src: add fib expression

This adds the 'fib' expression which can be used to
obtain the output interface from the route table based on either
source or destination address of a packet.

This can be used to e.g. add reverse path filtering:

 # drop if not coming from the same interface packet
 # arrived on
 # nft add rule x prerouting fib saddr . iif oif eq 0 drop

 # accept only if from eth0
 # nft add rule x prerouting fib saddr . iif oif eq "eth0" accept

 # accept if from any valid interface
 # nft add rule x prerouting fib saddr oif accept

Querying of address type is also supported.  This can be used
to e.g. only accept packets to addresses configured in the same
interface:
 # fib daddr . iif type local

Its also possible to use mark and verdict map, e.g.:
 # nft add rule x prerouting meta mark set 0xdead fib daddr . mark type vmap {
   blackhole : drop,
   prohibit : drop,
   unicast : accept
 }

Signed-off-by: Florian Westphal <fw@strlen.de>

2 files changed, 36 insertions, 0 deletions

diff --git a/tests/py/inet/fib.t b/tests/py/inet/fib.t
new file mode 100644
index 00000000..9ee282ab
--- /dev/null
+++ b/tests/py/inet/fib.t
@@ -0,0 +1,14 @@
+:prerouting;type filter hook prerouting priority 0
+
+*ip;test-ip;prerouting
+*ip6;test-ip6;prerouting
+
+fib saddr . daddr oif lo;fail
+fib iif . oif . daddr oif lo;fail
+fib mark oif lo;fail
+fib saddr . iif oif ne 0;ok;fib saddr . iif oif != 0
+fib saddr . iif oifname "lo";ok
+
+fib daddr . iif type local;ok
+fib daddr . iif type vmap { blackhole : drop, prohibit : drop, unicast : accept };ok
+fib daddr . oif type local;fail
diff --git a/tests/py/inet/fib.t.payload b/tests/py/inet/fib.t.payload
new file mode 100644
index 00000000..f5258165
--- /dev/null
+++ b/tests/py/inet/fib.t.payload
@@ -0,0 +1,22 @@
+# fib saddr . iif oif ne 0
+ip test-ip prerouting
+  [ fib saddr . iif oif => reg 1 ]
+  [ cmp neq reg 1 0x00000000 ]
+
+# fib saddr . iif oifname "lo"
+ip test-ip prerouting
+  [ fib saddr . iif oifname => reg 1 ]
+  [ cmp eq reg 1 0x00006f6c 0x00000000 0x00000000 0x00000000 ]
+
+# fib daddr . iif type local
+ip test-ip prerouting
+  [ fib daddr . iif type => reg 1 ]
+  [ cmp eq reg 1 0x00000002 ]
+
+# fib daddr . iif type vmap { blackhole : drop, prohibit : drop, unicast : accept }
+__map%d test-ip b
+__map%d test-ip 0
+        element 00000006  : 0 [end]     element 00000008  : 0 [end]     element 00000001  : 0 [end]
+ip test-ip prerouting
+  [ fib daddr . iif type => reg 1 ]
+  [ lookup reg 1 set __map%d dreg 0 ]