how to search for the best route from userspace in netlink?

how to search for the best route from userspace in netlink?

[Dave Taht]

Windows has the "RtmGetMostSpecificDestination" call:
https://docs.microsoft.com/en-us/windows/win32/rras/search-for-the-best-route

In particular, I wanted to search for the best route, AND pick up the
PMTU from that (if it existed)
for older UDP applications like dnssec[1] and newer ones like QUIC[2].
The alternatives, which
basically include probing perpetually and/or picking really low
values, seem increasingly less than
optimal.

Put in a wrapper around bpf[3]'s lpm calls? Create a new netlink message?

[1] https://github.com/dns-violations/dnsflagday/issues/125
[2] https://tools.ietf.org/html/draft-ietf-quic-transport-22#section-14.1
[3] https://github.com/torvalds/linux/blob/master/kernel/bpf/lpm_trie.c#L164

-- 

Dave Täht
CTO, TekLibre, LLC
http://www.teklibre.com
Tel: 1-831-205-9740

Re: how to search for the best route from userspace in netlink?

[David Ahern]

On 9/2/19 4:07 PM, Dave Taht wrote:
> Windows has the "RtmGetMostSpecificDestination" call:
> https://docs.microsoft.com/en-us/windows/win32/rras/search-for-the-best-route
> 
> In particular, I wanted to search for the best route, AND pick up the
> PMTU from that (if it existed)
> for older UDP applications like dnssec[1] and newer ones like QUIC[2].

RTM_GETROUTE with data for the route lookup. See iproute2 code as an
example.

Re: how to search for the best route from userspace in netlink?

[Dave Taht]

On Mon, Sep 2, 2019 at 8:13 PM David Ahern <dsahern@gmail.com> wrote:
>
> On 9/2/19 4:07 PM, Dave Taht wrote:
> > Windows has the "RtmGetMostSpecificDestination" call:
> > https://docs.microsoft.com/en-us/windows/win32/rras/search-for-the-best-route
> >
> > In particular, I wanted to search for the best route, AND pick up the
> > PMTU from that (if it existed)
> > for older UDP applications like dnssec[1] and newer ones like QUIC[2].
>
> RTM_GETROUTE with data for the route lookup. See iproute2 code as an
> example.

Yes. I really didn't describe my thinking very well. It's coping with
pmtu better
in the case of a more increasingly udp'd and tunneled internet. tcp
(being kernel based)
will do the probe and cache that attribute of the path, udp does not.
A udp based app with root privs could be setting it after figuring it
out,  a userspace one cannot.

for more detail from server-al sides of that philosophical debate,
please see the links I posted
originally.



-- 

Dave Täht
CTO, TekLibre, LLC
http://www.teklibre.com
Tel: 1-831-205-9740