[FS#714] ARP/Broadcast does not reach other clients in the same WLAN

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FS#714 - ARP/Broadcast does not reach other clients in the same WLAN
User who did this - Catalin Patulea (cpatulea)

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+1 seeing this on OpenWrt 19.07.2 on a Buffalo WZR-600DHP.

With default settings ("Isolate clients" *unchecked*), clients cannot 'ping' each other and Chromecast (which relies on multicast) is very unreliable. The interface and hostapd parameters are:

# grep isolate /var/run/hostapd-phy0.conf
ap_isolate=1
# ( cd /sys/devices/virtual/net/br-lan/lower_wlan0/brport; g
rep -H . multicast_to_unicast hairpin_mode; )
multicast_to_unicast:1
hairpin_mode:1

For a simple ping test, using a third device as 802.11 sniffer, we can see ARP broadcast sent by client1 (.10):
3937  0.022 IntelCor_e5:dd:5a   Broadcast     ARP 130 Who has 192.168.1.36? Tell 192.168.1.10

these are lost, there is no reply (.36), I think due to 802.11 powersave (?). Client 2 is probably in powersave, as most 802.11 clients usually are, and only AP knows the right moment to transmit frames (just after beacon frame, or PS-Poll). Client 1 always gets unlucky and sends when C2 is sleeping.


When setting the following settings:
config interface 'lan'
        option multicast_to_unicast '0'
config wifi-iface 'default_radio0'
        option isolate '0'

then the interface and hostapd parameters are:
~# grep isolate /var/run/hostapd-phy0.conf

# ( cd /sys/devices/virtual/net/br-lan/lower_wlan0/brport; g
rep -H . multicast_to_unicast hairpin_mode; )
multicast_to_unicast:0
hairpin_mode:0

now the clients can reach each other, by AP re-transmit the frames (from looking at 802.11 "Transmitter address" and "Receiver address" fields):

[c1->ap] 14378	2.156	192.168.1.10		192.168.1.36		62	ICMP	186	Echo (ping) request  id=0x0ca6, seq=1/256, ttl=62 (no response found!)
[ap->c2] 14380	0.000	192.168.1.10		192.168.1.36		62	ICMP	186	Echo (ping) request  id=0x0ca6, seq=1/256, ttl=62 (reply in 14382)
[c2->ap] 14382	0.001	192.168.1.36		192.168.1.10		64	ICMP	186	Echo (ping) reply    id=0x0ca6, seq=1/256, ttl=64 (request in 14380)
[ap->c1] 14384	0.000	192.168.1.36		192.168.1.10		64	ICMP	186	Echo (ping) reply    id=0x0ca6, seq=1/256, ttl=64

I think AP re-transmit is necessary for reliable transmission, since only AP is aware of client powersave state, can buffer frames, etc.

The fact that re-transmit does not work with default settings ("Isolate clients" disabled) seems like a significant bug to me, wish the bug "Priority" would be increased.
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More information can be found at the following URL:
https://bugs.openwrt.org/index.php?do=details&task_id=714#comment9216

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