!==
!== SSLeay.txt for Samba release 2.2.0-alpha3 24 Mar 2001
!==
Contributor: Christian Starkjohann <cs@obdev.at>
Date: May 29, 1998
Status:
Subject: Compiling and using samba with SSL support
============================================================================
What is SSL and SSLeay?
=======================
SSL (Secure Socket Layer) is a protocol for encrypted and authenticated data
transport. It is used by secure web servers for shopping malls, telebanking
and things like that.
SSLeay is a free implementation of the SSL protocol. It is available from
ftp://ftp.psy.uq.oz.au/pub/Crypto/SSL/
The current version while these lines are written is 0.9.0. Encryption is
plagued by legal problems of all kinds. For a discussion of these please
read the documentation of SSLeay, which is available at
http://www.psy.uq.edu.au/~ftp/Crypto/
To compile samba with SSL support, you must first compile and install SSLeay.
SSLeay consists of a library (which can be linked to other applications like
samba) and several utility programs needed for key generation, certification
etc. SSLeay installs to /usr/local/ssl/ by default.
Compiling samba with SSLeay
===========================
1. Get and install SSLeay. The rest of this documentation assumes that you
have installed it at the default location, which is /usr/local/ssl/.
I have used SSLeay 0.9.0, but samba will probably also work with other
versions (but not with versions older than 0.6).
2. Modify the Makefile. At the end of the configurable section you can find
the SSL definitions. You can find them quickly by searching for SSL_ROOT.
Unremark the definitions and modify SSL_ROOT if necessary.
3. Compile and install as usual.
Configuring SSL in samba
========================
Before you configure SSL, you should know the basics of cryptography and how
SSL relates to all of this. A basic introduction can be found further down in
this document. The following variables in the "[global]" section of the
configuration file are used to configure SSL:
ssl = yes
This variable enables or disables the entire SSL mode. If it is set to
"no", the SSL enabled samba behaves exactly like the non-SSL samba. If set
to "yes", it depends on the variables "ssl hosts" and "ssl hosts resign"
whether an SSL connection will be required.
ssl hosts =
ssl hosts resign = 192.168.
These two variables define whether samba will go into SSL mode or not. If
none of them is defined, samba will allow only SSL connections. If the
"ssl hosts" variable lists hosts (by IP-address, IP-address range, net
group or name), only these hosts will be forced into SSL mode. If the
"ssl hosts resign" variable lists hosts, only these hosts will NOT be
forced into SSL mode. The syntax for these two variables is the same as
for the "hosts allow" and "hosts deny" pair of variables, only that the
subject of the decision is different: It's not the access right but
whether SSL is used or not. See the man page of smb.conf (section about
"allow hosts") for details. The above example requires SSL connections
from all hosts outside the local net (which is 192.168.*.*).
ssl CA certDir = /usr/local/ssl/certs
This variable defines where to look up the Certification Autorities. The
given directory should contain one file for each CA that samba will trust.
The file name must be the hash value over the "Distinguished Name" of the
CA. How this directory is set up is explained later in this document. All
files within the directory that don't fit into this naming scheme are
ignored. You don't need this variable if you don't verify client
certificates.
ssl CA certFile = /usr/local/ssl/certs/trustedCAs.pem
This variable is a second way to define the trusted CAs. The certificates
of the trusted CAs are collected in one big file and this variable points
to the file. You will probably only use one of the two ways to define your
CAs. The first choice is preferable if you have many CAs or want to be
flexible, the second is perferable if you only have one CA and want to
keep things simple (you won't need to create the hashed file names). You
don't need this variable if you don't verify client certificates.
ssl server cert = /usr/local/ssl/certs/samba.pem
This is the file containing the server's certificate. The server _must_
have a certificate. The file may also contain the server's private key.
See later for how certificates and private keys are created.
ssl server key = /usr/local/ssl/private/samba.pem
This file contains the private key of the server. If this variable is not
defined, the key is looked up in the certificate file (it may be appended
to the certificate). The server _must_ have a private key and the
certificate _must_ match this private key.
ssl client cert = /usr/local/ssl/certs/smbclient.pem
The certificate in this file is used by smbclient if it exists. It's needed
if the server requires a client certificate.
ssl client key = /usr/local/ssl/private/smbclient.pem
This is the private key for smbclient. It's only needed if the client
should have a certificate.
ssl require clientcert = yes
If this variable is set to "yes", the server will not tolerate connections
from clients that don't have a valid certificate. The directory/file
given in "ssl CA certDir" and "ssl CA certFile" will be used to look up
the CAs that issued the client's certificate. If the certificate can't be
verified positively, the connection will be terminated.
If this variable is set to "no", clients don't need certificates. Contrary
to web applications you really _should_ require client certificates. In
the web environment the client's data is sensitive (credit card numbers)
and the server must prove to be trustworthy. In a file server environment
the server's data will be sensitive and the clients must prove to be
trustworthy.
ssl require servercert = yes
If this variable is set to "yes", the smbclient will request a certificate
from the server. Same as "ssl require clientcert" for the server.
ssl ciphers = ???
This variable defines the ciphers that should be offered during SSL
negotiation. You should not set this variable unless you know what you do.
ssl version = ssl2or3
This enumeration variable defines the versions of the SSL protocol that
will be used. "ssl2or3" allows dynamic negotiation of SSL v2 or v3, "ssl2"
results SSL v2, "ssl3" results in SSL v3 and "tls1" results in TLS v1. TLS
(Transport Layer Security) is the (proposed?) new standard for SSL. The
default value is "ssl2or3".
ssl compatibility = no
This variable defines whether SSLeay should be configured for bug
compatibility with other SSL implementations. This is probably not
desirable because currently no clients with SSL implementations other than
SSLeay exist.
Running samba with SSLeay
=========================
Samba is started as usual. The daemon will ask for the private key's pass
phrase before it goes to background if the private key has been encrypted.
If you start smbd from inetd, this won't work. Therefore you must not encrypt
your private key if you run smbd from inetd.
Windows clients will try to connect to the SSL enabled samba daemon and they
will fail. This can fill your log with failed SSL negotiation messages. To
avoid this, you can either not run nmbd (if all clients use DNS to look up
the server), which will leave the Windows machine unaware of the server, or
list all (local) Windows machines in the "ssl hosts resign" variable.
About certificates
==================
Secure samba servers will not be set up for public use as it is the case with
secure web servers. Most installations will probably use it for distributed
offices that use parts of the internet for their intranet, for access to a
web server that's physically hosted by the provider or simply for teleworking.
All these applications work with a known group of users that can easily agree
on a certification authority. The CA can be operated by the company and the
policy for issuing certificates can be determined by the company. If samba is
configured to verify client certificates, it (currently) only verifies
whether a valid certificate exists. It does not verify any of the data within
the certificate (although it prints some of the data to the log file).
Which clients are available that support SSL?
=============================================
Currently there are only smbclient which is part of the samba package and
Sharity. Shariy versions newer than 0.14 in the beta branch and 1.01 in the
main branch can be compiled with SSLeay. Sharity is a CIFS/SMB client
implementation for Unix. It is a commercial product, but it is available in
source code and the demo-mode allows access to the first three layers of the
mounted directory hierarchy. Licenses for universities and students are free.
Sharity is available at
http://www.obdev.at/Products/Sharity.html
###########################################################################
Basics about Cryptography and SSL(eay)
###########################################################################
There are many good introductions to cryptography. I assume that the reader
is familiar with the words "encryption", "digital signature" and RSA. If you
don't know these terms, please read the cryptography FAQ part 6 and 7, which
is posted to the usenet newsgroup sci.crypt. It is also available from
ftp://rtfm.mit.edu/pub/usenet/news.answers/cryptography-faq
and
http://www.cis.ohio-state.edu/hypertext/faq/usenet/cryptography-faq
I'll concentrate on the questions specific to SSL and samba here.
What is a certificate?
======================
A certificate is issued by an issuer, usually a "Certification Authority"
(CA), who confirms something by issuing the certificate. The subject of this
confirmation depends on the CA's policy. CAs for secure web servers (used for
shopping malls etc.) usually only attest that the given public key belongs the
the given domain name. Company-wide CAs might attest that you are an employee
of the company, that you have permissions to use a server or whatever.
What is an X.509 certificate technically?
=========================================
Technically, the certificate is a block of data signed by the certificate
issuer (the CA). The relevant fields are:
- unique identifier (name) of the certificate issuer
- time range during that the certificate is valid
- unique identifier (name) of the certified subject
- public key of the certified subject
- the issuer's signature over all of the above
If this certificate should be verified, the verifier must have a table of the
names and public keys of trusted CAs. For simplicity, these tables are lists
of certificates issued by the respective CAs for themselves (self-signed
certificates).
What are the implications of this certificate structure?
========================================================
- Because the certificate contains the subject's public key, the
certificate and the private key together are all that's needed to encrypt
and decrypt.
- To verify certificates, you need the certificates of all CAs you trust.
- The simplest form of a dummy-certificate is one that's signed by the
subject itself.
- A CA is needed. The client can't simply issue local certificates for
servers it trusts because the server determines which certificate it
presents.
###########################################################################
Setting up files and directories for SSLeay
###########################################################################
The first thing you should do is to change your PATH environment variable to
include the bin directory of SSLeay. E.g.:
PATH=$PATH:/usr/local/ssl/bin
Then you should set up SSLeay's random number generator. The state of this
random number generator is held in the file ".rnd" in your home directory. To
set a reasonable random seed, you need random data. Create a random file with
cat >/tmp/rfile.txt
Then type random keys on your keyboard for about one minute. Then type the
EOF character (^D) to terminate input. You may also use your favorite editor
to create the random file, of course. Now you can create a dummy key to
initialize the random number generator:
ssleay genrsa -rand /tmp/rfile.txt > /dev/null
rm -f /tmp/rfile.txt
Don't forget to delete the file /tmp/rfile.txt. It's more or less equivalent
to your private key!
How to create a keypair
=======================
This is done with 'genrsa' for RSA keys and 'gendsa' for DSA keys. For an RSA
key with 512 bits which is written to the file "key.pem" type:
ssleay genrsa -des3 512 > key.pem
You will be asked for a pass phrase to protect this key. If you don't want to
protect your private key with a pass phrase, just omit the parameter "-des3".
If you want a different key size, replace the parameter "512". You really
should use a pass phrase.
If you want to remove the pass phrase from a key use:
ssleay rsa -in key.pem -out newkey.pem
And to add or change a pass phrase:
ssleay rsa -des3 -in key.pem -out newkey.pem
How to create a dummy certificate
=================================
If you still have your keypair in the file "key.pem", the command
ssleay req -new -x509 -key key.pem -out cert.pem
will write a self-signed dummy certificate to the file "cert.pem". This can
be used for testing or if only encryption and no certification is needed.
Please bear in mind that encryption without authentication (certification)
can never be secure. It's open to (at least) "man-in-the-middle" attacks.
How to create a certificate signing request
===========================================
You must not simply send your keypair to the CA for signing because it
contains the private key which _must_ be kept secret. A signing request
consists of your public key and some additional information you want to have
bound to that key by the certificate. If you operate a secure web server,
this additional information will (among other things) contain the URL of
your server in the field "Common Name". The certificate signing request is
created from the keypair with the following command (assuming that the key
pair is still in "key.pem"):
ssleay req -new -key key.pem -out csr.pem
This command will ask you for the information which must be included in the
certificate and will write the signing request to the file "csr.pem". This
signing request is all the CA needs for signing, at least technically. Most
CAs will demand bureaucratic material and money, too.
How to set up a Certification Authority (CA)
============================================
Being a certification authority requires a database that holds the CA's
keypair, the CA's certificate, a list of all signed certificates and other
information. This database is kept in a directory hierarchy below a
configurable starting point. The starting point must be configured in the
ssleay.conf file. This file is at /usr/local/ssl/lib/ssleay.conf if you have
not changed the default installation path.
The first thing you should do is to edit this file according to your needs.
Let's assume that you want to hold the CA's database at the directory
"/usr/local/ssl/CA". Change the variable "dir" in section "CA_default" to
this path. You may also want to edit the default settings for some variables,
but the values given should be OK. This path is also contained in the shell
script CA.sh, which should be at "/usr/local/ssl/bin/CA.sh". Change the path
in the shell script:
CATOP=/usr/local/ssl/CA
CAKEY=./cakey.pem # relative to $CATOP/
CACERT=./cacert.pem # relative to $CATOP/private/
Then create the directory "/usr/local/ssl/CA" and make it writable for the
user that operates the CA. You should also initialize SSLeay as CA user (set
up the random number generator). Now you should call the shell script CA.sh
to set up the initial database:
CA.sh -newca
This command will ask you whether you want to use an existing certificate or
create one. Just press enter to create a new key pair and certificate. You
will be asked the usual questions for certificates: the country, state, city,
"Common Name", etc. Enter the appropriate values for the CA. When CA.sh
finishes, it has set up a bunch of directories and files. A CA must publish
it's certificate, which is in the file "/usr/local/ssl/CA/cacert.pem".
How to sign a certificate request
=================================
After setting up the CA stuff, you can start signing certificate requests.
Make sure that the SSLeay utilities know where the configuration file is.
The default is compiled in, if you don't use the default location, add the
parameter "-config <cfg-file>". Make also sure that the configuration file
contains the correct path to the CA database. If all this is set up properly,
you can sign the request in the file "csr.pem" with the command:
ssleay ca -policy policy_anything -days 365 -infiles csr.pem >cert.pem
The resulting certificate (and additional information) will be in "cert.pem".
If you want the certificate to be valid for a period different from 365 days,
simply change the "-days" parameter.
How to install a new CA certificate
===================================
Whereever a certificate must be checked, the CA's certificate must be
available. Let's take the common case where the client verifies the server's
certificate. The case where the server verfies the client's certificate works
the same way. The client receives the server's certificate, which contains
the "Distinguished Name" of the CA. To verify whether the signature in this
certificate is OK, it must look up the public key of that CA. Therefore each
client must hold a database of CAs, indexed by CA name. This database is best
kept in a directory where each file contains the certificate of one CA and is
named after the hashvalue (checksum) of the CA's name. This section describes
how such a database is managed technically. Whether or not to install (and
thereby trust) a CA is a totally different matter.
The client must know the directory of the CA database. This can be configured.
There may also be a configuration option to set up a CA database file which
contains all CA certs in one file. Let's assume that the CA database is kept
in the directory "/usr/local/ssl/certs". The following example assumes that
the CA's certificate is in the file "cacert.pem" and the CA is known as
"myCA". To install the certificate, do the following:
cp cacert.pem /usr/local/ssl/cers/myCA.pem
cd /usr/local/ssl/certs
ln -s myCA.pem `ssleay x509 -noout -hash < myCA.pem`.0
The last command creates a link from the hashed name to the real file.
From now on all certificates signed by the myCA authority will be accepted by
clients that use the directory "/usr/local/ssl/certs/" as their CA certificate
database.