Questions on Netlink sockets

Questions on Netlink sockets

[Bhaskar Upadhyayula]

Hello,

I want to check with the group if this is right mailing list to post
my question.

Here is what I am doing.

+ From a user-space program, mimicking the behavior of creating a VLAN
interface when we execute the following ip command:
    $ ip link add name <vlan-intf-name> link <parent-intf-name> type
vlan id <id>

Example: $ ip link add name dummy.110 link dummy type vlan id 110

+ My program open's a netlink socket and sends information to kernel
(interface names, vlan tags etc). My code more or less follows the
code in iproute2/ip/iplink.c

+ My code is not creating VLAN interface as executed by the 'ip link
...' command.

I have a few questions around this and can post my code, if required.

But please let me know if this is right mailing list or should I post
on netdev list (which I feel is for advanced network questions).

Thx.

Questions on Netlink sockets

[Rami Rosen]

Hi, Bhaskar,

Indeed the netdev mailing list is too advanced for this.

Do you get any error as a result of sending this netlink message ?
(I mean error in userspace or in kernel; if there is an error message
in the kernel it can be viewed by looking at the end of dmesg output)

Further more, I suggest that you will post the code here. The concept
itself is right, It should work, you either build the netlink message
incorrectly or send it incorrectly, so it seems.

Regards,
Rami Rosen
http://ramirose.wix.com/ramirosen


On 18 August 2015 at 01:55, Bhaskar Upadhyayula <bu.kernel@gmail.com> wrote:
> Hello,
>
> I want to check with the group if this is right mailing list to post
> my question.
>
> Here is what I am doing.
>
> + From a user-space program, mimicking the behavior of creating a VLAN
> interface when we execute the following ip command:
>     $ ip link add name <vlan-intf-name> link <parent-intf-name> type
> vlan id <id>
>
> Example: $ ip link add name dummy.110 link dummy type vlan id 110
>
> + My program open's a netlink socket and sends information to kernel
> (interface names, vlan tags etc). My code more or less follows the
> code in iproute2/ip/iplink.c
>
> + My code is not creating VLAN interface as executed by the 'ip link
> ...' command.
>
> I have a few questions around this and can post my code, if required.
>
> But please let me know if this is right mailing list or should I post
> on netdev list (which I feel is for advanced network questions).
>
> Thx.
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies

Questions on Netlink sockets

[Bhaskar Upadhyayula]

Hi Rami,

Thanks for the response. With the help of a colleague, I was able to
debug the issue. I was not *nesting* the attributes and not adjusting
the attribute lengths properly. I am still figuring out how nesting
works exactly and the need for it.

If anyone is interested, attaching the working code (inline as well,
if attachment is not permitted on kernelnewbies).

Thanks.

PS: I am humbled to see a reply from you. I read Chapter 2 from your
book to understand netlink sockets.

=======
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>

#define MAX_PAYLOAD 2048

/* All globals initialized to 0 */
struct sockaddr_nl src, dst;
struct iovec iov;
struct msghdr msg;
int sock_fd;

struct {
    struct nlmsghdr  n;
    struct ifinfomsg i;
    char             buf[MAX_PAYLOAD];
} req;

#define NLMSG_TAIL(nmsg) \
        ((struct rtattr *) (((void *) (nmsg)) + NLMSG_ALIGN((nmsg)->nlmsg_len)))

/* Function from libnetlink.h */
int addattr_l(struct nlmsghdr *n, int maxlen, int type, const void *data,
          int alen)
{
    int len = RTA_LENGTH(alen);
    struct rtattr *rta;

    if (NLMSG_ALIGN(n->nlmsg_len) + RTA_ALIGN(len) > maxlen) {
        fprintf(stderr, "addattr_l ERROR: message exceeded bound of
%d\n",maxlen);
        return -1;
    }
    rta = NLMSG_TAIL(n);
    rta->rta_type = type;
    rta->rta_len = len;
    memcpy(RTA_DATA(rta), data, alen);
    n->nlmsg_len = NLMSG_ALIGN(n->nlmsg_len) + RTA_ALIGN(len);
    return 0;
}

void usage()
{
    fprintf(stderr, "Incorrect usage\n");
    fprintf(stderr, "./nl <vlan_intf_name> <intf_name> <vlan_id>\n");
}

void dumpData(const char *note, const char *msg, unsigned int len)
{
    // no error checks
    unsigned int i;

    //fprintf(stderr, "\n%s(%u):\n", note, len);

    for(i=0; i<len; ++i) {
        fprintf(stderr, "%02X ",msg[i]);

        if (i>0 && i%15 == 0)
            fprintf(stderr, "\n");
    }

    fprintf(stderr,"\n");
}

void main(int argc, char *argv[])
{
    char *type, *intf_name, *vlan_intf_name;
    __u16   vlan_id;
    int     ifindex;

    if (argc != 4) {
        usage();
        return;
    }

    vlan_intf_name = argv[1];
    intf_name = argv[2];
    vlan_id = atoi(argv[3]);

    fprintf(stderr, "%s, %s, %d\n", vlan_intf_name, intf_name, vlan_id);
    sock_fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
    if (sock_fd < 0)
        return;

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

    /* Details of source end of socket */
    src.nl_family   = AF_NETLINK;
    src.nl_pid      = getpid();

    bind(sock_fd, (struct sockaddr *)&src, sizeof(src));

    /* Details of destination side of socket */
    dst.nl_family   = AF_NETLINK;

    req.n.nlmsg_len = NLMSG_SPACE(sizeof(struct ifinfomsg));
    req.n.nlmsg_pid = getpid();
    req.n.nlmsg_type = RTM_NEWLINK;
    req.n.nlmsg_flags = NLM_F_REQUEST | NLM_F_CREATE ;//| NLM_F_EXCL;

    ifindex = if_nametoindex(intf_name);

    req.i.ifi_family = AF_UNSPEC;
    //req.i.ifi_index  = ifindex;
    struct rtattr *linkinfo = NLMSG_TAIL(&req.n);
    addattr_l(&req.n, sizeof(req), IFLA_LINKINFO, NULL, 0);
    addattr_l(&req.n, sizeof(req), IFLA_INFO_KIND, "vlan", 4);
    struct rtattr * data = NLMSG_TAIL(&req.n);
    addattr_l(&req.n, sizeof(req), IFLA_INFO_DATA, NULL, 0);
    addattr_l(&req.n, sizeof(req), IFLA_VLAN_ID, &vlan_id, 2);
    data->rta_len = (void *)NLMSG_TAIL(&req.n) - (void *)data;
    linkinfo->rta_len = (void *)NLMSG_TAIL(&req.n) - (void *)linkinfo;
    addattr_l(&req.n, sizeof(req), IFLA_LINK, &ifindex, 4);
    addattr_l(&req.n, sizeof(req), IFLA_IFNAME, vlan_intf_name,
strlen(vlan_intf_name)+1);

    /* send msg to kernel */
    if( send(sock_fd, &req, req.n.nlmsg_len, 0) < 0)
       fprintf(stderr, "failed write\n");

    //dumpData("Dst socket", (char *)msg.msg_name, msg.msg_namelen);
    dumpData("Netlink msg", (char *)&req.n, req.n.nlmsg_len);

    close(sock_fd);
}

=======

On Mon, Aug 17, 2015 at 9:18 PM, Rami Rosen <roszenrami@gmail.com> wrote:
> Hi, Bhaskar,
>
> Indeed the netdev mailing list is too advanced for this.
>
> Do you get any error as a result of sending this netlink message ?
> (I mean error in userspace or in kernel; if there is an error message
> in the kernel it can be viewed by looking at the end of dmesg output)
>
> Further more, I suggest that you will post the code here. The concept
> itself is right, It should work, you either build the netlink message
> incorrectly or send it incorrectly, so it seems.
>
> Regards,
> Rami Rosen
> http://ramirose.wix.com/ramirosen
>
>
> On 18 August 2015 at 01:55, Bhaskar Upadhyayula <bu.kernel@gmail.com> wrote:
>> Hello,
>>
>> I want to check with the group if this is right mailing list to post
>> my question.
>>
>> Here is what I am doing.
>>
>> + From a user-space program, mimicking the behavior of creating a VLAN
>> interface when we execute the following ip command:
>>     $ ip link add name <vlan-intf-name> link <parent-intf-name> type
>> vlan id <id>
>>
>> Example: $ ip link add name dummy.110 link dummy type vlan id 110
>>
>> + My program open's a netlink socket and sends information to kernel
>> (interface names, vlan tags etc). My code more or less follows the
>> code in iproute2/ip/iplink.c
>>
>> + My code is not creating VLAN interface as executed by the 'ip link
>> ...' command.
>>
>> I have a few questions around this and can post my code, if required.
>>
>> But please let me know if this is right mailing list or should I post
>> on netdev list (which I feel is for advanced network questions).
>>
>> Thx.
>>
>> _______________________________________________
>> Kernelnewbies mailing list
>> Kernelnewbies at kernelnewbies.org
>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
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Questions on Netlink sockets

[Rami Rosen]

Hi, Bhaskar,

Thanks for posting the code.

>PS: I am humbled to see a reply from you. I read Chapter 2 from your
>book to understand netlink sockets.

I hope you enjoyed it and find it useful!

>+ My code is not creating VLAN interface as executed by the 'ip link
>...' command.

Can you elaborate a bit about this: which difference do you see between creating
a VLAN with
ip link add name eth2.200  link eth2 type vlan id 200

And running your program (let's call your executable "vlan_nl")
in this way:
./vlan_nl eth1.100 eth1 100



Regards,
Rami Rosen
http://ramirose.wix.com/ramirosen

Questions on Netlink sockets

[Bhaskar Upadhyayula]

Hi Rami,

Sorry for the delay in response.

In the buggy code, the missing lines were the following:

115     data->rta_len = (void *)NLMSG_TAIL(&req.n) - (void *)data;
116     linkinfo->rta_len = (void *)NLMSG_TAIL(&req.n) - (void *)linkinfo;

Above code is to adjust the length for nested attributes. I missed it
and the constructed packet was messed up.

'dmesg' was not throwing any errors and I was stuck, trying to debug.

Thx.


On Wed, Aug 19, 2015 at 3:28 AM, Rami Rosen <roszenrami@gmail.com> wrote:
> Hi, Bhaskar,
>
> Thanks for posting the code.
>
>>PS: I am humbled to see a reply from you. I read Chapter 2 from your
>>book to understand netlink sockets.
>
> I hope you enjoyed it and find it useful!
>
>>+ My code is not creating VLAN interface as executed by the 'ip link
>>...' command.
>
> Can you elaborate a bit about this: which difference do you see between creating
> a VLAN with
> ip link add name eth2.200  link eth2 type vlan id 200
>
> And running your program (let's call your executable "vlan_nl")
> in this way:
> ./vlan_nl eth1.100 eth1 100
>
>
>
> Regards,
> Rami Rosen
> http://ramirose.wix.com/ramirosen