Re: sendmsg and IP_PKTINFO

[Neil Brown]

On Tuesday February 18, [e-mail] wrote:
> 
> 	Hello,
> 
> Neil Brown wrote:
> 
> > Note that the in_pktinfo is described as "some information about the
> > incoming packet".  In particular ipi_ifindex is "the unique index of
> > the interface the packets was received on".
> >
> > i.e. it is more about the incoming than the outgoing packet.
> 
> 	Yes, because when set as socket option you can receive
> pktinfo with recvmsg. But IP_PKTINFO can be used also with sendmsg.
> Just forget about interfaces, i.e. use something like this in cmsg:
> 
> .ipi = {
> 	.ipi_ifindex = 0,
> 	.ipi_spec_dst = local_ip,
> },
> 

Certainly IP_PKTINFO can be used with sendmsg.  The doesn't change the
fact that the documentation clearly says that the information is
information about an incoming packet.

Also, th only documentation I can find does not specify that a value
of 0 means "no specific interface is selected".  So while I can see
from the code that what you suggest would work in practice, it would
be nice if this were clearly documented somewhere.

NeilBrown