Re: IP_PKTINFO, sendmsg(2) and routing.

[Dennis Ferguson <dennis.c.ferguson%gmail.com@localhost>]

> On Feb 8, 2019, at 20:34, Roy Marples <roy%marples.name@localhost> wrote:
> 
> On 08/02/2019 19:16, David Young wrote:
>> On Fri, Feb 08, 2019 at 05:19:40PM +0000, Roy Marples wrote:
>>> Hi List
>>> 
>>> Whilst traking down a dhcpcd issue for Jared it occured to me that
>>> my use of IP_PKTINFO might not be ideal for NetBSD. Here's the
>>> scenario:
>>> 
>>> interface a: 192.168.0.101/24
>>> interface b: 192.168.0.102/24
>>> 
>>> The route to 192.168.0.255 belongs on interface a.
>>> 
>>> If I send a UDP packet to 192.168.0.1 it will by default leave by
>>> interface a.
>>> My question is this - should IP_PKTINFO allow the same message to
>>> leave by interface b - ie effectively bypassing the routing table?
>> I think so, but I think that if you want to send a packet that both has
>> a particular source address and transmits on a particular interface,
>> IP_PKTINFO may not be adequate?
> 
> Well, our implementation of it is not adequate as it stands, especially if you share the same address across interfaces.
> 
> I suppose it's also a security question because a non privileged process can set IP_PKTINFO but only root can create routes.
> It's works on Linux (my primary use case) because each interface address has it's own prefix route and the lowest metric wins. Root has already created the routes, so it's an expression of choice.

It should send the packet out the IP_PKTINFO interface if packet is sent
with MSG_DONTROUTE/SO_DONTROUTE. If this isn’t working it should probably
be made to, routing protocols in general depend on being able to do that.

Dennis Ferguson