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doc:appunti:linux:sa:spamassassin_private_dnsbl [2020/02/17 17:02]
niccolo created
doc:appunti:linux:sa:spamassassin_private_dnsbl [2020/02/19 15:11] (current)
niccolo [Python script to manage the dynamic zone]
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-====== 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>​
 +$TTL 900     ; 15 minutes
 +bl.rigacci.org ​  IN SOA  ns1.rigacci.org. postmaster.rigacci.org. (
 +                1       ; serial
 +                3600    ; refresh (1 hour)
 +                600     ; retry (5 minutes)
 +                432000 ​ ; expire (5 days)
 +                10      ; minimum (10 seconds)
 +                )
 +
 +                NS      ns1.rigacci.org.
 +                NS      ns2.rigacci.org.
 +
 +$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 ​       CUSTOM_DNSBL ​   eval:​check_rbl('​bl-rigacci','​bl.rigacci.org.'​)
 +describe ​     CUSTOM_DNSBL ​   Entries listed in bl.rigacci.org RBL
 +score         ​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}}** 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>​
doc/appunti/linux/sa/spamassassin_private_dnsbl.1581955350.txt.gz ยท Last modified: 2020/02/17 17:02 by niccolo