GENETLINK(4)

💅 GENETLINK — like, the generic Netlink page, bestie

okay so this is from the Kernel Interfaces Manual (section 4, iykyk), and it's literally all about genetlink — Generic Netlink. buckle up, it's cuter than it sounds 🎀

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✨ NAME ✨

genetlink — Generic Netlink. that's it, that's the vibe 🦋

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✨ SYNOPSIS ✨

#include <netlink/netlink.h>
#include <netlink/netlink_generic.h>

int
socket(AF_NETLINK, SOCK_DGRAM, NETLINK_GENERIC);

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✨ DESCRIPTION ✨

okay so NETLINK_GENERIC is a "container" family — think of it as the group chat that other families get added to 👑. it's used for dynamic registration of other families that belong to all the various subsystems.

here's the tea 🍓: those subsystems hand over a string family name when they register, and in exchange they get a dynamically-allocated family id. then applications use those allocated family identifiers to actually get at the functions that subsystem provides over netlink. and yes, there are standard methods for turning a string family name into a family id — no cap. a similar mechanism works for the notification groups that those families provide, too ✨.

all generic netlink families share a common header:

struct genlmsghdr {
        uint8_t         cmd;            /* command within the family */
        uint8_t         version;        /* ABI version for the cmd */
        uint16_t        reserved;       /* reserved: set to 0 */
};

the family id is encoded in the nlmsg_type of the base netlink header. the cmd field is the command identifier within the family, and the version field is the command version. slay 💅

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✨ METHODS ✨

the generic Netlink framework provides the base family, GENL_ID_CTRL ("nlctrl"), with a fixed family id. this family is literally the one you use to list the details of all the registered families 🫶.

the framework supports the following messages:

🌸 CTRL_CMD_GETFAMILY 🌸

fetches a single family — or all registered families, depending on the NLM_F_DUMP flag. each family gets reported as a CTRL_CMD_NEWFAMILY message ⭐.

the kernel recognises these filters:

      CTRL_ATTR_FAMILY_ID     (uint16_t) current family id assigned by kernel
      CTRL_ATTR_FAMILY_NAME   (string) family name

🌸 TLVs 🌸

• CTRL_ATTR_FAMILY_ID — (uint16_t) the dynamically-assigned family identifier.
• CTRL_ATTR_FAMILY_NAME — (string) the family name.
• CTRL_ATTR_HDRSIZE — (uint32_t) family mandatory header size (typically 0).
• CTRL_ATTR_MAXATTR — (uint32_t) maximum attribute number that's valid for the family.
• CTRL_ATTR_OPS — (nested) the list of operations supported by the family. the attribute consists of a list of nested TLVs, with attribute values monotonically incremented, starting from 0. each TLV contains the following attributes:
  • CTRL_ATTR_OP_ID — operation (message) number.
  • CTRL_ATTR_OP_FLAGS — operation flags. the following flags are supported:
    • GENL_ADMIN_PERM — requires elevated permissions
    • GENL_CMD_CAP_DO — operation is a modification request
    • GENL_CMD_CAP_DUMP — operation is a get/dump request
• CTRL_ATTR_MCAST_GROUPS — (nested) the list of notification groups supported by the family. this attribute consists of a list of nested TLVs, with attribute values monotonically incremented, starting from 0. each TLV contains the following attributes:
  • CTRL_ATTR_MCAST_GRP_ID — group id that can be used in NETLINK_ADD_MEMBERSHIP setsockopt(2).
  • CTRL_ATTR_MCAST_GRP_NAME — (string) human-readable name of the group.

🌸 Groups 🌸

the following groups are defined:

      "notify"        Notifies on family registrations/removal.

so like… "notify" tells you when families get registered or removed. main character energy honestly 💀

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✨ SEE ALSO ✨

• netlink(4)

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✨ HISTORY ✨

the NETLINK_GENERIC protocol family appeared in FreeBSD 13.2 🌈.

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✨ AUTHORS ✨

the netlink was implemented by Alexander Chernikov <melifaro@FreeBSD.org>. it was derived from the Google Summer of Code 2021 project by Ng Peng Nam Sean. iconic behavior honestly 👑

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ngl bestie, if you made it this far you deserve a little treat 🍓 — go read netlink(4) next, it's giving required reading 🧚

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and that's the whole page, periodt 💜 — this cutie came to you from Debian, dated November 1, 2022. bye bestie, go write some netlink code 🦋✨