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Filename : ct_helpers.nft
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#!/usr/sbin/nft -f

# This example file shows how to use ct helpers in the nftables framework.
# Note that nftables includes interesting improvements compared to how this
# was done with iptables, such as loading multiple helpers with a single rule
# This script is meant to be loaded with `nft -f <file>`
# You require linux kernel >= 4.12 and nft >= 0.8
# For up-to-date information please visit https://wiki.nftables.org

# Using ct helpers is an important security feature when doing stateful
# firewalling, since it mitigate certain networking attacks.
# More info at: https://home.regit.org/netfilter-en/secure-use-of-helpers/


flush ruleset
table inet filter {
	# declare helpers of this table
	ct helper ftp-standard {
		type "ftp" protocol tcp;
		l3proto inet
	}
	ct helper sip-5060 {
		type "sip" protocol udp;
		l3proto inet
	}
	ct helper tftp-69 {
		type "tftp" protocol udp
		l3proto inet
	}

	chain input {
		type filter hook input priority 0; policy drop;
		ct state established,related accept

		# assign a single helper in a single rule
		tcp dport 21 ct helper set "ftp-standard"

		# assign multiple helpers in a single rule
		ct helper set udp dport map {
	                        69 : "tftp-69", \
		                5060 : "sip-5060" }
	}
}