Rtnetlink allows the kernel's routing tables to be read and
altered. It is used within the kernel to communicate between
various subsystems, though this usage is not documented here, and
for communication with user-space programs. Network routes, IP
addresses, link parameters, neighbor setups, queueing
disciplines, traffic classes and packet classifiers may all be
controlled through NETLINK_ROUTE sockets. It is based on netlink
messages; see netlink(7) for more information.
Routing attributes
Some rtnetlink messages have optional attributes after the
initial header:
struct rtattr {
unsigned short rta_len; /* Length of option */
unsigned short rta_type; /* Type of option */
/* Data follows */
};
These attributes should be manipulated using only the RTA_*
macros or libnetlink, see rtnetlink(3).
Messages
Rtnetlink consists of these message types (in addition to
standard netlink messages):
RTM_NEWLINK, RTM_DELLINK, RTM_GETLINK
Create, remove, or get information about a specific
network interface. These messages contain an ifinfomsg
structure followed by a series of rtattr structures.
struct ifinfomsg {
unsigned char ifi_family; /* AF_UNSPEC */
unsigned short ifi_type; /* Device type */
int ifi_index; /* Interface index */
unsigned int ifi_flags; /* Device flags */
unsigned int ifi_change; /* change mask */
};
ifi_flags contains the device flags, see netdevice(7);
ifi_index is the unique interface index (since Linux 3.7,
it is possible to feed a nonzero value with the
RTM_NEWLINK message, thus creating a link with the given
ifindex); ifi_change is reserved for future use and should
be always set to 0xFFFFFFFF.
Routing attributes
rta_type Value type Description
─────────────────────────────────────────────────────────
IFLA_UNSPEC - unspecified
IFLA_ADDRESS hardware address interface L2 address
IFLA_BROADCAST hardware address L2 broadcast address
IFLA_IFNAME asciiz string Device name
IFLA_MTU unsigned int MTU of the device
IFLA_LINK int Link type
IFLA_QDISC asciiz string Queueing discipline
IFLA_STATS see below Interface Statistics
The value type for IFLA_STATS is struct rtnl_link_stats
(struct net_device_stats in Linux 2.4 and earlier).
RTM_NEWADDR, RTM_DELADDR, RTM_GETADDR
Add, remove, or receive information about an IP address
associated with an interface. In Linux 2.2, an interface
can carry multiple IP addresses, this replaces the alias
device concept in 2.0. In Linux 2.2, these messages
support IPv4 and IPv6 addresses. They contain an
ifaddrmsg structure, optionally followed by rtattr routing
attributes.
struct ifaddrmsg {
unsigned char ifa_family; /* Address type */
unsigned char ifa_prefixlen; /* Prefixlength of address */
unsigned char ifa_flags; /* Address flags */
unsigned char ifa_scope; /* Address scope */
unsigned int ifa_index; /* Interface index */
};
ifa_family is the address family type (currently AF_INET
or AF_INET6), ifa_prefixlen is the length of the address
mask of the address if defined for the family (like for
IPv4), ifa_scope is the address scope, ifa_index is the
interface index of the interface the address is associated
with. ifa_flags is a flag word of IFA_F_SECONDARY for
secondary address (old alias interface), IFA_F_PERMANENT
for a permanent address set by the user and other
undocumented flags.
Attributes
rta_type Value type Description
─────────────────────────────────────────────────────────────
IFA_UNSPEC - unspecified
IFA_ADDRESS raw protocol address interface address
IFA_LOCAL raw protocol address local address
IFA_LABEL asciiz string name of the interface
IFA_BROADCAST raw protocol address broadcast address
IFA_ANYCAST raw protocol address anycast address
IFA_CACHEINFO struct ifa_cacheinfo Address information
RTM_NEWROUTE, RTM_DELROUTE, RTM_GETROUTE
Create, remove, or receive information about a network
route. These messages contain an rtmsg structure with an
optional sequence of rtattr structures following. For
RTM_GETROUTE, setting rtm_dst_len and rtm_src_len to 0
means you get all entries for the specified routing table.
For the other fields, except rtm_table and rtm_protocol, 0
is the wildcard.
struct rtmsg {
unsigned char rtm_family; /* Address family of route */
unsigned char rtm_dst_len; /* Length of destination */
unsigned char rtm_src_len; /* Length of source */
unsigned char rtm_tos; /* TOS filter */
unsigned char rtm_table; /* Routing table ID;
see RTA_TABLE below */
unsigned char rtm_protocol; /* Routing protocol; see below */
unsigned char rtm_scope; /* See below */
unsigned char rtm_type; /* See below */
unsigned int rtm_flags;
};
rtm_type Route type
───────────────────────────────────────────────────────────
RTN_UNSPEC unknown route
RTN_UNICAST a gateway or direct route
RTN_LOCAL a local interface route
RTN_BROADCAST a local broadcast route (sent as a
broadcast)
RTN_ANYCAST a local broadcast route (sent as a
unicast)
RTN_MULTICAST a multicast route
RTN_BLACKHOLE a packet dropping route
RTN_UNREACHABLE an unreachable destination
RTN_PROHIBIT a packet rejection route
RTN_THROW continue routing lookup in another table
RTN_NAT a network address translation rule
RTN_XRESOLVE refer to an external resolver (not
implemented)
rtm_protocol Route origin
───────────────────────────────────────
RTPROT_UNSPEC unknown
RTPROT_REDIRECT by an ICMP redirect
(currently unused)
RTPROT_KERNEL by the kernel
RTPROT_BOOT during boot
RTPROT_STATIC by the administrator
Values larger than RTPROT_STATIC are not interpreted by
the kernel, they are just for user information. They may
be used to tag the source of a routing information or to
distinguish between multiple routing daemons. See
<linux/rtnetlink.h> for the routing daemon identifiers
which are already assigned.
rtm_scope is the distance to the destination:
RT_SCOPE_UNIVERSE global route
RT_SCOPE_SITE interior route in the
local autonomous system
RT_SCOPE_LINK route on this link
RT_SCOPE_HOST route on the local host
RT_SCOPE_NOWHERE destination doesn't exist
The values between RT_SCOPE_UNIVERSE and RT_SCOPE_SITE are
available to the user.
The rtm_flags have the following meanings:
RTM_F_NOTIFY if the route changes, notify the user via
rtnetlink
RTM_F_CLONED route is cloned from another route
RTM_F_EQUALIZE a multipath equalizer (not yet implemented)
rtm_table specifies the routing table
RT_TABLE_UNSPEC an unspecified routing table
RT_TABLE_DEFAULT the default table
RT_TABLE_MAIN the main table
RT_TABLE_LOCAL the local table
The user may assign arbitrary values between
RT_TABLE_UNSPEC and RT_TABLE_DEFAULT.
Attributes
rta_type Value type Description
────────────────────────────────────────────────────────────────
RTA_UNSPEC - ignored
RTA_DST protocol address Route destination address
RTA_SRC protocol address Route source address
RTA_IIF int Input interface index
RTA_OIF int Output interface index
RTA_GATEWAY protocol address The gateway of the route
RTA_PRIORITY int Priority of route
RTA_PREFSRC protocol address Preferred source address
RTA_METRICS int Route metric
RTA_MULTIPATH Multipath nexthop data br
(see below).
RTA_PROTOINFO No longer used
RTA_FLOW int Route realm
RTA_CACHEINFO struct rta_cacheinfo (see linux/rtnetlink.h)
RTA_SESSION No longer used
RTA_MP_ALGO No longer used
RTA_TABLE int Routing table ID; if set,
rtm_table is ignored
RTA_MARK int
RTA_MFC_STATS struct rta_mfc_stats (see linux/rtnetlink.h)
RTA_VIA struct rtvia Gateway in different AF
(see below)
RTA_NEWDST protocol address Change packet destination
address
RTA_PREF char RFC4191 IPv6 router
preference (see below)
RTA_ENCAP_TYPE short Encapsulation type for
lwtunnels (see below)
RTA_ENCAP Defined by RTA_ENCAP_TYPE
RTA_EXPIRES int Expire time for IPv6
routes (in seconds)
RTA_MULTIPATH contains several packed instances of struct
rtnexthop together with nested RTAs (RTA_GATEWAY):
struct rtnexthop {
unsigned short rtnh_len; /* Length of struct + length
of RTAs */
unsigned char rtnh_flags; /* Flags (see
linux/rtnetlink.h) */
unsigned char rtnh_hops; /* Nexthop priority */
int rtnh_ifindex; /* Interface index for this
nexthop */
}
There exist a bunch of RTNH_* macros similar to RTA_* and
NLHDR_* macros useful to handle these structures.
struct rtvia {
unsigned short rtvia_family;
unsigned char rtvia_addr[0];
};
rtvia_addr is the address, rtvia_family is its family
type.
RTA_PREF may contain values ICMPV6_ROUTER_PREF_LOW,
ICMPV6_ROUTER_PREF_MEDIUM, and ICMPV6_ROUTER_PREF_HIGH
defined incw <linux/icmpv6.h>.
RTA_ENCAP_TYPE may contain values LWTUNNEL_ENCAP_MPLS,
LWTUNNEL_ENCAP_IP, LWTUNNEL_ENCAP_ILA, or
LWTUNNEL_ENCAP_IP6 defined in <linux/lwtunnel.h>.
Fill these values in!
RTM_NEWNEIGH, RTM_DELNEIGH, RTM_GETNEIGH
Add, remove, or receive information about a neighbor table
entry (e.g., an ARP entry). The message contains an ndmsg
structure.
struct ndmsg {
unsigned char ndm_family;
int ndm_ifindex; /* Interface index */
__u16 ndm_state; /* State */
__u8 ndm_flags; /* Flags */
__u8 ndm_type;
};
struct nda_cacheinfo {
__u32 ndm_confirmed;
__u32 ndm_used;
__u32 ndm_updated;
__u32 ndm_refcnt;
};
ndm_state is a bit mask of the following states:
NUD_INCOMPLETE a currently resolving cache entry
NUD_REACHABLE a confirmed working cache entry
NUD_STALE an expired cache entry
NUD_DELAY an entry waiting for a timer
NUD_PROBE a cache entry that is currently reprobed
NUD_FAILED an invalid cache entry
NUD_NOARP a device with no destination cache
NUD_PERMANENT a static entry
Valid ndm_flags are:
NTF_PROXY a proxy arp entry
NTF_ROUTER an IPv6 router
The rtattr struct has the following meanings for the
rta_type field:
NDA_UNSPEC unknown type
NDA_DST a neighbor cache n/w layer destination address
NDA_LLADDR a neighbor cache link layer address
NDA_CACHEINFO cache statistics
If the rta_type field is NDA_CACHEINFO, then a struct
nda_cacheinfo header follows.
RTM_NEWRULE, RTM_DELRULE, RTM_GETRULE
Add, delete, or retrieve a routing rule. Carries a struct
rtmsg
RTM_NEWQDISC, RTM_DELQDISC, RTM_GETQDISC
Add, remove, or get a queueing discipline. The message
contains a struct tcmsg and may be followed by a series of
attributes.
struct tcmsg {
unsigned char tcm_family;
int tcm_ifindex; /* interface index */
__u32 tcm_handle; /* Qdisc handle */
__u32 tcm_parent; /* Parent qdisc */
__u32 tcm_info;
};
Attributes
rta_type Value type Description
────────────────────────────────────────────────────────────────
TCA_UNSPEC - unspecified
TCA_KIND asciiz string Name of queueing discipline
TCA_OPTIONS byte sequence Qdisc-specific options follow
TCA_STATS struct tc_stats Qdisc statistics
TCA_XSTATS qdisc-specific Module-specific statistics
TCA_RATE struct tc_estimator Rate limit
In addition, various other qdisc-module-specific
attributes are allowed. For more information see the
appropriate include files.
RTM_NEWTCLASS, RTM_DELTCLASS, RTM_GETTCLASS
Add, remove, or get a traffic class. These messages
contain a struct tcmsg as described above.
RTM_NEWTFILTER, RTM_DELTFILTER, RTM_GETTFILTER
Add, remove, or receive information about a traffic
filter. These messages contain a struct tcmsg as
described above.
