rtnetlink - macros to manipulate rtnetlink messages
Standard C library (
libc,
-lc)
#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 manipulated only 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 nonzero.
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 nonstandard Linux extensions.
This manual page is incomplete.
Creating a rtnetlink message to set the MTU of a device:
#include <linux/rtnetlink.h>
...
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(req.if));
req.nh.nlmsg_flags = NLM_F_REQUEST;
req.nh.nlmsg_type = RTM_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(req.nh.nlmsg_len));
rta->rta_type = IFLA_MTU;
rta->rta_len = RTA_LENGTH(sizeof(mtu));
req.nh.nlmsg_len = NLMSG_ALIGN(req.nh.nlmsg_len) +
RTA_LENGTH(sizeof(mtu));
memcpy(RTA_DATA(rta), &mtu, sizeof(mtu));
send(rtnetlink_sk, &req, req.nh.nlmsg_len, 0);
netlink(3),
netlink(7),
rtnetlink(7)