tpasswd --passwd-conf /etc/tpasswd.conf -u test to set a password for user test. This password file holds the username, a password verifier and the dependency to the SRP parameters.

GnuTLSSRPPasswdConfFile

Set to the SRP password.conf file for SRP ciphersuites

GnuTLSSRPPasswdConfFile FILEPATH 

Default: none
Context: server config, virtual host

Takes an absolute or relative path to an SRP password.conf file. This is the same format as used in libsrp. You can generate such file using the command srptool --create-conf /etc/tpasswd.conf. This file holds the SRP parameters and is associate with the password file (the verifiers depends on these parameters).

GnuTLSPriorities

Set the allowed ciphers, key exchange algorithms, MACs and compression methods

GnuTLSPriorities NORMAL:+CIPHER_0:+CIPHER_1:...:+CIPHER_N

Default: none
Context: server config, virtual host

Takes a semi-colon separated list of ciphers, key exchange methods Message authentication codes and compression methods to enable. The allowed keywords are specified in the gnutls_priority_init() function of GnuTLS.

Full details can be found at the GnuTLS documentation. In brief you can specify a set of ciphersuites from the choices:

NONE

The empty list.

EXPORT

A list with all the supported cipher combinations including the EXPORT strength algorithms.

PERFORMANCE

A list with all the secure cipher combinations sorted in terms of performance.

NORMAL

A list with all the secure cipher combinations sorted with respect to security margin (subjective term).

SECURE

A list with all the secure cipher combinations including the 256-bit ciphers sorted with respect to security margin.

Additionally you can add or remove algorithms using the + and ! prefixes respectively.

For example, in order to disable the ARCFOUR cipher from the NORMAL set you can use the string NORMAL:!ARCFOUR-128

Other options such as the protocol version and the compression method can be specified using the VERS- and COMP- prefixes.

So in order to remove or add a specific TLS version from the NORMAL set, use NORMAL:!VERS-SSL3.0. And to enable zlib compression use NORMAL:+COMP-DEFLATE.

However it is recommended not to add compression at this level. With the NONE set, in order to be usable, you have to specify a complete set of combinations of protocol versions, cipher algorithms (AES-128-CBC), key exchange algorithms (RSA), message authentication codes (SHA1) and compression methods (COMP-NULL).

You can find a list of all supported Ciphers, Versions, MACs, etc. by running gnutls-cli --list.

The special keyword %COMPAT will disable some security features such as protection against statistical attacks to ciphertext data in order to achieve maximum compatibility (some broken mobile clients need this).

GnuTLSP11Module

Load this PKCS #11 module.

GnuTLSP11Module PATH_TO_LIBRARY

Default: none
Context: server config

Load this PKCS #11 provider module, instead of the system defaults. May occur multiple times to load multiple modules.

GnuTLSPIN

Set the PIN to be used to access encrypted key files or PKCS #11 objects.

GnuTLSPIN XXXXXX

Default: none
Context: server config, virtual host

Takes a string to be used as a PIN for the protected objects in a security module, or as a key to be used to decrypt PKCS #8, PKCS #12, or openssl encrypted keys.

GnuTLSSRKPIN

Set the SRK PIN to be used to unlaccess the TPM.

GnuTLSSRKPIN XXXXXX

Default: none
Context: server config, virtual host

Takes a string to be used as a PIN for the protected objects in the TPM module.

GnuTLSExportCertificates

Export the PEM encoded certificates to CGIs

GnuTLSExportCertificates [off|on|SIZE]

Default: off
Context: server config, virtual host

This directive configures exporting the full certificates of the server and the client to CGI scripts via the SSL_SERVER_CERT and SSL_CLIENT_CERT environment variables. The exported certificates will be PEM-encoded (if X.509) or ASCII-armored (if OpenPGP) up to the size given. The type of the certificate will be exported in SSL_SERVER_CERT_TYPE and SSL_CLIENT_CERT_TYPE.

SIZE should be an integer number of bytes, or may be written with a trailing K to indicate kibibytes. off means the same thing as 0, in which case the certificates will not be exported to the environment. on is an alias for 16K. If a non-zero size is specified for this directive, but a certificate is too large to fit in the buffer, then the corresponding environment variable will contain the fixed string GNUTLS_CERTIFICATE_SIZE_LIMIT_EXCEEDED.

With GnuTLSExportCertificates enabled, mod_gnutls exports the same environment variables to the CGI process as mod_ssl.

GnuTLSProxyEngine

Enable TLS proxy connections for this virtual host

GnuTLSProxyEngine [on|off]

Default: off
Context: virtual host

This directive enables support for TLS proxy connections for a virtual host.

GnuTLSProxyCAFile

Set to the PEM encoded Certificate Authority Certificate

GnuTLSProxyCAFile FILEPATH

Default: none
Context: server config, virtual host

Takes an absolute or relative path to a PEM encoded certificate to use as a Certificate Authority when verifying certificates provided by proxy back end servers. This file may contain a list of trusted authorities. If not set, verification of TLS back end servers will always fail due to lack of a trusted CA.

GnuTLSProxyCRLFile

Set to the PEM encoded Certificate Revocation List

GnuTLSProxyCRLFile FILEPATH

Default: none
Context: server config, virtual host

Takes an absolute or relative path to a PEM encoded Certificate Revocation List to use when verifying certificates provided by proxy back end servers. The file may contain a list of CRLs.

GnuTLSProxyCertificateFile

Set to the PEM encoded Client Certificate

GnuTLSProxyCertificateFile FILEPATH

Default: none
Context: server config, virtual host

Takes an absolute or relative path to a PEM encoded X.509 certificate to use as this Server's End Entity (EE) client certificate for TLS client authentication in proxy TLS connections. If you need to supply certificates for intermediate Certificate Authorities (iCAs), they should be listed in sequence in the file, from EE to the iCA closest to the root CA. Optionally, you can also include the root CA's certificate as the last certificate in the list.

If not set, TLS client authentication will be disabled for TLS proxy connections. If set, GnuTLSProxyKeyFile must be set as well to provide the matching private key.

GnuTLSProxyKeyFile

Set to the PEM encoded Private Key

GnuTLSProxyKeyFile FILEPATH

Default: none
Context: server config, virtual host

Takes an absolute or relative path to the Private Key matching the certificate configured using the GnuTLSProxyCertificateFile directive. This key cannot currently be password protected.

Security Warning:
This private key must be protected. It is read while Apache is still running as root, and does not need to be readable by the nobody or apache user.

GnuTLSProxyPriorities

Set the allowed ciphers, key exchange algorithms, MACs and compression methods for proxy connections

GnuTLSProxyPriorities NORMAL:+CIPHER_0:+CIPHER_1:...:+CIPHER_N

Default: none
Context: server config, virtual host

This option is used to set the allowed ciphers, key exchange algorithms, MACs and compression methods for proxy connections. It takes the same parameters as GnuTLSPriorities. Required if GnuTLSProxyEngine is On.

Configuration Examples

Simple Standard SSL Example

The following is an example of standard SSL Hosting, using one IP Addresses for each virtual host

 # Load the module into Apache.
 LoadModule gnutls_module modules/mod_gnutls.so
 GnuTLSCache gdbm /var/cache/www-tls-cache
 GnuTLSCacheTimeout 500
 # With normal SSL Websites, you need one IP Address per-site.
 Listen 1.2.3.1:443
 Listen 1.2.3.2:443
 Listen 1.2.3.3:443
 Listen 1.2.3.4:443
 <VirtualHost 1.2.3.1:443>
 GnuTLSEnable on
 GnuTLSPriorities NONE:+AES-128-CBC:+3DES-CBC:+ARCFOUR-128:+RSA:+DHE-RSA:+DHE-DSS:+SHA1:+MD5:+COMP-NULL
 DocumentRoot /www/site1.example.com/html
 ServerName site1.example.com:443
 GnuTLSCertificateFile conf/ssl/site1.crt
 GnuTLSKeyFile conf/ss/site1.key
 </VirtualHost>
 <VirtualHost 1.2.3.2:443>
 # This virtual host enables SRP authentication
 GnuTLSEnable on
 GnuTLSPriorities NORMAL:+SRP
 DocumentRoot /www/site2.example.com/html
 ServerName site2.example.com:443
 GnuTLSSRPPasswdFile conf/ssl/tpasswd.site2
 GnuTLSSRPPasswdConfFile conf/ssl/tpasswd.site2.conf
 </VirtualHost>
 <VirtualHost 1.2.3.3:443>
 # This server enables SRP, OpenPGP and X.509 authentication.
 GnuTLSEnable on
 GnuTLSPriorities NORMAL:+SRP:+SRP-RSA:+SRP-DSS
 DocumentRoot /www/site3.example.com/html
 ServerName site3.example.com:443
 GnuTLSCertificateFile conf/ssl/site3.crt
 GnuTLSKeyFile conf/ss/site3.key
 GnuTLSClientVerify ignore
 GnuTLSPGPCertificateFile conf/ss/site3.pub.asc
 GnuTLSPGPKeyFile conf/ss/site3.sec.asc
 GnuTLSSRPPasswdFile conf/ssl/tpasswd.site3
 GnuTLSSRPPasswdConfFile conf/ssl/tpasswd.site3.conf
 </VirtualHost>
 <VirtualHost 1.2.3.4:443>
 GnuTLSEnable on
 # %COMPAT disables some security features to enable maximum compatibility with clients.
 GnuTLSPriorities NONE:+AES-128-CBC:+ARCFOUR-128:+RSA:+SHA1:+MD5:+COMP-NULL:%COMPAT
 DocumentRoot /www/site4.example.com/html
 ServerName site4.example.com:443
 GnuTLSCertificateFile conf/ssl/site4.crt
 GnuTLSKeyFile conf/ss/site4.key
 </VirtualHost>

Server Name Indication Example

mod_gnutls can also use "Server Name Indication", as specified in RFC 3546. This allows hosting many SSL Websites, with a Single IP Address. Currently all the recent browsers support this standard. Here is an example, using SNI: ` `

 # Load the module into Apache.
 LoadModule gnutls_module modules/mod_gnutls.so
 # With normal SSL Websites, you need one IP Address per-site.
 Listen 1.2.3.1:443
 # This could also be 'Listen *:443',
 # just like '*:80' is common for non-https
 # No caching. Enable session tickets. Timeout is still used for
 # ticket expiration.
 GnuTLSCacheTimeout 600
 # This tells apache, that for this IP/Port combination, we want to use
 # Name Based Virtual Hosting. In the case of Server Name Indication,
 # it lets mod_gnutls pick the correct Server Certificate.
 NameVirtualHost 1.2.3.1:443
 <VirtualHost 1.2.3.1:443>
 GnuTLSEnable on
 GnuTLSSessionTickets on
 GnuTLSPriorities NORMAL
 DocumentRoot /www/site1.example.com/html
 ServerName site1.example.com:443
 GnuTLSCertificateFile conf/ssl/site1.crt
 GnuTLSKeyFile conf/ss/site1.key
 </VirtualHost>
 <VirtualHost 1.2.3.1:443>
 GnuTLSEnable on
 GnuTLSPriorities NORMAL
 DocumentRoot /www/site2.example.com/html
 ServerName site2.example.com:443
 GnuTLSCertificateFile conf/ssl/site2.crt
 GnuTLSKeyFile conf/ss/site2.key
 </VirtualHost>
 <VirtualHost 1.2.3.1:443>
 GnuTLSEnable on
 GnuTLSPriorities NORMAL
 DocumentRoot /www/site3.example.com/html
 ServerName site3.example.com:443
 GnuTLSCertificateFile conf/ssl/site3.crt
 GnuTLSKeyFile conf/ss/site3.key
 </VirtualHost>
 <VirtualHost 1.2.3.1:443>
 GnuTLSEnable on
 GnuTLSPriorities NORMAL
 DocumentRoot /www/site4.example.com/html
 ServerName site4.example.com:443
 GnuTLSCertificateFile conf/ssl/site4.crt
 GnuTLSKeyFile conf/ss/site4.key
 </VirtualHost>

Performance Issues

mod_gnutls by default uses conservative settings for the server. You can fine tune the configuration to reduce the load on a busy server. The following examples do exactly this:

 # Load the module into Apache.
 LoadModule gnutls_module modules/mod_gnutls.so
 # Using 4 memcache servers to distribute the SSL Session Cache.
 GnuTLSCache memcache "mc1.example.com mc2.example.com mc3.example.com mc4.example.com"
 GnuTLSCacheTimeout 600
 Listen 1.2.3.1:443
 NameVirtualHost 1.2.3.1:443
 <VirtualHost 1.2.3.1:443>
 GnuTLSEnable on
 # Here we disable the Perfect forward secrecy ciphersuites (DHE)
 # and disallow AES-256 since AES-128 is just fine.
 GnuTLSPriorities NORMAL:!DHE-RSA:!DHE-DSS:!AES-256-CBC:%COMPAT
 DocumentRoot /www/site1.example.com/html
 ServerName site1.example.com:443
 GnuTLSCertificateFile conf/ssl/site1.crt
 GnuTLSKeyFile conf/ss/site1.key
 </VirtualHost>
 <VirtualHost 1.2.3.1:443>
 GnuTLSEnable on
 # Here we instead of disabling the DHE ciphersuites we use
 # Diffie Hellman parameters of smaller size than the default (2048 bits).
 # Using small numbers from 768 to 1024 bits should be ok once they are
 # regenerated every few hours.
 # Use "certtool --generate-dh-params --bits 1024" to get those
 GnuTLSDHFile /etc/apache2/dh.params
 GnuTLSPriorities NORMAL:!AES-256-CBC:%COMPAT
 DocumentRoot /www/site2.example.com/html
 ServerName site2.example.com:443
 GnuTLSCertificateFile conf/ssl/site2.crt
 GnuTLSKeyFile conf/ss/site2.key
 </VirtualHost>

Environment Variables

mod_gnutls exports the following environment variables to scripts. These are compatible with mod_ssl.

HTTPS

Can be on or off

SSL_VERSION_LIBRARY

The version of the GnuTLS library

SSL_VERSION_INTERFACE

The version of this module

SSL_PROTOCOL

The SSL or TLS protocol name (such as TLS 1.0 etc.)

SSL_CIPHER

The SSL or TLS cipher suite name

SSL_COMPRESS_METHOD

The negotiated compression method (NULL or DEFLATE)

SSL_SRP_USER

The SRP username used for authentication (only set when GnuTLSSRPPasswdFile and GnuTLSSRPPasswdConfFile are configured).

SSL_CIPHER_USEKEYSIZE & SSL_CIPHER_ALGKEYSIZE

The number if bits used in the used cipher algorithm.

This does not fully reflect the security level since the size of RSA or DHE key exchange parameters affect the security level too.

SSL_DH_PRIME_BITS

The number if bits in the modulus for the DH group, if DHE or static DH is used.

This will not be set if DH is not used.

SSL_CIPHER_EXPORT

True or False. Whether the cipher suite negotiated is an export one.

SSL_SESSION_ID

The session ID negotiated in this session. Can be the same during client reloads.

SSL_CLIENT_V_REMAIN

The number of days until the client's certificate is expired.

SSL_CLIENT_V_START

The activation time of client's certificate.

SSL_CLIENT_V_END

The expiration time of client's certificate.

SSL_CLIENT_S_DN

The distinguished name of client's certificate in RFC2253 format.

SSL_CLIENT_I_DN

The SSL or TLS cipher suite name

SSL_CLIENT_S_AN%

These will contain the alternative names of the client certificate (% is a number starting from zero).

The values will be prepended by DNSNAME:, RFC822NAME: or URI: depending on the type.

If it is not supported the value UNSUPPORTED will be set.

SSL_SERVER_M_SERIAL

The serial number of the server's certificate.

SSL_SERVER_M_VERSION

The version of the server's certificate.

SSL_SERVER_A_SIG

The algorithm used for the signature in server's certificate.

SSL_SERVER_A_KEY

The public key algorithm in server's certificate.

SSL_SERVER_CERT

The PEM-encoded (X.509) or ASCII-armored (OpenPGP) server certificate (see the GnuTLSExportCertificates directive).

SSL_SERVER_CERT_TYPE

The certificate type can be X.509 or OPENPGP.

SSL_CLIENT_CERT

The PEM-encoded (X.509) or ASCII-armored (OpenPGP) client certificate (see the GnuTLSExportCertificates directive).

SSL_CLIENT_CERT_TYPE

The certificate type can be X.509 or OPENPGP.