selftests: seg6: add test for post-encap SID route lookup

Add a selftest for the SEG6_IPTUNNEL_TABLE attribute, which selects the FIB
table for the post-encap SID route lookup. This looks up the route for the
first SID, the outer destination of the encapsulated packet.

Two routers provide L3 VPN services over an IPv6 underlay. Each router uses
a separate VRF per tenant, with default blackhole routes (IPv4 and IPv6)
that drop unmatched traffic. Tenant traffic is encapsulated, then
decapsulated with an End.DT46.
The encap routes are installed in the tenant VRF, but the routes that match
the first SIDs live in a separate underlay table (500). The "lookup 500"
attribute points the lookup there rather than to the VRF.

The test covers both the input path, where forwarded host traffic triggers
encapsulation, and the output path, where a router originates traffic from
its own loopback inside a VRF.
With the "lookup" attribute, traffic reaches its destination on both paths.
Without it, on the input path the lookup stays in the VRF and hits the
blackhole, and on the output path it falls through to the main table, which
has no matching route.

Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://patch.msgid.link/20260711162907.6521-3-andrea.mayer@uniroma2.it
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions