TTL = 0 is overwritten to 1 when switching network interface in run time

[shihyu...@gmail.com]

Hi all:

I have encountered an issue regarding TTL and switching network interface in run time. The symptom is that even with TTL=0, if the network interface is switched during run, the LCM message will "leak" into local network instead of being confined in the local host. This can be reproduced as following:

1. Have host A connect to a local network via wifi
2. Start a LCM publisher on host A with TTL = 0.
3. Run lcm-spy on host B, which is also connected to the same local network. It should not be seeing messages published by host A since TTL was set to 0 on host A.
4. Keep the publisher on host A running while disabling wifi, and then connect host A to the same local network via ethernet instead.
   
5. lcm-spy on host B (and actually all other hosts on the local network) will start receiving messages from host A even though TTL was set to 0 on host A.
   

This is potentially dangerous in that somehow TTL=0 is not enforced when switching network interface.
Does anyone knows what might be the cause of this and how to prevent this behavior?

Thanks!

Shih-Yuan