rtnetlink - Linux routing socket

#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);

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.

TABLE

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.

TABLE

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.

TABLE

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;
};

TABLE

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.

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