Rtnetlink (7) Linux Manual Page

rtnetlink – Linux IPv4 routing socket

Synopsis

#include <asm/types.h>

#include <linux/if_link.h>

#include <linux/netlink.h>

#include <linux/rtnetlink.h>

#include <sys/socket.h>
rtnetlink_socket = socket(AF_NETLINK, int socket_type, NETLINK_ROUTE);

Description

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: unsigned int rtm_flags;
}
;

rtm_typeRoute type

RTN_UNSPECunknown route

RTN_UNICASTa gateway or
direct route

RTN_LOCALa local interface route

RTN_BROADCAST
a local broadcast
route(sent as a broadcast)

RTN_ANYCAST
a 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_XRESOLVE
refer to an external resolver(not implemented)

rtm_protocolRoute origin

RTPROT_UNSPECunknown

RTPROT_REDIRECT
by an ICMP redirect(currently unused)

RTPROT_KERNELby the kernel

RTPROT_BOOTduring boot

RTPROT_STATICby the administrator 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_typeRoute type

        RTN_UNSPECunknown route

            RTN_UNICASTa gateway or
    direct route

        RTN_LOCALa local interface route

            RTN_BROADCAST
                a local broadcast
                route(sent as a broadcast)

                    RTN_ANYCAST
    a 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_XRESOLVE
    refer to an external resolver(not implemented)

        rtm_protocolRoute origin

    RTPROT_UNSPECunknown

    RTPROT_REDIRECT
    by 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_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.

Versions

rtnetlink is a new feature of Linux 2.2.

Bugs

This manual page is incomplete.

Colophon

This page is part of release 5.10 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

rtnetlink (7) Linux Manual Page