# Not Seeing Multicast

## SDNick484

I recently set up a new laptop running Gentoo and am trying to see some multicast traffic which I know is present on my network however I can't seem to see any with the new system.  I know the traffic is present as I see it when running the same Python program on my old laptop (also running Gentoo) on the same network.  Even running "# tcpdump -i wlan0 host 239.255.255.250" on the new laptop does not reveal any traffic on the new laptop whereas the old laptop is seeing a ton.

At the kernel level, I'm configured with:

CONFIG_IP_MULTICAST=y

Both laptops are running iwlwifi albeit on different cards (Intel Wireless 8265 on the new, Intel Centrino Advanced-N 6205 on old)

Based on some Google searches, I've set the following to no avail:

net.ipv4.icmp_echo_ignore_broadcasts = 0

net.ipv4.conf.wlan0.rp_filter = 0

I don't have much multicast experience so any suggestions would be appreciated.   At this point I suspect it may be some sort of routing or grouping issue.

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## SDNick484

I've made a little progress in that I manually joined a multicast group and can see some traffic on the new laptop.  The python code referenced earlier is available here.

i.e. After running:

```
#  addr add 239.255.255.250/32 dev wlan0 autojoin

```

I can now see some (but not all) multicast traffic on that address

# tcpdump -i wlan0 host 239.255.255.250

I see other UDP packets, but not the ones I'm looking for:

```
# tcpdump -i wlan0 -n host 239.255.255.250

dropped privs to tcpdump

tcpdump: verbose output suppressed, use -v or -vv for full protocol decode

listening on wlan0, link-type EN10MB (Ethernet), capture size 262144 bytes

16:35:17.513019 IP 192.168.1.217.41281 > 239.255.255.250.1900: UDP, length 125

16:35:17.921516 IP 192.168.1.217.41281 > 239.255.255.250.1900: UDP, length 125

^C

2 packets captured

2 packets received by filter

0 packets dropped by kernel

```

On the old computer, I see the following:

```
# tcpdump -i wlan0 -vvn host 255.255.255.255

dropped privs to tcpdump

tcpdump: listening on wlan0, link-type EN10MB (Ethernet), capture size 262144 bytes

16:35:08.511946 IP (tos 0x0, ttl 255, id 44258, offset 0, flags [none], proto UDP (17), length 203)

    192.168.1.200.49154 > 255.255.255.255.6666: [udp sum ok] UDP, length 175

16:35:09.331148 IP (tos 0x0, ttl 255, id 43624, offset 0, flags [none], proto UDP (17), length 203)

    192.168.1.230.49154 > 255.255.255.255.6666: [udp sum ok] UDP, length 175

16:35:09.945341 IP (tos 0x0, ttl 255, id 52558, offset 0, flags [none], proto UDP (17), length 216)

    192.168.1.248.49154 > 255.255.255.255.6667: [udp sum ok] UDP, length 188

^C

3 packets captured

4 packets received by filter

0 packets dropped by kernel

```

However running the exact same command on the new PC at the same time doesn't show anything.

```
# tcpdump -i wlan0 host 255.255.255.255

dropped privs to tcpdump

tcpdump: verbose output suppressed, use -v or -vv for full protocol decode

listening on wlan0, link-type EN10MB (Ethernet), capture size 262144 bytes

^C

0 packets captured

0 packets received by filter

0 packets dropped by kernel

```

Filtering on port (i.e. #tcpdump -i wlan0 port 49154) doesn't help either.

----------

## mike155

Please look at the output of

```
ip link show wlan0
```

Is MULTICAST enabled?

Please look at he output of

```
netstat -g
```

Is wlan0 member of 'all-systems.mcast.net'?

----------

## SDNick484

 *mike155 wrote:*   

> Please look at the output of
> 
> ```
> ip link show wlan0
> ```
> ...

 

# ifconfig wlan0 | grep MUL

wlan0: flags=4419<UP,BROADCAST,RUNNING,PROMISC,MULTICAST>  mtu 1500

# ip link show wlan0

6: wlan0: <BROADCAST,MULTICAST,PROMISC,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DORMANT group default qlen 1000

    link/ether 3c:6a:a7:34:90:04 brd ff:ff:ff:ff:ff:ff

# netstat -g | grep wlan0

wlan0           1      239.255.255.0

wlan0           1      239.255.255.250

wlan0           1      all-systems.mcast.net

wlan0           1      ff02::1:ff00:a1e

wlan0           1      ff02::1:ff25:1e8b

wlan0           1      ff02::1:ff34:9004

wlan0           1      ip6-allnodes.lan

wlan0           1      ff01::1

----------

## mike155

The output of 'ip link' and 'netstat' looks  good!

How is your router / WLAN Access Point configured? Is it configured to use multicast-to-unicast conversion? Or is it configured to send multicast traffic as multicast packets at a low (mandatory) data rate? If the latter is the case: what are the settings? Is your new laptop able to receive the low data rate packets?

See https://wirelesslywired.com/2019/05/02/multicast-over-wireless, for example.

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## SDNick484

 *mike155 wrote:*   

> The output of 'ip link' and 'netstat' looks  good!
> 
> How is your router / WLAN Access Point configured? Is it configured to use multicast-to-unicast conversion? Or is it configured to send multicast traffic as multicast packets at a low (mandatory) data rate? If the latter is the case: what are the settings? Is your new laptop able to receive the low data rate packets?
> 
> See https://wirelesslywired.com/2019/05/02/multicast-over-wireless, for example.

 

Thank you for the confirmation and suggestion; I'll take a look tonight.  I'm using an older LEDE/OpenWRT build and don't recall putting in any modifications around multicast.  I'll dig through the config tonight to see if I can find more details on what's implemented.

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## SDNick484

 *mike155 wrote:*   

> The output of 'ip link' and 'netstat' looks  good!
> 
> How is your router / WLAN Access Point configured? Is it configured to use multicast-to-unicast conversion? Or is it configured to send multicast traffic as multicast packets at a low (mandatory) data rate? If the latter is the case: what are the settings? Is your new laptop able to receive the low data rate packets?
> 
> See https://wirelesslywired.com/2019/05/02/multicast-over-wireless, for example.

 

I confirmed this afternoon that when I hook the laptop up over a wired link, the UDP multicast packets show up in my python script so this is definitely WLAN related and my kernel conig is correct.    This definitely leads credence to where you were heading, and I plan to st up a multicast-to-unicast conversion.  I went ahead and updated to a current OpenWRT build and will be trying to set udpxy up tonight.  It's still surprising to me that my other laptop doesn't have issues seeing these packets over WiFi.

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## wjb

You've probably checked, but firewall settings maybe?

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## SDNick484

 *wjb wrote:*   

> You've probably checked, but firewall settings maybe?

 

Thanks but yeah, no firewalls in place on either the old or new systems.

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## mike155

Which frequency bands (2.4 GHz or 5 GHz) do your laptops use? If they use different frequency bands, switch your new laptop to the frequency band of the old laptop and retry.

Please take a look at this page. The first answer lists many reasons why multicast over wireless could fail. Maybe it's one of those problems.

----------

