UDP packet gets checksummed incorrectly by the network interface

[flux77]

Describe the bug
When running a DHCP server on the Raspberry Pi, the output from sudo tcpdump -n udp port 67 -e -vv shows that there are packets coming from the Raspberry Pi that have bad checksums. Specifically, the DHCP clients never receive the DHCPOFFER packet.

Once the offloading of checksums has been disabled (using sudo ethtool --offload eth0 tx off) there are no more packets with bad checksums (i.e. there is no longer any problem). That is why I suspect that there is an issue with some drivers in the kernel.

Because I am a beginner at networking, I have not reached the stage where I know how to produce a minimal example of packets. I hope you'll find it permissible for me to refer you to my question on the Raspberry Pi Stack Exchange.

Should you require further information, I will be happy to provide it.

Expected behaviour
The UDP packet has the correct checksum.

Actual behaviour
The UDP packet has a bad checksum.

System

• Which model of Raspberry Pi? e.g. Pi3B+, PiZeroW
  Raspberry Pi 3 B
• Which OS and version (cat /etc/rpi-issue)?

Raspberry Pi reference 2018-06-27
Generated using pi-gen, https://github.com/RPi-Distro/pi-gen, 0d34fb94398a4bd79711f21d6a0b5d35fe5f4a76, stage5

• Which firmware version (vcgencmd version)?

Jun  7 2018 15:31:38 
Copyright (c) 2012 Broadcom
version 4800f08a139d6ca1c5ecbee345ea6682e2160881 (clean) (release)

• Which kernel version (uname -a)?
  Linux rpi 4.14.50-v7+ #1122 SMP Tue Jun 19 12:26:26 BST 2018 armv7l GNU/Linux

[pelwell]

Some checksumming issues have been found and fixed since the kernel version you have now. Please run sudo rpi-update to install the latest kernel, then reboot and retest.

[6by9]

Please confirm that this is on a 3B and not a 3B+ - they have different network chips and therefore this is critical to debugging the issue.

4.14.50 is 7 months old and there have been a far number of fixes to both LAN drivers since then, although not apparently directly in this area. Can you update your kernel to the latest 4.14 kernel please? (The latest release from the Raspbian repos should be from mid December IIRC)

After updating, please provide the complete hex dump from tcpdump for these nominally corrupt packets.

There's also something strange about the MAC address that your computer is using. The first 3 octets of a MAC address should be registered to the manufacturer. f4:8e:38 is not a registered manufacturer, and I'd want to check whether it is actually valid (there are rules to dictate unicast vs multicast vs broadcast based on the MAC address, and I can't recall them off the top of my head).

[flux77]

@6by9 I can confirm that this is on a 3B (according to the PCB markings: "Raspberry Pi 3 Model B V1.2").

The output from lshw says that the network interface is a "RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller" by "Realtek Semiconductor Co., Ltd.".

I will post updates once I update the kernel.

[flux77]

@pelwell @6by9 I've upgraded the kernel using sudo apt-get install raspberrypi-kernel.

uname -a shows: Linux rpi 4.14.79-v7+ #1159 SMP Sun Nov 4 17:50:20 GMT 2018 armv7l GNU/Linux.

The issue is fixed. Thank you for your help.