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I have the current network configuration in my linux single board computer:


~# ifconfig

eth0      Link encap:Ethernet  HWaddr 00:05:B7:EE:88:0A  
          inet addr:192.168.0.20  Bcast:192.168.0.255  Mask:255.255.255.0
          inet6 addr: fe80::205:b7ff:feee:880a/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:258814 errors:0 dropped:0 overruns:0 frame:0
          TX packets:358552 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:18117399 (17.2 MiB)  TX bytes:122851279 (117.1 MiB)
          Memory:d0400000-d047ffff 

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:3380889 errors:0 dropped:0 overruns:0 frame:0
          TX packets:3380889 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:956796465 (912.4 MiB)  TX bytes:956796465 (912.4 MiB)

~# ip route

default via 192.168.0.1 dev eth0 
192.168.0.0/24 dev eth0  proto kernel  scope link  src 192.168.0.20

239.255.42.21 dev eth0  scope link

The latter multicast route made my network unstable, meaning that, randomly i can no more reach any local network using unicast route, when having this problem I use this command to check from where the issue came:

print route get 192.168.0.x 

and the result of this command is:

print route get 192.168.0.21
multicast 192.168.0.21 dev eth0 src 192.168.0.20 

cache <mc>

However when I don't have this random issue, the same latter command shows:

ip route get 192.168.0.21

192.168.0.21 dev eth0  src 192.168.0.20 

    cache

I don't know how to troubleshoot this problem - I tried creating a TCP server on the other machine using netcat, so when having the problem, I tried launching strace nc 192.168.0.21 12345 to show what happens and what I get is Network unreachable,

I used also tcpdump to check what happens behind, but I do not see any useful information better then "Network Unreachable"

Any help please?

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  • What are you using multicast address 239.255.42.21 for? Online references say it has something to do with GPS (GNSS) on public transport networks or something?
    – Spiff
    Jul 11, 2022 at 3:06
  • Yes, am using it exactly for that reason for an ITxPT protocol Jul 11, 2022 at 10:09

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