[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
Support for TLS (Transport Layer Security), formerly known as SSL (Secure Sockets Layer), is implemented by making use of the OpenSSL library or the GnuTLS library (Exim requires GnuTLS release 1.0 or later). There is no cryptographic code in the Exim distribution itself for implementing TLS. In order to use this feature you must install OpenSSL or GnuTLS, and then build a version of Exim that includes TLS support (see section Including TLS/SSL encryption support). You also need to understand the basic concepts of encryption at a managerial level, and in particular, the way that public keys, private keys, and certificates are used.
RFC 3207 defines how SMTP connections can make use of encryption. Once a connection is established, the client issues a STARTTLS command. If the server accepts this, the client and the server negotiate an encryption mechanism. If the negotiation succeeds, the data that subsequently passes between them is encrypted.
Exim's ACLs can detect whether the current SMTP session is encrypted or not, and if so, what cipher suite is in use, whether the client supplied a certificate, and whether or not that certificate was verified. This makes it possible for an Exim server to deny or accept certain commands based on the encryption state.
Warning: Certain types of firewall and certain anti-virus products can disrupt TLS connections. You need to turn off SMTP scanning for these products in order to get TLS to work.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
Early implementations of encrypted SMTP used a different TCP port from normal SMTP, and expected an encryption negotiation to start immediately, instead of waiting for a STARTTLS command from the client using the standard SMTP port. The protocol was called "ssmtp" or "smtps", and port 465 was allocated for this purpose.
This approach was abandoned when encrypted SMTP was standardised, but there are
still some legacy clients that use it. Exim supports these clients by means of
the tls_on_connect_ports
global option. Its value must be a list of port
numbers; the most common use is expected to be:
tls_on_connect_ports = 465 |
The port numbers specified by this option apply to all SMTP connections, both
via the daemon and via inetd. You still need to specify all the ports that
the daemon uses (by setting daemon_smtp_ports
or local_interfaces
or
the -oX
command line option) because tls_on_connect_ports
does not add
an extra port - rather, it specifies different behaviour on a port that is
defined elsewhere.
There is also a -tls-on-connect
command line option. This overrides
tls_on_connect_ports
; it forces the legacy behaviour for all ports.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
The first TLS support in Exim was implemented using OpenSSL. Support for GnuTLS followed later, when the first versions of GnuTLS were released. To build Exim to use GnuTLS, you need to set
USE_GNUTLS=yes |
in Local/Makefile, in addition to
SUPPORT_TLS=yes |
You must also set TLS_LIBS and TLS_INCLUDE appropriately, so that the include files and libraries for GnuTLS can be found.
There are some differences in usage when using GnuTLS instead of OpenSSL:
tls_verify_certificates
option must contain the name of a file, not the
name of a directory (for OpenSSL it can be either).
tls_dhparam
option is ignored, because early versions of GnuTLS had no
facility for varying its Diffie-Hellman parameters. I understand that this has
changed, but Exim has not been updated to provide this facility.
$tls_peerdn
variable.
tls_require_ciphers
options (the global option and the smtp
transport
option).
tls_require_ciphers
options operate differently, as described in the
sections Requiring specific ciphers in OpenSSL and Requiring specific ciphers in GnuTLS.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
GnuTLS uses RSA and D-H parameters that take a substantial amount of time to compute. It is unreasonable to re-compute them for every TLS session. Therefore, Exim keeps this data in a file in its spool directory, called ‘gnutls-params’. The file is owned by the Exim user and is readable only by its owner. Every Exim process that start up GnuTLS reads the RSA and D-H parameters from this file. If the file does not exist, the first Exim process that needs it computes the data and writes it to a temporary file which is renamed once it is complete. It does not matter if several Exim processes do this simultaneously (apart from wasting a few resources). Once a file is in place, new Exim processes immediately start using it.
For maximum security, the parameters that are stored in this file should be recalculated periodically, the frequency depending on your paranoia level. Arranging this is easy in principle; just delete the file when you want new values to be computed. However, there may be a problem. The calculation of new parameters needs random numbers, and these are obtained from ‘/dev/random’. If the system is not very active, ‘/dev/random’ may delay returning data until enough randomness (entropy) is available. This may cause Exim to hang for a substantial amount of time, causing timeouts on incoming connections.
The solution is to generate the parameters externally to Exim. They are stored
in ‘gnutls-params’ in PEM format, which means that they can be generated
externally using the certtool
command that is part of GnuTLS.
To replace the parameters with new ones, instead of deleting the file
and letting Exim re-create it, you can generate new parameters using
certtool
and, when this has been done, replace Exim's cache file by
renaming. The relevant commands are something like this:
# rm -f new-params # touch new-params # chown exim:exim new-params # chmod 0400 new-params # certtool --generate-privkey --bits 512 >new-params # echo "" >>new-params # certtool --generate-dh-params --bits 1024 >> new-params # mv new-params gnutls-params |
If Exim never has to generate the parameters itself, the possibility of stalling is removed.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
There is a function in the OpenSSL library that can be passed a list of cipher
suites before the cipher negotiation takes place. This specifies which ciphers
are acceptable. The list is colon separated and may contain names like
DES-CBC3-SHA. Exim passes the expanded value of tls_require_ciphers
directly to this function call. The following quotation from the OpenSSL
documentation specifies what forms of item are allowed in the cipher string:
Each cipher string can be optionally preceded by one of the characters ‘!’, ‘-’ or ‘+’.
If none of these characters is present, the string is interpreted as a list of ciphers to be appended to the current preference list. If the list includes any ciphers already present they will be ignored: that is, they will not be moved to the end of the list.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
The GnuTLS library does not have a combined function like OpenSSL. Instead, it allows the caller to specify separate lists of key-exchange methods, main cipher algorithms, and MAC algorithms. Unfortunately, these lists are numerical, and the library does not have a function for turning names into numbers. Consequently, the list of recognized names has to be built into the application.
At present, Exim permits only the list of main cipher algorithms to be
changed. The tls_require_ciphers
option is in the same format as for
OpenSSL. Exim searches each item for the name of available algorithm. For
example, if the list contains RSA_AES_SHA then AES is recognized.
The cipher algorithms list starts out with a default set of algorithms. If
the first item in tls_require_ciphers
does not start with an
exclamation mark, all the default items are deleted. Thus, only those specified
can be used. If the first item in tls_require_ciphers
does start with
an exclamation mark, the defaults are left on the list.
Then, any item that starts with an exclamation mark causes the relevant algorithms to be removed from the list, and any item that does not start with an exclamation mark causes the relevant algorithms to be added to the list. Thus,
tls_require_ciphers = !RSA_ARCFOUR_SHA |
allows all the defaults except those that use ARCFOUR, whereas
tls_require_ciphers = AES : 3DES |
allows only cipher suites that use AES and 3DES. The currently recognized algorithms are: AES_256, AES_128, AES (both of the preceding), 3DES, and ARCFOUR_128. Unrecognized algorithms are ignored. In a server, the order of the list is unimportant; the server will advertise the availability of all the relevant cipher suites. However, in a client, the order of the list specifies a preference order for the algorithms. The first one in the client's list that is also advertised by the server is tried first. The default order is as listed above.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
When Exim has been built with TLS support, it advertises the availability of
the STARTTLS command to client hosts that match tls_advertise_hosts
,
but not to any others. The default value of this option is unset, which means
that STARTTLS is not advertised at all. This default is chosen because you
need to set some other options in order to make TLS avaliable, and also it is
sensible for systems that want to use TLS only as a client.
If a client issues a STARTTLS command and there is some configuration problem in the server, the command is rejected with a 454 error. If the client persists in trying to issue SMTP commands, all except QUIT are rejected with the error
554 Security failure |
If a STARTTLS command is issued within an existing TLS session, it is rejected with a 554 error code.
To enable TLS operations on a server, you must set tls_advertise_hosts
to
match some hosts. You can, of course, set it to * to match all hosts.
However, this is not all you need to do. TLS sessions to a server won't work
without some further configuration at the server end.
It is rumoured that all existing clients that support TLS/SSL use RSA encryption. To make this work you need to set, in the server,
tls_certificate = /some/file/name tls_privatekey = /some/file/name |
These options are, in fact, expanded strings, so you can make them depend on
the identity of the client that is connected if you wish. The first file
contains the server's X509 certificate, and the second contains the private key
that goes with it. These files need to be readable by the Exim user, and must
always be given as full path names. They can be the same file if both the
certificate and the key are contained within it. If tls_privatekey
is not
set, or if its expansion is forced to fail or results in an empty string, this
is assumed to be the case. The certificate file may also contain intermediate
certificates that need to be sent to the client to enable it to authenticate
the server's certificate.
If you do not understand about certificates and keys, please try to find a source of this background information, which is not Exim-specific. (There are a few comments below in section Certificates and all that.)
Note: These options do not apply when Exim is operating as a client -
they apply only in the case of a server. If you need to use a certificate in an
Exim client, you must set the options of the same names in an smtp
transport.
With just these options, an Exim server will be able to use TLS. It does not require the client to have a certificate (but see below for how to insist on this). There is one other option that may be needed in other situations. If
tls_dhparam = /some/file/name |
is set, the SSL library is initialized for the use of Diffie-Hellman ciphers with the parameters contained in the file. This increases the set of cipher suites that the server supports. See the command
openssl dhparam |
for a way of generating this data. At present, tls_dhparam
is used only
when Exim is linked with OpenSSL. It is ignored if GnuTLS is being used.
The strings supplied for these three options are expanded every time a client
host connects. It is therefore possible to use different certificates and keys
for different hosts, if you so wish, by making use of the client's IP address
in $sender_host_address
to control the expansion. If a string expansion is
forced to fail, Exim behaves as if the option is not set.
The variable $tls_cipher
is set to the cipher suite that was negotiated for
an incoming TLS connection. It is included in the Received: header of an
incoming message (by default - you can, of course, change this), and it is
also included in the log line that records a message's arrival, keyed by
"X=", unless the tls_cipher
log selector is turned off. The encrypted
condition can be used to test for specific cipher suites in ACLs.
The ACLs that run for subsequent SMTP commands can check the name of the cipher suite and vary their actions accordingly. The cipher suite names are those used by OpenSSL. These may differ from the names used elsewhere. For example, OpenSSL uses the name DES-CBC3-SHA for the cipher suite which in other contexts is known as TLS_RSA_WITH_3DES_EDE_CBC_SHA. Check the OpenSSL documentation for more details.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
If you want an Exim server to request a certificate when negotiating a TLS
session with a client, you must set either tls_verify_hosts
or
tls_try_verify_hosts
. You can, of course, set either of them to * to
apply to all TLS connections. For any host that matches one of these options,
Exim requests a certificate as part of the setup of the TLS session. The
contents of the certificate are verified by comparing it with a list of
expected certificates. These must be available in a file or,
for OpenSSL only (not GnuTLS), a directory, identified by
tls_verify_certificates
.
A file can contain multiple certificates, concatenated end to end. If a directory is used (OpenSSL only), each certificate must be in a separate file, with a name (or a symbolic link) of the form <hash>.0, where <hash> is a hash value constructed from the certificate. You can compute the relevant hash by running the command
openssl x509 -hash -noout -in /cert/file |
where ‘/cert/file’ contains a single certificate.
The difference between tls_verify_hosts
and tls_try_verify_hosts
is
what happens if the client does not supply a certificate, or if the certificate
does not match any of the certificates in the collection named by
tls_verify_certificates
. If the client matches tls_verify_hosts
, the
attempt to set up a TLS session is aborted, and the incoming connection is
dropped. If the client matches tls_try_verify_hosts
, the (encrypted) SMTP
session continues. ACLs that run for subsequent SMTP commands can detect the
fact that no certificate was verified, and vary their actions accordingly. For
example, you can insist on a certificate before accepting a message for
relaying, but not when the message is destined for local delivery.
When a client supplies a certificate (whether it verifies or not), the value of
the Distinguished Name of the certificate is made available in the variable
$tls_peerdn
during subsequent processing of the message.
Because it is often a long text string, it is not included in the log line or
Received: header by default. You can arrange for it to be logged, keyed by
"DN=", by setting the tls_peerdn
log selector, and you can use
received_header_text
to change the Received: header. When no
certificate is supplied, $tls_peerdn
is empty.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
Certificate issuing authorities issue Certificate Revocation Lists (CRLs) when
certificates are revoked. If you have such a list, you can pass it to an Exim
server using the global option called tls_crl
and to an Exim client using
an identically named option for the smtp
transport. In each case, the value
of the option is expanded and must then be the name of a file that contains a
CRL in PEM format.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
The tls_cipher
and tls_peerdn
log selectors apply to outgoing SMTP
deliveries as well as to incoming, the latter one causing logging of the
server certificate's DN. The remaining client configuration for TLS is all
within the smtp
transport.
It is not necessary to set any options to have TLS work in the smtp
transport. If Exim is built with TLS support, and TLS is advertised by a
server, the smtp
transport always tries to start a TLS session. However,
this can be prevented by setting hosts_avoid_tls
(an option of the
transport) to a list of server hosts for which TLS should not be used.
If you do not want Exim to attempt to send messages unencrypted when an attempt
to set up an encrypted connection fails in any way, you can set
hosts_require_tls
to a list of hosts for which encryption is mandatory. For
those hosts, delivery is always deferred if an encrypted connection cannot be
set up. If there are any other hosts for the address, they are tried in the
usual way.
When the server host is not in hosts_require_tls
, Exim may try to deliver
the message unencrypted. It always does this if the response to STARTTLS is
a 5xx code. For a temporary error code, or for a failure to negotiate a TLS
session after a success response code, what happens is controlled by the
tls_tempfail_tryclear
option of the smtp
transport. If it is false,
delivery to this host is deferred, and other hosts (if available) are tried. If
it is true, Exim attempts to deliver unencrypted after a 4xx response to
STARTTLS, and if STARTTLS is accepted, but the subsequent TLS
negotiation fails, Exim closes the current connection (because it is in an
unknown state), opens a new one to the same host, and then tries the delivery
unencrypted.
The tls_certificate
and tls_privatekey
options of the smtp
transport provide the client with a certificate, which is passed to the server
if it requests it. If the server is Exim, it will request a certificate only if
tls_verify_hosts
or tls_try_verify_hosts
matches the client. Note:
These options must be set in the smtp
transport for Exim to use TLS when it
is operating as a client. Exim does not assume that a server certificate (set
by the global options of the same name) should also be used when operating as a
client.
If tls_verify_certificates
is set, it must name a file or,
for OpenSSL only (not GnuTLS), a directory, that contains a collection of
expected server certificates. The client verifies the server's certificate
against this collection, taking into account any revoked certificates that are
in the list defined by tls_crl
.
If
tls_require_ciphers
is set on the smtp
transport, it must contain a
list of permitted cipher suites. If either of these checks fails, delivery to
the current host is abandoned, and the smtp
transport tries to deliver to
alternative hosts, if any.
All the TLS options in the smtp
transport are expanded before use, with
$host
and $host_address
containing the name and address of the server to
which the client is connected. Forced failure of an expansion causes Exim to
behave as if the relevant option were unset.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
Exim sends multiple messages down the same TCP/IP connection by starting up an entirely new delivery process for each message, passing the socket from one process to the next. This implementation does not fit well with the use of TLS, because there is quite a lot of state information associated with a TLS connection, not just a socket identification. Passing all the state information to a new process is not feasible. Consequently, Exim shuts down an existing TLS session before passing the socket to a new process. The new process may then try to start a new TLS session, and if successful, may try to re-authenticate if AUTH is in use, before sending the next message.
The RFC is not clear as to whether or not an SMTP session continues in clear after TLS has been shut down, or whether TLS may be restarted again later, as just described. However, if the server is Exim, this shutdown and reinitialization works. It is not known which (if any) other servers operate successfully if the client closes a TLS session and continues with unencrypted SMTP, but there are certainly some that do not work. For such servers, Exim should not pass the socket to another process, because the failure of the subsequent attempt to use it would cause Exim to record a temporary host error, and delay other deliveries to that host.
To test for this case, Exim sends an EHLO command to the server after closing down the TLS session. If this fails in any way, the connection is closed instead of being passed to a new delivery process, but no retry information is recorded.
There is also a manual override; you can set hosts_nopass_tls
on the
smtp
transport to match those hosts for which Exim should not pass
connections to new processes if TLS has been used.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
In order to understand fully how TLS works, you need to know about certificates, certificate signing, and certificate authorities. This is not the place to give a tutorial, especially as I do not know very much about it myself. Some helpful introduction can be found in the FAQ for the SSL addition to Apache, currently at
http://www.modssl.org/docs/2.7/ssl_faq.html#ToC24 |
Other parts of the modssl documentation are also helpful, and have links to further files. Eric Rescorla's book, SSL and TLS, published by Addison-Wesley (ISBN 0-201-61598-3), contains both introductory and more in-depth descriptions. Some sample programs taken from the book are available from
http://www.rtfm.com/openssl-examples/ |
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
The file named by tls_certificate
may contain more than one
certificate. This is useful in the case where the certificate that is being
sent is validated by an intermediate certificate which the other end does
not have. Multiple certificates must be in the correct order in the file.
First the host's certificate itself, then the first intermediate
certificate to validate the issuer of the host certificate, then the next
intermediate certificate to validate the issuer of the first intermediate
certificate, and so on, until finally (optionally) the root certificate.
The root certificate must already be trusted by the recipient for
validation to succeed, of course, but if it's not preinstalled, sending the
root certificate along with the rest makes it available for the user to
install if the receiving end is a client MUA that can interact with a user.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
You can create a self-signed certificate using the req command provided with OpenSSL, like this:
openssl req -x509 -newkey rsa:1024 -keyout file1 -out file2 \ -days 9999 -nodes |
‘file1’ and ‘file2’ can be the same file; the key and the certificate are
delimited and so can be identified independently. The -days
option
specifies a period for which the certificate is valid. The -nodes
option is
important: if you do not set it, the key is encrypted with a passphrase
that you are prompted for, and any use that is made of the key causes more
prompting for the passphrase. This is not helpful if you are going to use
this certificate and key in an MTA, where prompting is not possible.
A self-signed certificate made in this way is sufficient for testing, and may be adequate for all your requirements if you are mainly interested in encrypting transfers, and not in secure identification.
However, many clients require that the certificate presented by the server be a user (also called "leaf" or "site") certificate, and not a self-signed certificate. In this situation, the self-signed certificate described above must be installed on the client host as a trusted root certification authority (CA), and the certificate used by Exim must be a user certificate signed with that self-signed certificate.
For information on creating self-signed CA certificates and using them to sign user certificates, see the General implementation overview chapter of the Open-source PKI book, available online at http://ospkibook.sourceforge.net/.
[ << ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
This document was generated on January, 24 2008 using texi2html 1.78.