Sunday, December 30, 2018

Making a Mailserver - Spam Blocking, Revisited

In an earlier post I described implementing spamassassin with exim4. The information there still holds true, but the technique of simply implementing "spamd" has not been enough to hold back spammers who have my email address. My email address was harvested in both the LinkedIn and hacks. In the last years the targeted spam has increased noticeably.

I started to train my desktop email client to pick out spam and it does a decent job, so I weathered the deluge for some time. However, plenty of spam still gets through and when my desktop email client is not open I have plenty of junk to pick through on my mobile devices.

Finally, I've taken the time to sharpen up my exim4 defenses.


In the rest of this post, I'll be answering these questions:
  1. Does spamassassin support Domain Name System Blacklists (DNSBL)?
  2. How do I integrate blocklist (DNSBL) checks in exim4?
  3. How do I block hosts that are not really mailservers?
  4. How do I block on reverse DNS failures?
  5. How do I allow specific hosts to skip checking by exim4 and spamd?
  6. How do I verify these measures are working properly?
The answers to these questions are straightforward, but took quite a bit of research time and verification. Point 6 isn't separately addressed; each section that follows will talk about the ways that I verified the spam mitigations were working.

spamassassin and exim4

It turns out that spamassassin (spamd) supports DNSBL by default. I actually discovered this after going through the process of integrating checking in exim4. The difference is that you can kill spam outright with exim4 integration, but spamd will use it as part of the point calculation when determining how 'spammy' an email is.

There's a downside then to spamd: while the blocklist from spamhaus has a very high accuracy, being on the blocklist doesn't guarantee that spamd will calculate enough points to junk the email.

It's possible to change the point value of addresses that are on DNSBLs by adding a "score URIBL_BLACK <value>" line to the spamassassin config file. You also need to ensure that perl's Net::DNS is installed. To check if that is installed, try "perl -MNet::DNS -e 1" and the command should execute with no errors.

One unanswered question I have is whether the perl module takes care of the DNS lookup and server to use, or whether your server needs to have a a spamhaus friendly DNS server in /etc/resolv.conf - see the spamassassin DNSBL discussion below.

Verify spamassassin is using DNSBL

To verify whether DNSBL is being used by spamassassin, check the log for the presence of URIBL_BLACK. This could be the syslog logfile depending on the system, not the exim4 logs:


This means that this particular message was found in a blocklist. Of course you are not going to see that present on every email that is checked.

Integrate the spamhaus blocklist into exim4

The instructions that I'll provide here are not specific to spamhaus, it's just the service I decided to try. It's free to a point, someone like me with relatively low volumes of email will be able to use the service unimpeded. There is a performance hit on your own server while doing the DNS lookup on spamhaus though. The speed of the lookup will slow your mail delivery of legitimate email by milliseconds.

It's supposedly possible to download a blocklist and do local lookups on that, but setting that up is more complex and requires frequent downloads of large lists, so it seems of small reward for a lot of work if you are not handling much of email.

My exim4 config is broken into separate config elements, which is a fairly normal thing to do, but you may find the files to place this config will differ depending on your system.

Enable "DNSBLS" as exim4 refers to it, in your custom macro file (/etc/exim4/conf.d/main/00-custom_macros for example):


Configure the deny option or leave it at a warning level (/etc/exim4/conf.d/acl/30_exim4-config_check_rcpt).

# Check against classic DNS "black" lists (DNSBLs) which list
# sender IP addresses
# message = X-Warning: $sender_host_address is listed at $dnslist_domain ($dnslist_value: $dnslist_text)
  message = Failed sender validation
  log_message = michael DENY - $sender_host_address is listed at $dnslist_domain ($dnslist_value: $dnslist_text)

Notice above that I've commented out the default "warn" and "message". I've also added a custom log_message so the "michael DENY" sticks out. I know when I see that log that it's taking action on something I did. Also I dumbed down the message to not be too helpful to spammers, not that I think they're reading the SMTP rejection reasons!

When I initially implemented this, I never saw the rule being triggered. The reason was because the spamhaus lookup always failed to return any record. If your mailserver is configured to do lookups via a major DNS servers like, or, the spamhaus lookups don't work. I'm not going to go into why they don't work here, but suffice to say that the major DNS providers don't want to know about these queries.

Unfortunately, that means finding a DNS server that will help you with your inquiries, or running a DNS server on the local box (or in your local network). If you refer back to my previous post on setting up a nameserver, then you can simply add the following snippets to the existing setup (/etc/bind/named.conf.options):

acl "trusted" {
        <other trusted IPs>;
options {

        allow-query { any; };
        allow-recursion { trusted; };
        allow-query-cache { trusted; };

I sincerely encourage you to check the bind documentation yourself. Don't go adding random config from the internet to highly sensitive services without understanding what each setting means. Just a little explanation, the acl restricts DNS queries for domains other than in the local zones to a list of permitted hosts, including localhost, so that exim4 and other local services can use this server to resolve IPs.

Naturally, you need to ensure that /etc/resolv.conf has "nameserver" or another DNSBL friendly server configured.

Back to the exim config. Without customising, i.e. using the default settings, you'll get a new header on the email message. The email won't be dropped by this rule,  but you will see in the email (or in the exim4 rejectlog if dropped elsewhere) an X-Warning header. Note it below:

Envelope-from: <>
Envelope-to: <>
P Received: from ([])
by with esmtp (Exim 4.84_2)
(envelope-from <>)
id 1gdFOD-00008q-90
for; Sat, 29 Dec 2018 15:14:57 +0100
I Message-ID: <4EAF76134D97CA28C910752BF1AC4EAF@KSP94W150>
F From: "" <>
T To: <>
Subject: ***wonderful spam***
Date: 29 Dec 2018 07:01:51 -0400
MIME-Version: 1.0
Content-Type: text/plain;
Content-Transfer-Encoding: 7bit
X-Priority: 3
X-MSMail-Priority: Normal
X-Mailer: Microsoft Outlook Express 6.00.2900.5512
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5512
X-Warning: is listed at (,
X-Spam-Score: 20.0 (++++++++++++++++++++)

You'll likely see the same warning in the exim4 mainlog:

2018-12-29 15:14:57 [] Warning: is listed at (,

Once you see this log or the custom deny log I suggested, you know that the lookup is working.

In the end, I ditched doing the spamhaus lookup when I realised that spamassassin was also doing it. I commented out the "CHECK_RCPT_IP_DNSBLS =" entry and left the 30_exim4-config_check_rcpt config just in case I wanted to switch it on again.

Blocking mailservers that aren't

Mail delivery is entirely dependent on DNS records. A mailserver without forward and reverse DNS entries is of doubtful reputation. It's a safe bet that any host sending you email that doesn't have complete DNS records is not a legitimate mailserver and should be ignored.

My default exim4 install does not hold mailservers or the sender addresses to strict standards. Organisations that deal with large volumes of emails, for large numbers of users, will receive legitimate email from such badly configured mail clients and mailservers.

I can say with a high degree of certainty that I should not be receiving email from weird hosts or senders with domains that don't exist or accept email. If I discard emails from such servers and senders, I might have a handful of people over some number of years that have a problem emailing me. The upsides to configuring my mailserver to be strict on these points outweigh the potential downsides.

There are three useful options. In the custom macros file (/etc/exim4/conf.d/main/00-custom_macros) you may elect to enable the following ACLs.
# Denied in /etc/exim4/conf.d/acl/30_exim4-config_check_rcpt
# Denied in /etc/exim4/conf.d/acl/40_exim4-config_check_data
# Denied in /etc/exim4/conf.d/acl/30_exim4-config_check_rcpt

In the configuration files, you'll see that two of the methods are already deny, once the option is enabled (as above).

There's a helpful overview of many exim4 ACL options here and here. My descriptions below paraphrase them.

CHECK_RCPT_VERIFY_SENDER verifies that the sender of the message (RCPT TO) has a DNS entry. This is disabled by default, but when enabled will deny by default. Note that I added a custom log log message:


# Deny unless the sender address can be verified.
# This is disabled by default so that DNSless systems don't break. If
# your system can do DNS lookups without delay or cost, you might want
# to enable this feature.
# This feature does not work in smarthost and satellite setups as
# with these setups all domains pass verification. See spec.txt chapter
# 39.31 with the added information that a smarthost/satellite setup
# routes all non-local e-mail to the smarthost.
  message = Sender verification failed
  log_message = michael DENY - Sender verification failed
  !acl = acl_local_deny_exceptions
  !verify = sender

To date I have not seen this logged, so I can't verify that it's doing anything. It is possible that one of the other ACLs denies the email first.

CHECK_RCPT_REVERSE_DNS is the ACL that actually checks whether the mailserver has a reverse DNS entry.


# Warn if the sender host does not have valid reverse DNS.
# If your system can do DNS lookups without delay or cost, you might want
# to enable this.
# If sender_host_address is defined, it's a remote call. If
# sender_host_name is not defined, then reverse lookup failed. Use
# this instead of !verify = reverse_host_lookup to catch deferrals
# as well as outright failures.
# message = X-Host-Lookup-Failed: Reverse DNS lookup failed for $sender_host_address (${if eq{$host_lookup_failed}{1}{failed}{deferred}})
  message = Sender validation failure
  log_message = michael DENY - Reverse DNS check failed
condition = ${if and{{def:sender_host_address}{!def:sender_host_name}}\

Reverse check logs will now appear as so:

2018-12-30 07:17:37 H=([]) [] F=<> rejected RCPT <>: michael DENY - Reverse DNS check failed

CHECK_DATA_VERIFY_HEADER_SENDER verifies that the sender is valid in at least one of the "Sender:", "Reply-To:", or "From:" header lines.


# require that there is a verifiable sender address in at least
# one of the "Sender:", "Reply-To:", or "From:" header lines.
  message = No verifiable sender address in message headers
  log_message = michael DENY - No verifiable sender address in message headers
  !acl = acl_local_deny_exceptions
  !verify = header_sender

This condition is generally rare to see in the logs, but will look as so:

2018-12-30 09:00:31 1gdW1P-0003gC-Fx ( [x.x.x.x] X=TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256 F=<> rejected after DATA: michael DENY - No verifiable sender address in message headers: syntax error in 'From:' header when scanning for sender: malformed address: <> may not follow  in " <>"

After a day of monitoring logs I found that the only instance was against a message that I wanted to receive. A friend was sending me emails directly from a host that sent an automated daily digest. I decided to whitelist the domain:

# cat /etc/exim4/sender_local_deny_exceptions

Skip checking friendly hosts

My mailserver relays email for a couple of other servers I have on the internet. These host websites with contact forms that can change the "From" header to use the email address of the person who filled out the form.

In this case there will be various validation checks that fail.

018-12-29 17:51:59 no IP address found for host (during SMTP connection from ( [x.x.x.x])
2018-12-29 17:51:59 ( [x.x.x.x] sender verify fail for <>: Unrouteable address
2018-12-29 17:51:59 ( [x.x.x.x] X=TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128 F=<> rejected RCPT <>: michael-today UNKNOWN - Sender verification failed: Sender verify failed

You also need to give local server applications the ability to send email through exim4. For example, if you had a python script that generated an email at the of the script, you might see something like this in mainlog:

2018-12-30 23:44:56 1gdjpI-0005VU-Dg H=localhost ( [::1] F=<> rejected after DATA: michael DENY - No verifiable sender address in message headers: there is no valid sender in any header line

To add trusted hosts, including localhost, the solution is simple:

# cat /etc/exim4/host_local_deny_exceptions

In Conclusion

Don't waste your time integrating DNSBL into exim4 unless you do not want spamassassin to check for you.

So far so good. After a day of monitoring, not one piece of spam slipped through. I did discover that one email that I wanted was denied but due to the fact that the sender was someone firing email out from a machine not correctly setup to be a mailserver (I don't believe the sender cares enough to set that up).

This result was stunning when considering I could reduce spam not totally eliminate it, as appears to be the case after just 24 hours. One should expect to lose the odd email but careful review of the exim4 mainlog and rejectlog will help identify and whitelist desired 'special case' email senders.

Wednesday, January 17, 2018

How to get the SSL/TLS certificate chain right

After installing Firefox and Chrome on a new PC, I noticed that I was getting "issuer unknown" (Firefox: SEC_ERROR_UNKNOWN_ISSUER) errors on a website that I was checking connectivity against. The website was one that I was recently put in charge of and the organisation had a paid-for COMODO certificate for a Wordpress install.

On other computers there were no such errors reported by Firefox or Chrome for this website, so I initially missed the significance of the problem.


Note: "Certificate" should be read as "public certificate". The private cert or key is not discussed in this document.
Websites that support HTTPS require a valid SSL/TLS certificate or the client will receive certificate warnings from the application or browser. Happily, your community organisation or personal website can get by with a free certificate courtesy of Let's Encrypt. At the same time, the website needs to provide a certificate chain, which essentially informs the client (your browser) about the identity of the host that signed your certificate.

Likewise, the certificate of the host that signed the previous host's certificate needs to be provided. This recursive method of providing the certificate of the previous "intermediary" signer continues until the root certificate authority (CA) is reached. The root CA certificate does not need to be provided because it should be specifically trusted on the client software or browser or the whole infrastructure of trust is useless. Browsers and other software will ship with root CA certificates, or you can manually add them if necessary.

Any intermediary CA needs to be included in the certificate chain, but the root CA should not be included.

Incomplete, Contains anchor

I turned to Qualys SSL Labs to see whether I could obtain a head start on the problem. I saw warnings, which told me where to look but didn't help in identifying exactly what was wrong.

"This server's certificate chain is incomplete. Grade capped to B."

And later in the report:

"Chain issues - Incomplete, Contains anchor"

 I had somewhere to investigate at least - the certificate chain.

Also worth noting was that curl also complained of certificate problems on my desktop even if Firefox and Chrome did not.

Examining the Certificate Chain

Note: is a placeholder for the real URL I was investigating.
openssl is the obvious tool to turn to for seeing the nitty-gritty of an SSL/TLS session. I was able to easily view the certificate chain:

$ openssl s_client -connect
Certificate chain
 0 s:/OU=Domain Control Validated/OU=Hosted by webgo GmbH/OU=PositiveSSL/
   i:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Domain Validation Secure Server CA

 1 s:/C=SE/O=AddTrust AB/OU=AddTrust External TTP Network/CN=AddTrust External CA Root
   i:/C=SE/O=AddTrust AB/OU=AddTrust External TTP Network/CN=AddTrust External CA Root

 2 s:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Certification Authority
   i:/C=SE/O=AddTrust AB/OU=AddTrust External TTP Network/CN=AddTrust External CA Root
To the untrained eye, the above output looks incomprehensible. But with a little understanding and research the problem can be clearly seen. The chain consists of three certificates (0,1,2) issued by the website. Each certificate has a (s)erver that the certificate belongs to and an (i)ssuer that signed the certificate for that server.

The first certificate (0) is for and it was issued by COMODO RSA Domain Validation Secure Server CA

The second certificate (1) is expected to be for the COMODO issuer in the first certificate, but it is not. The certificate is for some other server AddTrust External CA Root, who is oddly also the issuer (a so called "self signed" certificate). Things are broken to bits from this point.

The third certificate (2) is completely superfluous because the second cert in this chain should not be there. In this third cert, we see it is for the server COMODO RSA Certification Authority and has been signed by the issuer AddTrust External CA Root.

Fixing the Mess

The solution was to provide a correct certificate chain. The first certificate (our certificate) was valid, but since it was signed by "COMODO RSA Domain Validation Secure Server CA" that needs to be the next public certificate found in the chain.

I first checked whether this was part of a typical Firefox CA set. The shipped CA certs can be viewed either on the Mozilla website or via the options->preferences of Firefox itself:

Yeah that's in German, sorry, but the English version will look basically the same.

In this case, notice that in the above screenshot "COMODO RSA Domain Validation Secure Server CA"  is actually in my certificate list in Firefox. I realised afterwards that at some point I had clicked through the invalid certificate warnings and added the certificate to my Firefox certificate store, to be trusted for next time. That's why I only noticed the problem after installing a new PC with Firefox and Chrome.

Just to repeat myself, the "COMODO RSA Domain Validation Secure Server CA" certificate is not part of the default suite of certificates trusted by Firefox. I needed to download the public certificate from COMODO here and tell apache2 (the site's web server) to use that certificate and only that certificate as part of the certificate chain.

Briefly, this meant configuring these apache2 settings ...

SSLCertificateKeyFile /etc/apache2/ssl.key/
SSLCertificateFile /etc/apache2/ssl.crt/www.
SSLCertificateChainFile /etc/apache2/

... ensuring that SSLCertificateKeyFile  contained only the private key of the server, SSLCertificateFile contained only the public certificate associated with the aforementioned private key and that SSLCertificateChainFile contained only the public certificate for "COMODO RSA Domain Validation Secure Server CA". If you have multiple layers of signing, you need to add each intermediary CA to this file, in the correct order. 

This works because the certificate for the intermediary "COMODO RSA Domain Validation Secure Server CA" is issued by "COMODO RSA Certification Authority" and that root CA is part of the shipped set of certificates for Firefox, Chrome, curl, openssl and any other SSL/TLS client you care to name.

The certificate chain now looks like this, (see earlier openssl command syntax):
Certificate chain

 0 s:/OU=Domain Control Validated/OU=Hosted by webgo GmbH/OU=PositiveSSL/
   i:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Domain Validation Secure Server CA

 1 s:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Domain Validation Secure Server CA
   i:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Certification Authority

In the first certificate, the (i)ssuer name is the (s)erver name of the second certificate. The second validates the first. There is no need for a third certificate because the issuer of the second certificate is part of an established set of known root Certificate Authorities.