Port 546 is DHCPv6 Client, UDP.
UDP
low risk
App-IDdhcpv6
DHCPv6 client port — receives address and configuration offers from a DHCPv6 server.
Palo Alto
Create the service object for this port, then allow the App-ID on it. Restricting the rule to the port's service object keeps the rule working on a non-default port and stops the App-ID matching on other ports. Replace the placeholders with your own rule name and zones.
set service udp-546 protocol udp port 546
set rulebase security rules <rule-name> from <from-zone> to <to-zone> source any destination any application dhcpv6 service udp-546 action allow
Risk and exposure
Rated low risk. Even so, only expose this port to the networks and hosts that need it, and keep it patched.
Check it
- Linux
ss -ulnp | grep :546 - nmap
nmap -sU -p 546 <host>