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    • Booting Linux Kernel and Root Filesystem over Ethernet with Yocto (TFTP + NFS)

    When developing embedded Linux systems, constant flashing of SD cards or eMMC devices quickly becomes tedious.
    A much faster and more flexible approach is to boot your device entirely over Ethernet — loading the kernel via TFTP and mounting the root filesystem via NFS.

    This network-based setup is ideal during development and testing, especially when working with Yocto, because every change made on the host system is immediately visible on the target board.

    #Why Boot Over Ethernet

    Using TFTP and NFS for booting provides several major advantages:

    • ⚡ Instant updates: Edit files or rebuild the rootfs without reflashing storage
    • 🧩 Separation of kernel and rootfs: Test different kernel versions easily
    • 🧠 Easy debugging: Logs and files remain on the host
    • 💾 No flash wear: Protect limited write-endurance devices
    • 🔁 Rapid iteration: Perfect for development and continuous testing setups

    #1. Preparing the Yocto Build Output

    After building your Yocto image (for example, core-image-minimal), locate the generated files:

     1$BUILDDIR/tmp/deploy/images/<machine>/

    You’ll need:

    • The kernel image (zImage or Image)
    • The device tree blob (.dtb)
    • The root filesystem archive (.rootfs.tar.xz)

    #2. Setting Up the Workstation (TFTP + NFS Server)

    Install the required services:

     1sudo apt update
 2sudo apt install nfs-kernel-server tftpd-hpa

    #Configure the NFS export

     1sudo mkdir -p /nfs
 2sudo chmod 777 /nfs

    Add this line to /etc/exports:

     1/nfs *(rw,sync,no_root_squash,no_subtree_check)

    Apply the export configuration:

     1sudo exportfs -r

    #Configure the TFTP server

     1sudo mkdir -p /tftpboot
 2sudo chmod 777 /tftpboot
 3sudo cp zImage /tftpboot/
 4sudo cp board.dtb /tftpboot/

    Verify /etc/default/tftpd-hpa contains:

     1TFTP_DIRECTORY="/tftpboot"
 2TFTP_ADDRESS=":69"
 3TFTP_OPTIONS="--secure"

    Restart the service:

     1sudo systemctl restart tftpd-hpa

    #3. Configure the Host Ethernet Interface

    Assign a static IP to your workstation that will act as both the TFTP and NFS server.
    For example:

     1nmcli con add type ethernet ifname eth0 ip4 192.168.0.1/24

    If it already exists:

     1nmcli con mod eth0 ipv4.addresses 192.168.0.1/24 ipv4.method manual
 2nmcli con up eth0

    #4. Deploy the Yocto Root Filesystem

    Extract the root filesystem archive into the exported NFS directory:

     1sudo tar xpf $BUILDDIR/tmp/deploy/images/<machine>/core-image-minimal-<machine>.rootfs.tar.xz -C /nfs

    Now your board can mount /nfs as its root filesystem.

    #5. Configure U-Boot for Network Boot

    At the U-Boot prompt, set the necessary environment variables:

     1setenv serverip 192.168.0.1
 2setenv ipaddr 192.168.0.100
 3setenv bootfile zImage
 4setenv fdtfile board.dtb
 5setenv bootargs "console=ttyS0,115200 root=/dev/nfs rw nfsroot=${serverip}:/nfs,v3,tcp ip=dhcp"
 6
 7tftp ${loadaddr} ${bootfile}
 8tftp ${fdtaddr} ${fdtfile}
 9bootz ${loadaddr} - ${fdtaddr}

    If configured correctly, you’ll see output similar to:

     1IP-Config: eth0 hardware address ...
 2Looking up 192.168.0.1
 3Sending DHCP requests ...
 4VFS: Mounted root (nfs filesystem)

    Your board is now running completely from the network.

    #6. Adding Packages to the Root Filesystem

    To extend your rootfs, modify your Yocto configuration file:

     1$BUILDDIR/conf/local.conf

    Append the desired package, for example:

     1IMAGE_INSTALL:append = " dropbear"

    Rebuild the image:

     1bitbake core-image-minimal

    Extract the updated rootfs again to /nfs and reboot your board.
    You can now connect via SSH:

     1ssh root@192.168.0.100

    #7. Choosing Kernel Variants in Yocto

    Yocto uses virtual package providers, meaning different kernel recipes can fulfill virtual/kernel.
    To see which kernel is currently used:

     1bitbake -vn virtual/kernel

    If you want to switch to a different kernel:

     1PREFERRED_PROVIDER_virtual/kernel = "linux-custom"

    Then verify again:

     1bitbake -vn virtual/kernel

    #8. Useful BitBake Commands

    Here are some helpful Yocto commands for everyday work:

    | Purpose | Command | |----------|----------| | List available tasks | bitbake -c listtasks virtual/kernel | | Run a specific task | bitbake -c menuconfig virtual/kernel | | Force rebuild | bitbake -f virtual/kernel | | Fetch all sources | bitbake --runall=fetch world | | List available packages | bitbake -s |

    Explore more recipes at layers.openembedded.org.

    #9. Summary

    Booting over Ethernet using TFTP and NFS is a cornerstone of efficient embedded development:

    • No local storage needed
    • Immediate updates and faster iterations
    • Centralized kernel and rootfs management

    This setup is the foundation of most professional embedded Linux workflows — including our own at Mitkov Systems GmbH, where fast iteration and reliable testing environments are key to high-quality product development.

    #💡 Next Steps

    In future articles, we’ll show how to:

    • Automate U-Boot network booting
    • Use Yocto SDKs for seamless application development directly on NFS-mounted root filesystems

    Stay tuned — or contact us at mail@mitkov-systems.de if you’d like to discuss how we can help accelerate your embedded Linux development.

    Mitkov Systems GmbH
    Specialists in Embedded Systems, IoT, and Industrial Connectivity

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    Martin Mitkov

    Martin is a founder and CEO of Mitkov Systems GmbH.

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