What version of RHEL are you using? If you are using 7 or higher then you can just use the firewalld service and firewall-cmd to enable it.
NOTE: Make sure that all your host firewall rules aren't already using iptables.
if not already, enable firewalld
# systemctl start firewalld
Then start firewalld
# systemctl start firewalld
Just add the ports/services that you want to allow in:
# firewall-cmd --add-service=snmp --perm
(note: the --perm option makes it permanent after reloading)
Now just reload the firewall rules and you should be good to go.
# firewall-cmd --reload
To doublecheck that it was added, just run:
# firewall-cmd --list-services
You should now see "snmp" as one of the services.
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