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jayanthi_41101's avatar
jayanthi_41101
Icon for Nimbostratus rankNimbostratus
Jun 12, 2017

Use DNS server list for DNS resolution through F5 LTM irule

We already use DNS resolution using one specific DNS server in RESOLV::lookup. I am trying to figure out a solution where i can use a list of DNS servers and not finding any resources for it.

 

Can someone help me with it?

 

Following is a sample i got off devcentral:

 

Select the first returned IP address as the destination IP (inherits the destination port from the client's destination port).

 

when RULE_INIT { set static::dns_vs my_dns_vs } when CLIENT_ACCEPTED { Get IP(s) for hostname against 4.2.2.1 name server set ips [RESOLV::lookup @$static::dns_vs -a ";] Log result. If there are multiple IP's it could be a TCL list like {1.1.1.1 2.2.2.2 3.3.3.3}. log local0. "Looked up and found $ips, parsed first element: [lindex $ips 0]" Check if the first list element was empty if {$ips eq ""}{ Input wasn't an IP address, take some default action? } else { Select the IP node [lindex $ips 0] } }

 

application delivery
LTM

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  • tcastaing_23984's avatar
    tcastaing_23984
    Icon for Nimbostratus rankNimbostratus
    Dec 18, 2017

    Here is a working solution that is choosing randomly a DNS server from the specified list.

     FQDN to resolve
set FQDN_TO_RESOLVE "www.example.com"

 DNS server list
set DNS_SERVER_LIST {1.1.1.1 2.2.2.2 3.3.3.3}

 Select a DNS server from the list
set SELECTED_DNS_SERVER "[lindex $DNS_SERVER_LIST [expr {int(rand()*[llength $DNS_SERVER_LIST])}]]"

 Perform the Lookup
set FQDN_RESOLVED_IP "[lindex [RESOLV::lookup @$SELECTED_DNS_SERVER -a $FQDN_TO_RESOLVE] 0]"