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<div id="preamble"><h1>Apache Module mod_ssl_ct</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="/en/mod/mod_ssl_ct.html" title="English"> en </a></p>
</div>
<table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>Implementation of Certificate Transparency (RFC 6962)
</td></tr>
<tr><th><a href="module-dict.html#ModuleIdentifier">Module�Identifier:</a></th><td>ssl_ct_module</td></tr>
<tr><th><a href="module-dict.html#SourceFile">Source�File:</a></th><td>mod_ssl_ct.c</td></tr></table>
<h3>Summary</h3>
<p>This module provides an implementation of Certificate Transparency, in
conjunction with <code class="module"><a href="/mod/mod_ssl.html">mod_ssl</a></code> and command-line tools from the
open source project. The goal of Certificate Transparency is to expose the
use of server certificates which are trusted by browsers but were mistakenly
or maliciously issued. More information about Certificate Transparency is
available at <a href="http://www.certificate-transparency.org/">
http://www.certificate-transparency.org/</a>. Key terminology used in
this documentation:</p>
<dl>
<dt>Certificate log</dt>
<dd>A certificate log, referred to simply as <q>log</q> in this documentation,
is a network service to which server certificates have been submitted. A
user agent can confirm that the certificate of a server which it accesses
has been submitted to a log which it trusts, and that the log itself has
not been tampered with.</dd>
<dt>Signed Certificate Timestamp (SCT)</dt>
<dd>This is an acknowledgement from a log that it has accepted a valid
certificate. It is signed with the log's public key. One or more SCTs
is passed to clients during the handshake, either in the ServerHello
(TLS extension), certificate extension, or in a stapled OCSP response.</dd>
</dl>
<p>This implementation for Apache httpd provides these features for TLS
servers and proxies:</p>
<ul>
<li>Signed Certificate Timestamps (SCTs) can be obtained from logs
automatically and, in conjunction with any statically configured SCTs, sent
to aware clients in the ServerHello (during the handshake).</li>
<li>SCTs can be received by the proxy from origin servers in the ServerHello,
received can be partially validated on-line and optionally queued for off-line
audit.</li>
<li>The proxy can be configured to disallow communication with an origin
server which does not provide an SCT which passes on-line validation.</li>
</ul>
<p>Configuration information about logs can be defined statically in the web
server configuration or maintained in a SQLite3 database. In the latter case,
<code class="module"><a href="/mod/mod_ssl_ct.html">mod_ssl_ct</a></code> will reload the database periodically, so any
site-specific infrastructure for maintaining and propagating log configuration
information does not have to also restart httpd to make it take effect.</p>
<div class="note">This module is experimental for the following reasons:
<ul>
<li>Insufficient test and review</li>
<li>Reliance on an unreleased version of OpenSSL (1.0.2) for basic
operation</li>
<li>Incomplete <a href="#audit">off-line audit capability</a></li>
</ul>
<p>Configuration mechanisms, format of data saved for off-line audit, and
other characteristics are subject to change based on further feedback and
testing.</p>
</div>
</div>
<div id="quickview"><h3 class="directives">Directives</h3>
<ul id="toc">
<li><img alt="" src="/images/down.gif" /> <a href="#ctserverhellosctlimit">CTServerHelloSCTLimit</a></li>
</ul>
<h3>Topics</h3>
<ul id="topics">
<li><img alt="" src="/images/down.gif" /> <a href="#logging">Logging CT status in the access log</a></li>
</ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
<div class="section">
<h2><a name="server" id="server">Server processing overview</a></h2>
<p>Servers need to send SCTs to their clients. SCTs in a certificate
extension or stapled OCSP response will be sent without any special program
logic. This module handles sending SCTs configured by the administrator or
received from configured logs.</p>
<p>The number of SCTs sent in the ServerHello (i.e., not including those in a
certificate extension or stapled OCSP response) can be limited by the
<code class="directive"><a href="#ctserverhellosctlimit">CTServerHelloSCTLimit</a></code>
directive.</p>
<p>For each server certificate, a daemon process maintains an SCT list to be
sent in the ServerHello, created from statically configured SCTs as well as
those received from logs. Logs marked as untrusted or with a maximum valid
timestamp before the present time will be ignored. Periodically the daemon
will submit certificates to a log as necessary (due to changed log
configuration or age) and rebuild the concatenation of SCTs.</p>
<p>The SCT list for a server certificate will be sent to any client that
indicates awareness in the ClientHello when that particular server certificate
is used.</p>
<div class="section">
<h2><a name="proxy" id="proxy">Proxy processing overview</a></h2>
<p>The proxy indicates Certificate Transparency awareness in the ClientHello
by including the <em>signed_certificate_timestamp</em> extension. It can
recognize SCTs received in the ServerHello, in an extension in the certificate
for an origin server, or in a stapled OCSP response.</p>
<p>On-line verification is attempted for each received SCT:</p>
<ul>
<li>For any SCT, the timestamp can be checked to see if it is not yet valid
based on the current time as well as any configured valid time interval for
the log.</li>
<li>For an SCT from a log for which a public key is configured, the server
signature will be checked.</li>
</ul>
<p>If verification fails for at least one SCT and verification was not
successful for at least one SCT, the connection is aborted if
<code class="directive"><a href="#ctproxyawareness">CTProxyAwareness</a></code> is set to
<em>require</em>.</p>
<p>Additionally, the server certificate chain and SCTs are stored for off-line
verification if the <code class="directive"><a href="#ctauditstorage">CTAuditStorage</a></code>
directive is configured.</p>
<p>As an optimization, on-line verification and storing of data from the
server is only performed the first time a web server child process receives
the data. This saves some processing time as well as disk space. For typical
reverse proxy setups, very little processing overhead will be required.</p>
<div class="section">
<h2><a name="logconf" id="logconf">Log configuration</a></h2>
<p>Servers and proxies use different information about logs for their processing.
This <em>log configuration</em> can be set in two ways:</p>
<ul>
<li>Create a log configuration database using <code class="program"><a href="/programs/ctlogconfig.html">ctlogconfig</a></code>,
and configure the path to that database using the <code class="directive"><a href="#ctlogconfig">
CTLogConfig</a></code> directive. This method of configuration supports
dynamic updates; <code class="module"><a href="/mod/mod_ssl_ct.html">mod_ssl_ct</a></code> will re-read the database at
intervals. Additionally, the off-line audit program <code>ctauditscts</code>
can use this configuration to find the URL of logs.</li>
<li>Configure information about logs statically using the <code class="directive"><a href="#ctstaticlogconfig">CTStaticLogConfig</a></code> directive. As with all other
directives, the server must be restarted in order to pick up changes to the
directives.</li>
</ul>
<p>The information that can be configured about a log using either mechanism is
described below:</p>
<dl>
<dt>log id</dt>
<dd>The log id is the SHA-256 hash of the log's public key, and is part of
every SCT. This is a convenient way to identify a particular log when
configuring valid timestamp ranges or certain other information.</dd>
<dt>public key of the log</dt>
<dd>A proxy must have the public key of the log in order to check the
signature in SCTs it receives which were obtained from the log.</dd>
<dd>This is a mechanism to distrust or restore trust in a particular log,
for whatever reason (including simply avoiding interaction with the
log in situations where it is off-line).</dd>
<dd>When configured, the proxy will check that timestamps from SCTs
are within the valid range.</dd>
<dt>log URL</dt>
<dd>The URL of the log (for its API) is required by a server in order to
submit server certificates to the log. The server will submit
each server certificate in order to obtain an SCT for each log with a
configured URL, except when the log is also marked as distrusted or the
current time is not within any configured valid timestamp range.
<br />
The log URL is also needed by off-line auditing of SCTs received by a
proxy.</dd>
</dl>
<p>Generally, only a small subset of this information is configured for a
particular log. Refer to the documentation for the <code class="directive"><a href="#ctstaticlogconfig">CTStaticLogConfig</a></code> directive and the
<code class="program"><a href="/programs/ctlogconfig.html">ctlogconfig</a></code> command for more specific information.</p>
<div class="section">
<h2><a name="logging" id="logging">Logging CT status in the access log</a></h2>
<p>Both proxy and server modes set the <code>SSL_CT_PEER_STATUS</code>
variable to indicate if the peer is CT-aware.</p>
<p>Proxy mode sets the <code>SSL_PROXY_SCT_SOURCES</code> variable to
indicate whether and where SCTs were obtained (ServerHello, certificate
extension, etc.).</p>
<p>These variables can be logged with the <code>%{<em>varname</em>}e</code>
<div class="section">
<h2><a name="audit" id="audit">Off-line audit for proxy</a></h2>
<p>Experimental support for this is implemented in the <code>ctauditscts</code>
command (in the httpd source tree, not currently installed), which itself
<em>certificate-transparency</em> open source project. <code>ctauditscts</code>
can parse data for off-line audit (enabled with the <code class="directive"><a href="#ctauditstorage">
not provide a way to identify audit failures.</p>
<p>Here are rough notes for using <code>ctauditscts</code>:</p>
<ul>
directory within the <em>certificate-transparency</em> tools.</li>
directory.</li>
<li>Run <code>ctauditscts</code>, passing the value of the
<code class="directive">CTAuditStorage</code> directive and, optionally, the path to
the log configuration database. The latter will be used to look up log URLs
by log id.</li>
</ul>
<p>The data saved for audit can also be used by other programs; refer to the
<code>ctauditscts</code> source code for details.</p>
</div>
<div class="directive-section"><h2><a name="CTAuditStorage" id="CTAuditStorage">CTAuditStorage</a> <a name="ctauditstorage" id="ctauditstorage">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Existing directory where data for off-line audit will be stored</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CTAuditStorage <em>directory</em></code></td></tr>
</table>
<p>The <code class="directive">CTAuditStorage</code> directive sets the name of a
directory where data will be stored for off-line audit. If <em>directory</em>
is not absolute then it is assumed to be relative to <code class="directive"><a href="/mod/core.html#defaultruntimedir">
DefaultRuntimeDir</a></code>.</p>
<p>If this directive is not specified, data will not be stored for off-line
audit.</p>
<p>The directory will contain files named <code><em>PID</em>.tmp</code> for
active child processes and files named <code><em>PID</em>.out</code> for exited
child processes. These <code>.out</code> files are ready for off-line audit.
The experimental command <code>ctauditscts</code> (in the httpd source tree, not
currently installed) interfaces with <em>certificate-transparency</em> tools to
perform the audit.</p>
</div>
<div class="directive-section"><h2><a name="CTLogClient" id="CTLogClient">CTLogClient</a> <a name="ctlogclient" id="ctlogclient">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Location of certificate-transparency log client tool</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CTLogClient <em>executable</em></code></td></tr>
</table>
<p><em>executable</em> is the full path to the log client tool, which is
certificate-transparency</a> open source project.</p>
<p>An alternative implementation could be used to retrieve SCTs for a
server certificate as long as the command-line interface is equivalent.</p>
<p>If this directive is not configured, server certificates cannot be
submitted to logs in order to obtain SCTs; thus, only admin-managed
SCTs will be provided to clients.</p>
</div>
<div class="directive-section"><h2><a name="CTLogConfigDB" id="CTLogConfigDB">CTLogConfigDB</a> <a name="ctlogconfigdb" id="ctlogconfigdb">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Log configuration database supporting dynamic updates</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CTLogConfigDB <em>filename</em></code></td></tr>
</table>
<p>The <code class="directive">CTLogConfigDB</code> directive sets the name of a database
containing configuration about known logs. If <em>filename</em> is not absolute
then it is assumed to be relative to
<p>Refer to the documentation for the <code class="program"><a href="/programs/ctlogconfig.html">ctlogconfig</a></code> program,
which manages the database.</p>
</div>
<div class="directive-section"><h2><a name="CTMaxSCTAge" id="CTMaxSCTAge">CTMaxSCTAge</a> <a name="ctmaxsctage" id="ctmaxsctage">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Maximum age of SCT obtained from a log, before it will be
refreshed</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CTMaxSCTAge <em>num-seconds</em></code></td></tr>
</table>
<p>Server certificates with SCTs which are older than this maximum age will
be resubmitted to configured logs. Generally the log will return the same SCT
as before, but that is subject to log operation. SCTs will be refreshed as
necessary during normal server operation, with new SCTs returned to clients
as they become available.</p>
</div>
<div class="directive-section"><h2><a name="CTProxyAwareness" id="CTProxyAwareness">CTProxyAwareness</a> <a name="ctproxyawareness" id="ctproxyawareness">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Level of CT awareness and enforcement for a proxy
</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CTProxyAwareness <em>oblivious|aware|require</em></code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
</table>
<p>This directive controls awareness and checks for valid SCTs for a
proxy. Several options are available:</p>
<dl>
<dt>oblivious</dt>
<dd>The proxy will neither ask for nor examine SCTs. Certificate
Transparency processing for the proxy is completely disabled.</dd>
<dt>aware</dt>
<dd>The proxy will perform all appropriate Certificate Transparency
processing, such as asking for and examining SCTs. However, the
proxy will not disallow communication if the origin server does
not provide any valid SCTs.</dd>
<dt>require</dt>
<dd>The proxy will abort communication with the origin server if it
does not provide at least one SCT which passes on-line validation.</dd>
</dl>
</div>
<div class="directive-section"><h2><a name="CTSCTStorage" id="CTSCTStorage">CTSCTStorage</a> <a name="ctsctstorage" id="ctsctstorage">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Existing directory where SCTs are managed</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CTSCTStorage <em>directory</em></code></td></tr>
</table>
<p>The <code class="directive">CTSCTStorage</code> directive sets the name of a
directory where SCTs and SCT lists will will be stored. If <em>directory</em>
is not absolute then it is assumed to be relative to <code class="directive"><a href="/mod/core.html#defaultruntimedir">
DefaultRuntimeDir</a></code>.</p>
<p>A subdirectory for each server certificate contains information relative
to that certificate; the name of the subdirectory is the SHA-256 hash of the
certificate.</p>
<p>The certificate-specific directory contains SCTs retrieved from configured
logs, SCT lists prepared from statically configured SCTs and retrieved SCTs,
and other information used for managing SCTs.</p>
</div>
<div class="directive-section"><h2><a name="CTServerHelloSCTLimit" id="CTServerHelloSCTLimit">CTServerHelloSCTLimit</a> <a name="ctserverhellosctlimit" id="ctserverhellosctlimit">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Limit on number of SCTs that can be returned in
ServerHello</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CTServerHelloSCTLimit <em>limit</em></code></td></tr>
</table>
<p>This directive can be used to limit the number of SCTs which can be
returned by a TLS server in ServerHello, in case the number of configured
logs and statically-defined SCTs is relatively high.</p>
<p>Typically only a few SCTs would be available, so this directive is only
needed in special circumstances.</p>
<p>The directive does not take into account SCTs which may be provided in
certificate extensions or in stapled OCSP responses.</p>
</div>
<div class="directive-section"><h2><a name="CTStaticLogConfig" id="CTStaticLogConfig">CTStaticLogConfig</a> <a name="ctstaticlogconfig" id="ctstaticlogconfig">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Static configuration of information about a log</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CTStaticLogConfig <em>log-id|-</em> <em>public-key-file|-</em>
<em>1|0|-</em> <em>min-timestamp|-</em> <em>max-timestamp|-</em>
<em>log-URL|-</em></code></td></tr>
</table>
<p>This directive is used to configure information about a particular log.
This directive is appropriate when configuration information changes rarely.
If dynamic configuration updates must be supported, refer to the
<code class="directive"><a href="#ctlogconfigdb">CTLogConfigDB</a></code> directive.</p>
<p>Each of the six fields must be specified, but usually only a small
amount of information must be configured for each log; use <em>-</em> when no
information is available for the field. The fields are defined as follows:</p>
<dl>
<dt><em>log-id</em></dt>
<dd>This is the id of the log, which is the SHA-256 hash of the log's
public key.</dd>
<dt><em>public-key-file</em></dt>
<dd>This is the name of a file containing the PEM encoding of the log's
public key. If the name is not absolute, then it is assumed to be relative
<dt><em>trust</em></dt>
<dd>This is a generic <q>trust</q> flag. Set this field to <em>0</em> to
distrust this log.</dd>
<dt><em>min-timestamp</em></dt>
<dd>SCTs received from this log by the proxy are invalid if the timestamp
is older than this value.</dd>
<dt><em>max-timestamp</em></dt>
<dd>SCTs received from this log by the proxy are invalid if the timestamp
is newer than this value.</dd>
<dt><em>log-URL</em></dt>
<dd>This is the URL of the log, for use in submitting server certificates
and in turn obtaining an SCT to be sent to clients.</dd>
</dl>
<h3>See also</h3>
<ul>
<li><a href="#logconf">Log configuration</a> contains more general information
about the fields which can be configured with this directive.</li>
</ul>
</div>
<div class="directive-section"><h2><a name="CTStaticSCTs" id="CTStaticSCTs">CTStaticSCTs</a> <a name="ctstaticscts" id="ctstaticscts">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Static configuration of one or more SCTs for a server certificate
</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CTStaticSCTs <em>certificate-pem-file</em> <em>sct-directory</em></code></td></tr>
</table>
<p>This directive is used to statically define one or more SCTs corresponding
to a server certificate. This mechanism can be used instead of or in
addition to dynamically obtaining SCTs from configured logs. Any changes to
the set of SCTs for a particular server certificate will be adopted dynamically
without the need to restart the server.</p>
<p><em>certificate-pem-file</em> refers to the server certificate in PEM
format. If the name is not absolute, then it is assumed to be relative to
<p><em>sct-directory</em> should contain one or more files with extension
<code>.sct</code>, representing one or more SCTs corresponding to the
server certificate. If <em>sct-directory</em> is not absolute, then it is
assumed to be relative to <code class="directive"><a href="/mod/core.html#serverroot">ServerRoot</a></code>.</p>
<p>If <em>sct-directory</em> is empty, no error will be raised.</p>
<p>This directive could be used to identify directories of SCTs maintained by
other infrastructure, provided that they are saved in binary format with
file extension <em>.sct</em></p>
</div>
</div>
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