--- name: "rtnetlink" section: "7" platform: "linux" package: "man-pages" version: "6.19-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/rtnetlink/7" --- # rtnetlink(7) Linux routing socket ## NAME rtnetlink - Linux routing socket ## SYNOPSIS ``` #include #include #include #include ``` **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)](/netlink/7.md) 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)](/rtnetlink/3.md). ### 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)](/netdevice/7.md); *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 a mask specifying which bits of *ifi\_flags* should be applied to the device. | | | | | --- | --- | --- | | 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 | - ``` struct ifa_cacheinfo { __u32 ifa_prefered; // Preferred lifetime remaining, in seconds __u32 ifa_valid; // Valid lifetime remaining, in seconds __u32 cstamp; // Creation timestamp, in centiseconds __u32 tstamp; // Update timestamp, in centiseconds }; ``` - *ifa\_valid* cannot be zero, and *ifa\_prefered* cannot be greater than *ifa\_valid*. A value of **UINT32\_MAX** represents an infinite lifetime. - **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 *\* 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** | | Route metrics (see below). | | **RTA\_MULTIPATH** | | Multipath nexthop data (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\_METRICS** contains an array of *struct rtattr* with their corresponding attributes: - | | | | | --- | --- | --- | | Attributes | | **rta\_type** | Value type | Description | | **RTAX\_UNSPEC** | - | unspecified | | **RTAX\_LOCK** | \_\_u32 | Bit field indicating which RTAX\_\* attributes are locked. | | **RTAX\_MTU** | \_\_u32 | Maximum Transmission Unit for this route. | | **RTAX\_WINDOW** | \_\_u32 | Maximum size of the receive window for this route. | | **RTAX\_RTT** | \_\_u32 | Estimated round-trip time for this route. | | **RTAX\_RTTVAR** | \_\_u32 | Estimated round-trip time variation for this route. | | **RTAX\_SSTHRESH** | \_\_u32 | Slow start threshold to use for this route. | | **RTAX\_CWND** | \_\_u32 | Maximum size of the congestion window for this route. | | **RTAX\_ADVMSS** | \_\_u32 | Maximum Segment Size to advertise for this route. | | **RTAX\_REORDERING** | \_\_u32 | Initial reordering level of packets for this route. | | **RTAX\_HOPLIMIT** | \_\_u32 | Hop limit (TTL) to use for this route. | | **RTAX\_INITCWND** | \_\_u32 | Initial congestion window to use for this route. | | **RTAX\_FEATURES** | \_\_u32 | Features to enable for this route specifically. | | **RTAX\_RTO\_MIN** | \_\_u32 | Minimum Retransmission TimeOut to use for this route. | | **RTAX\_INITRWND** | \_\_u32 | Initial size of the receive window for this route. | | **RTAX\_QUICKACK** | \_\_u32 | Use quick ack for this route. | | **RTAX\_CC\_ALGO** | asciiz string | Congestion Control algorithm to use for this route. | | **RTAX\_FASTOPEN\_NO\_COOKIE** | \_\_u32 | Allow TCP Fast Open without cookie. | - Metrics that are locked with **RTAX\_LOCK** take precedence over the values normally used by the kernel (computed or assigned by a sysctl or [setsockopt(2)](/setsockopt/2.md)). Therefore, some metrics, like **RTAX\_RTO\_MIN**, have no effect unless their bit is set in **RTAX\_LOCK**. - **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 *\*. - **RTA\_ENCAP\_TYPE** may contain values **LWTUNNEL\_ENCAP\_MPLS**, **LWTUNNEL\_ENCAP\_IP**, **LWTUNNEL\_ENCAP\_ILA**, or **LWTUNNEL\_ENCAP\_IP6** defined in *\*. - **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)](/cmsg/3.md), [**rtnetlink**(3)](/rtnetlink/3.md), [**ip**(7)](/ip/7.md), [**netlink**(7)](/netlink/7.md)