Unlocking Linux Netlink Netdev Protocol: A Step-by-Step Guide to Retrieving Queue Information for Netdevs/Interfaces in Operstate Down

Are you tired of scratching your head over the complexities of Linux Netlink Netdev protocol? Do you struggle to get queue information for netdevs/interfaces when they’re in operstate down? Worry no more! In this comprehensive guide, we’ll delve into the world of Netlink Netdev protocol and provide you with clear, direct instructions on how to overcome this challenge.

What is Linux Netlink Netdev Protocol?

Before we dive into the nitty-gritty, let’s take a step back and understand what Linux Netlink Netdev protocol is all about. Netlink is a socket-based interface used for communication between the kernel and user-space processes. The Netdev family, specifically, is responsible for handling network device-related events and queries. It’s a fundamental component of the Linux kernel, enabling us to manage and interact with network interfaces.

The Operstate Down Conundrum

When a network interface is in operstate down, it means the interface is not operational, and no traffic can flow through it. In this state, retrieving queue information can be a challenge. The usual methods, such as querying the interface through ip link show or ethtool, won’t work since the interface is down. This is where Netlink Netdev protocol comes to the rescue!

Understanding Netlink Messages

Netlink messages are the fundamental units of communication between the kernel and user-space processes. They’re composed of a header, followed by a payload. The header contains essential information, such as the message type, flags, and sequence number. The payload, on the other hand, carries the actual data.

In the context of Netdev protocol, we’re interested in the NLMSG_INFO message, which carries information about a specific network device. This message is sent in response to a query from a user-space process.

struct nlmsghdr {
    __u32 nlmsg_len;    /* Length of message */
    __u16 nlmsg_type;   /* Message type */
    __u16 nlmsg_flags;  /* Additional flags */
    __u32 nlmsg_seq;   /* Sequence number */
    __u32 nlmsg_pid;   /* Sending process port ID */
};

Retrieving Queue Information using Netlink Netdev Protocol

To retrieve queue information for netdevs/interfaces in operstate down, we’ll use the rtnl_link_get+ and rtnl_link_message functions from the libnetlink library. These functions allow us to send a query to the kernel and receive the response.

Setting up the Environment

Before we start coding, make sure you have the following dependencies installed:

• libnetlink-dev (for development)
• libnetlink3 (for runtime)

On Ubuntu-based systems, you can install these dependencies using:

sudo apt-get install libnetlink-dev libnetlink3

Coding the Solution

Create a new C file, e.g., queue_info.c, and add the following code:

#include <linux/rtnetlink.h>
#include <libnetlink.h>
#include <stdio.h>
#include <string.h>

#define BUF_SIZE 1024

int main() {
    struct rtnl_link *link;
    struct nlmsghdr *nh;
    struct ifinfomsg *ifm;
    char buf[BUF_SIZE];
    int nl_sock, rc;

    /* Create a netlink socket */
    nl_sock = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
    if (nl_sock < 0) {
        perror("socket");
        return 1;
    }

    /* Initialize the netlink message header */
    nh = (struct nlmsghdr *)buf;
    nh->nlmsg_len = NLMSG_LENGTH(sizeof(struct ifinfomsg));
    nh->nlmsg_type = RTM_GETLINK;
    nh->nlmsg_flags = NLM_F_REQUEST;
    nh->nlmsg_seq = time(NULL);

    /* Add the interface index or name to the message */
    ifm = (struct ifinfomsg *)NLMSG_DATA(nh);
    ifm->ifi_family = AF_UNSPEC;
    ifm->ifi_index = if_nametoindex("eth0"); /* Replace with your interface name */

    /* Send the query to the kernel */
    rc = send(nl_sock, nh, nh->nlmsg_len, 0);
    if (rc < 0) {
        perror("send");
        return 1;
    }

    /* Receive the response */
    rc = recv(nl_sock, buf, BUF_SIZE, 0);
    if (rc < 0) {
        perror("recv");
        return 1;
    }

    /* Parse the response */
    for (nh = (struct nlmsghdr *)buf; NLMSG_OK(nh, rc); nh = NLMSG_NEXT(nh, rc)) {
        if (nh->nlmsg_type == NLMSG_DONE)
            break;

        if (nh->nlmsg_type == NLMSG_ERROR)
            continue;

        ifm = (struct ifinfomsg *)NLMSG_DATA(nh);
        printf("Interface: %s\n", if_indextoname(ifm->ifi_index, buf));

        /* Get the queue information */
        struct rtnl_link_stats *stats = IFLA_STATS_LINK((struct ifinfomsg *)NHMSG_DATA(nh));
        printf("Queue length: %u\n", stats->rx_queue_len);
        printf("Queue max length: %u\n", stats->rx_max_queue_len);
    }

    close(nl_sock);
    return 0;
}

Compile the code using:

gcc -o queue_info queue_info.c -l rtnl -l netlink

Running the Program

Run the program using:

./queue_info

This will display the queue information for the specified interface (in this case, eth0). Make sure to replace eth0 with your desired interface name.

Conclusion

In this article, we’ve explored the world of Linux Netlink Netdev protocol and learned how to retrieve queue information for netdevs/interfaces in operstate down. By using the rtnl_link_get+ and rtnl_link_message functions, we can query the kernel for information about specific network devices, even when they’re not operational.

Remember to adapt the provided code to your specific needs and environment. With this knowledge, you’ll be well-equipped to tackle complex network-related tasks in Linux.

Function	Description
rtnl_link_get+	Sends a query to the kernel for information about a specific network device
rtnl_link_message	Parses the response from the kernel and extracts the relevant information
NLMSG_INFO	The Netlink message type used to carry information about a network device
RTM_GETLINK	The Netlink message type used to request information about a network device

I hope you found this article informative and helpful. If you have any questions or need further clarification, feel free to ask in the comments!

References

• Netlink(7) – Linux manual page
• rtnetlink(3) – Linux manual page
• Linux Netlink and rtnetlink

Frequently Asked Question

Get ready to dive into the world of Linux Netlink Netdev protocol and explore the secrets of obtaining queue information for netdevs/interfaces in operstate down!