rtnetlink(7)

NAME
rtnetlink - Linux 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:

    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
    IFLA_PERM_ADDRESS | hardware address | hardware address provided by device (since Linux 5.5)

    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 Linux 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.

VERSIONS
rtnetlink is a new feature of Linux 2.2.

BUGS
This manual page is incomplete.

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