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--- doc:appunti:linux:sa:spamassassin_private_dnsbl [2020/02/17 17:02] – created niccolo
+++ doc:appunti:linux:sa:spamassassin_private_dnsbl [2021/10/08 10:45] (current) – [Python script to manage the dynamic zone] niccolo
@@ Line 1: / Line 1: @@
-====== How to use a private DNS Blocklist with SpamAssassin ======
+====== 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>
+$TTL 900     ; 15 minutes
+bl.rigacci.org   IN SOA  ns1.rigacci.org. postmaster.rigacci.org. (
+       ; serial
+    ; refresh (1 hour)
+     ; retry (5 minutes)
+  ; expire (5 days)
+      ; minimum (10 seconds)
+                )
+                NS      ns1.rigacci.org.
+                NS      ns2.rigacci.org.
+$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>