dovecot as proxy and verification of the backends certificate

[Timo Sirainen]

On 11 Oct 2015, at 20:04, Heiko Schlittermann <hs at schlittermann.de> wrote:
> 
> Hello,
> 
> I'm using a dovecot as proxy, connecting to one or more backends.
> The backends use X.509 certificates.
> 
> The proxy's passdb returns
> 
>    extra fields:
>    user=foo
>    proxy
>    host=backend1.<domain>
>    ssl=yes
>    nopassword=y
> 
> Thus the proxy connects to the backend but can't verify the backends
> certificate. 
> 
> The following comment suggests using ssl_client_ca_file for that.
> 
>    # Directory and/or file for trusted SSL CA certificates. These are used only
>    # when Dovecot needs to act as an SSL client (e.g. imapc backend). The
>    # directory is usually /etc/ssl/certs in Debian-based systems and the file is
>    # /etc/pki/tls/cert.pem in RedHat-based systems.
>    #ssl_client_ca_dir = 
>    #ssl_client_ca_file =
>    ssl_client_ca_file = /tmp/certs/ca-local.pem
> 
> But that does not work! Instead I've to use  ssl_ca
> 
>    # PEM encoded trusted certificate authority. Set this only if you intend to use
>    # ssl_verify_client_cert=yes. The file should contain the CA certificate(s)
>    # followed by the matching CRL(s). (e.g. ssl_ca = </etc/ssl/certs/ca.pem)
>    # ssl_ca = 
>    ssl_ca = </tmp/certs/ca-local.pem
> 
> Bug or feature? Mainly I'm asking because the comments do not indicate
> that I should have used ssl_ca for this type of operation (dovecot as a
> SSL client)

It's a missing feature. I updated http://wiki2.dovecot.org/PasswordDatabase/ExtraFields/Proxy about this. I'm thinking that once login-common code uses lib-ssl-iostream instead of the duplicated SSL code this gets fixed more or less automatically. Note sure if that'll happen for v2.3 or not.