exim4/doc/openssl.txt

OpenSSL
=======

The OpenSSL Project documents their supported releases at
<https://www.openssl.org/policies/releasestrat.html>.  The Exim
Maintainers are unwilling to try to support Exim built with a
version of a critical security library which is unmaintained.

Thus as versions of OpenSSL become unsupported by OpenSSL, they become
unsupported by Exim.  Exim might build with older releases of OpenSSL,
but that's risky behaviour.

If your operating system vendor continues to ship an older version of
OpenSSL and is diligently backporting security fixes, and they support
Exim, then they will be backporting fixes to their packages of Exim too.
If you wish to stick purely to packages of OpenSSL, then stick to
packages of Exim too.

If someone maintains "backports", that is worth exploring too.

Note that a number of OSes use Exim with GnuTLS, not OpenSSL.

Otherwise, assuming that your operating system has old OpenSSL, and you
wish to use current Exim with OpenSSL, then you need to build and
install your own, without interfering with the system libraries.
Fortunately, this is easy.

So this only applies if you build Exim yourself.


Insecure versions and ciphers
-----------------------------

Email delivery to MX hosts is usually done with automatic fallback to
plaintext if TLS could not be negotiated.  There are good historical reasons
for this.  You can and should avoid it by using DNSSEC for signing your DNS
and publishing TLSA records, to enable "DANE" security.  This signals to
senders that they should be able to verify your certificates, and that they
should not fallback to cleartext.

In the absence of DANE, trying to increase the security of TLS by removing
support for older generations of ciphers and protocols will actually _lower_
the security, because the clients fallback to plaintext and retry anyway.  As
a result, you should give serious thought to enabling older features which are
no longer default in OpenSSL.

The examples below explicitly enable ssl3 and weak ciphers.

We don't like this, but reality doesn't care and is messy.


Build
-----

Extract the current source of OpenSSL.  Change into that directory.

This assumes that `/opt/openssl` is not in use.  If it is, pick
something else.  `/opt/exim/openssl` perhaps.

If you pick a location shared amongst various local packages, such as
`/usr/local` on Linux, then the new OpenSSL will be used by all of those
packages.  If that's what you want, great!  If instead you want to
ensure that only software you explicitly set to use the newer OpenSSL
will try to use the new OpenSSL, then stick to something like
`/opt/openssl`.

    ./config --prefix=/opt/openssl --openssldir=/etc/ssl  \
        -L/opt/openssl/lib -Wl,-R/opt/openssl/lib         \
        enable-ssl-trace shared                           \
        enable-ssl3 enable-ssl3-method enable-weak-ssl-ciphers
    make
    make install

On some systems, the linker uses `-rpath` instead of `-R`; on such systems,
replace the parameter starting `-Wl` with: `-Wl,-rpath,/opt/openssl/lib`.
There are more variations on less common systems.

You now have an installed OpenSSL under /opt/openssl which will not be
used by any system programs.

When you copy `src/EDITME` to `Local/Makefile` to make your build edits,
choose the pkg-config approach in that file, but also tell Exim to add
the relevant directory into the rpath stamped into the binary:

    PKG_CONFIG_PATH=/opt/openssl/lib/pkgconfig

    SUPPORT_TLS=yes
    USE_OPENSSL_PC=openssl
    LDFLAGS+=-ldl -Wl,-rpath,/opt/openssl/lib

The -ldl is needed by OpenSSL 1.0.2+ on Linux and is not needed on most
other platforms.  The LDFLAGS is needed because `pkg-config` doesn't know
how to emit information about RPATH-stamping, but we can still leverage
`pkg-config` for everything else.

Then build Exim:

    make
    sudo make install


Confirming
----------

Run:

    exim -d-all+expand --version

and look for the `Library version: OpenSSL:` lines.

To look at the libraries _probably_ found by the linker, use:

    ldd $(which exim)		# most platforms
    otool -L $(which exim)	# MacOS

although that does not correctly handle restrictions imposed upon
executables which are setuid.

If the `chrpath` package is installed, then:

    chrpath -l $(which exim)

will show the DT_RPATH stamped into the binary.

Your `binutils` package should come with `readelf`, so an alternative
is to run:

    readelf -d $(which exim) | grep RPATH

It is important to use `RPATH` and not `RUNPATH`!

The gory details about `RUNPATH` (skip unless interested):
The OpenSSL library might be opened indirectly by some other library
which Exim depends upon.  If the executable does have `RUNPATH` then
that will inhibit using either of `RPATH` or `RUNPATH` from the
executable for finding the OpenSSL library when that other library tries
to load it.
In fact, if the intermediate library has a `RUNPATH` stamped into it,
then this will block `RPATH` too, and will create problems with Exim.
If you're in such a situation, and those libraries were supplied to you
instead of built by you, then you're reaching the limits of sane
repairability and it's time to prioritize rebuilding your mail-server
hosts to be a current OS release which natively pulls in an
upstream-supported OpenSSL, or stick to the OS releases of Exim.


Very Advanced
-------------

You can not use $ORIGIN for portably packing OpenSSL in with Exim with
normal Exim builds, because Exim is installed setuid which causes the
runtime linker to ignore $ORIGIN in DT_RPATH.

_If_ following the steps for a non-setuid Exim, _then_ you can use:

    EXTRALIBS_EXIM=-ldl '-Wl,-rpath,$$ORIGIN/../lib'

The doubled `$$` is needed for the make(1) layer and the quotes needed
for the shell invoked by make(1) for calling the linker.

Note that this is sufficiently far outside normal that the build-system
doesn't support it by default; you'll want to drop a symlink to the lib
directory into the Exim release top-level directory, so that lib exists
as a sibling to the build-$platform directory.