I have no experience with SNMP, but
google has a few interesting hits which may be of use.
FreeBSD has
bsnmpd(1) in the base system. The
net-mgmt/net-snmp/ also looks promising.
As for the firewall,
services(5) is of help:
Code:
$ grep -i snmp /etc/services
snmp 161/tcp
snmp 161/udp
snmptrap 162/tcp snmp-trap
snmptrap 162/udp snmp-trap
synotics-relay 391/tcp #SynOptics SNMP Relay Port
synotics-relay 391/udp #SynOptics SNMP Relay Port
snmp-tcp-port 1993/tcp #cisco SNMP TCP port
snmp-tcp-port 1993/udp #cisco SNMP TCP port