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SSL server requirements
-----------------------
In order to let any mail client (not just fetchmail) verify server
certificates properly, so that users can be sure their connection is not
eavesdropped, there are several requirements that need to be fulfilled.
1. Match certificate and DNS names:
The server certificate's "common name" or "subject alternative name"
must match the name by which clients are connecting. Avoid the use of
wildmats if possible, not all clients support them (fetchmail does).
This may sound trivial, but for load balancing and failover setups,
it may not be obvious.
2. Provide the *full* certificate chain
Many SSL documents tell you to install the server certificate,
silently assuming that it were directly signed by a trusted
root certification authority (CA).
If your server certificate is not directly signed by the root
certification authority (root CA), then you are using intermediate
CA. In this case, you *MUST* (as per the TLS standard) provide *ALL*
intermediate certificates.
If you fail to provide intermediate certificates, clients can only
connect if the end user overrides/disables security warnings in
his/her software, and this disables the detection of eavesdroppers.
The intermediate CA certificates must be issued after the server's
certificate in proper order, that is:
first the intermediate CA cert that signed the servers' certificate,
then the intermedate CA cert that signed the previous intermediate CA,
and all the way back to the root CA cert (which you should omit).
You can optionally add the root CA certificate, but this is
redundant, as the client needs to have that installed anyways (see 3
below) in its store of trusted root certification authorities in
order to verify certificates that this root CA has signed.
For software that does not offer "chain certificate" options, but
that supports reading certificates in PEM format, it is
usually sufficient to concatenate all the certs in proper order
(again, from server to root).
3. Provide the *root* CA's certificate separately.
Provide the root CA's certificate in a place where your end users
will quickly and easily find it, or provide a link to it. Depending
on which mail software your clients use, it may not be pre-installed,
and users require this root CA to verify your SSL server certificate,
and possibly intermediate certificates.
This is particularly important if you're using local self-signed
certificates, as these are never preinstalled into end-users clients.
Your technical support team should have the finger prints of this
root CA readily available at least in MD5 and SHA1 formats and offer
to clients and be ready to answer client questions as to the
fingerprints (for verification) and installation in commonly used
clients.
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