Netlink notes for kernel developers

General guidance

Attribute enums

Older families often define “null” attributes and commands with value of 0 and named unspec. This is supported (type: unused) but should be avoided in new families. The unspec enum values are not used in practice, so just set the value of the first attribute to 1.

Message enums

Use the same command IDs for requests and replies. This makes it easier to match them up, and we have plenty of ID space.

Use separate command IDs for notifications. This makes it easier to sort the notifications from replies (and present them to the user application via a different API than replies).

Answer requests

Older families do not reply to all of the commands, especially NEW / ADD commands. User only gets information whether the operation succeeded or not via the ACK. Try to find useful data to return. Once the command is added whether it replies with a full message or only an ACK is uAPI and cannot be changed. It’s better to err on the side of replying.

Specifically NEW and ADD commands should reply with information identifying the created object such as the allocated object’s ID (without having to resort to using NLM_F_ECHO).

NLM_F_ECHO

Make sure to pass the request info to genl_notify() to allow NLM_F_ECHO to take effect. This is useful for programs that need precise feedback from the kernel (for example for logging purposes).

Support dump consistency

If iterating over objects during dump may skip over objects or repeat them - make sure to report dump inconsistency with NLM_F_DUMP_INTR. This is usually implemented by maintaining a generation id for the structure and recording it in the seq member of struct netlink_callback.

Netlink specification

Documentation of the Netlink specification parts which are only relevant to the kernel space.

Globals

kernel-policy

Defines whether the kernel validation policy is global i.e. the same for all operations of the family, defined for each operation individually - per-op, or separately for each operation and operation type (do vs dump) - split. New families should use per-op (default) to be able to narrow down the attributes accepted by a specific command.

checks

Documentation for the checks sub-sections of attribute specs.

unterminated-ok

Accept strings without the null-termination (for legacy families only). Switches from the NLA_NUL_STRING to NLA_STRING policy type.

max-len

Defines max length for a binary or string attribute (corresponding to the len member of struct nla_policy). For string attributes terminating null character is not counted towards max-len.

The field may either be a literal integer value or a name of a defined constant. String types may reduce the constant by one (i.e. specify max-len: CONST - 1) to reserve space for the terminating character so implementations should recognize such pattern.

min-len

Similar to max-len but defines minimum length.