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After a new Kernel e.g. 4.4.0-131 is installed on our Ubuntu 16.04 LTS
clients, unneeded (waste) 14 bytes paddings are unexpectedly present
after PPPoE payload in client's Ethernet frames which are sent towards a
NAS access router. The content of the waste padding is not all zero
filled bytes, but rather parts of memory, sometimes with readable text.
Sometimes the waste padding is more than 14 bytes. For example, a PPPoED
PADI Frame with "amf/application/12" (18 bytes) shown as padding in
Wireshark:
0000 ff ff ff ff ff ff 00 0a cd 2a ea 9f 81 00 00 07 ÿÿÿÿÿÿ..Í*ê.....
0010 88 63 11 09 00 00 00 0c 01 01 00 00 01 03 00 04 .c..............
0020 d9 1f 00 00 61 6d 66 2f 61 70 70 6c 69 63 61 74 Ù...amf/applicat
0030 69 6f 6e 2f 31 32 ion/12
It is a critical bug of Kernel 4.4.0-131 and some previous releases
(since July 2018).
The affected client Ethernet frames are large enough (>64 bytes), and
thus they do not need to be padded. Padding should be included and is
required in Ethernet frames only to achieve the minimum 64 byte size of
an Ethernet frame sent on wire. For a packet larger than e.g. 100 bytes
no padding is needed in Ethernet frame. However we see such 14 Byte
paddings also in large client Ethernet Frames (in PPPoE/PPP/IP session
packets), with payload size of several hundred bytes (200, 300, 400
bytes and more).
Our Ubuntu 16.04 LTS Clients are always updated via apt update / apt
upgrade early enough, after a new Ubuntu update is released in Internet.
Our statistics after analyzing the collected archived pcap traces
(Clients with Ubuntu 16.04 LTS and the latest Kernel updates installed)
can help to identify the date when the kernel bug first appeared:
>From 15.Jul.2018 till 29.Jul.2018 - OK, still no waste padding
10.07.2018 - 14 byte padding (PADI etc) - a kernel BUG !
12.07.2018 - 14 byte padding (PADI etc) - a kernel BUG !
13.07.2018 - 14 byte padding (PADI etc) - a kernel BUG !
19.07.2018 - 14 byte padding (PADI etc) - a kernel BUG !
A kernel version affected with this BUG:
@client:~$ uname -a
Linux client 4.4.0-131-generic #157-Ubuntu SMP Thu Jul 12 15:51:36 UTC 2018
x86_64 x86_64 x86_64 GNU/Linux
@client:~$ lsb_release -rd
Description: Ubuntu 16.04.5 LTS
Release: 16.04
The kernel bug does not occur on Ubuntu 16.04 LTS Client and waste
paddings (14 byte or 18 byte) are not present in Client frames, if an
old Kernel e.g. 4.4.0-116 (of 12-Feb-2018) is installed on the client
and activated at boot time.
This old kernel 4.4.0-116 is still without the padding bug:
@client:~$ uname -a
Linux client 4.4.0-116-generic #140-Ubuntu SMP Mon Feb 12 21:23:04 UTC 2018
x86_64 x86_64 x86_64 GNU/Linux
A PADI packet with unneeded padding is included as attachment. It was
captured via tcpdump directly on the Ubuntu Client. It is a padding
inserted by Ubuntu, not by ethernet HW or driver. If it were padding
included by Ethernet adapter or driver, we could not capture it on the
same host and we could not see such real padding for this client packet
in Wireshark.
The issue affects both PPPoED client packets (PADI, PADR) and session
PPPoE/PPP/IP client packets. We expect no padding is included after
PPPoE payload in Ethernet Frames if the packets are not small (>64 byte)
and thus are not required to be padded for Ethernet. Such correct
behavior (without waste paddings) can be observed on Ubuntu 16.04 LTS
with old Kernel versions released before June 2018, like e.g. 4.4.0-116.
Please implement the fix in the next Kernel update.
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
** Tags: bot-comment
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Waste padding after PPPoE payload
https://bugs.launchpad.net/bugs/1784684
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