I just fact-checked my statement about enclosing the list in single-quotes.  The man page for openssl ciphers specifies only a colon-separated list.  The enclosing in single quotes may just be community habit rather than an actual requirement.

-Andy


On 9/4/2019 5:04 AM, Andrew Swartz wrote:


On 9/4/2019 4:04 AM, Gary Bowling wrote:


That's excellent info Andy, many thanks for that!! I'm going to have to go back and read it about 10 times and possibly go read the referenced material too!


Questions, I think you are saying that I can put either 'HIGH:-SSLv3' in the tlsserverciphers file (and also in the dovecot.conf file) or I can do openssl ciphers -v 'HIGH:-SSLv3' > tlsserverciphers to put the full individual ciphers in the list?

Yes, that is correct. But if you use the individual ciphers, the format is the same (i.e. enclosed in single quotes, separated by colons).  I think the only advantage of using the individual ciphers is that you get to put them in the order you prefer.  The advantage of the macros is that if you upgrade to openssl 1.1.1 which has TLSv1.3, then you automatically get the new ciphers (without having to manually regenerate the cipher list).

Can I also put the full individual ciphers in the dovecot.conf? I probably wouldn't, but just curious.

Yes, you can (enclosed in single-quotes, colon separated).

I understand the info about the client/server negotiation. But then you talk about other servers, I suppose the server to server delivery over smtp. In that scenario, does the sending server send the list of ciphers and the receiving server match that to what it has and pick the first overlapping cipher to use?


With smpt server-to-server, the sending server is considered the "client" and the receiving server is considered the "server". Thus the two files: //var/qmail/control/tlsserverciphers/ for when you are the server and //var/qmail/control/tlsclientciphers/ for when you are the client/. /I have the former file as a link to the latter.  I see no reason to use a different list for outgoing mail (when you are the client) than for incoming mail (when you are the server).  Also, I link clientcert.pem to servercert.pem (for the same logic).

In the case of dovecot, if you specify a cipher list and also a min protocol, I'm assuming it won't use a cipher for something lower than the specified protocol, even if it's in the list? Maybe it doesn't offer up a cipher that doesn't meet the min protocol spec?


I believe that that is wrong.  If you configure as such, it will use the SSLv3 protocol with TLSv1.2 ciphers.  I'm only 80% sure of this answer.

For my server, I'm not sure I care whether I receive mail from a Centos 5 server. I realize many here are still using them, but it's been out of support for a while so it should be either patched or upgraded. I guess bottom line is I need to try something like the following:


tlsservercipher contains ''ECDHE:DHE:-SSLv3'   (without the quotes?)

Yes, but use a single-quote at beginning and end.

toaster.conf (in the /etc/dovecot/ dir) contains

ssl_cipher_list = ECDHE:DHE:-SSLv3

use a single-quote at beginning and end

ssl_min_protocol = TLSv1.2

Yes.  Those are good, secure settings.

Then I need to watch logs to see if I have problems. I'm guessing problems would show up in both the dovecot.log and the /var/log/qmail/smtp or /var/log/qmail/send logs.

Yes.

Thanks, Gary



-Andy



On 9/4/2019 1:46 AM, Andrew Swartz wrote:
Some background:

During the TLS negotiation, the client gives the server a list of ciphers which it supports, then from that list the server chooses which one to use.

The server's cipher list is a list, in order of preference, of the ciphers it will use (from the client's list).  If there is no overlap between what the client offers and what the server requires, then the connection fails.

The server dose not use the cipher list itself, but rather just passes the list to openssl when it requests establishment of the TLS connection.  Therefore essentially all servers/clients use the same format cipherlist.

The next thing to know is that the list can specify individual ciphers or macros like "TLSv1.2".  Most people do not specify individual ciphers but rather just use the macros.

There is no right or wrong for a cipher list, as the most appropriate list is the one which best meets your security requirements.

The cipherlist "builds" a list of ciphers:

'ALL' adds all of the ciphers (including those with no encrpytion).

'ALL:-SSLv2' adds all the ciphers and then removes all of the SSLv2 ciphers.

A reasonable cipherlist is:
'HIGH:-SSLv3'

If you want "perfect forward secrecy", try this:
'ECDHE:DHE:-SSLv3'
This will yield a subset of the TLSv1.2 ciphers which has the elliptic-curve diffie-hellman-ephemerel ciphers first and then standard diffie-hellman-ephemerel ciphers after that.

If you put that into openssl ciphers ( openssl ciphers -v 'HIGH:-SSLv3') you will note that you only get TLSv1.2 ciphers.  That is because an important concept is the difference between ciphers and protocols.  TLS 1.0 and 1.1 updated the protocol but added no new ciphers.  (you can confirm this by comparing "openssl ciphers -v 'SSLv3' | md5sum" to "openssl ciphers -v 'TLSv1' | md5sum"; you'll get an error if you do it with TLSv1.1 because it does not even have a list of ciphers).

But note that older servers, such as centos 5, will not be able to connect to you (if you use 'ECDHE:DHE:-SSLv3') because their old version of openssl does not support TLSv1.2.  In that case, for STARTTLS, it will fail, which will default to smtp transmission as cleartext.  SMTP is somewhat forgiving, as a failed STARTTLS connection will fall back to cleartext, whereas most other TLS protocols will fail to connect.

This is a segway into the related topic of "protocols".  Many servers (like dovecot) have separate a setting for "TLS cipherlist" and "TLS protocol".  The protocol is the algorithm for establishing the connection, and it is independent of the ciphers.  You should avoid the SSLv3 or TLSv1 protocols, as the these protocols have been found to have weaknesses in how they negotiate the connection (completely unrelated to the strength of the ciphers).

This manpage is a good explanation of all the macros and has examples at the end:
https://www.openssl.org/docs/man1.0.2/man1/ciphers.html

People with older versions of openssl (i.e. Centos 5) cannot do TLSv1.2 and will have no choice but to use ciphers/protocols with known weaknesses, and then hope that the other servers do not try to force a certain level of cipher/protocol.  That is not supposed to happen (per smtp/STARTTLS protocol), but I know for a fact that does:  I finally decided to upgrade from centos-5 because an important mail server started refusing to receive mail from mine, with a complaint about not accepting the SSLv3 ciphers.  I think it was Outlook Server, but I'm not sure.

Hope this helps.

-Andy

PS: Someone running the old version of openssl will need to put '-SSLv2" at the end of the cipherlist, whereas the newer version no longer supports it so it doesn't require removing it.  And NO ONE should be using the SSLv2 protocol, as hacking it is trivial.







On 9/3/2019 1:22 PM, CarlC Internet Services Service Desk wrote:
Actually, doing the openssl ciphers > /var/qmail/control/tlsservercipher is a starting point.

After I did that, I then ran my server through some tests. I happen to use OpenVAS [which tool you want to use to find insecure SSL connections is up to you]. It was able to tell me which ciphers to disable and why. Whichever product you use to test the SSL should be one that’s up to date [or can be brought up to date]. For example, I run the tests against my email server every week [for example, I test against port 25, 465 and 587]. In my case, I also use OpenVAS to test the HTTPS side as well.

If you’re using dovecot, you will want to also put the ssl_cipher_list in /etc/dovecot/dovecot.conf as well as the ssl_protocols list. This protects your IMAPS and POP3S protocols. Again, OpenVAS is set to run against those protocols as well.

Carl

*From:*Gary Bowling [mailto:g...@gbco.us]
*Sent:* Tuesday, September 03, 2019 03:35 PM
*To:* qmailtoaster-list@qmailtoaster.com
*Subject:* Re: [qmailtoaster] SSL Problem Dovecot

Thanks for that Carl. I'm running openssl-1.0.2k-16.el7_6.1.x86_64

Pretty much everything about my server is continuously updated stock Centos 7. Currently at CentOS Linux release 7.6.1810 (Core)

I do have epel installed, which updates some things and the qmt repo. That's it, and I'm a stickler for NOT installing anything that isn't done through yum and those repos. I've done this long enough to know that it's much easier to maintain, migrate to a new server, etc. is you're running everything in a managed way. So installing the repos and doing yum installs is pretty much the only way anything ever changes on my server, sans config files.

Would be very interested in knowing not only the proper tlsservercipher file for this type of server, but also how to create/recreate it if it's a command done from openssl. Looks like you can create it with the command.

openssl ciphers > /var/qmail/control/tlsservercipher

But what I'm reading is that your advice is to NOT do that due to security concerns. So what would you recommend?

Thanks, Gary

On 9/3/2019 3:28 PM, CarlC Internet Services Service Desk wrote:

    Your real problem is that this file is different based on which
    CentOS you’re on [or should I say, which openssl is loaded]. If you
    have CentOS 7, with openssl 1.0.2k, you can tune this file to
    include each cipher you want [the file can actually be 10+ lines
    long wrapped]. This is so you can remove all the “hacked” ciphers,
    especially to force your clients security to remain high. If your
    running openssl 0.9.x, you don’t get the newer TLS ciphers you need
    to be secure.

    Using the default is way too low, and if you do, you will where
    someone gets hacked over a ‘free’ WiFi connection [because you had
    SSL 3.0/TLS 1.0 on].

    Carl

    *From:*Gary Bowling [mailto:g...@gbco.us]
    *Sent:* Tuesday, September 03, 2019 02:58 PM
    *To:* qmailtoaster-list@qmailtoaster.com
<mailto:qmailtoaster-list@qmailtoaster.com>
    *Subject:* Re: [qmailtoaster] SSL Problem Dovecot

    So this may be an issue of the tlsserverciphers file. Some times
    it's interesting not knowing what your doing! haha

    I guess the question I have is.. What is the proper tlsserverciphers
    for a qmailtoaster with a letsencrypt certificate. If that even
    makes sense.

    And what is the proper way to actually do it. I've read multiple
    things on various forums, including here.

    One says to do:

    echo
"!EDH:!DHE:!RC4:!ADH:!DSS:HIGH:+AES128:+AES256-SHA256:+AES128-SHA256:+SHA:!3DES:!NULL:!aNULL:!eNULL"
     > /var/qmail/control/tlsserverciphers

    One says to do:

    openssl ciphers 'MEDIUM:HIGH:!SSLv2:!MD5:!RC4:!3DES' >
    /var/qmail/control/tlsserverciphers

    yet another says to create a sym link to the servercert.pem file.

    ln -sf /var/qmail/control/servercert.pem
    /var/qmail/control/tlsserverciphers

    I guess it has to do with how tight you want security to be and
    maybe tlsserverciphers can contain various forms of how to define
    that. Just looking for what "most" people would use for an up to
    date Centos 7 server.

    Thanks, Gary

    On 9/3/2019 11:04 AM, Gary Bowling wrote:

        I had to get a new cert for my server, which I installed
        yesterday. Now I'm having problems with certain clients logging
        in. I get the following error in the dovecot.log.

        TLS handshaking: SSL_accept() failed: error:1408A10B:SSL
        routines: ssl3_get_client_hello:wrong version number

        Any help would be appreciated.

        Thanks, Gary

        --         ____________________
        Gary Bowling
        ____________________

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