[Yocto-BBB] 12. Transferring files over TFTP

Translated from the Vietnamese original; wording may still be rough. Read the original →

In the previous post I showed you how to transfer files over UART; in this post I show you how to transfer files over TFTP. (u-boot supports TFTP for loading files)

1. What is TFTP?

TFTP (Trivial File Transfer Protocol) is a very simple file transfer protocol running over UDP (port 69).

Some key characteristics:

Simpler than FTP: no authentication, no username/password needed.
Uses UDP instead of TCP → fast and light, but without FTP's reliability guarantees.
Common use: often used in embedded environments, especially for loading firmware, kernels or rootfs over the network (for example: U-Boot uses TFTP to download images from the host to the board).
Limitations: no security (no encryption, no elaborate access control), only suitable for local networks or trusted environments.
How it works: the client sends a read request (RRQ) or write request (WRQ) for a file to the server, then the data is sent in blocks (512 bytes), each block acknowledged (ACK).

--> In short: TFTP = a quick, light way to copy files over the network in embedded systems, usually so the board can fetch the kernel/initramfs from the PC during boot.

2. Building a yocto image with TFTP support

In this post I am not touching u-boot yet; I transfer files over TFTP in userspace. So we have to add a package that supports it

I continue editing the core-image-minimal.bbappend used in the previous post

bash

inherit extrausers
TMPDIR = "${TOPDIR}/tmp"
# User "root" has password set to "test" in the image.
# printf "%q" $(mkpasswd -m sha256crypt test)
# \$5\$1ywTDE4jLgPDskTp\$yK5Ap.2xjc5gYeFQ4MGvR6C0VzA4VDSMIFkQ5.TJO84
PASSWD = "\$5\$y9Aeg5ctwntRHo/g\$CAKtoTfQg7VPGfVAMGo5ZG/0GJLn3AD0JdoQ.i0dDFC"
EXTRA_USERS_PARAMS = "\
    usermod -p '${PASSWD}' root; \
    "
# Add for UART file transfer
IMAGE_INSTALL:append = " \
  lrzsz \
"

# Add support for TFTP file transfer
IMAGE_INSTALL:append = " \
  tftp-hpa \
  net-tools \
  iproute2 \
"

Here I add 3 packages

tftp-hpa: provides the tftp command to transfer files
net-tools: provides many commands such as ifconfig, route, ss, netstat
iproute2: provides ip addr, ip link, ip route, ip neigh, ss, ...

(ping: already included in the lightweight busybox)

Okay, that's enough; build and flash.

Oh, at this point your build fails, right? Read the FIX below

HOW TO FIX THE BUILD ERROR

Since it is a bit long, I put it in this toggle list :v. I also found it interesting, so I am writing it up.

The build error message is

bash

Missing or unbuildable dependency chain was: ['tftp-hpa']
ERROR: Required build target 'core-image-minimal' has no buildable providers.
Missing or unbuildable dependency chain was: ['core-image-minimal', 'tftp-hpa']

This means it cannot find a recipes to build the tftp-hpa package.

So how did I know to add tftp-hpa in the first place? I wandered through some forums to see which package people add to send/receive over TFTP, and added it.

Now I went searching again. I searched the following line on google

bash

tftp-hpa yocto

The first result was this page, which is exactly what I needed

bash

https://layers.openembedded.org/layerindex/recipe/49880/

Reading the description, you see the recipe that builds tftp-hpa belongs to the meta-networking layer. So we need to add it to build-bbb/conf/bblayers.conf. This layer is in yocto-bbb/meta-openembedded

After adding it and rebuilding, we get the next error

bash

zk47@ltu:~/Learning/yocto-bbb/build-bbb$ bitbake core-image-minimal
WARNING: Layer meta-bbb should set LAYERSERIES_COMPAT_meta-bbb in its conf/layer.conf file to list the core layer names it is compatible with.
ERROR: Layer 'networking-layer' depends on layer 'meta-python', but this layer is not enabled in your configuration

Summary: There was 1 WARNING message.
Summary: There was 1 ERROR message, returning a non-zero exit code.

This one is easier to fix. networking-layer depends on meta-python, so we have to add that meta-layer too.

You can edit it by hand or use bitbake-layers add-layer <path-to-layer>. In the end my build-bbb/conf/bblayers.conf is as follows:

bash

# POKY_BBLAYERS_CONF_VERSION is increased each time build/conf/bblayers.conf
# changes incompatibly
POKY_BBLAYERS_CONF_VERSION = "2"

BBPATH = "${TOPDIR}"
BBFILES ?= ""

BBLAYERS ?= " \
  /home/zk47/Learning/yocto-bbb/poky/meta \
  /home/zk47/Learning/yocto-bbb/poky/meta-poky \
  /home/zk47/Learning/yocto-bbb/poky/meta-yocto-bsp \
  /home/zk47/Learning/yocto-bbb/meta-openembedded/meta-oe \
  /home/zk47/Learning/yocto-bbb/meta-arm/meta-arm-toolchain \
  /home/zk47/Learning/yocto-bbb/meta-arm/meta-arm \
  /home/zk47/Learning/yocto-bbb/meta-ti/meta-ti-bsp \
  /home/zk47/Learning/yocto-bbb/meta-ti/meta-ti-extras \
  /home/zk47/Learning/yocto-bbb/meta-bbb \
  /home/zk47/Learning/yocto-bbb/meta-openembedded/meta-python \
  /home/zk47/Learning/yocto-bbb/meta-openembedded/meta-networking \
  "

3. Plug in Ethernet and configure the network

To start, connect Ethernet from the board directly to the Host, your PC.

3.1 Host side configuration

Check ifconfig for the name of the ethernet interface

ifconfig

bash

zk47@ltu:~$ ifconfig
enx207bd232cd61: flags=4163  mtu 1500
        inet 192.168.10.1  netmask 255.255.255.0  broadcast 0.0.0.0
        inet6 fe80::baa9:5f6d:2de1:8159  prefixlen 64  scopeid 0x20
        ether 20:7b:d2:32:cd:61  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 8974  bytes 1983414 (1.9 MB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 8974  bytes 1983414 (1.9 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

wlp0s20f3: flags=4163  mtu 1500
        inet 192.168.1.19  netmask 255.255.255.0  broadcast 192.168.1.255
        inet6 2402:9d80:854:e290:49de:1a3b:524c:e455  prefixlen 64  scopeid 0x0
        inet6 2402:9d80:854:e290:9d08:178:e63:f837  prefixlen 64  scopeid 0x0
        inet6 fe80::f19b:6180:2774:2711  prefixlen 64  scopeid 0x20
        ether 10:91:d1:9e:bb:8c  txqueuelen 1000  (Ethernet)
        RX packets 434632  bytes 492104577 (492.1 MB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 116294  bytes 46891184 (46.8 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

So my ethernet interface is enx207bd232cd61. We run the following command to set a static ip and bring the interface up

bash

sudo ip addr add 192.168.10.1/24 dev enx207bd232cd61
sudo ip link set enx207bd232cd61 up

3.2 Board side configuration

Similarly on the BBB

bash

ip addr add 192.168.10.2/24 dev eth0
ip link set eth0 up

3.3 Ping test

After setting up the static ips, remember to test with ping to see how the connection is

bash

ping 192.168.10.1   # from BBB
ping 192.168.10.2   # from PC

4. Configure TFTP on the host

4.1 Install and change the default folder

bash

sudo apt install tftpd-hpa
sudo systemctl enable --now tftpd-hpa

Change the default TFTP folder

bash

sudo nano /etc/default/tftpd-hpa

We change it to the following content. TFTP_DIRECTORY is simply the Share folder between the board and the host for sending and receiving files

bash

# /etc/default/tftpd-hpa

TFTP_USERNAME="tftp"
TFTP_DIRECTORY="/home/zk47/Learning/TFTP"
TFTP_ADDRESS=":69"
TFTP_OPTIONS="--secure"

4.2 Local host test

On the host we test whether TFTP works properly

Remember to change the permissions of the folder and the mode of the new file

bash

sudo chmod 777 /home/zk47/Learning/TFTP
touch /home/zk47/Learning/TFTP/testfile
chmod 666 testfile

From another folder, type the command

bash

zk47@ltu:~/Learning/Empty$ ls
zk47@ltu:~/Learning/Empty$ tftp 127.0.0.1
tftp> get testfile
tftp> q
zk47@ltu:~/Learning/Empty$ ls
testfile

TFTP works nicely for the local test (loopback address 127.0.0.1)

5. Sending and receiving files over TFTP

On the host you have created testfile and tested locally. On the beaglebone board you do much the same, but with the real address of the host server.

bash

root@beaglebone:~# tftp 192.168.10.1
tftp> get testfile
tftp> q
root@beaglebone:~# ls
testfile

So the transfer from the host to the board works.

Normally tftp also supports the put command, but for now the host side is still set to not allow creating new files. However, testfile on the host already has write permission.

So we can edit testfile and put it back to the host.

bash

root@beaglebone:~# ls
testfile
root@beaglebone:~# cat testfile
root@beaglebone:~# echo "Content from beaglebone black" > testfile
root@beaglebone:~# ls
testfile
root@beaglebone:~# cat testfile
Content from beaglebone black
root@beaglebone:~# tftp 192.168.10.1
tftp> put testfile
tftp> q
root@beaglebone:~#

And the content on the host has been updated

bash

zk47@ltu:~/Learning/TFTP$ cat testfile
Content from beaglebone black

Good luck with the hands-on !!!

In the next post I will write about RDP with the Weston backend on the Beaglebone Black. Hope you keep supporting.