28.3. DNS Relay

When Abilis acts as DNS resolver, it seems a DNS server to other IP stations while it simply forwards the requests to real DNS servers. Their responses are forwarded back to the original requesters.

[Tip]Tip

Refer to chapter Section 28.3.1, “Appendix - How DNS relay works” to know more information about DNS realy functioning.

The example below shows how to configure the DNS Resouce as Relay.

[17:12:51] ABILIS_CPX:s p dns act:yes relay:yes

COMMAND EXECUTED

[17:13:12] ABILIS_CPX:d p dns

RES:Dns - Not Saved (SAVE CONF), Not Refreshed (INIT) -------------------------
       ------------------------------------------------------------------------
Run    DESCR:Domain_Name_System
       LOG:NO           ACT:YES
       udp-locport:53   SRCADD:OUT-IP                      TOS:0-N
       wdir:C:\APP\DNS\
       - Resolver -------------------------------------------------------------
       SERVERS:AUTO
       PRIMARY:062.094.000.001   SECONDARY:062.094.000.002
       DELAY:5                   RTY:1                    
       CACHE:YES                 cache-size:500
       - Relay/Server ---------------------------------------------------------
       RELAY:NO                  relay-size:500            RELAY-TOUT:10  
       RELAY-BLACKLIST:YES       RELAY-BLACKLIST-BYPASS:#
       SERVER:NO
       IPSRC:*                   IPSRCLIST:#

Filters on the IP addresses allowed to the DNS service can be applied by combining the parameters IPSRC and IPSRCLIST.

In the following example, the “DN_Allowedlist will be created and the range of IP from 192.168.1.10 to 192.168.1.50 will be added; otherwise, the address 192.168.1.100 will be set as the IP address Source.

[17:46:50] ABILIS_CPX:list create DNS_Allowed ir IP_Addresses_allowed_to_DNS_service

COMMAND EXECUTED

[17:48:04] ABILIS_CPX:a list:dns_allowed 192.168.1.10:192.168.1.50

COMMAND EXECUTED

[17:48:19] ABILIS_CPX:d list:dns_allowed

- Not Saved (SAVE CONF) -------------------------------------------------------

LIST:DNS_Allowed          - IR
     IP_Addresses_allowed_to_DNS_service
     192.168.001.010:192.168.001.050

[17:49:20] ABILIS_CPX:s p dns ipsrc:192.168.1.100 ipsrclist:dns_allowed

COMMAND EXECUTED

[17:49:23] ABILIS_CPX:d p dns

RES:Dns - Not Saved (SAVE CONF), Not Refreshed (INIT) -------------------------
       ------------------------------------------------------------------------
Run    DESCR:Domain_Name_System
       LOG:NO           ACT:YES
       udp-locport:53   SRCADD:OUT-IP                      TOS:0-N
       wdir:C:\APP\DNS\
       - Resolver -------------------------------------------------------------
       SERVERS:AUTO
       PRIMARY:062.094.000.001   SECONDARY:062.094.000.002
       DELAY:5                   RTY:1
       CACHE:YES                 cache-size:500
       - Relay/Server ---------------------------------------------------------
       RELAY:YES                 relay-size:500            RELAY-TOUT:10
       RELAY-BLACKLIST:YES       RELAY-BLACKLIST-BYPASS:#
       SERVER:NO
       IPSRC:192.168.001.100     IPSRCLIST:DNS_Allowed
[Caution]Caution

To activate the changes made on the upper case parameters, execute the initialization command init res:dns; while to set act the changes made on the lowercase parameters a save conf and an Abilis restart are required (i.e. with warm start command).

28.3.1. Appendix - How DNS relay works

The Abilis CPX DNS Resolver first requests the primary DNS Server.

  • if it doesn't get any response, it forwards the same query to both primary and secondary server after 500 milliseconds;

  • if no response is received, another request is sent to both servers after 1 second;

  • in case of no answer, the same procedure is repeated after 1 second and after 2 seconds;

  • if the DNS Resolver doesn't get any answer within 4 seconds, it terminates the procedure.

This procedure is repeated for a number of times equal to the value set in the RTY parameter. The delay (in seconds) between two subsequent queries can be set with the DELAY parameter.