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<div class="moz-cite-prefix">>> because common configurations
of fail2ban [...] absolutely will ban your IP [...] after multiple
connection attempts and no successful mail transfer<br>
<br>
I would consider this a heavy misconfiguration. Please explain to
me what incomplete SMTP connections have in common with spammers,
virus/worm/trojan compromised hosts or open relay searching bots.
Those bad senders WANT to <u>successfully</u> deliver mails to
you. They will never abort the connection on purpose. For example:
bots which search for open relays ALWAYS try to send mails with a
foreign sender and recipient domain. That's how you discover them.
But as suggested, the Atlas SMTP check should not send E-Mails at
all, not even send MAIL FROM: or RCPT TO: command.<br>
<br>
You will not achieve mitigation of inbound spam/malware/phishing
traffic by blocking IP addresses of hosts from incomplete SMTP
sessions. Usually, incomplete SMTP sessions indicate a
misconfiguration. For example: forced TLS enabled, but expired
certificate or no matching cipher suites. But that is no reason to
ban the senders! I think you have to go a little bit deeper in
your logs and consider why mailtransfer was not successfull,
before blocking ip addresses.<br>
<br>
I am no expert for fail2ban, but as far is i know, i searches for
failed login attempts. So that affects mostly authenticated SMTP
connections (client E-Mail submission on tcp/465 or tcp/587),
right? This should not concern us here.<br>
<br>
I work with enterprise mailgateway solutions for years (mostly
Proofpoint), but i have never encountered what you describe.<br>
<br>
Reject or throttle because of too much connections at the same
time? Yes.<br>
Reject or throttle because of too much non-existing recipient
adresses? Yes.<br>
Reject or throttle because both sender and recipient domain is
foreign? Yes.<br>
Reject or throttle because of bad reputation (known spammer or TOR
exit node ip address)? Yes.<br>
<br>
But not because of incomplete SMTP connections. From my point of
view, I can not confirm that this common behaviour.<br>
<br>
BR,<br>
Simon<br>
<br>
<br>
On 20.09.22 19:22, Eric Kuhnke wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAB69EHgBqegtm1YO-e2fTfrwkExzMYAemBuoubZGNLufSfW5bQ@mail.gmail.com">
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<div dir="ltr">
<div>I would discourage anyone from relying upon the data from
"probing" the MX and SMTP daemons for a domain name no matter
what port they run on, because common configurations of
fail2ban used with postfix and others absolutely will ban your
IP at the host operating system level (iptables or other)
after multiple connection attempts and no successful mail
transfer or authentication. <br>
</div>
<div><br>
</div>
<div>a probe of smtpd will look not much different from the many
things out there on the internet already maliciously probing
smtpd trying to find open relays.</div>
<div><br>
</div>
<div><br>
</div>
<div><br>
</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Tue, 20 Sept 2022 at 09:22,
Simon Brandt via ripe-atlas <<a
href="mailto:ripe-atlas@ripe.net" moz-do-not-send="true"
class="moz-txt-link-freetext">ripe-atlas@ripe.net</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div>
<div>Hi Michel,<br>
<br>
Currently, HTTP and SSL are separate measurements. But for
SMTP it will probably not work this way... Encryption is
not mandatory for SMTP and thus negotiated between client
and server every time on port 25. Ports 465 and 587 are
used for Client Email Submission. You could run some
measurements for these ports as well, but for the
beginning, i would focus on server2server communication.<br>
<br>
So here's what i think:<br>
<br>
What we could measure:<br>
- General reachability/availability of the e-mail service<br>
- Response time of the remote server: time between
connection establish and first SMTP response (220 service
ready)<br>
- Which enhanced status codes are offered by the server?<br>
- Forward/Reverse DNS matching?<br>
- SMTP banner matching the DNS name?<br>
- if not: what is it?<br>
- Does the remote server offer encryption (250-STARTTLS)<br>
- Which cipher settings are offered by the server (SSL/TLS
Version, Key Exchange Algorithms, Encryption Algorithms,
Hashing Algorithms)<br>
- alternatively: Which cipher setting has been
negotiated between probe and server?<br>
- Can we successfully establish a TLS connection?<br>
- Certificate Check: is the server-certificate valid and
does it match the hostname?<br>
- Forced Encryption: MTA-STS available and 'enforced' or
'testing' or 'report'?<br>
- Forced Authentication: DANE available and check
successfull?<br>
<br>
<br>
What we should not do:<br>
- send MAIL FROM: command<br>
- send RCPT TO: command<br>
- send DATA command<br>
- measure e-mail delivery/roundtrip time, etc.<br>
- Sending e-mails at all<br>
<br>
The Atlas probe should quit the connection after the data
is collected. An actual e-mail should not be send. The
target mailserver would count this session as "incomplete"
or "aborted" which is totally fine. If someone would want
to monitor what happens after a mailserver has
successfully accepted a testmail, he should better use a
monitoring service/solution. We measure the INTERnet, not
what comes after (Intra) :)<br>
<br>
I don't expect any "spam" problems. Since the Atlas probes
wouldn't send e-mails, there's nothing a spamfilter could
analyze. The only thing that could theoretically happen,
is that the probes ip addresses are flagged as bad by
services like Spamhaus etc. and thus be listed on
DNSBL/IPBL, but i really don't see a reason why that
should happen. There wouldn't be any activity originating
from the probes which could be classified as bad. IP
addresses are normally only blacklisted, if they send
unwanted mails like spam. Also: there are a lot of "check
you mailserver" or "check your SSL/TLS" websites. The RIPE
Atlas probes would behave the same way. No big deal.<br>
<br>
Maybe we can think of an "extended" SMTP measurement where
RIPE Atlas sends actual e-mails, but that would require
(in my opinion), that the person who is creating the
measurement somehow provides proof, to be the owner of the
target mailserver.<br>
<br>
<br>
BR,<br>
Simon<br>
<br>
<br>
On 15.09.22 12:03, Michel Stam wrote:<br>
</div>
<blockquote type="cite"> Hello Simon,
<div><br>
</div>
<div>Thank you for the suggestion.</div>
<div><br>
</div>
<div>I have a couple of questions to get a better idea:</div>
<div>
<ul>
<li>Can you maybe describe what a SMTP measurement
would look like?</li>
<ul>
<li>Simple EHLO/HELO</li>
<li>Sending an email to a designated address (which
would then validate that the SMTP server is
capable of relaying etc.)</li>
</ul>
<li>How would DNSBL or other spam prevention
techniques fit into this? </li>
<li>What would the result be? </li>
<ul>
<li>Delay until mail received</li>
<li>Response time by the actual mail server</li>
<li>Using the Received: headers to get a
“traceroute” like result.</li>
</ul>
<li>What about the more uncommon ports such as 565
(SMTP+SSL/TLS) or 587 (mail submission port).</li>
<li>How can we prevent this implementation from having
RIPE Atlas be identified as a spam bot network?</li>
</ul>
</div>
<div>
<div> </div>
<div>Best regards,</div>
<div><br>
</div>
<div>Michel</div>
<div><br>
<blockquote type="cite">
<div>On 3 Sep 2022, at 14:48, Simon Brandt via
ripe-atlas <<a
href="mailto:ripe-atlas@ripe.net"
target="_blank" moz-do-not-send="true"
class="moz-txt-link-freetext">ripe-atlas@ripe.net</a>>
wrote:</div>
<br>
<div>
<div> Hello,<br>
<br>
i'd like to start a discussion about having a
RIPE Atlas SMTP measurements.<br>
First of all: yes, i know there's a big obstacle
for such a measurement type. A lot of probes are
deployed using enduser internet-connections like
dsl, cable, etc. with dynamic/eyeball IP
addresses. Those IP spaces are usually blocked
by SMTP servers as approach to reduce spam
mails. For Example: by using blocklists like
Spamhaus PBL. So a proper SMTP measurement
wouldn't work.<br>
<br>
BUT we could have an easy way for RIPE Atlas
probe hosters to signalize, that their probe is
eligible for SMTP measurements:<br>
<br>
Step 1: enable "Simple DNS Entry"<br>
Step 2: create a matching reverse DNS record for
the probes IP address<br>
<br>
Everybody who is able so configure a reverse DNS
record for his probes IP address, is most likely
using a non-dynamic/non-home ip address space
e.g. a datacenter or office network. If an ISP
provides the option for his customers to
configure a reverse DNS record, it's most likely
a "business-customer" subnet which can be used
to run mailservers. After Step 1+2 are done, the
RIPE Atlas platform would easily be able to
verify if forward-confirmed reverse DNS is
successful, and if so, automatically enable that
probe for SMTP measurements. Alternatively:
probe hosters choose their own Forward-confirmed
reverse DNS name and submit it on the RIPE Atlas
website.<br>
<br>
Also: if we would have STMP measurements,
forward-confirmed reverse DNS should be
mandatory for anchors, or is it already?<br>
<br>
Why should we have SMTP measurements?<br>
<br>
Besides general reachability checks, we could
also evaluate SMTP response codes. But the most
important thing for me is this: the SMTP
protocol is old. Very old. From a security point
of view, it's absolutely outdated. Most security
features have been added years after the initial
RfC. Thus, those security features are optional.
Mandatory SMTP encryption is not provided by the
SMTP RfC. So both sides have to signalize, that
they are capable of encryption using the
STARTTLS command. An attacker
(man-in-the-middle) can perform a downgrade
attack by suppressing the STARTTLS command. So
both sides are forced to fallback and
communicate unencrypted. RIPE Atlas would be a
really good tool to identify such attacks, by
monitor/measure the (enhanced) status codes of a
target.<br>
<br>
But there's more!<br>
I see a two-sided model here. Either use the
RIPE Atlas SMTP measurements to monitor/measure
your own mailserver by alot of other RIPE probes
out there, OR probe hosters could run SMTP
measurements originating from their own probe to
find out, if their own IP address is currently
blocked by other mailservers.<br>
<br>
<br>
What do you think?<br>
<br>
<br>
BR,<br>
Simon<br>
</div>
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