mod_ssl_ct - Apache HTTP Server Version 2.5

mod_ssl_ct - Apache HTTP Server Version 2.5
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Apache HTTP Server Version 2.5
Apache
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Version 2.5
Modules
Apache Module mod_ssl_ct
Available Languages:
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Description:
Implementation of Certificate Transparency (RFC 6962)
Status:
Extension
Module Identifier:
ssl_ct_module
Source File:
mod_ssl_ct.c
Summary
This module provides an implementation of Certificate Transparency, in
conjunction with
mod_ssl
and command-line tools from the
certificate-transparency
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
. Key terminology used in
this documentation:
Certificate log
A certificate log, referred to simply as
log
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.
Signed Certificate Timestamp (SCT)
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.
This implementation for Apache httpd provides these features for TLS
servers and proxies:
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).
SCTs can be received by the proxy from origin servers in the ServerHello,
in a certificate extension, and/or within stapled OCSP responses; any SCTs
received can be partially validated on-line and optionally queued for off-line
audit.
The proxy can be configured to disallow communication with an origin
server which does not provide an SCT which passes on-line validation.
Configuration information about logs can be defined statically in the web
server configuration or maintained in a SQLite3 database. In the latter case,
mod_ssl_ct
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.
This module is experimental for the following reasons:
Insufficient test and review
Reliance on an unreleased version of OpenSSL (1.0.2, Beta 3 or later) for
basic operation
Incomplete
off-line audit capability
Configuration mechanisms, format of data saved for off-line audit, and
other characteristics are subject to change based on further feedback and
testing.
Topics
Server processing overview
Proxy processing overview
Log configuration
Storing SCTs in a form consumable by mod_ssl_ct
Logging CT status in the access log
Off-line audit for proxy
Directives
CTAuditStorage
CTLogClient
CTLogConfigDB
CTMaxSCTAge
CTProxyAwareness
CTSCTStorage
CTServerHelloSCTLimit
CTStaticLogConfig
CTStaticSCTs
Bugfix checklist
httpd changelog
Known issues
Report a bug
See also
Comments
Server processing overview
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.
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
CTServerHelloSCTLimit
directive.
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.
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.
Proxy processing overview
The proxy indicates Certificate Transparency awareness in the ClientHello
by including the
signed_certificate_timestamp
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.
On-line verification is attempted for each received SCT:
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.
For an SCT from a log for which a public key is configured, the server
signature will be checked.
If verification fails for at least one SCT and verification was not
successful for at least one SCT, the connection is aborted if
CTProxyAwareness
is set to
require
Additionally, the server certificate chain and SCTs are stored for off-line
verification if the
CTAuditStorage
directive is configured.
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.
Log configuration
Servers and proxies use different information about logs for their processing.
This
log configuration
can be set in two ways:
Create a log configuration database using
ctlogconfig
and configure the path to that database using the
CTLogConfig
directive. This method of configuration supports
dynamic updates;
mod_ssl_ct
will re-read the database at
intervals. Additionally, the off-line audit program
ctauditscts
can use this configuration to find the URL of logs.
Configure information about logs statically using the
CTStaticLogConfig
directive. As with all other
directives, the server must be restarted in order to pick up changes to the
directives.
The information that can be configured about a log using either mechanism is
described below:
log id
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.
public key of the log
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.
A server must have the public key of the log in order to submit certificates
to it.
general trust/distrust setting
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).
minimum and/or maximum valid timestamps
When configured, the proxy will check that timestamps from SCTs
are within the valid range.
log URL
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.
The log URL is also needed by off-line auditing of SCTs received by a
proxy.
Generally, only a small subset of this information is configured for a
particular log. Refer to the documentation for the
CTStaticLogConfig
directive and the
ctlogconfig
command for more specific information.
Storing SCTs in a form consumable by mod_ssl_ct
mod_ssl_ct
allows you to configure SCTs statically
using the
CTStaticSCTs
directive. These must be
in binary form, ready to send to a client.
Sample code in the form of a Python script to build an SCT in the correct
format from data received from a log can be found in
Tom Ritter's ct-tools
repository
. Refer to
write-sct.py
Logging CT status in the access log
Proxy and server modes set the
SSL_CT_PROXY_STATUS
and
SSL_CT_CLIENT_STATUS
variables, respectively, to indicate
if the corresponding peer is CT-aware.
Proxy mode sets the
SSL_CT_PROXY_SCT_SOURCES
variable to
indicate whether and where SCTs were obtained (ServerHello, certificate
extension, etc.).
These variables can be logged with the
%{
varname
}e
format of
mod_log_config
Off-line audit for proxy
Experimental support for this is implemented in the
ctauditscts
command, which itself relies on the
verify_single_proof.py
tool in the
certificate-transparency
open source project.
ctauditscts
can parse data for off-line audit (enabled with the
CTAuditStorage
directive) and invoke
verify_single_proof.py
Here are rough notes for using
ctauditscts
Create a
virtualenv
using the
requirements.txt
file
from the
certificate-transparency
project and run the following steps
with that
virtualenv
activated.
Set
PYTHONPATH
to include the
python
directory within the
certificate-transparency
tools.
Set
PATH
to include the
python/ct/client/tools
directory.
Run
ctauditscts
, passing the value of the
CTAuditStorage
directive and, optionally, the path to
the log configuration database. The latter will be used to look up log URLs
by log id.
The data saved for audit can also be used by other programs; refer to the
ctauditscts
source code for details on processing the data.
CTAuditStorage
Directive
Description:
Existing directory where data for off-line audit will be stored
Syntax:
CTAuditStorage
directory
Default:
none
Context:
server config
Status:
Extension
Module:
mod_ssl_ct
The
CTAuditStorage
directive sets the name of a
directory where data will be stored for off-line audit. If
directory
is not absolute then it is assumed to be relative to
DefaultRuntimeDir
If this directive is not specified, data will not be stored for off-line
audit.
The directory will contain files named
PID
.tmp
for
active child processes and files named
PID
.out
for exited
child processes. These
.out
files are ready for off-line audit.
The experimental command
ctauditscts
(in the httpd source tree, not
currently installed) interfaces with
certificate-transparency
tools to
perform the audit.
CTLogClient
Directive
Description:
Location of certificate-transparency log client tool
Syntax:
CTLogClient
executable
Default:
none
Context:
server config
Status:
Extension
Module:
mod_ssl_ct
executable
is the full path to the log client tool, which is
normally file
cpp/client/ct
(or
ct.exe
) within the
source tree of the
certificate-transparency
open source project.
An alternative implementation could be used to retrieve SCTs for a
server certificate as long as the command-line interface is equivalent.
If this directive is not configured, server certificates cannot be
submitted to logs in order to obtain SCTs; thus, only admin-managed
SCTs or SCTs in certificate extensions will be provided to clients.
CTLogConfigDB
Directive
Description:
Log configuration database supporting dynamic updates
Syntax:
CTLogConfigDB
filename
Default:
none
Context:
server config
Status:
Extension
Module:
mod_ssl_ct
The
CTLogConfigDB
directive sets the name of a database
containing configuration about known logs. If
filename
is not absolute
then it is assumed to be relative to
ServerRoot
Refer to the documentation for the
ctlogconfig
program,
which manages the database.
CTMaxSCTAge
Directive
Description:
Maximum age of SCT obtained from a log, before it will be
refreshed
Syntax:
CTMaxSCTAge
num-seconds
Default:
1 day
Context:
server config
Status:
Extension
Module:
mod_ssl_ct
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.
CTProxyAwareness
Directive
Description:
Level of CT awareness and enforcement for a proxy
Syntax:
CTProxyAwareness
oblivious|aware|require
Default:
aware
Context:
server config, virtual host
Status:
Extension
Module:
mod_ssl_ct
This directive controls awareness and checks for valid SCTs for a
proxy. Several options are available:
oblivious
The proxy will neither ask for nor examine SCTs. Certificate
Transparency processing for the proxy is completely disabled.
aware
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.
require
The proxy will abort communication with the origin server if it
does not provide at least one SCT which passes on-line validation.
CTSCTStorage
Directive
Description:
Existing directory where SCTs are managed
Syntax:
CTSCTStorage
directory
Default:
none
Context:
server config
Status:
Extension
Module:
mod_ssl_ct
The
CTSCTStorage
directive sets the name of a
directory where SCTs and SCT lists will be stored. If
directory
is not absolute then it is assumed to be relative to
DefaultRuntimeDir
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.
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.
CTServerHelloSCTLimit
Directive
Description:
Limit on number of SCTs that can be returned in
ServerHello
Syntax:
CTServerHelloSCTLimit
limit
Default:
100
Context:
server config
Status:
Extension
Module:
mod_ssl_ct
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.
Typically only a few SCTs would be available, so this directive is only
needed in special circumstances.
The directive does not take into account SCTs which may be provided in
certificate extensions or in stapled OCSP responses.
CTStaticLogConfig
Directive
Description:
Static configuration of information about a log
Syntax:
CTStaticLogConfig
log-id|-
public-key-file|-
1|0|-
min-timestamp|-
max-timestamp|-
log-URL|-
Default:
none
Context:
server config
Status:
Extension
Module:
mod_ssl_ct
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
CTLogConfigDB
directive.
Each of the six fields must be specified, but usually only a small
amount of information must be configured for each log; use
when no
information is available for the field. For example, in support of a
server-only configuration (i.e., no proxy), the administrator might
configure only the log URL to be used when submitting server certificates
and obtaining a Signed Certificate Timestamp.
The fields are defined as follows:
log-id
This is the id of the log, which is the SHA-256 hash of the log's
public key, provided in hexadecimal format. This string is 64 characters
in length.
This field should be omitted when
public-key-file
is provided.
public-key-file
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
to
ServerRoot
trust/distrust
Set this field to
to distrust this log, or to otherwise avoid
using it for server certificate submission. Set this to
or
(the default) to treat the log normally.
min-timestamp
and
max-timestamp
A timestamp is a time as expressed in the number of milliseconds since the
epoch, ignoring leap seconds. This is the form of time used in Signed Certificate
Timestamps. This must be provided as a decimal number.
Specify
for one of the timestamps if it is unknown.
For example, when configuring the minimum valid timestamp for a log which remains
valid, specify
for
max-timestamp
SCTs received from this log by the proxy are invalid if the timestamp
is older than
min-timestamp
or newer than
max-timestamp
log-URL
This is the URL of the log, for use in submitting server certificates
and in turn obtaining an SCT to be sent to clients.
See also
Log configuration
contains more general information
about the fields which can be configured with this directive.
CTStaticSCTs
Directive
Description:
Static configuration of one or more SCTs for a server certificate
Syntax:
CTStaticSCTs
certificate-pem-file
sct-directory
Default:
none
Context:
server config
Status:
Extension
Module:
mod_ssl_ct
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.
certificate-pem-file
refers to the server certificate in PEM
format. If the name is not absolute, then it is assumed to be relative to
ServerRoot
sct-directory
should contain one or more files with extension
.sct
, representing one or more SCTs corresponding to the
server certificate. If
sct-directory
is not absolute, then it is
assumed to be relative to
ServerRoot
If
sct-directory
is empty, no error will be raised.
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
.sct
Available Languages:
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Copyright 2026 The Apache Software Foundation.
Licensed under the
Apache License, Version 2.0
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