Dear Maintainer, In the upstream version v2.3.1 ferm renamed SYNPROXY's timestamp option to timestamps, presumably following iptables' documentation[1]. However, the current iptables source[2] still calls the option timestamp and fails when `--timestamps` is passed: # iptables -j SYNPROXY --timestamps iptables v1.8.2 (nf_tables): unknown option "--timestamps" # iptables-legacy -j SYNPROXY --timestamps iptables v1.8.2 (legacy): unknown option "--timestamps" The option was renamed[3] back to `timestamp` in version 2.4.1, but Buster still comes with 2.4 which has this bug. [1]: https://git.netfilter.org/iptables/tree/extensions/libxt_SYNPROXY.man [2]: https://git.netfilter.org/iptables/plain/extensions/libxt_SYNPROXY.c [3]: https://github.com/MaxKellermann/ferm/pull/17