The protocol filter
The protocol filter specifies the protocols to which NAT applies. NAT applies only to packets of the specified protocol. The default protocol is all protocols. The protocol filter can be used in SNAT and DNAT rules.
Filtering packets by protocol
The following example shows how to apply a SNAT rule, rule 10, to TCP protocol packets. Only TCP packets have address translation performed.
vyatta@vyatta# set service nat source rule 10 protocol tcp