Synproxy

table ip foo {

    synproxy https-synproxy {
        mss 1460
        wscale 7
        timestamp sack-perm
    }

    synproxy other-synproxy {
        mss 1460
        wscale 5
    }

    chain pre {
        type filter hook prerouting priority raw; policy accept;

        tcp dport 8888 tcp flags syn notrack
    }

    chain bar {
        type filter hook forward priority filter; policy accept;

        ct state invalid,untracked synproxy name ip saddr map {
            192.168.1.0/24 : "https-synproxy", 
            192.168.2.0/24 : "other-synproxy",
        }
    }
}

Support for using synproxy objects in maps was added in nftables 0.9.3.