Manipulating the Networking Environment Using RTNETLINK

Is it possible to get the default gateway with out having to create a route? I am trying to get the default gateways ip with out having to add/delete to the routing table.

Please let me know.. and Thank You for this article it was great!

Using this tutorial, i crated a function to add the route, I believe I am populating all the necessary elements of the data structures. However, the route is getting added wrongly. For any kind of route, the function only add 0.0.0.0 route with mask 255.255.255.255 and gateway 0.0.0.0. It points to the correct interface that i specify in RTA_OIF.

Here is the function.

unsigned int rtm_add_v4 (unsigned int prefix,
unsigned char len, u_char tbl_index,
unsigned int oif,
u_char proto,
u_char rt_type,
struct rtnexthop *rtnh)
{
struct sockaddr_nl ra;
struct msghdr msg;
struct iovec iov;
char buf[8192];
int rtn;

struct nlmsghdr *nlm;
int nlml;
struct rtmsg *rt;
int rtl;
struct rtattr *rta;
rtsock_req_t rreq;

assert(rtm_initialized);

bzero(&rreq, sizeof(rreq));

rtl = sizeof(struct rtmsg);

rta = (struct rtattr *)rreq.buf;

rta->rta_type = RTA_DST;
rta->rta_len = sizeof(struct rtattr) + 4;

printf("Copying prefix 0x%08x\n", prefix);

bcopy(&prefix,(char *)rta+rta->rta_len,4);

rtl += rta->rta_len;

rta = (struct rtattr *)(((char *)rta) + rta->rta_len);

rta->rta_type = RTA_OIF;
rta->rta_len = sizeof(struct rtattr) + 4;

printf("Copying OIF: %d\n", oif);
bcopy(&oif, (char *)rta+sizeof(struct rtattr), 4);

rtl += rta->rta_len;

/* Setup the NETLINK Header */

rreq.nl.nlmsg_len = NLMSG_LENGTH(rtl);
rreq.nl.nlmsg_flags = NLM_F_REQUEST|NLM_F_CREATE;
rreq.nl.nlmsg_type = RTM_NEWROUTE;

/* Setup operation header */

rreq.rt.rtm_family = AF_INET;
rreq.rt.rtm_dst_len = len;
rreq.rt.rtm_table = tbl_index;
rreq.rt.rtm_protocol = proto;
rreq.rt.rtm_scope = RT_SCOPE_UNIVERSE;
rreq.rt.rtm_type = rt_type;

bzero(&ra, sizeof(ra));
ra.nl_family = AF_NETLINK;

bzero(&msg, sizeof(msg));

msg.msg_name = (void *)&ra;
msg.msg_namelen = sizeof(ra);

iov.iov_base = (void *)&rreq.nl;
iov.iov_len = rreq.nl.nlmsg_len;
msg.msg_iov = &iov;
msg.msg_iovlen = 1;

rtn = sendmsg(rtsock,&msg, 0);

if (rtn < 0)
{
printf("%s :", __FUNCTION__);
perror("sendmsg");
printf("\n");
return 0;
} else {
return 1;
}

return 0;

}

Here is the routing table before route add

[root@iLinux-Nilesh route]# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
10.2.2.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1
172.19.57.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
192.168.60.0 0.0.0.0 255.255.255.0 U 0 0 0 eth3
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
0.0.0.0 172.19.57.1 0.0.0.0 UG 0 0 0 eth0

Here is the sample run of the test program that uses this function.

Enter the route to be added: 172.21.1.1
Addr: 0xac150101
Enter prefix len: 32
len: 32
Enter the oif: 2
Copying prefix 0xac150101
Copying OIF: 2
ROUTE ADDED SUCCESSFULLY
[root@iLinux-Nilesh route]#

Routing table after route add program.

[root@iLinux-Nilesh route]# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 0.0.0.0 255.255.255.255 UH 0 0 0 eth0 <<<<<
10.2.2.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1
172.19.57.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
192.168.60.0 0.0.0.0 255.255.255.0 U 0 0 0 eth3
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
0.0.0.0 172.19.57.1 0.0.0.0 UG 0 0 0 eth0
[root@iLinux-Nilesh route]#

Any idea what is wrong? Function in test program is invoked as below.

rc = rtm_add_v4(addr.s_addr,len,RT_TABLE_MAIN,oif,RTPROT_STATIC,RTN_UNICAST,NULL);

Hello Asanga...
I am newbie to Linux. As far as u have googled i found only your material for a sample.With my understanding on ur illustration I have made the below module to get the destination address and the gateway address when i give "route add -host 192.168.2.45 gw 202.34.2.1"
I get the gateway address as 192.168.2.45 from the module. but i expect the gateway to be 202.34.2.1...

Please help me on this regard.
Thanks in advance...

/* Read message from kernel */
recv(sock_fd,nlh,size, 0);
printf(" Received message payload: %s\n",
NLMSG_DATA(nlh));

rtp = (struct rtmsg*)NLMSG_DATA(nlh);

rtap =(struct rtattr*) RTM_RTA(rtp);
rtl = RTM_PAYLOAD(nlh);
for(;RTA_OK(rtap,rtl);rtap = RTA_NEXT(rtap,rtl))
{

switch(rtap_rta_type)
{
case RTA_GATEWAY:
inet_ntop(AF_INET,RTA_DATA(rtap),gws,24);
printf("\n gateway address is %s",gws);
break;

case RTA_DST:
inet_ntop(AF_INET,RTA_DATA(rtap),dsts,24);
printf("\n destination address is %s",dsts);
break;

case RTA_SRC:
printf("\n received source address");
break;

default:
break;
}
}