[Bridge] [RFC] Do not activate promiscuous mode on wlan interfaces for bridging.

Luis Carlos Cobo luisca at cozybit.com
Wed Jun 11 18:06:00 PDT 2008


(Sorry if you received this mail multiple times, my email client had problems with the Cc's)

I have come across a 802.11 device that stops acknowledging frames addressed to
it when in promiscuous mode. The device can act as an AP with hostapd. However,
when the interface is attached to a bridge the lack of acknowledgments makes the
device non-functional as an AP.

It seems that it is not really necessary to activate promiscuous mode when
attaching a wlan interface to a bridge, and it causes unnecessary processing
overhead to any wlan device.

Case 1: Ethernet to ethernet
----------------------------

A <----> BRIDGE <----> B

The bridge needs to set promiscuous mode on both interfaces.

Case 2: Ethernet to 802.11 through AP
-------------------------------------

A <----> BRIDGE/AP -))  ((- B

The ethernet interface needs to be in promiscuous mode so that frames from A to
B are received by the bridge and then forwarded.

However, the wlan interface does _not_ need to be a promiscuous mode. A 802.11
frame from B to A will have:

ToDS
RA = BSSID = MAC_AP
TA = SA = MAC_B
DA = MAC_A

Frames are naturally addressed to the AP, so the wlan interface does not need to
be in promiscuous mode. Activating promiscuous mode would actually force the
device to deal with many frames it does not really have to deal with.

Case 3: Ethernet to 802.11 through IBSS / Managed STAs
------------------------------------------------------

A <----> BRIDGE/IBSS_STA_B -))  ((- IBSS_STA_C

or

A <----> BRIDGE/STA -))  ((- AP

In this case the frames on the wireless segment will not be naturally addressed
to the bridge, so activating promiscuous mode on the wireless interface would be
necessary for the bridge to work. However there are three problems here:

First as Jouni Malinen states [1], this would not comply with 802.11 standard,
which does not allow to use as SA a MAC address other than our own. If we do it
anyway, we will have to look for 802.11 ACKs to the MAC address we bridge to
control frame retransmissions. I do not think any device does this.

Second, such a bridge would be responsible for sending 802.11 ACKs for any
802.11 frame addressed to a MAC in the ethernet segment it bridges to. As acking
is usually implemented at MAC level I do not think any device supports this.

Third, on the managed STA case, the STA would have to fake an association with
the AP for A (and every other node in the ethernet segment it bridges to).  While
doable, it adds unnecessary ugly complexity.

Even with these problems the bridging might "work", but every frame on the wlan
segment from/to the bridged ethernet will have to be retransmitted the maximum
number of times since acks would not work properly.

Given this three cases, the best would be to consider case 3 to be not
supported (it is indeed not supported, AFAIK), and apply the patch below.

Please let me know what you think of this patch and whether it could interfere
with any functionality.

Nomenclature
RA = receiver address
TA = transmitter address
SA = source address
BSSID = basic service set identifier
<----> = Ethernet link
-)) ((- = 802.11 link

[1]: http://marc.info/?l=linux-wireless&m=117881727809631&w=2

Signed-off-by: Luis Carlos Cobo <luisca at cozybit.com>
---
 net/bridge/br_if.c |   13 +++++++++++--
 1 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/net/bridge/br_if.c b/net/bridge/br_if.c
index c2397f5..155bd90 100644
--- a/net/bridge/br_if.c
+++ b/net/bridge/br_if.c
@@ -135,7 +135,8 @@ static void del_nbp(struct net_bridge_port *p)
 
 	sysfs_remove_link(br->ifobj, dev->name);
 
-	dev_set_promiscuity(dev, -1);
+	if (!dev->ieee80211_ptr)
+		dev_set_promiscuity(dev, -1);
 
 	spin_lock_bh(&br->lock);
 	br_stp_disable_port(p);
@@ -389,7 +390,15 @@ int br_add_if(struct net_bridge *br, struct net_device *dev)
 		goto err2;
 
 	rcu_assign_pointer(dev->br_port, p);
-	dev_set_promiscuity(dev, 1);
+	/*
+	 * 802.11 interfaces working as Access Points do not need to be set to
+	 * promiscuous mode for bridging, as every frame is addressed to them.
+	 *
+	 * Bridging of 802.11 interfaces which are not in AP mode is not
+	 * supported.
+	 */
+	if (!dev->ieee80211_ptr)
+		dev_set_promiscuity(dev, 1);
 
 	list_add_rcu(&p->list, &br->port_list);
 
-- 
1.5.4.3



-- 
Luis Carlos Cobo Rus       GnuPG ID: 44019B60
cozybit Inc.




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