x/net/ipv4: an address as a result of routing table lookup for source-routing, source address selection

[anacrolix]

In addition to Dst and IfIndex, ControlMessage should contain a member corresponding to
ipi_spec_dst, from struct in_pktinfo. This is available when IP_PKTINFO is set.

It's necessary when the IP address that a packet is received on is required, rather than
the header destination address (which is what Dst represents).

[anacrolix]

Comment 1:

A patch is attached.

Attachments:

1. specdst.diff (1126 bytes)

[minux]

Comment 2:

Owner changed to @mikioh.

[davecheney]

Comment 3:

Can you please follow the contribution process described here,
http://golang.org/doc/contribute.html

[anacrolix]

Comment 4:

https://golang.org/cl/9727045

[mikioh]

Comment 5:

I want to understand why we have to have a ipi_spec_dst.
Can you give us some use cases  for both receiving, specifying ipi_spec_dst?

Status changed to Thinking.

[anacrolix]

Comment 6:

When the IP address that a packet is received on is required, rather than the header
destination address (which is what Dst represents). This occurs when you need to involve
the receiving interface address in the crafting of the response. For example in
responding to an SSDP M-SEARCH, one may infer the value for the response Location header
with respect to the searcher.

[anacrolix]

Comment 7:

When the IP address that a packet is received on is required, rather than the header
destination address (which is what Dst represents). This occurs when you need to involve
the receiving interface address in the crafting of the response. For example in
responding to an SSDP M-SEARCH, one may infer the value for the response Location header
with respect to the searcher.

[anacrolix]

Comment 8:

When the IP address that a packet is received on is required, rather than the header
destination address (which is what Dst represents). This occurs when you need to involve
the receiving interface address in the crafting of the response. For example in
responding to an SSDP M-SEARCH, one may infer the value for the response Location header
with respect to the searcher.

[anacrolix]

Comment 9:

When the IP address that a packet is received on is required, rather than the header
destination address (which is what Dst represents). This occurs when you need to involve
the receiving interface address in the crafting of the response. For example in
responding to an SSDP M-SEARCH, one may infer the value for the response Location header
with respect to the searcher.

[mikioh]

Comment 11:

I'm afraid that your use case is not suitable for this ancillary usage because, a) it's
a result of routing, also cached information, b) even though linux 3.x and beyond will
replace ipi/rt_spec_dst cache with fib_compute_path stuff it's still part of routing
stuff, c) seems like what you need is application-specific, adaptive source address
selection.
But, there's no reason to hesitate to review, so please proceed with your CL.

[anacrolix]

Comment 12:

a) Sure, I have no problem with that.
b) It's on my system: matt@dogfort:~/gopath/src/bitbucket.org/anacrolix/dms$ uname -a
Linux dogfort 3.5.0-28-generic #48-Ubuntu SMP Tue Apr 23 23:03:38 UTC 2013 x86_64 x86_64
x86_64 GNU/Linux
c) What? I need to know the address that received the packet. Since interfaces can have
multiple addresses, I can't use IfIndex. Dst was the intended destination, which in the
case of SSDP is a multicast group, hardly useful.

[mikioh]

Comment 13:

> I need to know the address that received the packet
I thought that you want to use that address for specifying originator's Loc/ID stuff
(e.g.,  source address in the application protocol) on outgoing packets. That means that
applications which don't have any help of multi6, shim6 or mif6-like super-duper
complicated source address selection and its negotiation/signaling stuff need such last
hop address on the reverse path of source-routing. Am I missing something?

[anacrolix]

Comment 14:

Yes, that's the Src address when a packet is received. This is the local address that
the packet was received on. 
"IP address that a packet is received on is required, rather than the header destination
address (which is what Dst represents)"

[mikioh]

Comment 15:

I think it might help to understand if you can say scoped "src" or "local" (e.g., IP
header's src or routed protocol's src) because "src" or "local" w/o scope doesn't make
sense when we should think of both routed and routing stuff at the same time.
Anyway before proceeding, please change the field name and its comment to:
Hop []net.IP // hop of source routing
then move down just after IfIndex.
Also pls change CL description to:
go.net/ipv4: add source routing support
and probably drop "see ip(7)".
Thanks.

[anacrolix]

Comment 16:

This doesn't make sense, I'm not sure you're understanding what this field represents.