Category: Email

Moving on from “should we do it?” (with the answer to most real-world scenarios being “yes, and as a bonus it can help block a lot of spambots“), here’s how to restrict several Internet services — Nginx, Apache, Postfix, and Dovecot — to TLSv1.2 or newer.

As usual, these are not complete guides for any of those servers; I’m assuming you already have them working fine (including TLS encryption), and just want to disable any TLS protocols lower than v1.2. (If you need to add TLS to a non-TLS server, see instructions for Nginx, Apache, Postfix, and Dovecot.)

Nginx:

In each virtual host’s server section — or, even better, if you’re using Let’s Encrypt, in /etc/letsencrypt/options-ssl-nginx.conf or its equivalent –, add the following (or replace any existing ssl_protocols entry):

ssl_protocols TLSv1.2 TLSv1.3;

Restart Nginx, and test it on SSL Labs. You should get something like this¹:

(Note: if you’re using a very old version of Nginx, it may not accept the “TLSv1.3” parameter and refuse to start; in such a case, remove it — or, better yet, upgrade your system. 🙂 )

Apache:

Similarly to Nginx, you can add one of the following to each Virtual Host, or to the global HTTPS configuration (typically in /etc/httpd/conf.d/ssl.conf), or, if using Let’s Encrypt, to /etc/letsencrypt/options-ssl-apache.conf :

SSLProtocol            +TLSv1.2 +TLSv1.3

or:

SSLProtocol             all -SSLv2 -SSLv3 -TLSv1 -TLSv1.1

Right now, they’ll do the same thing: allow TLSv1.2 and v1.3 only. Personally, I like the second version (which disables older protocols) better, for two reasons: 1) it’ll work even with some ancient Apache version that doesn’t recognize “TLSv1.3”, and 2) when future TLS versions are added, they’ll be enabled, making it more future-proof.

Again, you can test the new configuration on SSL Labs.

Postfix:

Add the following to /etc/postfix/main.cf (replacing any equivalent entries, if they exist).

smtp_tls_mandatory_protocols=!SSLv2, !SSLv3, !TLSv1, !TLSv1.1
smtpd_tls_mandatory_protocols=!SSLv2, !SSLv3, !TLSv1, !TLSv1.1
smtp_tls_protocols=!SSLv2, !SSLv3, !TLSv1, !TLSv1.1
smtpd_tls_protocols=!SSLv2, !SSLv3, !TLSv1, !TLSv1.1

After restarting Postfix, you can test its available protocols with Immuniweb’s SSL Security Test (specify something like yourhostname:25, or yourhostname:465 if not using STARTTLS).

Dovecot:

Add the following (or replace it if it exists) to your SSL configuration (typically /etc/dovecot/conf.d/10-ssl.conf):

ssl_min_protocol = TLSv1.2

Restart Dovecot, and test it on Immuniweb’s test (use something like yourhostname:143 , or yourhostname:993 if not using STARTTLS).

TLSv1.0, used not only for HTTPS but also for secure SMTP, IMAP, etc., has been around for a while — 1999, in fact –, and, while not “hopelessly broken” like SSL 2 and 3 are, there have been many successful attacks/exploits against it in the past two decades, and while most implementations (in terms of both servers and clients) have been patched to work around those, there may always be another one just around the corner. Also, TLSv1.0 came from a time where it was desirable to support as many different ciphers as possible, which means that, by default, some ciphers later found to be insecure may still be enabled.

Therefore, the end of TLSv1.0 has been talked about for a while, and, indeed, the main browser vendors — Google, Mozilla, Apple, and Microsoft — have announced that they’ll deprecate TLSv1.0 (and 1.1, which is little more than a revision of 1.0) by early 2020.

Now, until a few days ago I still had TLSv1.0 enabled on my servers (including Nginx, Postfix and Dovecot); my belief then being that some encryption was always better than no encryption, and besides we were mostly talking about public websites, that didn’t ask for user credentials or anything. Furthermore, any modern browsers/email clients default to secure protocols (TLSv1.2 or better), so there would never be, “in real life”, a situation where important/sensitive data (say, me logging on to my own mail server, or to one of my WordPresses) was being transmitted in an insecure way.

On the other hand… doesn’t the above mean that, “in real life”, nobody will really be using TLSv1.0 “legitimately”? Doesn’t it mean that setting up TLSv1.2 as a minimum will have no adverse consequences?

So I investigated it a little:

• in terms of browsers/clients, requiring TLSv1.2 or above means: 1) no Internet Explorer before version 11; 2) no Safari before 6.x; 3) no default Android browser before Android 4.4.x (but those users can still use Chrome or other non-default browsers). (You can check this out on SSL Labs). I don’t have a way to check email clients that old, but you can assume that anything released after the browsers just mentioned will be OK;
• in terms of my own server logs (mostly Nginx, but also Postfix, in terms of opportunistic encryption), apparently, only spambots and such are using TLSv1.0; any “legitimate” email servers use TLSv1.2 or 1.3. The same thing with my websites — any TLSv1.0 entries in my logs are always from some bot (and not one for a “well-known” search engine), not actual visitors. ¹

In short: you can probably move to requiring TLSv1.2 or above now (and, indeed, I’ve done so since a few days ago), unless you’re in some very peculiar situation — maybe your site needs to be accessed by users of a company whose IT policy includes not using software newer than 20 years old? Or you have a CEO who’s always refused to upgrade his beloved Symbian phone, but who’d flip out if he couldn’t access your company’s webmail with it? 🙂 Otherwise, my advice is: go TLSv1.2 and don’t look back.

However, and since this post is already quite long, I’ll leave the specific instructions for disabling TLS before 1.2 in Nginx, Apache, Postfix, and Dovecot for the next one…