Viktor Dukhovni <postfix-us...@dukhovni.org> writes: > On Wed, Oct 02, 2013 at 03:39:06PM -0400, Micah Anderson wrote: > >> From my understanding of the way postfix currently operates, there is no >> smtpd/stmp TLS setting that can be set that would provide a >> configuration that would result in a more 'hardened' configuration, >> without causing interoperability problems. If I am wrong, I'm very >> interested in knowing where. > > You get no benefit from hardening the Postfix SMTP server on port > 25 (tighter mandatory parameters on the submission port may work > for some). This has little to do with Postfix and everything to > do with the fact that SMTP servers accept messages from total > strangers (many of the clients don't support TLS at all).
Regarding tighter mandatory parameters on the submission port - any idea what these could reasonably be? For example, if I disable SSLv2/v3 am I going to cut off Outlook users? It would be nice if we had a good survey of clients that are still out there. I looked at some of my logs and found the following from a small sample over the last day: # zgrep 'TLS connection' /var/log/postfix.log* |grep 'TLS connection'|awk '{print $12, $15}' |sort | uniq -c |sort -nr 301849 TLSv1 DHE-RSA-AES256-SHA 109117 TLSv1 AES128-SHA 30032 TLSv1 RC4-SHA 14446 SSLv3 DHE-RSA-AES256-SHA 2532 TLSv1 AES256-SHA 1552 TLSv1 DHE-RSA-AES128-SHA 424 SSLv3 AES256-SHA 178 SSLv3 DHE-RSA-AES128-SHA 69 TLSv1 DES-CBC3-SHA 26 SSLv3 AES128-SHA 18 SSLv3 DES-CBC3-SHA 17 SSLv3 RC4-SHA but...the way this works: the server gets offered a list of ciphersuites from the client, and then the server picks a ciphersuite, so without knowing how the server picks its ciphersuites from the client, these results are not clear. The client is expected to offer ciphersuites in preferred order (https://tools.ietf.org/html/rfc5246#page-41), so if the server just chooses the client's earliest-listed ciphersuite that the server supports, then you have this situation. So that leaves us with the unanswered question of what does that translate into for restricting those paramters on the server? On a slightly other subject, I know that the smtps port 465 has already been reallocated as a port number, because it is considered deprecated, but I dont understand why. Providing a TLS-wrapped, from the beginning, port is better than offering STARTTLS. The STARTTLS offering is easily stripped by a MiTM, and I know that clients are supposed to handle that, but this seems incredibly brittle and prone to errors or manipulation, or minor configuration mistakes. XMPP threw out the TLS-wrapped connection as well, but they recently have brought it back because of this very reason.