The site weakdh.org explains how to fix postfix against the weak Diffie-Hellman attack called "logjam".
But don't I have to fix courier too? Or do I have to migrate to dovecot to be logjam-safe?
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I found this blog post that explains it quite well.
To speed this up, first check, if you already have good parameters in
/etc/ssl/certs/dhparams.pem check with
openssl dhparam -text -noout -in /etc/ssl/certs/dhparams.pem
if so copy them into
cp -a /etc/ssl/certs/dhparams.pem /etc/courier/dhparams.pem
otherwise generate with
openssl dhparam -out /etc/courier/dhparams.pem 4096
Courrier version 4.15 removes the TLS_DHCERTFILE parameter from imap, and pop3d configuration files. DH parameters, and DH parameters only, get read from the new TLS_DHPARAMS file (and the other functionaly of TLS_DHCERTFILE, for DSA certificates, is merged into TLS_CERTFILE). After upgrading, run the mkdhparams script to create a new TLS_DHPARAMS file.
So check your installed version with
apt-cache show courier-imap-ssl|grep Version
If you have at least Version 4.15, now edit
/etc/courier/imapd-ssl and set
check the connection with openssl version 1.0.2a.
openssl s_client -host <yourhost.org> -port 993
When using courier, you need to make sure that Diffie-Hellman parameters in
/etc/courier/dhparams.pem are generated with more than the default 768 bit. I guess 2048 or 4096 bit should do.
Instead of using
mkdhparams to generate
dhparams.pem (with just 768 bit by default!) you could do it that way:
openssl dhparam -out /etc/courier/dhparams.pem 2048 service courier-mta-ssl restart
Here's some info (in German) and some further reading about how to mitigate the Logjam-attack on Courier-MTA.