Module mod_ssl - Apache Interface to SSLeay

> | Apache Interface to SSLeay |

> # Frequently Asked Questions

> - What are the differences between mod_ssl and the original Apache-SSL?

>   This cannot be answered in short, because there are a lot of changes (see the `CHANGES` file in the mod_ssl distribution for detailed information). Most of them are internal changes and re-organizations of the source code. But the user visible changes are mainly the following:

>   - mod_ssl provides a complete documentation (this document) of all configuration directives, environment variables, etc. while Apache-SSL still has no such documentation although it exists for over three years now. Additionally mod_ssl provides answers to often occuring frequently asked questions (this list) in the Apache/SSL/SSLeay area. For instance mod_ssl gives detailed hints about how to setup a CA, how to create real a server Certificate, etc. Because these are the typical hurdles every user stumbles over.

>   - mod_ssl comes as clean and documented source code with the intent that only this way the user is able to re-view it for backdoors, security holes, etc. This is considered important for security-related software. It's absolutely incomprehensible to the author how Apache-SSL could exist for over three years while both the source code and the source patches were provided in an undocumented and partly unreadable format. For the mod_ssl package the source codes follows the Apache coding style, is logically ordered to follow the API phases and even the patches to the Apache source tree are annotated with descriptions to give the user a chance to re-view and understand them.

>   - mod_ssl can be easily applied to the Apache source tree because it provides an automated process for this task while Apache-SSL forces the user to fiddle with the `patch` and `cp` tools theirself. Just because security is not for amateurs haven't mean that user friendliness is not important. So it's a must to assist the user in applying the SSL-stuff to vanilla Apache sources.

>   - mod_ssl fixes a lot of bugs and inconsistencies in Apache-SSL. For Apache experts here are a few impressions: Apache-SSL wrote directly to `stderr` instead of the Apache error logfile; it messed up it's error messages with the SSLeay error messages, it directly patched the `SERVER_BASEVERSION` define instead of using the Apache 1.3 conforming `ap_add_version_component` function; it used the unsafe `sprintf` function instead of the robust `ap_snprintf`; it incorrectly spawned and killed the `gcache` auxiliary program and it totally failed to pass the ```gcc -Wall -Wshadow -Wpointer-arith -Wcast-align -Wmissing-prototypes -Wmissing-declarations -Wnested-externs -Winline`'' test (while Apache passes it) because of unclean code.

>     Additionally Apache-SSL it didn't provide a way to easily apply it to the Apache source tree (semi-manual editing and patching was required); it didn't seamlessly integrate into the new Apache 1.3 Autoconf-style Interface (APACI) under configuration time; it didn't automatically recognize the difference between an installed SSLeay and an out-of-the-source-only SSLeay; it didn't provide integration into the APACI installation process (`make install`); it used numbers 0 to 2 instead of reasonable names for the argument of `SSLVerifyClient` just because internally an `enum` was used and for the provided `%{version}c` construct of CustomLog it used the results "2", "3" under SSLeay 0.8 while under SSLeay 0.9 the results were "SSL2", "SSL3", etc. pp.

>   - mod_ssl adds new functionalities which are missing in Apache-SSL. A few selected points which give you an impression follow:
>     - mod_ssl provides a real dedicated SSL log file controlled by log level and the additional features that messages logged at the `error' level are automatically duplicated to the general Apache error log file. And occuring system and SSLeay error messages are automatically appended to mod_ssl messages. Additionally mod_ssl annotates deep-level SSLeay messages with more high-level hints.
>     - mod_ssl provides a completely new and enhanced handling of encrypted private key files. First the private keys are kept in a permanent memory pool (as SSLeay already does internally), so Apache now survives server restarts without falling down. Second the pass phrase dialog is a lot more user friendly and advanced: It uses a pass phrase reuse-algorithm to minimize the dialog, it recognizes wrong pass phrases and allows retries (but with a backoff time delay), etc. And additionally a minimal interface is provided to plug-in an external program for providing the pass phrase for special batch security situations.
>     - mod_ssl provides the `SSLCACertificateReqFile` directive which can be used to configure a different (from `SSLCACertificateFile`) set of CA Certifcates for the SSLv3 feature used by the clients to load CA Certifcates from the server for speeding up server authentication.
>     - mod_ssl replaced the "gcache" stuff of Apache-SSL (used for caching SSL sessions) with a more robust DBM-based solution, because the controlling of an external program cannot be done very reliable from within Apache. Additionally a "mutex" is now used to synchronize the inter-process access to this cache.
>     - mod_ssl provides support for the SSLeay+RSAref couple, i.e. mod_ssl supports the building with RSAref.
>     - mod_ssl provides a new SSLRequire directive which can be used to implement more granular access control based on arbitrary complex boolean expression.
>     - mod_ssl adds support for HTTPS to the Apache Proxy Module (mod_proxy).

>   For more details I really advice you to read the `CHANGES` file from the mod_ssl distribution.

> - What are the user visible incompatibilities to Apache-SSL?

>   - <IfModule apache_ssl.c> --> <IfModule mod_ssl.c> or <IfDefine MOD_SSL>
>     Because of the renamed source files, when you used <IfModule> containers inside your server config files to encapsulate Apache-SSL directives, you now have to use the filename `mod_ssl.c` instead. Although such changes are nasty for the users the new name directly follows the Apache module conventions. Additionally to let you distinguish between Stronghold and mod_ssl (both use `mod_ssl.c`) a define `MOD_SSL` is provided which can be used with `<IfDefine>` sections.

>   - Ben-SSL/1.18 --> mod_ssl/2.0.0 SSLeay/0.9.0b
>     Because the workhorse of SSL inside Apache is SSLeay and not mod_ssl, the SSLeay version is more important in the HTTP Server field. So both mod_ssl's and SSLeay's version is now added to the Server field.

>   - httpsd --> httpd -DSSL
>     The provided (and installed) default configuration is SSL-ready but has SSL-related directives encapsulated in `<IfDefine SSL>` sections. So, when you immediately want to fire up your httpd with SSL enabled you have to specify the `-DSSL` option on the command line. Or edit the configuration. Additionally the Apache server executable is again named `httpd` and not `httpsd`, because the intent is that the mod_ssl package only patches things inside Apache which are really necessary to patch.

>   - Skip first time initialization...
>     These and other verbose terminal messages of Apache-SSL were completely removed because its nasty to get those messages when a daemon is run (especially when its run from Boot scripts or Cron scripts). Instead the remaining useful ones are now printed to the dedicated SSL logfile.

>   - SSLEnable/SSLDisable --> SSLEngine on/off
>     The `SSLEnable` and `SSLDisable` directives were made obsolete by adding the simpler `SSEngine on/off` directive. The old directives are now deprecated but still can be used for backward-compatibility to Apache-SSL. Additionally the default is now `SSLEngine off` (formerly `SSLDisable`) instead of the Apache-SSL default of `SSLEngine on`. This breaks a little bit with Apache-SSL semantics, but doesn't hurt as much as it sounds. Because people run SSL inside a virtual hosts and thus already have a `SSLDisable` in their main (non-SSL) server. The difference is just that with mod_ssl one no longer has to add `SSLDisable` to all non-SSL virtual hosts (which is nasty especially for ISPs running thousends of virtual hosts). OTOH SSL is an additional feature, so the logic of `disabled per default and have to be enabled explicity' is more useful and what is expected.

> - Ok, does this mean I should avoid using Apache-SSL any longer?

>   No, it just means that you can use mod_ssl instead if you prefer maximum cleanness and up-to-date stuff. But beside some bugs Apache-SSL works great. Ben Laurie did and still does a great job in maintaining it. The big difference is just that Ben's goals are not really cleanness or user friendliness. So, as long as you don't get bothered by the inconsistencies and the fact that it's mostly impossible for you to re-view Apache-SSL's internals for security reasons you don't have to upgrade. Instead you should decide yourself if you already feel comfortable or not. If yes, stay with Apache-SSL - if not, move to mod_ssl or (even better) one of the commercial SSL solutions for Apache. Or in other words: No solution is better than another in general. Which one you should use depends mainly on your personal requirements.

> - On which Apache-SSL version is mod_ssl actually based?

>   The mod_ssl package was initially created by porting the Apache-SSL 1.17 stuff from Apache 1.2.6 to Apache 1.3b6. Because of conflicts with Ben Laurie's development cycle it then was re-assembled from scratch for Apache 1.3.0 by merging the old mod_ssl with the newer Apache-SSL 1.18. From this point mod_ssl lived its own life and changes with Apache-SSL releases were merged after they were released. In other words: mod_ssl is based on the latest Apache-SSL and always will contain all useful changes which will occur with Apache-SSL in the future.

> - Why is mod_ssl's version starting with 2.0.0?

>   Because initially the mod_ssl project was intended as a contribution to the Apache-SSL project from Ben Laurie. The idea was that mod_ssl formed Apache-SSL 2.0.0. But after Ralf S. Engelschall and Ben Laurie couldn't find a reasonable compromise in merging mod_ssl with Apache-SSL, the stuff was released as a new package named `mod_ssl'. But to still indicate that it's some `second generation' stuff, the first mod_ssl version was named 2.0.0.

> - How do I know which mod_ssl version is for which Apache version?

>   That's trivial: mod_ssl uses version strings of the syntax <mod_ssl-version>-<apache-version>, for instance `2.0.1-1.3.1`. This directly indicated that it's mod_ssl version 2.0.1 for Apache version 1.3.1. And this also means you only can apply this mod_ssl version to exactly this Apache version.

> - I want to run HTTP and HTTPS on the same machine. Is that possible?

>   There are two ways to do this: run two server daemons, or run both services from the same daemon. Unless there is a good reason to run two (like using a different product for secure/non-secure), it's usually simplest to run a single daemon and disable SSL on those virtual hosts that don't need it. If you wish to run two daemons you must make sure that they each only try to bind to their alloted ports (normally port 80 for non-secure and 443 for secure).

> - I know that HTTP is on port 80, but where is HTTPS?

>   You can run HTTPS on any port, but the standard specifies port 443, which is where any compliant browser will look by default. You can force your browser to look on a different port by specifying it in the URL like this (for port 666): `https://secure.server.dom:666/`

> - How can I speak HTTPS manually for testing purposes?

>   While you usually just use

>   `$ telnet localhost 80`
>   `GET / HTTP/1.0`

>   for simple testing the HTTP protocol of Apache, it's not such easy for HTTPS because of the SSL protocol between TCP and HTTP. But with the help of SSLeay's `s_client` program you can do a similar check even for HTTPS:

>   `$ s_client -host localhost -port 443`
>   `GET / HTTP/1.0`

>   Before the actual HTTP response you receive detailed information about the SSL handshake. For a more general command line client which directly understands both the HTTP and HTTPS scheme, can perform GET and POST methods, can use a proxy, supports byte ranges, etc. you should have a look at cURL. With it you can directly check if your Apache is running fine on Port 80 and 443 as following:

>   `$ curl http://localhost/`
>   `$ curl https://localhost/`

> - What are RSA private keys, CSR's and Certificates?

>   The RSA private key file is a digital file that you can use to decrypt messages sent to you. It has a public component which you distribute (via your Certificate file) which allows people to encrypt those messages to you. A Certificate Signing Request (CSR) is a digital file which contains your public key and your name. You send the CSR to a Certifying Authority (CA) to be converted into a real Certificate. A Certificate contains your RSA public key, your name, the name of the CA, and is digitally signed by your CA. Browsers that know the CA can verify the signature on that Certificate, thereby obtaining your RSA public key. That enables them to send messages which only you can decrypt.

> - Seems like there is a difference on startup between the original Apache and an SSL-aware Apache?

>   Yes, in general, starting Apache with a built-in mod_ssl is just like starting an unencumbered Apache, except for the fact that when you have a pass phrase on your SSL private key file. Then a startup dialog pops up asking you to enter the pass phrase.

>   To type in the pass phrase manually when starting the server can be problematic, for instance when starting the server from the system boot scripts. As an alternative to this situation you can follow the steps below under ``How can I get rid of the pass-phrase dialog on Apache startup time?''.

> - How can I create a dummy SSL server Certificate for testing purposes?

>   A Certificate does not have to be signed by a public CA. You can use your private key to sign the Certificate which contains your public key. You can install this Certificate into your server, and people using Netscape Navigator (not MSIE) will be able to connect after clicking OK to a warning dialogue. You can get MSIE to work, and your customers can eliminate the dialogue, by installing that Certificate manually into their browsers.

>   Just use the ```make certificate`'' command at the top-level directory of the Apache source tree right before installing Apache via ```make install`''. This creates a self-signed SSL Certificate which expires after 30 days and isn't encrypted (which means you don't need to enter a pass-phrase on Apache startup time).

>   BUT REMEMBER: YOU REALLY HAVE TO CREATE A REAL CERTIFICATE FOR THE LONG RUN! HOW THIS IS DONE IS DESCRIBED IN THE NEXT ANSWER.

> - Ok, I've got my server installed and not want to create a real SSL server Certificate for it. How do I do it?

>   Here is a step-by-step description:

>   1. Make sure SSLeay is really installed and in your `PATH`. But some commands even work ok when you just run the ```ssleay`'' program from within the SSLeay source tree as ```./apps/ssleay`''.

>   2. Create a RSA private key for your Apache server (will be Triple-DES encrypted and PEM formatted):

>      `$ ssleay genrsa -des3 -out server.key 1024`

>      Please backup this `server.key` file and remember the pass-phrase you had to enter at a secure location. You can see the details of this RSA private key via the command:

>      `$ ssleay rsa -noout -text -in server.key`

>      And you could create a decrypted PEM version (not recommended) of this RSA private key via:

>      `$ ssleay rsa -in server.key -out server.key.unsecure`

>   3. Create a Certificate Signing Request (CSR) for the server RSA private key (output will be PEM formatted):

>      `$ ssleay req -new -days 365 -key server.key -out server.csr`

>      You can see the details of this CSR via the command

>      `$ ssleay req -noout -text -in server.csr`

>   4. You now have to send this Certificate Signing Request (CSR) to a Certifying Authority (CA) for signing. The result is then a real Certificate which can be used for Apache. Here you have to options: First you can let the CSR sign by a commercial CA like Verisign or Thawte. Then you usually have to post the CSR into a web form, pay for the signing and await the signed Certificate you then can store into a server.crt file. For more information about commercial CAs have a look at the following locations:

>      - Verisign
>        http://digitalid.verisign.com/server/apacheNotice.htm
>      - Thawte Consulting
>        http://www.thawte.com/certs/server/request.html
>      - CertiSign Certificadora Digital Ltda.
>        http://www.certisign.com.br
>      - IKS GmbH
>        http://www.iks-jena.de/produkte/ca/
>      - Uptime Commerce Ltd.
>        http://www.uptimecommerce.com
>      - BelSign NV/SA
>        http://www.belsign.be

>      Second you can use your own CA and now have to sign the CSR yourself by this CA. Read the next answer in this FAQ on how to sign a CSR with your CA yourself. You can see the details of the received Certificate via the command:

>      `$ ssleay x509 -noout -text -in server.crt`

>   5. Now you have two files: `server.key` and `server.crt`. These now can be used as following inside your Apache's `httpd.conf` file:

>                 SSLCertificateFile    /path/to/this/server.crt
>                 SSLCertificateKeyFile /path/to/this/server.key

>      The `server.csr` file is no longer needed.

> - How can I create and use my own Certificate Authority (CA)?

>   The short answer is to use the `CA.sh` script provided by SSLeay. The long and manual answer is this:

>   1. Create a RSA private key for your CA (will be Triple-DES encrypted and PEM formatted):

>      `$ ssleay genrsa -des3 -out ca.key 1024`

>      Please backup this `ca.key` file and remember the pass-phrase you currently entered at a secure location. You can see the details of this RSA private key via the command

>      `$ ssleay rsa -noout -text -in ca.key`

>      And you can create a decrypted PEM version (not recommended) of this private key via:

>      `$ ssleay rsa -in ca.key -out ca.key.unsecure`

>   2. Create a self-signed CA Certificate (X509 structure) for the RSA key of the CA (output will be PEM formatted):

>      `$ ssleay req -new -x509 -days 365 -key ca.key -out ca.crt`

>      You can see the details of this Certificate via the command:

>      `$ ssleay x509 -noout -text -in ca.crt`

>   3. Prepare a script for signing which is needed because the ```ssleay ca`'' command has some strange requirements and the default SSLeay config doesn't allow one easily to use ```ssleay ca`'' directly. So a script named `ca.sign` is distributed with the mod_ssl distribution. Use this script for signing.

>   4. Now you can use this CA to sign CSR's in order to create real SSL Certificates for use inside an Apache webserver:

>      `$ ./ca.sign server.csr`

>      This signs the CSR and results in a `server.crt` file.

> - How can I change the pass-phrase on my private key file?

>   You simply have to read it with the old pass-phrase and write it again by specifying the new pass-phrase. You can accomplish this with the following commands:

>   `$ ssleay rsa -des3 -in server.key -out server.key.new`
>   `$ mv server.key.new server.key`

>   Here you're asked two times for a PEM pass-phrase. At the first prompt enter the old pass-phrase and at the second prompt enter the new pass-phrase.

> - How can I get rid of the pass-phrase dialog on Apache startup time?

>   The reason why this dialog pops up at startup and every re-start is that the RSA private key inside your server.key file is stored in encrypted format for security reasons. The pass-phrase is needed to be able to read and parse this file. When you can be sure that your server is secure enough you perform two steps:

>   1. Remove the encryption from the RSA private key (while preserving the original file):

>      `$ cp server.key server.key.org`
>      `$ ssleay rsa -in server.key.org -out server.key`

>   2. Make sure the server.key file is now only readable by root:

>      `$ chmod 400 server.key`

>   Now `server.key` will contain an unencrypted copy of the key. If you point your server at this file it will not prompt you for a pass-phrase. HOWEVER, if anyone gets this key they will be able to impersonate you on the net. PLEASE make sure that the permissions on that file are really such that only root or the web server user can read it (preferably get your web server to start as root but run as another server, and have the key readable only by root).

> - How do I verify that a private key matches its Certificate?

>   The private key contains a series of numbers. Two of those numbers form the "public key", the others are part of your "private key". The "public key" bits are also embedded in your Certificate (we get them from your CSR). To check that the public key in your cert matches the public portion of your private key, you need to view the cert and the key and compare the numbers. To view the Certificate and the key run the commands:

>   `$ ssleay x509 -noout -text -in server.crt`
>   `$ ssleay rsa -noout -text -in server.key`

>   The `modulus' and the `public exponent' portions in the key and the Certificate must match. But since the public exponent is usually 65537 and it's bothering comparing long modulus you can use the following approach:

>   `$ ssleay x509 -noout -modulus -in server.crt | ssleay md5`
>   `$ ssleay rsa -noout -modulus -in server.key | ssleay md5`

>   And then compare these really shorter numbers. With overwhelming probability they will differ if the keys are different. BTW, if I want to check to which key or certificate a particular CSR belongs you can compute

>   `$ ssleay req -noout -modulus -in server.csr | ssleay md5`

> - Why does my 2048-bit private key not work?

>   The private key sizes must be either 512 or 1024 for compatibility with certain web browsers. A keysize of 1024 bits is recommended because keys larger than 1024 bits are incompatible with some versions of Netscape Navigator and Microsoft Internet Explorer, and with other browsers that use RSA's BSAFE cryptography toolkit.

> - Why does my browser hang when I connect to my SSL-aware Apache server?

>   Because you used an URL of the form ```http://`'' instead of ```https:`''. Really! Also, if you see: ```SSL_Accept failed error:140760EB:SSL routines: SSL23_GET_CLIENT_HELLO:unknown protocol`'' in your Apache error logfile, it's for the same reason. This also happens the other way round, i.e. when you try to use ```https://`'' on a server that doesn't support SSL (on this port). Make sure you are connecting to a virtual server that supports SSL, which is probably the IP associated with your hostname, not localhost (127.0.0.1).

> - How can I use relative hyperlinks to switch between HTTP and HTTPS?

>   Usually you have to use fully-qualified hyperlinks because you have to change the URL scheme. But with the help of some URL manipulations through mod_rewrite you can achieve the same effect while you still can use relative URLs:

>           RewriteEngine on
>           RewriteRule   ^/(.*):SSL$   https://%{SERVER_NAME}/$1 [R,L]
>           RewriteRule   ^/(.*):NOSSL$ http://%{SERVER_NAME}/$1  [R,L]

>   This rewrite ruleset lets you use hyperlinks of the form

>           <a href="document.html:SSL">

> - Why has my webserver a higher load now that I run SSL there?

>   Because SSL uses strong cryptographic encryption and this needs a lot of number crunching. And because when you request a webpage via HTTPS even the images are transfered encrypted. There is dramatically more to do on the webserver, so the load increases.

> - What SSL Ciphers are supported by mod_ssl?

>   Usually just all SSL chipers which are supported by the used version of SSLeay (can depend on the way you built SSLeay). Typically this at least includes the following:

>   - RC4 with MD5
>   - RC4 with MD5 (export version restricted to 40-bit key)
>   - RC2 with MD5
>   - RC2 with MD5 (export version restricted to 40-bit key)
>   - IDEA with MD5
>   - DES with MD5
>   - Triple-DES with MD5

>   To determine the actual list of supported ciphers you can run the following command:

>   `$ ssleay ciphers -v`

> - Why cannot I use SSL with my name-based virtual hosts?

>   The reason is very technically. Actually it's some sort of a chicken and egg problem: The SSL protocol layer stays below the HTTP protocol layer and encapsulates HTTP. When an SSL connection (HTTPS) is established Apache/mod_ssl has to negotiate the SSL protocol parameters with the client. For this mod_ssl has to consult the configuration of the virtual server (for instance it has to look for the cipher suite, the server certificate, etc.). But in order to dispatch to the correct virtual server Apache has to know the `Host` HTTP header field. For this the HTTP request header has to be read. This cannot be done before the SSL handshake is finished. But the information is already needed at the SSL handshake phase. Bingo!