An entry in data-types.txt offers space for a name-value table. Even if
one would refer to ARPHRD_*, some names are not derived from the
respective macro name and thus not intuitive.

Signed-off-by: Phil Sutter <phil@nwl.cc>
---
 doc/data-types.txt         | 27 +++++++++++++++++++++++++++
 doc/primary-expression.txt |  2 --
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/doc/data-types.txt b/doc/data-types.txt
index e44308b5322cb..0b350effbea68 100644
--- a/doc/data-types.txt
+++ b/doc/data-types.txt
@@ -83,6 +83,33 @@ filter input iifname eth0
 filter input iifname "(eth0)"
 ----------------------------
 
+INTERFACE TYPE TYPE
+~~~~~~~~~~~~~~~~~~~
+[options="header"]
+|==================
+|Name | Keyword | Size | Base type
+|Interface type |
+iface_type|
+16 bit |
+integer
+|===================
+
+The interface type type is used with *meta iiftype/oiftype* expression. Its values correspond with respective ARPHRD_* defines in <linux/if_arp.h>.
+
+.The following keywords will automatically resolve into an interface type type with given value
+
+[options="header"]
+|==================
+|Keyword | Value
+| ether | 1
+| ppp | 512
+| ipip | 768
+| ipip6 | 769
+| loopback | 772
+| sit | 776
+| ipgre | 778
+|===================
+
 LINK LAYER ADDRESS TYPE
 ~~~~~~~~~~~~~~~~~~~~~~~
 [options="header"]
diff --git a/doc/primary-expression.txt b/doc/primary-expression.txt
index d5495e2c86291..bd80cd7f92fda 100644
--- a/doc/primary-expression.txt
+++ b/doc/primary-expression.txt
@@ -149,8 +149,6 @@ String value in the form HH:MM or HH:MM:SS. Values are expected to be less than
 Interface index (32 bit number). Can be specified numerically or as name of an existing interface.
 |ifname|
 Interface name (16 byte string). Does not have to exist.
-|iface_type|
-Interface type (16 bit number).
 |uid|
 User ID (32 bit number). Can be specified numerically or as user name.
 |gid|
-- 
2.51.0