packet(7) omits sll_protocol from required field list for outbound packets

[Graham Shaw <gdshaw-1YjpMCWsJptAfugRpC6u6w@public.gmane.org>]

The manpage packet(7) currently states that:

"When you send packets it is enough to specify sll_family, sll_addr, 
sll_halen, sll_ifindex."

This is incorrect: you also need to specify sll_protocol.

(The protocol specified when the socket is created is used for filtering 
inbound packets, but not for constructing outbound packets.)

I encountered this while researching a page for my website:

http://www.microhowto.info/howto/send_an_arbitrary_ethernet_frame_using_an_af_packet_socket_in_c.html

To empirically verify the behaviour I took my test code from the above 
page then changed it to use different values for the third argument to 
socket() and the sll_protocol field:

- socket created with ETH_P_ARP, packet sent with ETH_P_ARP:
     packet sent with EtherType of ETH_P_ARP
- socket created with ETH_P_ARP, sll_protocol==0:
     packet sent with EtherType of 0
- socket created with 0x88b5, sll_protocol==htons(ETH_P_ARP):
     packet sent with EtherType of ETH_P_ARP
- socket created with ETH_P_ARP, sll_protocol==htons(0x88b5):
     packet sent with EtherType of 0x88b5

This shows that leaving sll_protocol set to zero does not have the 
desired effect and that it needs to be set to the desired link-layer 
protocol.

There is code in the relevant kernel source file 
(net/packet/af_packet.c) which appears to inspect the value of the 
sll_protocol field and use it as the link-layer protocol number, however 
I am not sufficiently familiar with this subsystem to be fully confident 
of what is happening. The line in question is:

     proto   = saddr->sll_protocol;

In version 3.4 of the kernel this can be found in the functions 
packet_snd and tpacket_snd. In version 2.6.26 it is in packet_sendmsg.

Below is a patch that adds sll_protocol to the list of required fields. 
This may not be the whole truth, since it is not clear what role if any 
sll_protocol, sll_halen or sll_addr would play when the socket type is 
SOCK_RAW, however I'm confident it is more accurate than the page as it 
stands at present:

diff --git a/man7/packet.7 b/man7/packet.7
index 374f6da..fb09f15 100644
--- a/man7/packet.7
+++ b/man7/packet.7
@@ -160,7 +160,9 @@ When you send packets it is enough to specify
  .IR sll_family ,
  .IR sll_addr ,
  .IR sll_halen ,
-.IR sll_ifindex .
+.IR sll_ifindex
+and
+.IR sll_protocol .
  The other fields should be 0.
  .I sll_hatype
  and

Yours,

Graham Shaw
http://gdshaw.net/
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