The error message you received indicates that the “conntrack” match module is not available on your system. This is likely because the kernel option CONFIG_NETFILTER_XT_MATCH_CONNTRACK is not set in your system’s configuration.
To resolve the issue, you need to ensure that this option is enabled in your system’s kernel configuration and then rebuild the kernel. Once the kernel is rebuilt, you should be able to use the “conntrack” match module in your iptables rules.
Alternatively, you can try using a different match module, such as “state”, which should be available on most systems:
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT