[lkml]   [2003]   [Jan]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
Subjectdoubt in netlink sockets
 Please CC the answers/comments in response to this posting to me.

I read about netlink sockets from the URL :

I also tried the sample program given there to add a new route to the
routing table but it is not working(ie the program is compiling properly but
output is not coming). I tried to see the output by executing the
route -n
command to see whether the new route is being added but nothing got
added. I also downloaded iproute2 code and compared the given code in
the website with iproute2 code but found no differences. I am attaching
that file. Please execute and see whether it is
working. If it is not working please tell me what changes should be made.

//The file included below summarises the code for adding a route to the kernel routing table, the purpose of the code is to show the flow of control, however it may not complete in all respects.

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

struct rtnl_handle
int fd;
struct sockaddr_nl local;
struct sockaddr_nl peer;
__u32 seq;
__u32 dump;

// This code may not be complete in all respects, it just shows the flow of
// control, for more detailed information refer to the code of iproute2 and
// zebra packages.

// This function is to open the netlink socket as the name suggests.
int netlink_open(struct rtnl_handle* rth)
int addr_len;
memset(rth, 0, sizeof(rth));

// Creating the netlink socket of family NETLINK_ROUTE

if (rth->fd < 0) {
perror("cannot open netlink socket");
return -1;

memset(&rth->local, 0, sizeof(rth->local));
rth->local.nl_family = AF_NETLINK;
rth->local.nl_groups = 0;

// Binding the netlink socket
if (bind(rth->fd, (struct sockaddr*)&rth->local, sizeof(rth->local)) < 0)
perror("cannot bind netlink socket");
return -1;
addr_len = sizeof(rth->local);

if (getsockname(rth->fd, (struct sockaddr*)&rth->local, &addr_len) < 0)
perror("cannot getsockname");
return -1;

if (addr_len != sizeof(rth->local)) {
fprintf(stderr, "wrong address lenght %d\n", addr_len);
return -1;

if (rth->local.nl_family != AF_NETLINK) {
fprintf(stderr, "wrong address family %d\n", rth->local.nl_family);
return -1;
rth->seq = time(NULL);
return 0;

// This function does the actual reading and writing to the netlink socket
int rtnl_talk(struct rtnl_handle *rtnl, struct nlmsghdr *n, pid_t peer,
unsigned groups, struct nlmsghdr *answer)
int status;
struct nlmsghdr *h;
struct sockaddr_nl nladdr;

// Forming the iovector with the netlink packet.
struct iovec iov = { (void*)n, n->nlmsg_len };
char buf[8192];

// Forming the message to be sent.
struct msghdr msg = {
(void*)&nladdr, sizeof(nladdr),
&iov, 1,
NULL, 0,

// Filling up the details of the netlink socket to be contacted in the
// kernel.
memset(&nladdr, 0, sizeof(nladdr));
nladdr.nl_family = AF_NETLINK;
nladdr.nl_pid = peer;
nladdr.nl_groups = groups;

n->nlmsg_seq = ++rtnl->seq;
if (answer == NULL)
n->nlmsg_flags |= NLM_F_ACK;

// Actual sending of the message, status contains success/failure
status = sendmsg(rtnl->fd, &msg, 0);

if (status < 0)
return -1;

// This function forms the netlink packet to add a route to the kernel routing
// table
route_add(__u32* destination, __u32* gateway)
struct rtnl_handle rth;

// structure of the netlink packet.
struct {
struct nlmsghdr n;
struct rtmsg r;
char buf[1024];
} req;

char mxbuf[256];
struct rtattr * mxrta = (void*)mxbuf;
unsigned mxlock = 0;

memset(&req, 0, sizeof(req));

// Initialisation of a few parameters
req.n.nlmsg_len = NLMSG_LENGTH(sizeof(struct rtmsg));
req.n.nlmsg_flags = NLM_F_REQUEST|NLM_F_CREATE|NLM_F_EXCL;
req.n.nlmsg_type = RTM_NEWROUTE;
req.r.rtm_family = AF_INET;
req.r.rtm_table = RT_TABLE_MAIN;

req.r.rtm_protocol = RTPROT_BOOT;
req.r.rtm_scope = RT_SCOPE_UNIVERSE;
req.r.rtm_type = RTN_UNICAST;

mxrta->rta_type = RTA_METRICS;
mxrta->rta_len = RTA_LENGTH(0);

// RTA_DST and RTA_GW are the two esential parameters for adding a route,
// there are other parameters too which are not discussed here. For ipv4,
// the length of the address is 4 bytes.
addattr_l(&req.n, sizeof(req), RTA_DST, destination, 4);
addattr_l(&req.n, sizeof(req), RTA_GATEWAY, gateway, 4);

// opening the netlink socket to communicate with the kernel
if (netlink_open(&rth) < 0)
fprintf(stderr, "cannot open rtnetlink\n");

// sending the packet to the kernel.
if (rtnl_talk(&rth, &req.n, 0, 0, NULL) < 0)

return 0;

// This is the utility function for adding the parameters to the packet.
int addattr_l(struct nlmsghdr *n, int maxlen, int type, void *data, int alen)
int len = RTA_LENGTH(alen);
struct rtattr *rta;

if (NLMSG_ALIGN(n->nlmsg_len) + len > maxlen)
return -1;
rta = (struct rtattr*)(((char*)n) + NLMSG_ALIGN(n->nlmsg_len));
rta->rta_type = type;
rta->rta_len = len;
memcpy(RTA_DATA(rta), data, alen);
n->nlmsg_len = NLMSG_ALIGN(n->nlmsg_len) + len;
return 0;

void main()
__u32 dest=0x3209A8FE;
__u32 gw=0x1F09A8C0;
 \ /
  Last update: 2005-03-22 13:32    [W:0.044 / U:1.352 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site