Networking – DD-WRT and Chromecast: default WLAN to LAN bridge blocks UDP

I have DD-WRT running in gateway mode on a DIR-615 I1. I've updated to the newest available build.

Chromecast is a wireless-only device, for those unfamiliar. The device has been set up and operates normally using wireless devices associated with the router, but wired ethernet devices cannot communicate with Chromecast. Specifically, the Chrome plugin says "No Cast Devices Found."

AP Isolation is disabled.

I have sniffed packets between a laptop via both wired and wireless connections and Chromecast, and this is what I've found:

There are IGMP, TCP, UDP packets between Chromecast and a wireless interface on the laptop.

There are IGMP and TCP, but zero UDP packets exchanged between Chromecast and a wired ethernet interface on the same laptop.

Specifically, the wired interface does receive the following IGMP packet from the Chromecast:

1062    55.011807   192.168.1.145   239.255.255.250 IGMP    60  V2 Membership Report / Join group 239.255.255.250

… which is also observable on the wireless interface. Some packets on the wired ethernet interface are marked as having a bad ipv4 header checksum (0x0000), but I've read this is artifactual due to checksum offloading on the NIC.

Here is the (redacted) TCP stream observable on the wired interface:

GET /ssdp/device-desc.xml HTTP/1.1
Host: 192.168.1.145:8008
Connection: keep-alive
Cache-Control: max-age=0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/29.0.1547.49 Safari/537.36
DNT: 1
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8,fr;q=0.6

HTTP/1.1 200 OK
Application-URL: http://192.168.1.145:8008/apps/
Content-Type: application/xml
Content-Length: 1078


<?xml version="1.0"?>
<root xmlns="urn:schemas-upnp-org:device-1-0">
  <specVersion>
    <major>1</major>
    <minor>0</minor>
  </specVersion>
  <URLBase>http://192.168.1.145:8008</URLBase>
  <device>
    <deviceType>urn:dial-multiscreen-org:device:dial:1</deviceType>
    <friendlyName>Bedroom TV</friendlyName>
    <manufacturer>Google Inc.</manufacturer>
    <modelName>Eureka Dongle</modelName>
    <UDN>uuid:XXXXXXXXXXXXXXXXXXXXXXXXXXXXX</UDN>
    <iconList>
      <icon>
        <mimetype>image/png</mimetype>
        <width>98</width>
        <height>55</height>
        <depth>32</depth>
        <url>/setup/icon.png</url>
      </icon>
    </iconList>
    <serviceList>
      <service>
        <serviceType>urn:dial-multiscreen-org:service:dial:1</serviceType>
        <serviceId>urn:dial-multiscreen-org:serviceId:dial</serviceId>
        <controlURL>/ssdp/notfound</controlURL>
        <eventSubURL>/ssdp/notfound</eventSubURL>
        <SCPDURL>/ssdp/notfound</SCPDURL>
      </service>
    </serviceList>
  </device>
</root>

This is apparently not good enough for function, and the lack of UDP packets going in the ath0 –> eth1 (the WLAN and LAN interfaces on the DD-WRT router) direction is suspicious. Can anyone shed light on what may be happening? Bridge and route settings on the DD-WRT are all basically default.