Argthtjtr

I have a Raspberry Pi setup as a WiFi hotspot (it is a DHCP server and it has HOSTAPD running). It does not have an internet connection and isn't routing traffic. Clients only connect to its network when they want to communicate with it.

I have a python script on the Raspberry Pi that is sending a UDP message ~5 seconds.

I can see the packets being sent through Wireshark (which I have installed on the Raspberry Pi), but none of the clients that connect to the Pi's network can see those packets. It's like they are hitting a dead end somewhere before they get out of the Pi.

I should also mention the clients have their firewalls turned off.

Here's some screenshots from the Pi:
You can see Wireshark shows the packets being sent. You can also see BROADCAST is enabled on wlan0.

The client (a Windows 10 computer in this case) is also running Wireshark and you can see the network info for it too:

What am I missing in the network setup that is keeping this broadcast from reaching clients? If I connect the Raspberry Pi to a real router, the clients on that network can see the UDP messages fine. That makes me think there is something wrong with the self-hosted hostspot I setup.

Thanks for any insight you can provide.

The contents of the hostapd config:

interface=wlan0

driver=nl80211

#driver=rtl871xdrv

ssid=PI032378

channel=6

dtim_period=1

beacon_int=400

Here's the Wireshark info about the packets that are being sent, but not received:

Frame 162: 60 bytes on wire (480 bits), 60 bytes captured (480 bits) on interface 0

    Interface id: 0 (wlan0)

    Encapsulation type: Ethernet (1)

    Arrival Time: Feb 21, 2017 16:45:08.000647000 Central Standard Time

    [Time shift for this packet: 0.000000000 seconds]

    Epoch Time: 1487717108.000647000 seconds

    [Time delta from previous captured frame: 0.007711000 seconds]

    [Time delta from previous displayed frame: 0.000000000 seconds]

    [Time since reference or first frame: 1.299469000 seconds]

    Frame Number: 162

    Frame Length: 60 bytes (480 bits)

    Capture Length: 60 bytes (480 bits)

    [Frame is marked: False]

    [Frame is ignored: False]

    [Protocols in frame: eth:ethertype:ip:udp:data]

    [Coloring Rule Name: UDP]

    [Coloring Rule String: udp]

Ethernet II, Src: Shenzhen_0c:b4:25 (40:a5:ef:0c:b4:25), Dst: Broadcast (ff:ff:ff:ff:ff:ff)

    Destination: Broadcast (ff:ff:ff:ff:ff:ff)

    Source: Shenzhen_0c:b4:25 (40:a5:ef:0c:b4:25)

    Type: IPv4 (0x0800)

Internet Protocol Version 4, Src: 192.168.42.1, Dst: 192.168.42.255

    0100 .... = Version: 4

    .... 0101 = Header Length: 20 bytes (5)

    Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)

    Total Length: 46

    Identification: 0xa04d (41037)

    Flags: 0x02 (Don't Fragment)

    Fragment offset: 0

    Time to live: 64

    Protocol: UDP (17)

    Header checksum: 0xc420 [validation disabled]

    [Header checksum status: Unverified]

    Source: 192.168.42.1

    Destination: 192.168.42.255

    [Source GeoIP: Unknown]

    [Destination GeoIP: Unknown]

User Datagram Protocol, Src Port: 44661, Dst Port: 5003

    Source Port: 44661

    Destination Port: 5003

    Length: 26

    Checksum: 0x779c [unverified]

    [Checksum Status: Unverified]

    [Stream index: 3]

Data (18 bytes)

    Data: 426c756562657272793a5049303332333738

    [Length: 18]

Multicasts and broadcasts on 802.11 must be sent using a modulation scheme (data rate) that every client can receive. They must be one of the rates in the Basic Rate Set (mandatory rate set) for that network, and they must be encrypted with the group key and group cipher for that network.

Although you might have a multicast rate problem, if you're using encryption (i.e. WPA2-PSK), it's more likely to be an encryption problem. As a test, try turning off wireless encryption in your hostapd config and see if the problem goes away.

It could be that the network interface on the Raspberry Pi is calculating the UDP checksums incorrectly. You can check if the checksumming is offloaded to the network interface by running ethtool -k eth0 (orethtool -k wlan0, or whatever the network interface you are serving the DHCP server on) on the Raspberry Pi. If its output shows that the checksumming is on (i.e. tx-checksumming: on), you can try to disable checksumming:

sudo ethtool --offload eth0 tx off

I've had this issue, and there's a relevant question on the Raspberry Pi stackexchange.