Then why bring it up in the first place?
If you can take a RPI for $50 or whatever they cost and throw iptables onto it and get the same result w/o dealing with the licensing BS w/ Cisco or the price.... It doesn't make sense.
However a power user / prosumer not interested in the Asus Fanboy experience might come across this thread and find something useful through a google search.
Taking a Cisco FW config and being able to chop it down into this is impossible.
Code:
# Generated by iptables-save v1.8.7 on Sun Jan 23 20:51:32 2022
*mangle
:PREROUTING ACCEPT [41:49134]
:INPUT ACCEPT [39:49020]
:FORWARD ACCEPT [2:114]
:OUTPUT ACCEPT [21:1544]
:POSTROUTING ACCEPT [22:1474]
COMMIT
# Completed on Sun Jan 23 20:51:32 2022
# Generated by iptables-save v1.8.7 on Sun Jan 23 20:51:32 2022
*security
:INPUT ACCEPT [24:32068]
:FORWARD ACCEPT [2:114]
:OUTPUT ACCEPT [20:1360]
COMMIT
# Completed on Sun Jan 23 20:51:32 2022
# Generated by iptables-save v1.8.7 on Sun Jan 23 20:51:32 2022
*raw
:PREROUTING ACCEPT [41:49134]
:OUTPUT ACCEPT [21:1544]
:FORWARD - [0:0]
COMMIT
# Completed on Sun Jan 23 20:51:32 2022
# Generated by iptables-save v1.8.7 on Sun Jan 23 20:51:32 2022
*filter
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT DROP [0:0]
:PERMIT-FWD - [0:0]
:PERMIT-IN - [0:0]
:PERMIT-OUT - [0:0]
-A INPUT -j PERMIT-IN
-A FORWARD -j PERMIT-FWD
-A OUTPUT -j PERMIT-OUT
-A PERMIT-FWD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A PERMIT-FWD -m conntrack --ctstate NEW -j ACCEPT
-A PERMIT-FWD -j DROP
-A PERMIT-IN -i lo -j ACCEPT
-A PERMIT-IN -i br0 -j ACCEPT
-A PERMIT-IN -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A PERMIT-IN -j DROP
-A PERMIT-OUT -o lo -j ACCEPT
-A PERMIT-OUT -o br0 -j ACCEPT
-A PERMIT-OUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A PERMIT-OUT -m conntrack --ctstate NEW -j ACCEPT
-A PERMIT-OUT -j DROP
COMMIT
# Completed on Sun Jan 23 20:51:32 2022
# Generated by iptables-save v1.8.7 on Sun Jan 23 20:51:32 2022
*nat
:PREROUTING ACCEPT [0:0]
:INPUT ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
-A POSTROUTING -o nordlynx -j MASQUERADE
-A POSTROUTING -o bo0 -j MASQUERADE
COMMIT
# Completed on Sun Jan 23 20:51:32 2022
For even the pebcak user can figure this out with very little explanation and full protection offered by a $500 FW that can't (shouldn't) be used a router. This iptables configuration is quite basic and probably more secure than your initial lockdown configuration on a PIX. I've been running this now for years at this point and no issues or anything triggering a raised eyebrow for traffic that shouldn't be.