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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