Why Hardcoding Src MAC Address is a Bad Idea

Oct 13, 2025 by wbrum.org 137 views

Introduction

In the world of network programming, the source MAC (Media Access Control) address is a crucial piece of information that identifies a device on a network. In the context of the Linux kernel, the source MAC address is used to identify the sender of a packet. However, hardcoding the source MAC address in the kernel code can lead to several issues, including but not limited to, inflexibility, security vulnerabilities, and maintenance headaches. In this article, we will explore the importance of not hardcoding the source MAC address and how to modify the fd_xdp_tile.c file to read the MAC address from the interface table (MIB) instead.

Why Hardcoding Src MAC Address is a Bad Idea

Hardcoding the source MAC address in the kernel code can lead to several issues:

Inflexibility: Hardcoding the source MAC address makes it difficult to change or update the MAC address without modifying the kernel code. This can lead to maintenance headaches and make it challenging to keep the kernel code up-to-date.
Security Vulnerabilities: Hardcoding the source MAC address can expose the kernel to security vulnerabilities. If the hardcoded MAC address is known, an attacker can use it to launch a targeted attack on the device.
Limited Flexibility: Hardcoding the source MAC address limits the flexibility of the kernel code. It makes it difficult to support multiple devices or interfaces with different MAC addresses.

Modifying fd_xdp_tile.c to Read MAC Address from MIB

To modify the fd_xdp_tile.c file to read the MAC address from the interface table (MIB), we need to use the ethtool library to retrieve the MAC address of the interface. Here's an example of how to modify the fd_xdp_tile.c file:

#include <linux/ethtool.h>
#include <linux/mii.h>

// ...

struct fd_xdp_tile *tile = ...;

// ...

// Get the MAC address of the interface
struct ifreq ifr;
memset(&ifr, 0, sizeof(ifr));
strncpy(ifr.ifr_name, tile->name, IFNAMSIZ);
if (ioctl(fd, SIOCGIFHWADDR, &ifr) == 0) {
    tile->mac_addr = ifr.ifr_hwaddr.sa_data;
} else {
    // Handle error
}

// ...

In this modified code, we use the ethtool library to retrieve the MAC address of the interface. We create a struct ifreq object and set the ifr_name field to the name of the interface. We then use the ioctl function to retrieve the MAC address of the interface. If the ioctl function returns 0, we copy the MAC address to the tile->mac_addr field.

Benefits of Reading MAC Address from MIB

Reading the MAC address from the interface table (MIB) has several benefits:

Flexibility: Reading the MAC address from the MIB makes it easy to change or update the MAC address without modifying the kernel code.
Security: Reading the MAC address from the MIB reduces the risk of security vulnerabilities associated with hardcoding the MAC address.
Maintenance: Reading the MAC address from the MIB makes it easier to maintain the kernel code and keep it up-to-date.

Conclusion

In conclusion, hardcoding the source MAC address in the kernel code can lead to several issues, including inflexibility, security vulnerabilities, and maintenance headaches. Modifying the fd_xdp_tile.c file to read the MAC address from the interface table (MIB) using the ethtool library can help mitigate these issues. By reading the MAC address from the MIB, we can make the kernel code more flexible, secure, and maintainable.

Future Work

In the future, we can explore other ways to improve the kernel code, such as:

Using a more robust MAC address retrieval mechanism: We can use a more robust MAC address retrieval mechanism, such as the netlink interface, to retrieve the MAC address of the interface.
Implementing MAC address validation: We can implement MAC address validation to ensure that the MAC address is valid and not spoofed.
Supporting multiple devices or interfaces: We can modify the kernel code to support multiple devices or interfaces with different MAC addresses.

References

Appendix

Here is the complete modified fd_xdp_tile.c file:

#include <linux/ethtool.h>
#include <linux/mii.h>

struct fd_xdp_tile {
    char name[IFNAMSIZ];
    unsigned char mac_addr[6];
};

int fd_xdp_tile_init(struct fd_xdp_tile *tile, const char *name)
{
    // ...

    // Get the MAC address of the interface
    struct ifreq ifr;
    memset(&ifr, 0, sizeof(ifr));
    strncpy(ifr.ifr_name, name, IFNAMSIZ);
    if (ioctl(fd, SIOCGIFHWADDR, &ifr) == 0) {
        tile->mac_addr = ifr.ifr_hwaddr.sa_data;
    } else {
        // Handle error
    }

    // ...
}

int fd_xdp_tile_get_mac_addr(struct fd_xdp_tile *tile)
{
    return memcpy(tile->mac_addr, tile->mac_addr, 6);
}

Introduction

In our previous article, we discussed the importance of not hardcoding the source MAC address in the kernel code and how to modify the fd_xdp_tile.c file to read the MAC address from the interface table (MIB) using the ethtool library. In this article, we will answer some frequently asked questions (FAQs) related to this topic.

Q: Why is hardcoding the source MAC address a bad idea?

A: Hardcoding the source MAC address can lead to several issues, including inflexibility, security vulnerabilities, and maintenance headaches. It makes it difficult to change or update the MAC address without modifying the kernel code, which can lead to maintenance headaches and make it challenging to keep the kernel code up-to-date.

Q: What are the benefits of reading the MAC address from the MIB?

A: Reading the MAC address from the MIB has several benefits, including flexibility, security, and maintenance. It makes it easy to change or update the MAC address without modifying the kernel code, reduces the risk of security vulnerabilities associated with hardcoding the MAC address, and makes it easier to maintain the kernel code and keep it up-to-date.

Q: How do I modify the `fd_xdp_tile.c` file to read the MAC address from the MIB?

A: To modify the fd_xdp_tile.c file to read the MAC address from the MIB, you need to use the ethtool library to retrieve the MAC address of the interface. You can use the following code as a reference:

#include <linux/ethtool.h>
#include <linux/mii.h>

struct fd_xdp_tile *tile = ...;

// ...

// Get the MAC address of the interface
struct ifreq ifr;
memset(&ifr, 0, sizeof(ifr));
strncpy(ifr.ifr_name, tile->name, IFNAMSIZ);
if (ioctl(fd, SIOCGIFHWADDR, &ifr) == 0) {
    tile->mac_addr = ifr.ifr_hwaddr.sa_data;
} else {
    // Handle error
}

// ...

Q: What are some potential issues with reading the MAC address from the MIB?

A: Some potential issues with reading the MAC address from the MIB include:

Error handling: You need to handle errors that may occur when retrieving the MAC address from the MIB.
MAC address validation: You need to validate the MAC address to ensure that it is valid and not spoofed.
Support for multiple devices or interfaces: You need to modify the kernel code to support multiple devices or interfaces with different MAC addresses.

Q: How do I handle errors when retrieving the MAC address from the MIB?

A: You can handle errors when retrieving the MAC address from the MIB by using error codes and error messages. For example:

if (ioctl(fd, SIOCGIFHWADDR, &ifr) != 0) {
    // Handle error
    return -1;
}

Q: How do I validate the MAC address?

A: You can validate the MAC address by checking its format and ensuring that it is not spoofed. For example:

if (memcmp(tile->mac_addr, ifr.ifr_hwaddr.sa_data, 6) != 0) {
    // Handle error
    return -1;
}