Thursday, January 24, 2013

"Network is unreachable" on an embedded box

We've been working extensively with embedded boxes lately trying to migrate our platform over. One of the programs we've been trying to migrate today was a simple announcer program that sends out a UDP broadcast. On our first go, the program wouldn't work, saying that the "Network is unreachable" when doing a 'perror' after the call to 'sendto'. After much experimentation with various network binding flags, etc, which took the whole day, I went into overtime trying to figure out the problem. Eventually, I got to the point where I inspected the routing tables of the embedded system and was shocked to find .... there was no routing table entry for the default gateway, i.e. destination IP. I'm actually kind of ashamed of how long it took me to find this, but in my defense, I was dealing with a wild mixture of systems and had done a lot of network reconfiguration with the systems and some of the stuff was handled automatically for me by my Ubuntu box, which gave me some inconsistent results at times when experimenting.  For my own future reference, here's how I solved the problem:

  • Run 'route -n'
  • Ensure there's an entry that looks like the following :
    • Destination: Gateway: [the desired gateway, e.g.] Genmask: Flags: UG, Metric: 0, Ref: 0, Use: 0, Iface: eth0 (or eth2, or whatever you may have)
  • If the above entry doesn't exist, add it with the following command:
    • sudo route add -net netmask gw dev eth0
You should now have no problems with your UDP broadcasting. If you do, well, let me know how it goes, I'd love to hear your problem and your solution.

1 comment:

Stefan said...

Thank you,
this exactely solved my problem on GNOME ubuntu linux