rtnetlink - macros to manipulate rtnetlink messages


#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);"

int RTA_OK(struct rtattr *rta, int rtabuflen);

void *RTA_DATA(struct rtattr *rta);

unsigned int RTA_PAYLOAD(struct rtattr *rta);

struct rtattr *RTA_NEXT(struct rtattr *rta ", unsigned int " rtabuflen );

unsigned int RTA_LENGTH(unsigned int length);

unsigned int RTA_SPACE(unsigned int length);


All rtnetlink(7) messages consist of a netlink(7) message header and appended attributes. The attributes should be only manipulated using the macros provided here.

RTA_OK(rta, attrlen) returns true if rta points to a valid routing attribute; attrlen is the running length of the attribute buffer. When not true then you must assume there are no more attributes in the message, even if attrlen is non-zero.

RTA_DATA(rta) returns a pointer to the start of this attribute’s data.

RTA_PAYLOAD(rta) returns the length of this attribute’s data.

RTA_NEXT(rta, attrlen) gets the next attribute after rta. Calling this macro will update attrlen. You should use RTA_OK to check the validity of the returned pointer.

RTA_LENGTH(len) returns the length which is required for len bytes of data plus the header.

RTA_SPACE(len) returns the amount of space which will be needed in a message with len bytes of data.


These macros are non-standard Linux extensions.


This manual page is incomplete.


Creating a rtnetlink message to set the MTU of a device:

struct { struct nlmsghdr nh; struct ifinfomsg if; char attrbuf[512]; } req;

struct rtattr *rta; unsigned int mtu = 1000;

int rtnetlink_sk = socket(AF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE);

memset(&req, 0, sizeof(req)); req.nh.nlmsg_len = NLMSG_LENGTH(sizeof(struct ifinfomsg)); req.nh.nlmsg_flags = NLM_F_REQUEST; req.nh.nlmsg_type = RTML_NEWLINK; req.if.ifi_family = AF_UNSPEC; req.if.ifi_index = INTERFACE_INDEX; req.if.ifi_change = 0xffffffff; /* ???*/ rta = (struct rtattr *)(((char *) &req) + NLMSG_ALIGN(n->nlmsg_len)); rta->rta_type = IFLA_MTU; rta->rta_len = sizeof(unsigned int); req.n.nlmsg_len = NLMSG_ALIGN(req.n.nlmsg_len) + RTA_LENGTH(sizeof(mtu)); memcpy(RTA_DATA(rta), &mtu, sizeof(mtu)); send(rtnetlink_sk, &req, req.n.nlmsg_len);


netlink(3), netlink(7), rtnetlink(7)


This page is part of release 3.23 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at

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