source: mod_gnutls/doc/mod_gnutls_manual.mdwn @ 0cd8f3d

% `mod_gnutls` Manual

* * * * *

`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](https://mod.gnutls.org/).

* * * * *

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
========================

General Options
---------------

### 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 TLS Session Cache

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

Default: `GnuTLSCache none`\
Context: server config

This directive configures the TLS Session Cache for `mod_gnutls`.
This could be shared between machines of different architectures. If a
DBM cache is used, access is serialized using the `gnutls-cache`
mutex.

`dbm` (Requires Berkeley DBM)
:   Uses the default Berkeley DB backend of APR DBM to cache TLS
    Session data. 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 TLS Session data.

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

`memcache`
:   Uses memcached server(s) to cache TLS Session data.

    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 TLS 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 TLS Session Cache expiration

    GnuTLSCacheTimeout SECONDS

Default: `GnuTLSCacheTimeout 300`\
Context: server config

Sets the timeout for TLS Session Cache entries expiration. This value
is also used for OCSP responses if they do not contain a `nextUpdate`
time.

### 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 the server can
provide clients with tickets, to use on return. Tickets are an
alternative to using a session cache, mostly used for busy servers
with limited storage. For a pool of servers this option is not
recommended since the tickets are bound to the issuing server only.

If this option is set in the global configuration, virtual hosts
without a `GnuTLSSessionTickets` setting will use the global setting.

*Warning:* Currently the master key that protects the tickets is
generated only on server start, and there is no mechanism to roll over
the key. If session tickets are enabled it is highly recommened to
restart the server regularly to protect past sessions in case an
attacker gains access to server memory.

### GnuTLSClientVerify

Enable Client Certificate Verification

    GnuTLSClientVerify [ignore|request|require]

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

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

`ignore`
:   `mod_gnutls` will ignore the contents of any TLS 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`.

### 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.

### GnuTLSPriorities

Set the allowed protocol versions, 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 colon separated list of protocol version, 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.

Please refer to [the GnuTLS documentation](https://gnutls.org/manual/html_node/Priority-Strings.html#Priority-Strings)
for details. A few commonly used sets are listed below, note that
their exact meaning may change with GnuTLS versions.

`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).

`SECURE128`
:   A list with all the secure cipher suites that offer a security level
    of 128-bit or more.

`PFS`
:   Only cipher suites offering perfect forward secrecy (ECDHE and DHE),
    sorted by security margin.

You can add or remove algorithms using the `+` and `!` prefixes
respectively. For example, in order to use the `NORMAL` set but
disable TLS 1.0 and 1.1 you can use the string
`NORMAL:!VERS-TLS1.0:!VERS-TLS1.1`.

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

### 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 access 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`.

X.509 Certificate Authentication
--------------------------------

### 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.

### GnuTLSClientCAFile

Set the PEM encoded Certificate Authority list to use for X.509 base
client authentication

    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.

OpenPGP Certificate Authentication
----------------------------------

### 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.

### 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.

SRP Authentication
------------------

### 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).

TLS Proxy Configuration
-----------------------

### 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`.

OCSP Stapling Configuration
---------------------------

### GnuTLSOCSPStapling

Enable OCSP stapling for this (virtual) host.

    GnuTLSOCSPStapling [On|Off]

Default: *off*\
Context: server config, virtual host

OCSP stapling, formally known as the TLS Certificate Status Request
extension, allows the server to provide the client with an OCSP
response for its certificate during the handshake. This way the client
does not have to send an OCSP request to the CA to check the
certificate status, which offers privacy and performance advantages.

Using OCSP stapling has a few requirements:

* Caching OCSP responses requires a cache, so `GnuTLSCache` must not
  be `none`.
* `GnuTLSCertificateFile` must contain the issuer CA certificate in
  addition to the server certificate so responses can be verified.
* The certificate must either contain an OCSP access URI using HTTP,
  or `GnuTLSOCSPResponseFile` must be set.

OCSP cache updates are serialized using the `gnutls-ocsp` mutex.

### GnuTLSOCSPResponseFile

Read the OCSP response for stapling from this file instead of sending
a request over HTTP.

    GnuTLSOCSPResponseFile /path/to/response.der

Default: *empty*\
Context: server config, virtual host

The response file must be updated externally, for example using a cron
job. This option is an alternative to the server fetching OCSP
responses over HTTP. Reasons to use this option include:

* Performing OCSP requests separate from the web server, to prevent slow
  responses from stalling handshakes.
* The issuer CA uses an access method other than HTTP.
* Testing

### GnuTLSOCSPGraceTime

Replace cached OCSP responses this many seconds before they expire.

    GnuTLSOCSPGraceTime SECONDS

Default: *60*\
Context: server config, virtual host

A cached OCSP response should be updated a little before it expires to
account for potential clock skew between server, CA, and client, as
well as transmission time in corner cases. Note that a response
without a `nextUpdate` field will be subject to the default cache
lifetime.

### GnuTLSOCSPFailureTimeout

Wait this many seconds before retrying a failed OCSP request.

    GnuTLSOCSPFailureTimeout SECONDS

Default: *300*\
Context: server config, virtual host

Retries of failed OCSP requests must be rate limited to avoid
overloading both the server using mod_gnutls and the CA's OCSP
responder. A shorter value increases the load on both sides, a longer
one means that stapling will remain disabled for longer after a failed
request.

### GnuTLSOCSPSocketTimeout

Timeout for TCP sockets used to send OCSP requests

    GnuTLSOCSPFailureTimeout SECONDS

Default: *6*\
Context: server config, virtual host

Stalled OCSP requests must time out after a while to prevent stalling
the server too much. However, if the timeout is too short requests may
fail with a slow OCSP responder or high latency network
connection. This parameter allows you to adjust the timeout if
necessary.

Note that this is not an upper limit for the completion of an OCSP
request but a socket timeout. The connection will time out if there is
no activity (successful send or receive) at all for the configured
time.

* * * * *

Configuration Examples
======================

Simple Standard TLS Example
---------------------------

The following is an example of simple TLS 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

     # Without SNI you need one IP Address per-site.
     Listen 192.0.2.1:443
     Listen 192.0.2.2:443
     Listen 192.0.2.3:443
     Listen 192.0.2.4:443

     <VirtualHost 192.0.2.1:443>
         GnuTLSEnable on
         GnuTLSPriorities SECURE128
         DocumentRoot /www/site1.example.com/html
         ServerName site1.example.com:443
         GnuTLSCertificateFile conf/tls/site1.crt
         GnuTLSKeyFile conf/tls/site1.key
     </VirtualHost>

     <VirtualHost 192.0.2.2:443>
         # This virtual host enables SRP authentication
         GnuTLSEnable on
         GnuTLSPriorities NORMAL:+SRP
         DocumentRoot /www/site2.example.com/html
         ServerName site2.example.com:443
         GnuTLSSRPPasswdFile conf/tls/tpasswd.site2
         GnuTLSSRPPasswdConfFile conf/tls/tpasswd.site2.conf
     </VirtualHost>

     <VirtualHost 192.0.2.3:443>
         # This server enables SRP, OpenPGP and X.509 authentication.
         GnuTLSEnable on
         GnuTLSPriorities NORMAL:+SRP:+SRP-RSA:+SRP-DSS:+CTYPE-OPENPGP
         DocumentRoot /www/site3.example.com/html
         ServerName site3.example.com:443
         GnuTLSCertificateFile conf/tls/site3.crt
         GnuTLSKeyFile conf/tls/site3.key
         GnuTLSClientVerify ignore
         GnuTLSPGPCertificateFile conf/tls/site3.pub.asc
         GnuTLSPGPKeyFile conf/tls/site3.sec.asc
         GnuTLSSRPPasswdFile conf/tls/tpasswd.site3
         GnuTLSSRPPasswdConfFile conf/tls/tpasswd.site3.conf
     </VirtualHost>

     <VirtualHost 192.0.2.4:443>
         GnuTLSEnable on
         # %COMPAT disables some security features to enable maximum
         # compatibility with clients. Don't use this if you need strong
         # security.
         GnuTLSPriorities NORMAL:%COMPAT
         DocumentRoot /www/site4.example.com/html
         ServerName site4.example.com:443
         GnuTLSCertificateFile conf/tls/site4.crt
         GnuTLSKeyFile conf/tls/site4.key
     </VirtualHost>

Server Name Indication Example
------------------------------

`mod_gnutls` supports "Server Name Indication", as specified in
RFC 3546. This allows hosting many TLS websites with a single IP
address. All recent browsers support this standard. Here is an
example using SNI:

     # Load the module into Apache.
     LoadModule gnutls_module modules/mod_gnutls.so

     # SNI allows hosting multiple sites using one IP address. This
     # could also be 'Listen *:443', just like '*:80' is common for
     # non-HTTPS
     Listen 198.51.100.1:443

     <VirtualHost _default_:443>
         GnuTLSEnable on
         GnuTLSSessionTickets on
         GnuTLSPriorities NORMAL
         DocumentRoot /www/site1.example.com/html
         ServerName site1.example.com:443
         GnuTLSCertificateFile conf/tls/site1.crt
         GnuTLSKeyFile conf/tls/site1.key
     </VirtualHost>

     <VirtualHost _default_:443>
         GnuTLSEnable on
         GnuTLSPriorities NORMAL
         DocumentRoot /www/site2.example.com/html
         ServerName site2.example.com:443
         GnuTLSCertificateFile conf/tls/site2.crt
         GnuTLSKeyFile conf/tls/site2.key
     </VirtualHost>

     <VirtualHost _default_:443>
         GnuTLSEnable on
         GnuTLSPriorities NORMAL
         DocumentRoot /www/site3.example.com/html
         ServerName site3.example.com:443
         GnuTLSCertificateFile conf/tls/site3.crt
         GnuTLSKeyFile conf/tls/site3.key
     </VirtualHost>

     <VirtualHost _default_:443>
         GnuTLSEnable on
         GnuTLSPriorities NORMAL
         DocumentRoot /www/site4.example.com/html
         ServerName site4.example.com:443
         GnuTLSCertificateFile conf/tls/site4.crt
         GnuTLSKeyFile conf/tls/site4.key
     </VirtualHost>

OCSP Stapling Example
---------------------

This example uses an X.509 server certificate. The server will fetch
OCSP responses from the responder listed in the certificate and store
them im a memcached cache shared with another server.

     # Load the module into Apache.
     LoadModule gnutls_module modules/mod_gnutls.so
     GnuTLSCache memcache "192.0.2.1:11211 192.0.2.2:11211"
     GnuTLSCacheTimeout 600

     Listen 192.0.2.1:443

     <VirtualHost _default_:443>
         GnuTLSEnable          On
         GnuTLSPriorities      NORMAL
         DocumentRoot          /www/site1.example.com/html
         ServerName            site1.example.com:443
         GnuTLSCertificateFile conf/tls/site1.crt
         GnuTLSKeyFile         conf/tls/site1.key
         GnuTLSPriorities      NORMAL
         GnuTLSOCSPStapling    On
     </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`.