IMPORTANT NOTE: most Tor servers do not currently use nftables, as we still use the Ferm firewall wrapper, which only uses iptables. Still, we sometimes end up on machines that might have nftables and those instructions will be useful for that brave new future. See tpo/tpa/team#40554 for a followup on that migration.

Listing rules

nft -a list ruleset

The -a flag shows the handles which is useful to delete a specific rule.

Checking and applying a ruleset

This checks the ruleset of Puppet rule files as created by the puppet/nftables modules before applying it:

nft -c -I /etc/nftables/puppet -f /etc/nftables/puppet.nft

This is done by Puppet before actually applying the ruleset, which is done with:

nft -I /etc/nftables/puppet -f /etc/nftables/puppet.nft

The -I parameter stands for --includepath and tells nft to look for rules in that directory.

Inserting a rule to bypass a restriction

Say you have the chain INPUT in the table filter which looks like this:

table inet filter {
	chain INPUT {
		type filter hook input priority filter; policy drop;
		iifname "lo" accept
		ct state established,related accept
		ct state invalid drop
		tcp dport 22 accept
		reject
	}
}

.. and you want to temporarily give access to the web server on port 443. You would do a command like:

nft insert rule inet filter INPUT 'tcp dport 443 accept'

Or if you need to allow a specific IP, you could do:

nft insert rule inet filter INPUT 'ip saddr 192.0.2.0/24 accept'

Blocking a host

Similarly, assuming you have the same INPUT chain in the filter table, you could do this to block a host from accessing the server:

nft insert rule inet filter INPUT 'ip saddr 192.0.2.0/24 reject'

That will generate an ICMP response. If this is a DOS condition, you might rather avoid that and simply drop the packet with:

nft insert rule inet filter INPUT 'ip saddr 192.0.2.0/24 drop'

Deleting a rule

If you added a rule by hand in the above and now want to delete it, you first need to find the handle (with the -a flag to nft list ruleset) and then delete the rule:

nft delete rule inet filter INPUT handle 39

Be VERY CAREFUL with this step as using the wrong handle might lock you out of the server.

nftables

Listing rules

Checking and applying a ruleset

Inserting a rule to bypass a restriction

Blocking a host

Deleting a rule

Other documentation

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