<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body>
<div class="moz-cite-prefix">Hi Michel,<br>
<br>
<blockquote type="cite">That would would indeed mean a combination
of TCP and SSL measurement to achieve all 3 required functions.
Is it problematic if the result comes from multiple steps? If
so, can you explain how?</blockquote>
<blockquote type="cite">
<div class="">The intent of the measurement would be to validate
whether an SMTP server is:</div>
<ul class="MailOutline">
<li class="">reachable</li>
<li class="">responsive</li>
<li class="">capable of secured transmissions</li>
</ul>
</blockquote>
<br>
First, let's define the testmethod. In my opinion:<br>
<br>
- reachable<br>
3-way TCP Handshake with target on tcp/25 successful?<br>
<br>
- responsive<br>
when establishing and SMTP connection, does the smtp-server
signalize readiness of the service (SMTP 220)?<br>
<br>
- capable of secured transmissions<br>
when sending an EHLO, the server will tell us his features.
250-STARTTLS should be there.<br>
<br>
<br>
For all three checks, it's the easiest to use netcat.<br>
<br>
Reachability:<br>
<blockquote type="cite">$ nc -vz mahimahi.ripe.net 25<br>
mahimahi.ripe.net [193.0.19.114] 25 (smtp) open</blockquote>
<br>
Note, that we have not measured the response time. That's why you
wanted to use TCP Traceroute, right? We can also go with TCP
Traceroute here.<br>
<br>
<br>
Responsiveness (wait for 220):<br>
<blockquote type="cite">$ nc -C mahimahi.ripe.net 25<br>
220 mahimahi.ripe.net ESMTP Sat, 01 Oct 2022 15:25:22 +0200<br>
quit<br>
221 mahimahi.ripe.net closing connection<br>
</blockquote>
<br>
You might want to use the -w option here, to specify a timeout.<br>
<br>
<br>
capable of secured transmissions (send EHLO and check response):<br>
<blockquote type="cite">$ nc -C mahimahi.ripe.net 25<br>
220 mahimahi.ripe.net ESMTP Sat, 01 Oct 2022 15:54:04 +0200<br>
EHLO p123456.probes.atlas.ripe.net<br>
250-mahimahi.ripe.net Hello p123456.probes.atlas.ripe.net
[123.123.123.123]<br>
250-SIZE 52428800<br>
250-8BITMIME<br>
250-ETRN<br>
250-PIPELINING<br>
250-PIPE_CONNECT<br>
250-STARTTLS<br>
250 HELP</blockquote>
<br>
<br>
To check the Certificate validity and if encryption is indeed
successful, we can use OpenSSL:<br>
<blockquote type="cite">$ openssl s_client -starttls smtp -connect
mahimahi.ripe.net:25<br>
</blockquote>
(output to long, i stripped it)<br>
<br>
<br>
<br>
<blockquote type="cite">You’re correct, the current SSL
measurement does not support any form of STARTTLS, this is
something that would have to be considered for implementation. I
assume, much like with SMTP, similar cases could be made for
IMAP4/POP3 or XMPP.</blockquote>
Yeah, as far as i know, STARTTLS is also used for imap, pop3, xmpp
and ftp (ftps, not sftp).<br>
<br>
<br>
As i said before, there are additional e-mail security features
that we could check. There's MTA-STS, where we would have to
perform a combination of HTTP and SSL check. Also, there is DANE,
where we would perform a combination of DNS and SSL check
(including DNSSEC). But DANE can be used for other protocols as
well, not only SMTP. DNSSEC/DANE are perhaps worth a separate
check type.<br>
<br>
Last but no least, we should check for Forward-confirmed reverse
DNS and matching SMTP banner, which is a combination of DNS and
netcat check. This would be a reasonable part of every smtp
measurement.<br>
<br>
<br>
Please note, that the creator of the measurement should either
specify the exact mailserver FQDN, or the target Domain. In the
latter case, an MX record lookup has to be performed before the
measurement starts, not while the measurement is running.
Otherwise it could cause credit consumption trouble, if suddenly
multiple mx records are added the domain, while the measurement is
running.<br>
<br>
Oh, and please make the SMTP measurement IPv6 capable :)<br>
<br>
<br>
BR,<br>
Simon<br>
<br>
<br>
<br>
On 29.09.22 11:50, Michel Stam wrote:<br>
</div>
<blockquote type="cite"
cite="mid:2D29819A-6B48-44F8-A15A-2F04B2912F29@ripe.net">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
Hi Simon,
<div class=""><br class="">
</div>
<div class="">
<blockquote type="cite" class="">
<div class="">
<div class="moz-cite-prefix">>Can we achieve the first 2
items of this measurement by doing a TCP traceroute on
port 25?<br class="">
I would say no. Using TCP Traceroute, you can may check
for reachability/responsiveness of the host, but not the
actual service (smtp).<br class="">
</div>
</div>
</blockquote>
<div class=""><br class="">
</div>
<div class="">That would would indeed mean a combination of TCP
and SSL measurement to achieve all 3 required functions. Is it
problematic if the result comes from multiple steps? If so,
can you explain how?</div>
<div class=""><br class="">
</div>
<div class="">I just noticed that the SSL measurement offers a
time to connect, response time, certificates as well as SSL
alerts which may be leveraged, see here: <a
href="https://atlas.ripe.net/docs/apis/result-format/#version-4610"
class="moz-txt-link-freetext" moz-do-not-send="true">https://atlas.ripe.net/docs/apis/result-format/#version-4610</a>,
under "Version 4610 TLS (SSL) GET Cert”. TCP traceroute may
not be necessary in this case, although I understand it is
typically used to determine service availability.</div>
<br class="">
<blockquote type="cite" class="">
<div class="">
<div class="moz-cite-prefix">>Does the SSL measurement
cover the intended use cases?<br class="">
I would say no. Correct me if am am wrong. Usually (for
example HTTPS or LDAPS) the SSL/TLS encryption starts
right after the TCP 3-way Handshake was successfull. For
SMTP, that doesn't work. That's because regular SMTP
communication starts first, so both sides can negotiate if
SSL/TLS encryption is possible (via Enhanced SMTP Status
Codes). However, as far as i know, OpenSSL <u class="">does</u> support
SMTP and STARTTLS. So you could probably modify the
existing SSL measurement.<br class="">
<br class="">
Keep in mind that there's also MTA-STS and DANE, which are
really enhancing SMTPs security. A dedicated SMTP
measurement would be a good thing.<br class="">
</div>
</div>
</blockquote>
</div>
<div class=""><br class="">
<div>You’re correct, the current SSL measurement does not
support any form of STARTTLS, this is something that would
have to be considered for implementation. I assume, much like
with SMTP, similar cases could be made for IMAP4/POP3 or XMPP.</div>
<div>I would like to understand if there are particulars you are
looking for that need to be considered outside of STARTTLS
support?</div>
<div><br class="">
</div>
<div>Regards,</div>
<div><br class="">
</div>
<div>Michel</div>
<div><br class="">
</div>
<div><br class="">
</div>
<div>
<blockquote type="cite" class="">
<div class="">On 23 Sep 2022, at 17:08, <a
href="mailto:ripe.net@toppas.net"
class="moz-txt-link-freetext" moz-do-not-send="true">ripe.net@toppas.net</a>
wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<meta http-equiv="Content-Type" content="text/html;
charset=UTF-8" class="">
<div class="">
<div class="moz-cite-prefix">Hi Michel,<br class="">
<br class="">
>Are we monitoring the Internet or monitoring a
service using the proposed SMTP measurement?<br
class="">
First of all, we are monitoring the service of a
specific target. Same as http or ntp measurements,
just another protocol. But we also monitor the
Internet. Using an SMTP measurement, we could identify
censorship or discover Man-in-the-middle attacks
(downgrade attack by suppressing the STARTTLS
command).<br class="">
<br class="">
>Can we achieve the first 2 items of this
measurement by doing a TCP traceroute on port 25?<br
class="">
I would say no. Using TCP Traceroute, you can may
check for reachability/responsiveness of the host, but
not the actual service (smtp).<br class="">
<br class="">
>Does the SSL measurement cover the intended use
cases?<br class="">
I would say no. Correct me if am am wrong. Usually
(for example HTTPS or LDAPS) the SSL/TLS encryption
starts right after the TCP 3-way Handshake was
successfull. For SMTP, that doesn't work. That's
because regular SMTP communication starts first, so
both sides can negotiate if SSL/TLS encryption is
possible (via Enhanced SMTP Status Codes). However, as
far as i know, OpenSSL <u class="">does</u> support
SMTP and STARTTLS. So you could probably modify the
existing SSL measurement.<br class="">
<br class="">
Keep in mind that there's also MTA-STS and DANE, which
are really enhancing SMTPs security. A dedicated SMTP
measurement would be a good thing.<br class="">
<br class="">
BR,<br class="">
Simon<br class="">
<br class="">
<br class="">
<br class="">
On 23.09.22 16:04, Michel Stam wrote:<br class="">
</div>
<blockquote type="cite"
cite="mid:B5640B9C-ADC1-496C-8145-4D8C48EA8D5D@ripe.net"
class="">
<meta http-equiv="Content-Type" content="text/html;
charset=UTF-8" class="">
Hi everyone,
<div class=""><br class="">
</div>
<div class="">Great that this request sparked a good
discussion on the merits of a measurement, as well
as its potential impact on servers around the world.
Good to see this!</div>
<div class=""><br class="">
</div>
<div class="">So I’m going to do a quick recap here,
hoping that I capture the intent and the concerns
correctly. Please correct me if I err.</div>
<div class=""><br class="">
</div>
<div class="">The intent of the measurement would be
to validate whether an SMTP server is:</div>
<div class="">
<ul class="MailOutline">
<li class="">reachable</li>
<li class="">responsive</li>
<li class="">capable of secured transmissions</li>
</ul>
<div class=""><br class="">
</div>
</div>
<div class="">The concern is that such a check would
trigger one of a variety of anti spam measures in
place around the world, and/or cause undue traffic
to SMTP server operators.</div>
<div class=""><br class="">
</div>
<div class="">With this in mind, I am wondering: </div>
<div class="">
<ul class="MailOutline">
<li class="">Are we monitoring the Internet or
monitoring a service using the proposed SMTP
measurement? </li>
<li class="">Can we achieve the first 2 items of
this measurement by doing a TCP traceroute on
port 25?</li>
<li class="">Does the SSL measurement cover the
intended use cases?</li>
<ul class="">
<li class=""> Is it worth exploring STARTTLS
support as an extension and what would the
implications be?</li>
</ul>
</ul>
<div class=""><br class="">
</div>
</div>
<div class="">Have a good weekend!</div>
<div class=""><br class="">
</div>
<div class="">Best regards,</div>
<div class=""><br class="">
</div>
<div class="">Michel</div>
<div class="">
<div class=""><br class="">
<blockquote type="cite" class="">
<div class="">On 21 Sep 2022, at 00:11,
Avamander <<a
href="mailto:avamander@gmail.com"
class="moz-txt-link-freetext"
moz-do-not-send="true">avamander@gmail.com</a>>
wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<div dir="ltr" class="">> Making arguments
based upon extreme cases, assumptions, or
potential-for-collateral-damage is not
scientific. "I know one that even [...]"
Anecdotal evidence isn't scientific.
<div class=""><br class="">
</div>
<div class="">From the perspective of your
previous sentences that's kinda humorous.
"To avoid unnecessary costs incurred from
disruption of service, excessive traffic,
annoyances using up *my* time, and
countless other reasonable rationale from
*my* point of view." Because sure, a few
(hypothetical RIPE probe) connections are
exactly that, zero exaggeration, right?</div>
<div class=""><br class="">
</div>
<div class="">In the end such
fail2ban-fueled (or similar) behaviour I
initially addressed, is exactly a
non-scientific extreme-case
assumption-based approach. There are
better and even more standard ways. </div>
<div class=""><br class="">
</div>
<div class="">Crutch solutions out in the
wild shouldn't be a showstopper for
measuring the ecosystem. (That is already
quite neglected)</div>
<div class=""><br class="">
</div>
<div class="">> What _objective_
risk/benefit analysis are you basing your
opinions upon?<br class="">
<br class="">
And you? What's the implication here about
systems being as trigger-happy as
previously described?</div>
<div class=""><br class="">
</div>
<div class="">Because sure, at some point
rate limits make total sense, but
certainly not at the point where it would
ban any potential RIPE probes.</div>
<div class=""><br class="">
</div>
<div class="">> Are you a systems
administrator?</div>
<div class=""><br class="">
</div>
<div class="">Let's not get into such
measuring contests, even if it is the RIPE
Atlas mailing list.</div>
</div>
<br class="">
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Tue,
Sep 20, 2022 at 11:42 PM Paul
Theodoropoulos via ripe-atlas <<a
href="mailto:ripe-atlas@ripe.net"
class="moz-txt-link-freetext"
moz-do-not-send="true">ripe-atlas@ripe.net</a>>
wrote:<br class="">
</div>
<blockquote class="gmail_quote"
style="margin:0px 0px 0px
0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">
<div class=""> On 9/20/2022 10:45 AM,
Avamander wrote:<br class="">
<blockquote type="cite" class="">
<div dir="ltr" class="">Great to hear
it works for you, but the potential
unfortunate collateral from such a
blanket action is not really
RIPE Atlas' problem. There are more
fine-grained methods against
bruteforce attempts and open relay
probes, than triggering on a few
connections.</div>
</blockquote>
What _objective_ risk/benefit analysis
are you basing your opinions upon? Are
you a systems administrator? My
responsibility is to avoid unnecessary
costs incurred from disruption of
service, excessive traffic, annoyances
using up *my* time, and countless other
reasonable rationale from *my* point of
view. <br class="">
<br class="">
You suggest that it is "not really RIPE
Atlas' problem". That's very true. And
it is not really my problem if I bounce
yoinky, pointless probes of my server,
and ruthlessly block them from
contacting my server ever again. My
server, my choice, my wallet, nobody's
business but my own.<br class="">
<blockquote type="cite" class="">
<div dir="ltr" class="">
<div class="">Some webmasters ban
IP's for simply visiting a domain,
I know one that even dispatches an
email to your ISP's abuse@ address
upon visit. Should RIPE Atlas
probes then not probe any HTTP
servers? The answer is obviously
no, they shouldn't care.</div>
</div>
</blockquote>
Making arguments based upon extreme
cases, assumptions, or
potential-for-collateral-damage is not
scientific. "I know one that even [...]"
Anecdotal evidence isn't scientific.<br
class="">
<br class="">
Note, I run a probe myself. I don't
block any RIPE Atlas traffic on my
separate servers hosted on AWS, Oracle,
and GCE. <br class="">
<br class="">
<div class="">-- <br class="">
Paul Theodoropoulos<br class="">
<a href="https://www.anastrophe.com/"
target="_blank" class=""
moz-do-not-send="true">anastrophe.com</a></div>
</div>
-- <br class="">
ripe-atlas mailing list<br class="">
<a href="mailto:ripe-atlas@ripe.net"
target="_blank"
class="moz-txt-link-freetext"
moz-do-not-send="true">ripe-atlas@ripe.net</a><br
class="">
<a
href="https://mailman.ripe.net/"
rel="noreferrer" target="_blank"
class="moz-txt-link-freetext"
moz-do-not-send="true">https://mailman.ripe.net/</a><br
class="">
</blockquote>
</div>
-- <br class="">
ripe-atlas mailing list<br class="">
<a href="mailto:ripe-atlas@ripe.net"
class="moz-txt-link-freetext"
moz-do-not-send="true">ripe-atlas@ripe.net</a><br
class="">
<a class="moz-txt-link-freetext"
href="https://mailman.ripe.net/"
moz-do-not-send="true">https://mailman.ripe.net/</a><br
class="">
</div>
</blockquote>
</div>
<br class="">
</div>
<br class="">
<fieldset class="moz-mime-attachment-header"></fieldset>
</blockquote>
<br class="">
</div>
</div>
</blockquote>
</div>
<br class="">
</div>
</blockquote>
<br>
</body>
</html>