make a first pass at documenting the network offload support.

the ioctls are here, the offload header is next.

1 files changed, 39 insertions, 2 deletions

diff --git a/share/man/man4/tun.4 b/share/man/man4/tun.4
index f300f68568c..2668f68d8d7 100644
--- a/share/man/man4/tun.4
+++ b/share/man/man4/tun.4
@@ -1,4 +1,4 @@
-.\"	$OpenBSD: tun.4,v 1.47 2024/11/15 00:32:04 jsg Exp $
+.\"	$OpenBSD: tun.4,v 1.48 2024/11/17 00:51:04 dlg Exp $
 .\"
 .\" Copyright (c) 2003 Marcus D. Watts  All rights reserved.
 .\"
@@ -26,7 +26,7 @@
 .\" TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
 .\" USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd $Mdocdate: November 15 2024 $
+.Dd $Mdocdate: November 17 2024 $
 .Dt TUN 4
 .Os
 .Sh NAME
@@ -115,6 +115,42 @@ and must be set to the same values that the kernel provided.
 .It Dv TUNSIFMODE Fa int *
 is provided for backwards compatibility.
 The flags set must match what the kernel initialised them to.
+.Pp
+.It Dv TUNSCAP Fa struct tun_capabilities *
+Enable the prepending of network packets with a
+.Vt struct tun_hdr
+offload header,
+and set which interface offload capabilities that userland can
+handle on behalf of the kernel.
+.Bd -literal
+struct tun_capabilities {
+	uint32_t	tun_if_capabilities;
+};
+.Ed
+.Pp
+The
+.Fa tun_if_capabilities
+field is set with IFCAP values from
+.In net/if.h .
+.Pp
+The prepending of network packets with the a
+.Vt struct tun_hdr
+offload header is disabled,
+and the interface offload capabilities are reset when the device
+special file is closed.
+.Pp
+.It Dv TUNGCAP Fa struct tun_capabilities *
+Get which interface offload capabilities are currently enabled.
+If the interface has not been configured with
+.Dv TUNSCAP
+this command will fail with
+.Er ENODEV .
+.Pp
+.It Dv TUNDCAP
+Disable the prepending of network packets with the
+.Vt struct tun_hdr
+offload header,
+and clear interface offload capabilities.
 .El
 .Pp
 The generic ioctls
@@ -154,6 +190,7 @@ commands:
 .It Dv SIOCSIFADDR Fa uint8_t Ns [ETHER_ADDR_LEN]
 Get or set the link layer address (MAC address) of the interface.
 .El
+.\" .Ss Network offload support
 .Sh FILES
 .Bl -tag -width /dev/tun* -compact
 .It Pa /dev/tun*