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

[LEDE Bugs]

The following task has a new comment added:

FS#714 - ARP/Broadcast does not reach other clients in the same WLAN
User who did this - K2DLS (K2DLS)

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After upgrading my WRT1900AC to LEDE 17.01.4, I noticed that Chromecast had stopped working.  In searching for an answer I found this thread:

https://forum.lede-project.org/t/odhcpd-as-ipv4-dhcp-w-unbound-dns/12630/4

Even though I have not defined AP isolation, the contents of /tmp/run/hostapd-phy0.conf contains ap_isolate=1 in the definition for my 2.4 GHz interface.

ap_isolate=1 is also present in /tmp/run/hostapd-phy1.conf which is the definition for my 5 GHz interface.

As a workaround to the problem, I changed /sys/devices/virtual/net/br-lan/brif/wlan0/brport/hairpin_mode from 1 to 0.  Then, Chromecast began to work.

This appears to be a widespread issue which merits higher prioritization.
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More information can be found at the following URL:
https://bugs.openwrt.org/index.php?do=details&task_id=714#comment4567