netdev - Re: [PATCH] packet: Allow packets with only a header (but no payload)

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Message-ID: <CAFBinCBYv44ttGNyny11gEwNMJDwYzO=YYJC5scwOE-PN2nn4A@mail.gmail.com>
Date:	Fri, 31 Jul 2015 00:15:46 +0200
From:	Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:	Willem de Bruijn <willemb@...gle.com>
Cc:	Network Development <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	David Miller <davem@...emloft.net>, johann.baudy@...-log.net,
	paulus@...ba.org
Subject: Re: [PATCH] packet: Allow packets with only a header (but no payload)

On Wed, Jul 29, 2015 at 8:05 AM, Willem de Bruijn <willemb@...gle.com> wrote:
> Martin, to return to your initial statement that PPPoE PADI packets can
> have a zero payload: the PPPoE RFC states that PADI packets "MUST
> contain exactly one TAG of TAG_TYPE Service-Name, indicating the
> service the Host is requesting, and any number of other TAG types."
> (RFC 2516, 5.1). Is the observed behavior (no payload) perhaps
> incorrect?
As far as I can see you are right, but the real world seems to be different.
My ISP for example lists the PPPoE connection settings, but they are
nowhere mentioning the "service name".

I have also re-read pppd's source code again and that seems to confirm
what you are reading in the RFC: Leaving the service name away makes
seems to violate the RFC, but pppd still accepts those configurations.

> Even if it is, if this is breaking established userspace expectations,
> we should look into it. Ethernet specifies a minimum payload size of
> 46 on the wire, but perhaps that is handled with padding, so that
> 0 length should be valid within the stack. Also, there may be other
> valid uses of 0 length payload on top of link layers that are not Ethernet.
Good catch. I would also like to note that the documentation for
"hard_header_len" describes it as "Hardware header length". When the
purpose of this field we should check whether the documentation should
be updated to "Minimum hardware header length" -> that would mean the
condition has to be a "len < hard_header_len" instead of a "len <=
hard_header_len" (as it is now).

PS: I have also added the pppd maintainer (Paul Mackerras) to this
thread because I think he should know about this issue (and he can
probably provide more details if required).
As a quick summary for him: linux  >= 3.19 rejects PADI packets when
no service name is configured.


Regards,
Martin
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