This target marks packets so that the kernel will log every rule which match the packets as those traverse the tables, chains, rules. It can only be used in the raw table.

With iptables-legacy, a logging backend, such as ip(6)t_LOG or nfnetlink_log, must be loaded for this to be visible. The packets are logged with the string prefix: "TRACE: tablename:chainname:type:rulenum " where type can be "rule" for plain rule, "return" for implicit rule at the end of a user defined chain and "policy" for the policy of the built in chains.

With iptables-nft, the target is translated into nftables' meta nftrace expression. Hence the kernel sends trace events via netlink to userspace where they may be displayed using xtables-monitor --trace command. For details, refer to xtables-monitor(8).