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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?
Can I also put the full individual ciphers in the dovecot.conf? I
probably wouldn't, but just curious.
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?
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?
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?)
toaster.conf (in the /etc/dovecot/ dir) contains
ssl_cipher_list = ECDHE:DHE:-SSLv3
ssl_min_protocol = TLSv1.2
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.
Thanks, Gary
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:[email protected]]
*Sent:* Tuesday, September 03, 2019 03:35 PM
*To:* [email protected]
*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:[email protected]]
*Sent:* Tuesday, September 03, 2019 02:58 PM
*To:* [email protected]
<mailto:[email protected]>
*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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