Netfilter hooks

nftables uses mostly the same Netfilter infrastructure as legacy iptables. The hook infrastructure, Connection Tracking System, NAT engine, logging infrastructure, and userspace queueing remain the same. Only the packet classification framework is new.

Netfilter hooks into Linux networking packet flows

The following schematic shows packet flows through Linux networking:

Traffic flowing to the local machine in the input path sees the prerouting and input hooks. Then, the traffic that is generated by local processes follows the output and postrouting path.

If you configure your Linux box to behave as a router, do not forget to enable forwarding via:

echo 1 > /proc/sys/net/ipv4/ip_forward

Then packets that are not addressed to your local system will be seen from the forward hook. Such forwarded packets follow the path: prerouting, forward and postrouting.

In a major change from iptables, which predefines chains at every hook (i.e. INPUT chain in filter table), nftables predefines no chains at all. You must must explicitly create a base chain at each hook at which you want to filter traffic.

Ingress hook

The ingress hook was added in Linux kernel 4.2. Unlike the other netfilter hooks, the ingress hook is attached to a particular network interface.

You can use nftables with the ingress hook to enforce very early filtering policies that take effect even before prerouting. Do note that at this very early stage, fragmented datagrams have not yet been reassembled. So, for example, matching ip saddr and daddr works for all ip packets, but matching L4 headers like udp dport works only for unfragmented packets, or the first fragment.

The ingress hook provides an alternative to tc ingress filtering. You still need tc for traffic shaping/queue management.

Priority within hook

Within a given hook, Netfilter performs operations in order of increasing numerical priority. Each nftables base chain and flowtable is assigned a priority that defines its ordering among other base chains and flowtables and Netfilter internal operations at the same hook. For example, a chain on the prerouting hook with priority -300 will be placed before connection tracking operations.

The following table shows Netfilter priority values, check the nft manpage for reference.

nftables Families	Hooks	nft Keyword	Value	Netfilter Internal Priority	Description
	prerouting		-450	NF_IP_PRI_RAW_BEFORE_DEFRAG
inet, ip, ip6	prerouting		-400	NF_IP_PRI_CONNTRACK_DEFRAG	Packet defragmentation / datagram reassembly
inet, ip, ip6	all	raw	-300	NF_IP_PRI_RAW	Traditional priority of the raw table placed before connection tracking operation
			-225	NF_IP_PRI_SELINUX_FIRST	SELinux operations
inet, ip, ip6	prerouting, output		-200	NF_IP_PRI_CONNTRACK	Connection tracking processes run early in prerouting and output hooks to associate packets with tracked connections.
inet, ip, ip6	all	mangle	-150	NF_IP_PRI_MANGLE	Mangle operation
inet, ip, ip6	prerouting	dstnat	-100	NF_IP_PRI_NAT_DST	Destination NAT
inet, ip, ip6, arp, netdev	all	filter	0	NF_IP_PRI_FILTER	Filtering operation, the filter table
inet, ip, ip6	all	security	50	NF_IP_PRI_SECURITY	Place of security table, where secmark can be set for example
inet, ip, ip6	postrouting	srcnat	100	NF_IP_PRI_NAT_SRC	Source NAT
	postrouting		225	NF_IP_PRI_SELINUX_LAST	SELinux at packet exit
inet, ip, ip6	postrouting		300	NF_IP_PRI_CONNTRACK_HELPER	Connection tracking helpers, which identify expected and related packets.
inet, ip, ip6	input, postrouting		INT_MAX	NF_IP_PRI_CONNTRACK_CONFIRM	Connection tracking adds new tracked connections at final step in input & postrouting hooks.
bridge	prerouting	dstnat	-300	NF_BR_PRI_NAT_DST_BRIDGED
bridge	all	filter	-200	NF_BR_PRI_FILTER_BRIDGED
bridge			0	NF_BR_PRI_BRNF
bridge	output	out	100	NF_BR_PRI_NAT_DST_OTHER
bridge			200	NF_BR_PRI_FILTER_OTHER
bridge	postrouting	srcnat	300	NF_BR_PRI_NAT_SRC

Starting with nftables 0.9.6 you may set priority using keywords instead of numbers. (Note that the same keyword maps to different numerical priorities in the bridge family vs. the other families.) You can also specify priority as an integral offset from a keyword, i.e. mangle - 5 is equivalent to numerical priority -155.

It's possible to specify keyword priorities even in family/hook combinations where they don't make logical sense. Recall that the relative numerical ordering of priorities within a given hook is all that matters as far as Netfilter is concerned. Keep in mind that this relative ordering includes packet defragmentation, connection tracking and other Netfilter operations as well as your nftables base chains and flowtables.