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Re: How to Run High Capacity Tor Relays

Do you have suggestions on how to rewrite firewall rules without using
RELATED,ESTABLISHED? My primary goal is to prevent access to other
ports, which I believe can be done with:

iptables -A INPUT -p tcp --tcp-flags SYN,ACK,FIN,RST SYN -j DROP

if you had this ruleset _using_ conntrack:

-A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A INPUT -m conntrack --ctstate NEW -m multiport -p tcp --dports 22,443 -j ACCEPT
(I assume OUTPUT is unfiltered [ACCEPT])

I would rewrite it to the following if you have to _omit_ connection tracking and want to have unfiltered outbound traffic

-A INPUT -m multiport -p tcp --dports 22,443 -j ACCEPT
-A INPUT -p tcp --syn -j DROP (this is the short equivalent to your rule)
(INPUT policy is ACCEPT)

regarding udp rules: it depends on your udp needs and DNS settings.

However, this obviously doesn't cover udp. Also, my secondary goal is
to slow down port scanning of the machine. I'm guessing a simple SYN
filter rule like this might still allow other scan types to work
without issue. What else can be done to eliminate those?

I'm not sure why would you want to do that, as this gives no additional security, but if you want to block FIN and XMAS scan without connection tracking:

-A INPUT -p tcp --tcp-flags ALL FIN -m comment --comment "FIN Scan filtering" -j DROP

-A INPUT -p tcp --tcp-flags ALL FIN,PSH,URG -m comment --comment "XMAS Scan filtering" -j DROP

If you filter XMAS and FIN scans, nmap (-sX -sF) scans will always result in open|filtered regardles of their real state and the scan will take longer.

These rules are tested and triggered with nmap -sF/-sX
wrote also a rule for null scan (-sN) but didn't see any effect on scan time.