Developing New Socket Option on 4.10

[Haisang Wu]

Hello, all,

  I am writing for help on my freebsd kernel hacking. I am working on 4.10,
and intend to introduct a new socket option (say, SO_TAG_PAK)
for my own kernel. When sending out a packet, if this option is set, the
socket layer will produce a 20-byte tag, and finally this tag will be
prepended before IP header (may be done in ip_output() ). This is somewhat
like MPLS's shim layer, but this special packet with the tag will only be
sent over a logical tunnel to another system. When receiving a packet, the
IP layer will check whether the packet has this tag (through the
first integer of the tag), and pass the tag up (to applications).

  For future possible extensions, I introduced a flag, so_tag_pak_flag, into
"struct socket".  I modified sosetopt() to set so->so_tag_pak_flag=0x1 when
(sopt->sopt_name == SO_TAG_PAK). I will write a function so_tagpak( ) to
generate the 20-byte tag. This tag will be kept in the kernel
space, and later be prepended to the IP packet in function ip_output( ).

  I have a few questions below:

At sending direction:

1. Should I allocate a separate mbuf for this tag inside so_tagpak( )?
2. The function so_tagpak( ) should be called in the socket layer so the
socket can check the flag and produce the tag.
    Where should it be called?
3. In ip_output( ), can I prepend this tag (i.e., a separate mbuf) before
the IP header? Could you give me some suggestions on how to do so?

At receiving direction:

4. Can I detect the tag in ipintr( )? How will be the mbuf operation here?

  Thank you very much for your help!

Haisang