28.3. DNS Relay

28.3. DNS Relay
	Chapter 28. DNS - Domain Name System

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
	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_Allowed” list 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
	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.


28.2. DNS Resolver		28.4. DNS Server