rtnetlink(3) -- Linux man page



rtnetlink - Macros to manipuate rtnetlink messages  


#include <asm/types.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>
#include <sys/socket.h>

rtnetlink_socket = socket(PF_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 for 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 the message with len bytes of data.



Creating a rtnetlink message to set a 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(PF_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) + 
        rta->rta_type = IFLA_MTU;
        rta->rta_len = sizeof(unsigned int);
        req.n.nlmsg_len = NLMSG_ALIGN(req.n.nlmsg_len) + 
        memcpy(RTA_DATA(rta), &mtu, sizeof (mtu));
        send(rtnetlink_sk, &req, req.n.nlmsg_len); 



This manual page is lacking and incomplete.



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