Developer’s guide to Netlink and pyroute2
General netlink packet structure:
nlmsg packet:
+ header
+ data
Generic netlink message header:
nlmsg header:
+ uint32 length
+ uint16 type
+ uint16 flags
+ uint32 sequence number
+ uint32 pid
The length field is the length of all the packet, including data and header. The type field is used to distinguish different message types, commands etc. Please note, that there is no explicit protocol field – you choose a netlink protocol, when you create a socket.
The sequence number is very important. Netlink is an asynchronous protocol – it means, that the packet order doesn’t matter. But responses to a request are always marked with the same sequence number, so you can treat it as a cookie.
Please keep in mind, that a netlink request can initiate a cascade of events, and netlink messages from these events can carry sequence number == 0. E.g., it is so when you remove a primary IP addr from an interface, when promote_secondaries sysctl is set.
Beside of incapsulated headers and other protocol-specific data, netlink messages can carry NLA (netlink attributes). NLA structure is as follows:
NLA header:
+ uint16 length
+ uint16 type
NLA data:
+ data-specific struct
# optional:
+ NLA
+ NLA
+ ...
So, NLA structures can be nested, forming a tree.
Complete structure of a netlink packet:
nlmsg header:
+ uint32 length
+ uint16 type
+ uint16 flags
+ uint32 sequence number
+ uint32 pid
[ optional protocol-specific data ]
[ optional NLA tree ]
More information about netlink protocol you can find in the man pages.
RTNL is a netlink protocol, used to get and set information about different network objects – addresses, routes, interfaces etc.
RTNL protocol-specific data in messages depends on the object type. E.g., complete packet with the interface address information:
nlmsg header:
+ uint32 length
+ uint16 type
+ uint16 flags
+ uint32 sequence number
+ uint32 pid
ifaddrmsg structure:
+ unsigned char ifa_family
+ unsigned char ifa_prefixlen
+ unsigned char ifa_flags
+ unsigned char ifa_scope
+ uint32 ifa_index
[ optional NLA tree ]
NLA for this kind of packets can be of type IFA_ADDRESS, IFA_LOCAL etc. – please refer to the corresponding source.
Other objects types require different structures, sometimes really complex. All these structures are described in sources.
To simplify the development, pyroute2 provides an easy way to describe packet structure. As an example, you can take the ifaddrmsg description – pyroute2/netlink/rtnl/ifaddrmsg.py.
To describe a packet, you need to inherit from nlmsg class:
from pyroute2.netlink.generic import nlmsg
class foo_msg(nlmsg):
fields = ( ... )
nla_map = ( ... )
NLA are described in the same way, but the parent class should be nla, instead of nlmsg. And yes, it is important to use the proper parent class.
The fields attribute describes the structure of the protocol-specific data. It is a tuple of tuples, where each member contains a field name and its data format.
Field data format should be specified as for Python struct module. E.g., ifaddrmsg structure:
ifaddrmsg structure:
+ unsigned char ifa_family
+ unsigned char ifa_prefixlen
+ unsigned char ifa_flags
+ unsigned char ifa_scope
+ int ifa_index
should be described as follows:
class ifaddrmsg(nlmsg):
fields = (('family', 'B'),
('prefixlen', 'B'),
('flags', 'B'),
('scope', 'B'),
('index', 'I'))
Format strings are passed directly to the struct module, so you can use all the notations like >I, 16s etc. All fields are parsed from the stream separately, so if you want to explicitly fix alignemt, as if it were C struct, use the pack attribute:
class tstats(nla):
pack = "struct"
fields = (('version', 'H'),
('ac_exitcode', 'I'),
('ac_flag', 'B'),
...)
The nla_map attribute is a tuple of NLA descriptions. Each description is also a tuple in two different forms: either two fields, name and format, or three fields: type, name and format.
Please notice, that the format field is a string name of corresponding NLA class:
class ifaddrmsg(nlmsg):
...
nla_map = (('IFA_UNSPEC', 'hex'),
('IFA_ADDRESS', 'ipaddr'),
('IFA_LOCAL', 'ipaddr'),
...)
This code will create mapping, where IFA_ADDRESS NLA will be of type 1 and IFA_LOCAL – of type 2, etc. Both NLA will be decoded as IP addresses (class ipaddr). IFA_UNSPEC will be of type 0, and if it will be in the NLA tree, it will be just dumped in hex.
NLA class names are should be specified as strings, since they are resolved in runtime.
There are several pre-defined NLA types, that you will get with nla class:
Please refer to pyroute2/netlink/generic.py for details.
You can also make your own NLA descriptions:
class ifaddrmsg(nlmsg):
...
nla_map = (...
('IFA_CACHEINFO', 'cacheinfo'),
...)
class cacheinfo(nla):
fields = (('ifa_prefered', 'I'),
('ifa_valid', 'I'),
('cstamp', 'I'),
('tstamp', 'I'))
Custom NLA descriptions should be defined in the same class, where they are used.
Also, it is possible to use not autogenerated type numbers, as for ifaddrmsg, but specify them explicitly:
class iw_event(nla):
...
nla_map = ((0x8B00, 'SIOCSIWCOMMIT', 'hex'),
(0x8B01, 'SIOCGIWNAME', 'hex'),
(0x8B02, 'SIOCSIWNWID', 'hex'),
(0x8B03, 'SIOCGIWNWID', 'hex'),
...)
Here you can see custom NLA type numbers – 0x8B00, 0x8B01 etc. It is not permitted to mix these two forms in one class: you should use ether autogenerated type numbers (two fields tuples), or explicit numbers (three fields typles).
Netlink messages are represented by pyroute2 as dictionaries as follows:
{'header': {'pid': ...,
'length: ...,
'flags': ...,
'error': None, # if you are lucky
'type': ...,
'sequence_number': ...},
# fields attributes
'field_name1': value,
...
'field_nameX': value,
# nla tree
'attrs': [['NLA_NAME1', value],
...
['NLA_NAMEX', value],
['NLA_NAMEY', {'field_name1': value,
...
'field_nameX': value,
'attrs': [['NLA_NAME.... ]]}]]}
As an example, a message from the wireless subsystem about new scan event:
{'index': 4,
'family': 0,
'__align': 0,
'header': {'pid': 0,
'length': 64,
'flags': 0,
'error': None,
'type': 16,
'sequence_number': 0},
'flags': 69699,
'ifi_type': 1,
'event': 'RTM_NEWLINK',
'change': 0,
'attrs': [['IFLA_IFNAME', 'wlp3s0'],
['IFLA_WIRELESS',
{'attrs': [['SIOCGIWSCAN',
'00:00:00:00:00:00:00:00:00:00:00:00']]}]]}
Using high-level interfaces like IPRoute or IPDB, you will never need to manually construct and send netlink messages. But in the case you really need it, it is simple as well.
Having a description class, like ifaddrmsg from above, you need to:
The code:
from pyroute2.netlink import NLM_F_REQUEST
from pyroute2.netlink import NLM_F_ACK
from pyroute2.netlink import NLM_F_CREATE
from pyroute2.netlink import NLM_F_EXCL
from pyroute2.netlink.iproute import RTM_NEWADDR
from pyroute2.netlink.rtnl.ifaddrmsg import ifaddrmsg
##
# add an addr to an interface
#
# create the message
msg = ifaddrmsg()
# fill the protocol-specific fields
msg['index'] = index # index of the interface
msg['family'] = AF_INET # address family
msg['prefixlen'] = 24 # the address mask
msg['scope'] = scope # see /etc/iproute2/rt_scopes
# attach NLA -- it MUST be a list / mutable
msg['attrs'] = [['IFA_LOCAL', '192.168.0.1'],
['IFA_ADDRESS', '192.162.0.1']]
# fill generic netlink fields
msg['header']['sequence_number'] = nonce # an unique seq number
msg['header']['pid'] = os.getpid()
msg['header']['type'] = RTM_NEWADDR
msg['header']['flags'] = NLM_F_REQUEST |\
NLM_F_ACK |\
NLM_F_CREATE |\
NLM_F_EXCL
# encode the packet
msg.encode()
# send the buffer
nlsock.send(msg.buf.getvalue())
Please notice, that NLA list MUST be mutable.