You can use the adclient.ipv4.port.range.low and adclient.ipv4.port.range.high parameters to specify the low and high ends of the range of IP ports for adclient and adnisd to use. These parameters control the outbound connection port for both TCP and UDP connections.
By specifying an IP port range, you can then configure your firewall to allow traffic through that port range only.
The typical port number is between 1024 and 65535. Setting this parameter does require a restart of adclient.