Mangling packet headers

Mangle packet header fields

Since nft v0.6 nftables supports stateless payload mangling.

To mangle packet header fields you should create a rule to match the packet, match the desired header field and set a new value to it:

% nft add table mangle
% nft add chain mangle forward {type filter hook forward priority 0\;}
% nft add rule mangle forward tcp dport 8080 tcp dport set 80

The commands above create a table named mangle, a chain named forward, see Netfilter hooks, and a rule to mangle the destination port of packets over TCP from 8080 to 80. Keep in mind the interactions with conntrack, flows with mangled traffic must be untracked.

The rule below is another example, it matches packets heading to address 192.168.1.3 and modifies their Time to Live field:

% nft add rule mangle forward ip daddr 192.168.1.3 ip ttl set 2

For more information about packet headers to mangle check manpage nft(8), Matching packet header fields and Quick reference-nftables in 10 minutes.