On 2017-04-13 14:53:50 (+0200), Zbyszek Żółkiewski <t...@onefellow.com> wrote:
Wiadomość napisana przez Zbyszek Żółkiewski <t...@onefellow.com> w dniu
13.04.2017, o godz. 13:33:
Question: postfix 2.11: I have configured both RSA and ECDSA support
on the server (smtpd_tls_cert_file and smtpd_tls_eccert_file) and
support for ECDSA works great - however ECDSA is _never_ selected as
cipher for sending or receiving mails.
To check if it is properly configured i have disabled RSA support and
running server only with ECDSA and i confirm it works with gmail
servers for example (cipher ECDHE-ECDSA…).
Is there any way i can force postfix to first try ECDHE-ECDSA… and
then fallback to RSA? Note, i have tried custom tls_high_cipherlist
but no luck…
I think i found solution to this, by modifying default high list to:
tls_high_cipherlist = ECDSA:aNULL:-aNULL:ALL:!EXPORT:!LOW:!MEDIUM:+RC4:@STRENGTH
server now prefers ECDSA over RSA. Can someone cross-check if that is correct
solution for a problem and not pose any risk?
This poses an interoperability risk. You should carefully check your
maillogs for the ciphers you're excluding with this.
Try something like:
egrep "TLS connection established from.*with cipher" \
/var/log/maillog* | awk \
'{printf("%s %s %s %s\n", $12, $13, $14, $15)}' | \
sort | uniq -c | sort -n
This will give you a list of ciphers negotiated by occurence.
I would not recommend fiddling with the default TLS cipherlists unless
you have a very specific need.
Note that many senders will fall back to plain SMTP if they can't
negotiate TLS with you. I feel a little security is better than no
security at all.
Philip
--
Philip Paeps
Senior Reality Engineer
Ministry of Information
