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-rw-r--r--doc/primary-expression.txt6
1 files changed, 6 insertions, 0 deletions
diff --git a/doc/primary-expression.txt b/doc/primary-expression.txt
index 6eb9583a..34bcf2d3 100644
--- a/doc/primary-expression.txt
+++ b/doc/primary-expression.txt
@@ -274,6 +274,12 @@ fib_addrtype
# drop packets without a reverse path
filter prerouting fib saddr . iif oif missing drop
+In this example, 'saddr . iif' looks up routing information based on the source address and the input interface.
+oif picks the output interface index from the routing information.
+If no route was found for the source address/input interface combination, the output interface index is zero.
+In case the input interface is specified as part of the input key, the output interface index is always the same as the input interface index or zero.
+If only 'saddr oif' is given, then oif can be any interface index or zero.
+
# drop packets to address not configured on ininterface
filter prerouting fib daddr . iif type != { local, broadcast, multicast } drop