>> because common configurations of fail2ban [...] absolutely will ban your IP [...] after multiple connection attempts and no successful mail transfer

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 successfully 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.

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.

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.

I work with enterprise mailgateway solutions for years (mostly Proofpoint), but i have never encountered what you describe.

Reject or throttle because of too much connections at the same time? Yes.
Reject or throttle because of too much non-existing recipient adresses? Yes.
Reject or throttle because both sender and recipient domain is foreign? Yes.
Reject or throttle because of bad reputation (known spammer or TOR exit node ip address)? Yes.

But not because of incomplete SMTP connections. From my point of view, I can not confirm that this common behaviour.

BR,
Simon


On 20.09.22 19:22, Eric Kuhnke wrote:
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.

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.




On Tue, 20 Sept 2022 at 09:22, Simon Brandt via ripe-atlas <ripe-atlas@ripe.net> wrote:
Hi Michel,

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.

So here's what i think:

What we could measure:
- General reachability/availability of the e-mail service
- Response time of the remote server: time between connection establish and first SMTP response (220 service ready)
- Which enhanced status codes are offered by the server?
- Forward/Reverse DNS matching?
- SMTP banner matching the DNS name?
    - if not: what is it?
- Does the remote server offer encryption (250-STARTTLS)
- Which cipher settings are offered by the server (SSL/TLS Version, Key Exchange Algorithms, Encryption Algorithms, Hashing Algorithms)
    - alternatively: Which cipher setting has been negotiated between probe and server?
- Can we successfully establish a TLS connection?
- Certificate Check: is the server-certificate valid and does it match the hostname?
- Forced Encryption: MTA-STS available and 'enforced' or 'testing' or 'report'?
- Forced Authentication: DANE available and check successfull?


What we should not do:
- send MAIL FROM: command
- send RCPT TO: command
- send DATA command
- measure e-mail delivery/roundtrip time, etc.
- Sending e-mails at all

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) :)

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.

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,
Simon


On 15.09.22 12:03, Michel Stam wrote:
Hello Simon,

Thank you for the suggestion.

I have a couple of questions to get a better idea:
  • Can you maybe describe what a SMTP measurement would look like?
    • Simple EHLO/HELO
    • Sending an email to a designated address (which would then validate that the SMTP server is capable of relaying etc.)
  • How would DNSBL or other spam prevention techniques fit into this? 
  • What would the result be? 
    • Delay until mail received
    • Response time by the actual mail server
    • Using the Received: headers to get a “traceroute” like result.
  • What about the more uncommon ports such as 565 (SMTP+SSL/TLS) or 587 (mail submission port).
  • How can we prevent this implementation from having RIPE Atlas be identified as a spam bot network?
 
Best regards,

Michel

On 3 Sep 2022, at 14:48, Simon Brandt via ripe-atlas <ripe-atlas@ripe.net> wrote:

Hello,

i'd like to start a discussion about having a RIPE Atlas SMTP measurements.
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.

BUT we could have an easy way for RIPE Atlas probe hosters to signalize, that their probe is eligible for SMTP measurements:

Step 1: enable "Simple DNS Entry"
Step 2: create a matching reverse DNS record for the probes IP address

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.

Also: if we would have STMP measurements, forward-confirmed reverse DNS should be mandatory for anchors, or is it already?

Why should we have SMTP measurements?

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.

But there's more!
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.


What do you think?


BR,
Simon
--
ripe-atlas mailing list
ripe-atlas@ripe.net
https://lists.ripe.net/mailman/listinfo/ripe-atlas


--
ripe-atlas mailing list
ripe-atlas@ripe.net
https://lists.ripe.net/mailman/listinfo/ripe-atlas