rtnetlink, NETLINK_ROUTE - Linux IPv4 routing socket


#include <asm/types.h> #include <linux/netlink.h> #include <linux/rtnetlink.h> #include <sys/socket.h>

rtnetlink_socket = socket(AF_NETLINK, int socket_type, NETLINK_ROUTE);


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 only manipulated using the RTA_* macros or libnetlink, see rtnetlink(3).


Rtnetlink consists of these message types (in addition to standard netlink messages):
 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, ifi_change is reserved for future use and should be always set to 0xFFFFFFFF.
Routing attributes
rta_typevalue typedescription
IFLA_ADDRESShardware addressinterface L2 address
IFLA_BROADCASThardware addressL2 broadcast address.
IFLA_IFNAMEasciiz stringDevice name.
IFLA_MTUunsigned intMTU of the device.
IFLA_LINKintLink type.
IFLA_QDISCasciiz stringQueueing discipline.
IFLA_STATSsee below Interface Statistics.

The value type for IFLA_STATS is struct net_device_stats.

 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 rtaddr 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 */ 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.
rta_typevalue typedescription
IFA_ADDRESSraw protocol addressinterface address
IFA_LOCALraw protocol addresslocal address
IFA_LABELasciiz stringname of the interface
IFA_BROADCASTraw protocol addressbroadcast address.
IFA_ANYCASTraw protocol addressanycast address
IFA_CACHEINFOstruct ifa_cacheinfoAddress information.

 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 */ 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_typeRoute type
RTN_UNSPECunknown route
RTN_UNICASTa gateway or direct route
RTN_LOCALa local interface route
RTN_BROADCASTa local broadcast route (sent as a broadcast)
RTN_ANYCASTa local broadcast route (sent as a unicast)
RTN_MULTICASTa multicast route
RTN_BLACKHOLEa packet dropping route
RTN_UNREACHABLEan unreachable destination
RTN_PROHIBITa packet rejection route
RTN_THROWcontinue routing lookup in another table
RTN_NATa network address translation rule
RTN_XRESOLVErefer to an external resolver (not implemented)
rtm_protocolRoute origin.
RTPROT_REDIRECTby an ICMP redirect (currently unused)
RTPROT_KERNELby the kernel
RTPROT_BOOTduring boot
RTPROT_STATICby 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_SITEinterior route in the local autonomous system
RT_SCOPE_LINKroute on this link
RT_SCOPE_HOSTroute on the local host
RT_SCOPE_NOWHEREdestination 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_NOTIFYif the route changes, notify the user via rtnetlink
RTM_F_CLONEDroute is cloned from another route
RTM_F_EQUALIZEa multipath equalizer (not yet implemented)

rtm_table specifies the routing table
RT_TABLE_UNSPECan unspecified routing table
RT_TABLE_DEFAULTthe default table
RT_TABLE_MAINthe main table
RT_TABLE_LOCALthe local table

The user may assign arbitrary values between RT_TABLE_UNSPEC and RT_TABLE_DEFAULT.
rta_typevalue typedescription
RTA_DSTprotocol addressRoute destination address.
RTA_SRCprotocol addressRoute source address.
RTA_IIFintInput interface index.
RTA_OIFintOutput interface index.
RTA_GATEWAYprotocol addressThe gateway of the route
RTA_PRIORITYintPriority of route.
RTA_METRICSintRoute metric

Fill these values in!

 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_INCOMPLETEa currently resolving cache entry
NUD_REACHABLEa confirmed working cache entry
NUD_STALEan expired cache entry
NUD_DELAYan entry waiting for a timer
NUD_PROBEa cache entry that is currently reprobed
NUD_FAILEDan invalid cache entry
NUD_NOARPa device with no destination cache
NUD_PERMANENTa static entry

Valid ndm_flags are:
NTF_PROXYa proxy arp entry
NTF_ROUTERan IPv6 router

The rtaddr struct has the following meanings for the rta_type field:
NDA_UNSPECunknown type
NDA_DSTa neighbor cache n/w layer destination address
NDA_LLADDRa neighbor cache link layer address
NDA_CACHEINFOcache statistics.

If the rta_type field is NDA_CACHEINFO then a struct nda_cacheinfo header follows

 Add, delete or retrieve a routing rule. Carries a struct rtmsg
 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; };
rta_typevalue typeDescription
TCA_KINDasciiz stringName of queueing discipline
TCA_OPTIONSbyte sequenceQdisc-specific options follow
TCA_STATSstruct tc_statsQdisc statistics.
TCA_XSTATSqdisc specificModule-specific statistics.
TCA_RATEstruct tc_estimatorRate limit.

In addition various other qdisc module specific attributes are allowed. For more information see the appropriate include files.

 Add, remove or get a traffic class. These messages contain a struct tcmsg as described above.
 Add, remove or receive information about a traffic filter. These messages contain a struct tcmsg as described above.


rtnetlink is a new feature of Linux 2.2.


This manual page is incomplete.


cmsg(3), rtnetlink(3), ip(7), netlink(7)


This page is part of release 3.23 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at

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