packet.7: Add sll_protocol to list of required fields for outbound packets

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.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man7/packet.7

@@ -173,7 +173,9 @@ 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