rtnetlink(7) Miscellaneous Information Manual rtnetlink(7)

rtnetlink - Linux 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.

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).

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 (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).
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.
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
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_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.
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_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_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>.
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_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.
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_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.
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)

2024-05-02 Linux man-pages 6.9.1