GTM / DNS Restrict source ports from GTM when LB DNS to Microsoft servers

I have tested the following iRule and it seems to accomplish what you are looking for. Let me know if this works.

when RULE_INIT {

 On the line below, select which IP's are to be part of the SNAT list
set static::ip_list [list 10.1.20.254 ]
}
when CLIENT_ACCEPTED {
set random_snat_ip [lindex $static::ip_list [expr {int(rand()*[llength $static::ip_list])}]]

 On line below this one, replace "http_pool" with the correct pool name where traffic will be load balanced to
set available_pool_member [active_members -list demo_dns_pool]

set random_pool_member [lindex [lindex $available_pool_member [expr {int(rand()*[llength $available_pool_member])}]] 0]

 On line below, replace "80" with pool members listening port. Values of 50000, 49152 and 65535 represent "hint port", minimum port and maximum port, respectively.
set random_port [UDP::unused_port $random_pool_member 53 $random_snat_ip 50000 49152 65535 ]

snat $random_snat_ip $random_port
pool demo_dns_pool member $random_pool_member
}