Netlink specification support for legacy Generic Netlink families¶
This document describes the many additional quirks and properties
required to describe older Generic Netlink families which form
the genetlink-legacy
protocol level.
The spec is a work in progress, some of the quirks are just documented for future reference.
Specification (defined)¶
Attribute type nests¶
New Netlink families should use multi-attr
to define arrays.
Older families (e.g. genetlink
control family) attempted to
define array types reusing attribute type to carry information.
For reference the multi-attr
array may look like this:
[ARRAY-ATTR]
[INDEX (optionally)]
[MEMBER1]
[MEMBER2]
[SOME-OTHER-ATTR]
[ARRAY-ATTR]
[INDEX (optionally)]
[MEMBER1]
[MEMBER2]
where ARRAY-ATTR
is the array entry type.
array-nest¶
array-nest
creates the following structure:
[SOME-OTHER-ATTR]
[ARRAY-ATTR]
[ENTRY]
[MEMBER1]
[MEMBER2]
[ENTRY]
[MEMBER1]
[MEMBER2]
It wraps the entire array in an extra attribute (hence limiting its size
to 64kB). The ENTRY
nests are special and have the index of the entry
as their type instead of normal attribute type.
type-value¶
type-value
is a construct which uses attribute types to carry
information about a single object (often used when array is dumped
entry-by-entry).
type-value
can have multiple levels of nesting, for example
genetlink’s policy dumps create the following structures:
[POLICY-IDX]
[ATTR-IDX]
[POLICY-INFO-ATTR1]
[POLICY-INFO-ATTR2]
Where the first level of nest has the policy index as it’s attribute type, it contains a single nest which has the attribute index as its type. Inside the attr-index nest are the policy attributes. Modern Netlink families should have instead defined this as a flat structure, the nesting serves no good purpose here.
Other quirks (todo)¶
Structures¶
Legacy families can define C structures both to be used as the contents
of an attribute and as a fixed message header. The plan is to define
the structs in definitions
and link the appropriate attrs.
C Arrays¶
Legacy families also use binary
attributes to encapsulate C arrays. The
sub-type
is used to identify the type of scalar to extract.
attributes:
-
name: ports
type: binary
sub-type: u32
Multi-message DO¶
New Netlink families should never respond to a DO operation with multiple
replies, with NLM_F_MULTI
set. Use a filtered dump instead.
At the spec level we can define a dumps
property for the do
,
perhaps with values of combine
and multi-object
depending
on how the parsing should be implemented (parse into a single reply
vs list of objects i.e. pretty much a dump).