====== How to run a private DNSBL for SpamAssassin ======

Here we will present a recipe to create a personal **[[wp>Domain_Name_System-based_Blackhole_List|Domain Name System-based Blackhole List]]** to be used with **SpamAssassin**. This will enable you to assign a **custom SPAM score** to mails coming from **specific IP addresses** (at the moment only IPv4 addresses).

We will use the well-known mechanism of DNSBLs, so that the general-purpose DNS server software **Bind9** and **SpamAssassin** are used, just with some specific configuration. This allows us to combine our blackhole list with the traditional ones provided by e.g. **dnsbl.sorbs.net** and **zen.spamhaus.org**.

===== Configure the DNS =====

==== Dynamic updates using an HMAC-MD5 key ====

Our DNSBL zone will be **updated dynamically** on our **DNS server** using a Python script; to allow only authenticated queries we create a **DNS key**. To generate the key we run the command:

<code>
dnssec-keygen -a HMAC-MD5 -b 512 -n USER bl-rigacci-org_rndc-key
</code>

Notice that **bl-rigacci-org_rndc-key** is the //username// associated with the key. Two file will be created: one with the **.key** and one with the **.private** extension. Get the secret from the generated private file:

<code>
cat Kbl-rigacci-org_rndc-key.+157+27575.private
...
Key: rg2aizg+T6XkKkmpI42K7g==
...
</code>

Now create a file name **/etc/bind/bl-rigacci-org_rndc-key** containing the secret, be sure to set the file into **640 mode** and owned by **bind:bind**:

<file>
key "bl-rigacci-org_rndc-key" {
        algorithm hmac-md5;
        secret "rg2aizg+T6XkKkmpI42K7g==";
};
</file>

==== The dynamic zone ====

Your DNS server will manage a **dynamic zone** dedicated to the DNSBL service. Create a file **/var/cache/bind/bl.rigacci.org** owned by **bind:bind**:

<file>
$TTL 900     ; 15 minutes
bl.rigacci.org   IN SOA  ns1.rigacci.org. postmaster.rigacci.org. (
                1       ; serial
                3600    ; refresh (1 hour)
                600     ; retry (5 minutes)
                432000  ; expire (5 days)
                10      ; minimum (10 seconds)
                )

                NS      ns1.rigacci.org.
                NS      ns2.rigacci.org.

$ORIGIN bl.rigacci.org.
$TTL 900        ; 15 minutes
</file>

Add that zone to **/etc/bind/named.conf.local**. Suppose that **10.100.101.102** is your secondary DNS server, this is the snippet required:

<file>
//--------------------------------------------------------------
// Dynamic update zone for DNS Blackhole List.
//--------------------------------------------------------------
zone "bl.rigacci.org" {
    type master;
    allow-update { 127.0.0.1; 10.100.101.102; };
    allow-transfer { 127.0.0.1; 10.100.101.102; };
    file "bl.rigacci.org";
    max-journal-size 500k;
};
include "/etc/bind/bl-rigacci-org_rndc-key";
</file>

===== Configure SpamAssassin =====

To add a check against our DNSBL, just edit **/etc/spamassassin/local.cf** and add a section like this:

<file>
header     LOCAL_CUSTOM_DNSBL    eval:check_rbl('bl-rigacci','bl.rigacci.org.')
describe   LOCAL_CUSTOM_DNSBL    Entries listed in bl.rigacci.org RBL
score      LOCAL_CUSTOM_DNSBL    100.0
</file>

You can customize the **score** (default SPAM score is 5.0 in SpamAssassin) to match your requirements.

===== Python script to manage the dynamic zone =====

Finally we need a script to add, remove or query IP address into the DNSBL zone. We have written a **{{.:dnsbl-tool.txt|dnsbl-tool}}** (use this one if you have the **python3-dnspython** **2.0.0** library **{{.:dnsbl-tool_0.4.0.txt|dnsbl-tool_0.4.0}}**) which can be used as follow:

<code>
dnsbl-tool -a 192.168.10.1
Adding record type "A" for 1.10.168.192.bl.rigacci.org
</code>

<code>
dnsbl-tool -q 192.168.10.1
Address 192.168.10.1 is listed: 1.10.168.192.bl.rigacci.org => 127.0.0.1
</code>

<code>
dnsbl-tool -r 192.168.10.1
Removing record type "A" for 1.10.168.192.bl.rigacci.org
</code>

To query the entire zone from the DNS server, you can request an **AXFR** (zone transfer). For doing that, you must do it from an IP address listed into the **allow-transfer** declared into named.conf.local:

<code>
dig -tAXFR bl.rigacci.org
</code>