nlmsghdr

nlmsghdr is the standard header used by netlink messages in the Linux kernel and user-space netlink interfaces. It prefixes the payload of a netlink message and identifies how the message should be interpreted by the receiving end. The header is defined in linux/netlink.h and is used across various netlink families, such as NETLINK_ROUTE and NETLINK_GENERIC.

The structure consists of five fields. nlmsg_len is a 32-bit value that gives the total length of the message, including the header itself. nlmsg_type is a 16-bit value that designates the type of the payload within the specific netlink family (for example, RTM_ or NLMSG_ values). nlmsg_flags is a 16-bit field containing flags that control message handling, such as requests, multi-part messages, acknowledgments, or echo. nlmsg_seq is a 32-bit sequence number used to match requests with replies, and nlmsg_pid is a 32-bit identifier of the sending process (the port ID).

The header’s length field includes the header; the actual payload starts after the header and is interpreted according to nlmsg_type and the active netlink family. Multi-part messages use NLMSG_DONE and related conventions, and NLMSG_ERROR can indicate an error condition in a multipart exchange. To navigate within a message buffer, helper macros such as NLMSG_NEXT, NLMSG_DATA, NLMSG_PAYLOAD, NLMSG_LENGTH, NLMSG_ALIGN, and NLMSG_SPACE are commonly used to compute offsets and access the payload safely.

nlmsghdr plays a central role in kernel-to-user and user-to-kernel communication via netlink sockets. It provides a compact, extensible header that enables diverse subsystems to exchange control information and data with consistent framing.