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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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