Currently, there is an unused function TCP_LISTEN in src/libs/zbxsysinfo/common/net.c, which does approximately what is required, but not quite. In particular, it requires the port to be specified in uppercase hex. It also only tries to read from /proc/net/tcp, while it should also handle /proc/net/tcp6. Also, in net.tcp.listen, should an additional parameter be added to distinguish between IPv4, IPv6, and both?