TLS in a VirtualHost context

mod_tls TLS v1.2 and v1.3 implemented in memory-safe Rust via the rustls library Experimental mod_tls.c tls_module Available in version 2.4.52 and later

mod_tls is an alternative to mod_ssl for providing https to a server. It's feature set is a subset, described in more detail below. It can be used as a companion to mod_ssl, e.g. both modules can be loaded at the same time.

mod_tls, being written in C, used the Rust implementation of TLS named rustls via its C interface rustls-ffi. This gives memory safe cryptography and protocol handling at comparable performance.

It can be configured for frontend and backend connections. The configuration directive have been kept mostly similar to mod_ssl ones.

TLS in a VirtualHost context Listen 443 TLSEngine 443 <VirtualHost *:443> ServerName example.net TLSCertificate file_with_certificate.pem file_with_key.pem ... </VirtualHost>

The above is a minimal configuration. Instead of enabling mod_tls in every virtual host, the port for incoming TLS connections is specified.

You cannot mix virtual hosts with mod_ssl and mod_tls on the same port. It's either or. SNI and ALPN are supported. You may use several virtual hosts on the same port and a mix of protocols like http/1.1 and h2.

Feature Comparison with mod_ssl

The table below gives a comparison of feature between mod_ssl and mod_tls. If a feature of mod_ssl is no listed here, it is not supported by mod_tls. The one difference, probably most relevant is the lack for client certificate support in the current version of mod_tls.

Feature	mod_ssl	mod_tls	Comment
Frontend TLS	yes	yes
Backend TLS	yes	yes
TLS v1.3	yes*	yes	*)with recent OpenSSL
TLS v1.2	yes	yes
TLS v1.0	yes*	no	*)if enabled in OpenSSL
SNI Virtual Hosts	yes	yes
Client Certificates	yes	no
Machine Certificates for Backend	yes	yes
OCSP Stapling	yes	yes*	*)via mod_md
Backend OCSP check	yes	no*	*)stapling will be verified
TLS version to allow	min-max	min
TLS ciphers	exclusive list	preferred/suppressed
TLS cipher ordering	client/server	client/server
TLS sessions	yes	yes
SNI strictness	default no	default yes
Option EnvVars	exhaustive	limited*	*)see var list
Option ExportCertData	client+server	server
Backend CA	file/dir	file
Revocation CRLs	yes	no
TLS Renegotiation	yes*	no	*)in TLS v1.2
Encrypted Cert Keys	yes	no

TLS Protocols

mod_tls supports TLS protocol version 1.2 and 1.3. Should there ever be a version 1.4 and rustls supports it, it will be available as well.

In mod_tls, you configure the minimum version to use, never the maximum:

TLSProtocol TLSv1.3+

This allows only version 1.3 and whatever may be its successor one day when talking to your server or to a particular virtual host.

TLS Ciphers

The list of TLS ciphers supported in the rustls library, can be found here. All TLS v1.3 ciphers are supported. For TLS v1.2, only ciphers that rustls considers secure are available.

mod_tls supports the following names for TLS ciphers:

The IANA assigned name which uses `_` to separate parts. Example: TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384
The OpenSSL name, using `-` as separator (for 1.2). Example: ECDHE-ECDSA-AES256-SHA384. Such names often appear in documentation. `mod_tls` defines them for all TLS v1.2 ciphers. For TLS v1.3 ciphers, names starting with TLS13_ are also supported.
The IANA assigned identifier, which is a 16-bit numeric value. Example: 0xc024. You can use this in configurations as TLS_CIPHER_0xc024.

You can configure a preference for ciphers, which means they will be used for clients that support them. If you do not configure a preference, rustls will use the one that it considers best. This is recommended.

Should you nevertheless have the need to prefer one cipher over another, you may configure it like this:

TLSCiphersPrefer ECDHE-ECDSA-AES256-SHA384 # or several TLSCiphersPrefer ECDHE-ECDSA-AES256-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305

If you name a cipher that is unknown, the configuration will fail. If you name a cipher is not supported by rustls (or no longer supported in an updated version of rustls for security reasons), mod_tls will log a WARNING, but continue to work.

A similar mechanism exists, if you want to disable a particular cipher:

TLSCipherSuppress ECDHE-ECDSA-AES256-SHA384

A suppressed cipher will not longer be used. If you name a cipher that is unknown, the configuration will fail. If you name a cipher is not supported by rustls (or no longer supported in an updated version of rustls for security reasons), mod_tls will log a WARNING, but continue to work.

Virtual Hosts

mod_tls uses the SNI (Server Name Indicator) to select one of the configured virtual hosts that match the port being served. Should the client not provide an SNI, the first configured virtual host will be selected. If the client does provide an SNI (as all today's clients do), it must match one virtual host (ServerName or ServerAlias) or the connection will fail.

As with mod_ssl, you may specify ciphers and protocol versions for the base server (global) and/or individual virtual hosts that are selected via SNI by the client.

Listen 443 TLSEngine 443 <VirtualHost *:443> ServerName example1.net TLSCertificate example1-cert.pem ... </VirtualHost> <VirtualHost *:443> ServerName example2.net TLSCertificate example2-cert.pem ... TLSProtocol v1.3+ </VirtualHost>

The example above show different TLS settings for virtual hosts on the same port. This is supported. example1 can be contacted via all TLS versions and example2 only allows v1.3 or later.

ACME Certificates

ACME certificates via mod_md are supported, just as for mod_ssl. A minimal configuration:

Listen 443 TLSEngine 443 MDomain example.net <VirtualHost *:443> ServerName example.net ... </VirtualHost>

OCSP Stapling

mod_tls has no own implementation to retrieve OCSP information for a certificate. However, it will use such for Stapling if it is provided by mod_md. See mod_md's documentation on how to enable this.

TLS Variables

Via the directive TLSOptions, several variables are placed into the environment of requests and can be inspected, for example in a CGI script.

The variable names are given by mod_ssl. Note that these are only a subset of the many variables that mod_ssl exposes.

Variable	TLSOption	Description
SSL_TLS_SNI	*	the server name indicator (SNI) send by the client
SSL_PROTOCOL	*	the TLS protocol negotiated
SSL_CIPHER	*	the name of the TLS cipher negotiated
SSL_VERSION_INTERFACE	StdEnvVars	the module version
SSL_VERSION_LIBRARY	StdEnvVars	the rustls-ffi version
SSL_SECURE_RENEG	StdEnvVars	always `false`
SSL_COMPRESS_METHOD	StdEnvVars	always `false`
SSL_CIPHER_EXPORT	StdEnvVars	always `false`
SSL_CLIENT_VERIFY	StdEnvVars	always `false`
SSL_SESSION_RESUMED	StdEnvVars	either `Resumed` if a known TLS session id was presented by the client or `Initial` otherwise
SSL_SERVER_CERT	ExportCertData	the selected server certificate in PEM format

The variable SSL_SESSION_ID is intentionally not supported as it contains sensitive information.

Client Certificates

While rustls supports client certificates in principle, parts of the infrastructure to make use of these in a server are not offered.

Among these features are: revocation lists, inspection of certificate extensions and the matched issuer chain for OCSP validation. Without these, revocation of client certificates is not possible. Offering authentication without revocation is not considered an option.

Work will continue on this and client certificate support may become available in a future release.

TLSEngine defines on which address+port the module shall handle incoming connections. TLSEngine [address:]port server config

This is set on a global level, not in individual VirtualHosts. It will affect all VirtualHost that match the specified address/port. You can use TLSEngine several times to use more than one address/port.

Example TLSEngine 443

The example tells mod_tls to handle incoming connection on port 443 for all listeners.

TLSCertificate adds a certificate and key (PEM encoded) to a server/virtual host. TLSCertificate cert_file [key_file] server config virtual host

If you do not specify a separate key file, the key is assumed to also be found in the first file. You may add more than one certificate to a server/virtual host. The first certificate suitable for a client is then chosen.

The path can be specified relative to the server root.

TLSProtocol specifies the minimum version of the TLS protocol to use. TLSProtocol version+ TLSProtocol v1.2+ server config virtual host

The default is `v1.2+`. Settings this to `v1.3+` would disable TLSv1.2.

TLSCiphersPrefer defines ciphers that are preferred. TLSCiphersPrefer cipher(-list) server config virtual host

This will not disable any ciphers supported by `rustls`. If you specify a cipher that is completely unknown, the configuration will fail. If you specify a cipher that is known but not supported by `rustls`, a warning will be logged but the server will continue.

Example TLSCiphersPrefer ECDHE-ECDSA-AES256-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305

The example gives 2 ciphers preference over others, in the order they are mentioned.

TLSCiphersSuppress defines ciphers that are not to be used. TLSCiphersSuppress cipher(-list) server config virtual host

This will not disable any unmentioned ciphers supported by `rustls`. If you specify a cipher that is completely unknown, the configuration will fail. If you specify a cipher that is known but not supported by `rustls`, a warning will be logged but the server will continue.

Example TLSCiphersSuppress ECDHE-ECDSA-CHACHA20-POLY1305

The example removes a cipher for use in connections.

TLSHonorClientOrder determines if the order of ciphers supported by the client is honored TLSHonorClientOrder on|off TLSHonorClientOrder on server config virtual host

TLSHonorClientOrder determines if the order of ciphers supported by the client is honored.

TLSOptions enables SSL variables for requests. TLSOptions [+|-]option server config virtual host directory .htaccess

TLSOptions is analog to SSLOptions in mod_ssl. It can be set per directory/location and `option` can be:

`StdEnvVars`: adds more variables to the requests environment, as forwarded for example to CGI processing and other applications.
`ExportCertData`: adds certificate related variables to the request environment.
`Defaults`: resets all options to their default values.

Adding variables to a request environment adds overhead, especially when certificates need to be inspected and fields extracted. Therefore most variables are not set by default.

You can configure TLSOptions per location or generally on a server/virtual host. Prefixing an option with `-` disables this option while leaving others unchanged. A `+` prefix is the same as writing the option without one.

The `Defaults` value can be used to reset any options that are inherited from other locations or the virtual host/server.

Example <Location /myplace/app> TLSOptions Defaults StdEnvVars ... </Location> TLSProxyEngine enables TLS for backend connections. TLSProxyEngine on|off server config virtual host proxy section

TLSProxyEngine is analog to SSLProxyEngine in mod_ssl.

This can be used in a server/virtual host or Proxy section to enable the module for outgoing connections using mod_proxy.

TLSProxyCA sets the root certificates to validate the backend server with. TLSProxyCA file.pem server config virtual host proxy section

TLSProxyProtocol specifies the minimum version of the TLS protocol to use in proxy connections. TLSProxyProtocol version+ TLSProxyProtocol v1.2+ server config virtual host proxy section

The default is `v1.2+`. Settings this to `v1.3+` would disable TLSv1.2.

TLSProxyCiphersPrefer defines ciphers that are preferred for a proxy connection. TLSProxyCiphersPrefer cipher(-list) server config virtual host proxy section

TLSProxyCiphersSuppress defines ciphers that are not to be used for a proxy connection. TLSProxyCiphersSuppress cipher(-list) server config virtual host proxy section

TLSProxyMachineCertificate adds a certificate and key file (PEM encoded) to a proxy setup. TLSProxyMachineCertificate cert_file [key_file] server config virtual host proxy section

The certificate is used to authenticate against a proxied backend server.

If you do not specify a separate key file, the key is assumed to also be found in the first file. You may add more than one certificate to a proxy setup. The first certificate suitable for a proxy connection to a backend is then chosen by rustls.

The path can be specified relative to the server root.

TLSStrictSNI enforces exact matches of client server indicators (SNI) against host names. TLSStrictSNI on|off TLSStrictSNI on server config

Client connections using SNI will be unsuccessful if no match is found.

TLSSessionCache specifies the cache for TLS session resumption. TLSSessionCache cache-spec server config

This uses a cache on the server side to allow clients to resume connections.

You can set this to `none` or define a cache as in the SSLSessionCache directive of mod_ssl.

If not configured, `mod_tls` will try to create a shared memory cache on its own, using `shmcb:tls/session-cache` as specification. Should that fail, a warning is logged, but the server continues.