Netlink notes for kernel developers¶
Older families often define "null" attributes and commands with value
0 and named
unspec. This is supported (
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
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).
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
Make sure to pass the request info to genl_notify() to allow
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
This is usually implemented by maintaining a generation id for the
structure and recording it in the
seq member of struct netlink_callback.
Documentation of the Netlink specification parts which are only relevant to the kernel space.
Defines if the kernel validation policy is per operation (
or for the entire family (
global). New families should use
(default) to be able to narrow down the attributes accepted by a specific
Documentation for the
checks sub-sections of attribute specs.
Accept strings without the null-termination (for legacy families only).
Switches from the
NLA_STRING policy type.
Defines max length for a binary or string attribute (corresponding
len member of struct nla_policy). For string attributes terminating
null character is not counted towards
The field may either be a literal integer value or a name of a defined
constant. String types may reduce the constant by one
max-len: CONST - 1) to reserve space for the terminating
character so implementations should recognize such pattern.
max-len but defines minimum length.