On Sat, 18 Jun 2005, Thomas Graf wrote:

* Tom?? Macek [ref] 2005-06-18 20:55

quoted

The 'rtm_dst_len = 16' should mean the mask of the route I'm looking for, correct?

Yes.

quoted

The whole code before sending the packet is below:


     /* Create Socket */
     if((sock = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE)) < 0)
         perror("Socket Creation: ");

     /* Initialize the buffer */
     memset(msgBuf, 0, BUFSIZE);

     /* point the header and the msg structure pointers into the buffer */
     nlMsg = (struct nlmsghdr *)msgBuf;
     rtMsg = (struct rtmsg *)NLMSG_DATA(nlMsg);
     rtMsg->rtm_family = AF_INET;
     rtMsg->rtm_dst_len = 16;

     /* Fill in the nlmsg header*/
     nlMsg->nlmsg_len = NLMSG_LENGTH(sizeof(struct rtmsg)); // Length of message.
     nlMsg->nlmsg_type = RTM_GETROUTE;   // Get the routes from kernel routing table .
     nlMsg->nlmsg_flags = NLM_F_REQUEST;   // The message is a request for dump.
     nlMsg->nlmsg_seq = msgSeq++;   // Sequence of the message packet.
     nlMsg->nlmsg_pid = getpid();   // PID of process sending the request.

     char *cp;
     unsigned int xx[4]; int i = 0;
     unsigned char *ap = (unsigned char *)xx;
     for (cp = argv[1], i = 0; *cp; cp++) {
         if (*cp <= '9' && *cp >= '0') {
             ap[i] = 10*ap[i] + (*cp-'0');
             continue;
         }
         if (*cp == '.' && ++i <= 3)
             continue;
         return -1;
     }

     NetlinkAddAttr(nlMsg, sizeof(nlMsg), RTA_DST, &xx, 4);

This looks good but your NetlinkAddAttr is bogus, it should
be something like this:

int nl_msg_append_tlv(struct nlmsghdr *n, int type, void *data, size_t len)
{
	int tlen;
	struct rtattr *rta;

	tlen = NLMSG_ALIGN(n->nlmsg_len) + RTA_LENGTH(NLMSG_ALIGN(len));

	rta = (struct rtattr *) NLMSG_TAIL(n);
	rta->rta_type = type;
	rta->rta_len = RTA_LENGTH(NLMSG_ALIGN(len));
	memcpy(RTA_DATA(rta), data, len);
	n->nlmsg_len = tlen;

	return 0;
}

Your code is missing various alignment requirements. I can't tell
whether this is the last bug. I recommend you to read ip/iproute.c
in the iproute2 source or give libnl a second chance.

The code now works this way:
[root@localhost route]# route
1.1.1.0         *               255.255.255.0   U     0      0        0 eth0
3.3.0.0         *               255.255.0.0     U     0      0        0 eth1
default         meric           0.0.0.0         UG    0      0        0 eth0

[root@localhost route]# ./a.out 2.2.2.2 16
Destination             Gateway         Interface               Source          Netmask
2.2.2.2         213.250.192.33          eth0            255.255.255.255

[root@localhost route]# ./a.out 1.1.1.2 16
Destination             Gateway         Interface               Source          Netmask
1.1.1.2         *.*.*.*                         eth0            255.255.255.255

[root@localhost route]# ./a.out 3.3.3.2 16
Destination             Gateway         Interface               Source          Netmask
3.3.3.2         *.*.*.*                         eth1            255.255.255.255

so it returns the route, where the data would go, if their DST address would be the one given by the argv[1] with mask argv[2].
I don't know now, if we understood to each other and if this is you thought it should be. 
If I will write on the command line './a.out 3.3.0.0 16', it should print the line like this if the record is present:
3.3.0.0         *               255.255.0.0     U     0      0        0 eth1

if I would write './a.out 3.3.3.1 32' it MUST print nothing! :)

Thank you for help