Manipulating the Networking Environment Using RTNETLINK

The sample code presented here focuses on three of the operations that can be performed on the routing table:

All three samples use a similar main() function that calls a set of subfunctions to form RTNETLINK messages and send, receive and process the received messages. To simplify the explanation, no error handling is considered. These samples perform on the IP version 4 environment of the system (AF_INET). Here is the main() function:

int main(int argc, char *argv[])
{

  // open socket
  fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);

  // setup local address & bind using
  // this address
  bzero(&la, sizeof(la));
  la.nl_family = AF_NETLINK;
  la.nl_pid = getpid();
  bind(fd, (struct sockaddr*) &la, sizeof(la));


  // sub functions to create RTNETLINK message,
  // send over socket, receive reply & process
  // message
  form_request();
  send_request();
  recv_reply();
  read_reply();

  // close socket
  close(fd);
}

Similar to the above function, the two functions that perform the socket communications are almost common to all the samples. These two functions simply send a formed message to the kernel and receive messages sent by the kernel. Exceptions here are the set_routing_table and mon_routing_table samples. In set_routing_table, a receive phase is not considered. In the mon_routing_table, a send phase is not present as it attempts to monitor only the state of the routing environment to see what is being changed. This information is mulitcast by the kernel to all the RTNETLINK sockets that are in the appropriate receiving state.

First, here's the code for send_request():

void send_request()
{
  // create the remote address
  // to communicate
  bzero(&pa, sizeof(pa));
  pa.nl_family = AF_NETLINK;

  // initialize & create the struct msghdr supplied
  // to the sendmsg() function
  bzero(&msg, sizeof(msg));
  msg.msg_name = (void *) &pa;
  msg.msg_namelen = sizeof(pa);

  // place the pointer & size of the RTNETLINK
  // message in the struct msghdr
  iov.iov_base = (void *) &req.nl;
  iov.iov_len = req.nl.nlmsg_len;
  msg.msg_iov = &iov;
  msg.msg_iovlen = 1;

  // send the RTNETLINK message to kernel
  rtn = sendmsg(fd, &msg, 0);
}

And, here's the recv_reply():

void recv_reply()
{
  char *p;

  // initialize the socket read buffer
  bzero(buf, sizeof(buf));

  p = buf;
  nll = 0;

  // read from the socket until the NLMSG_DONE is
  // returned in the type of the RTNETLINK message
  // or if it was a monitoring socket
  while(1) {
    rtn = recv(fd, p, sizeof(buf) - nll, 0);

    nlp = (struct nlmsghdr *) p;

    if(nlp->nlmsg_type == NLMSG_DONE)
      break;

    // increment the buffer pointer to place
    // next message
    p += rtn;

    // increment the total size by the size of
    // the last received message
    nll += rtn;

    if((la.nl_groups & RTMGRP_IPV4_ROUTE)
                      == RTMGRP_IPV4_ROUTE)
      break;
  }
}

The above functions and the following ones use a set of globally defined variables. These are used for all the socket operations as well as for forming and processing RTNETLINK messages:

// buffer to hold the RTNETLINK request
struct {
  struct nlmsghdr nl;
  struct rtmsg    rt;
  char            buf[8192];
} req;

// variables used for
// socket communications
int fd;
struct sockaddr_nl la;
struct sockaddr_nl pa;
struct msghdr msg;
struct iovec iov;
int rtn;

// buffer to hold the RTNETLINK reply(ies)
char buf[8192];

// RTNETLINK message pointers & lengths
// used when processing messages
struct nlmsghdr *nlp;
int nll;
struct rtmsg *rtp;
int rtl;
struct rtattr *rtap;

The get_routing_table sample retrieves the main routing table of the IPv4 environment. The form_request() function is as follows:

void form_request()
{
  // initialize the request buffer
  bzero(&req, sizeof(req));

  // set the NETLINK header
  req.nl.nlmsg_len
             = NLMSG_LENGTH(sizeof(struct rtmsg));
  req.nl.nlmsg_flags = NLM_F_REQUEST | NLM_F_DUMP;
  req.nl.nlmsg_type = RTM_GETROUTE;

  // set the routing message header
  req.rt.rtm_family = AF_INET;
  req.rt.rtm_table = RT_TABLE_MAIN;
}

The received message for the RTNETLINK request in the buf variable to retrieve the routing table is processed by the read_reply() function. Here is the code of this function:

void read_reply()
{
  // string to hold content of the route
  // table (i.e. one entry)
  char dsts[24], gws[24], ifs[16], ms[24];

  // outer loop: loops thru all the NETLINK
  // headers that also include the route entry
  // header
  nlp = (struct nlmsghdr *) buf;
  for(;NLMSG_OK(nlp, nll);nlp=NLMSG_NEXT(nlp, nll))
  {

    // get route entry header
    rtp = (struct rtmsg *) NLMSG_DATA(nlp);

    // we are only concerned about the
    // main route table
    if(rtp->rtm_table != RT_TABLE_MAIN)
      continue;

    // init all the strings
    bzero(dsts, sizeof(dsts));
    bzero(gws, sizeof(gws));
    bzero(ifs, sizeof(ifs));
    bzero(ms, sizeof(ms));

    // inner loop: loop thru all the attributes of
    // one route entry
    rtap = (struct rtattr *) RTM_RTA(rtp);
    rtl = RTM_PAYLOAD(nlp);
    for(;RTA_OK(rtap, rtl);rtap=RTA_NEXT(rtap,rtl))
    {
      switch(rtap->rta_type)
      {
        // destination IPv4 address
        case RTA_DST:
          inet_ntop(AF_INET, RTA_DATA(rtap),
                                     dsts, 24);
          break;

        // next hop IPv4 address
        case RTA_GATEWAY:
          inet_ntop(AF_INET, RTA_DATA(rtap),
                                     gws, 24);
          break;

        // unique ID associated with the network
        // interface
        case RTA_OIF:
          sprintf(ifs, "%d",
                   *((int *) RTA_DATA(rtap)));
        default:
          break;
      }
    }
    sprintf(ms, "%d", rtp->rtm_dst_len);

    printf("dst %s/%s gw %s if %s\n",
                          dsts, ms, gws, ifs);
  }
}

The set_routing_table sample sends an RTNETLINK request to insert an entry to the routing table. The route entry that is inserted is a host route (32-bit network prefix) to a private IP address (192.168.0.100) through interface number 2. These values are defined in the variables dsts (destination IP address), ifcn (interface number) and pn (prefix length). You can run the get_routing_table sample to get an idea about the interface numbers and the IP network in your system. Here's the form_request():

void form_request()
{
  // attributes of the route entry
  char dsts[24] = "192.168.0.100";
  int ifcn = 2, pn = 32;

  // initialize RTNETLINK request buffer
  bzero(&req, sizeof(req));

  // compute the initial length of the
  // service request
  rtl = sizeof(struct rtmsg);

  // add first attrib:
  // set destination IP addr and increment the
  // RTNETLINK buffer size
  rtap = (struct rtattr *) req.buf;
  rtap->rta_type = RTA_DST;
  rtap->rta_len = sizeof(struct rtattr) + 4;
  inet_pton(AF_INET, dsts,
     ((char *)rtap) + sizeof(struct rtattr));
  rtl += rtap->rta_len;

  // add second attrib:
  // set ifc index and increment the size
  rtap = (struct rtattr *) (((char *)rtap)
            + rtap->rta_len);
  rtap->rta_type = RTA_OIF;
  rtap->rta_len = sizeof(struct rtattr) + 4;
  memcpy(((char *)rtap) + sizeof(struct rtattr),
           &ifcn, 4);
  rtl += rtap->rta_len;

  // setup the NETLINK header
  req.nl.nlmsg_len = NLMSG_LENGTH(rtl);
  req.nl.nlmsg_flags = NLM_F_REQUEST | NLM_F_CREATE;
  req.nl.nlmsg_type = RTM_NEWROUTE;

  // setup the service header (struct rtmsg)
  req.rt.rtm_family = AF_INET;
  req.rt.rtm_table = RT_TABLE_MAIN;
  req.rt.rtm_protocol = RTPROT_STATIC;
  req.rt.rtm_scope = RT_SCOPE_UNIVERSE;
  req.rt.rtm_type = RTN_UNICAST;
  // set the network prefix size
  req.rt.rtm_dst_len = pn;
}

The mon_routing_table sample reads the RTNETLINK messages received when other processes change the system's main routing table. This function will use the same read_reply() function to process the messages. The main() function requires a slight change. Because this operation involves listening to multicast messages of the kernel, the local address to which we bind, it also must include the two flags RTMGRP_IPV4_ROUTE and RTMGRP_NOTIFY. Here is the required change:

la.nl_groups = RTMGRP_IPV4_ROUTE | RTMGRP_NOTIFY;

Once mon_routing_table is executed, run a route add or a route del command from another shell prompt to see the results.