UDP Broadcast Traffic Not Received

Question

This is likely to be more of a networking question than a C++ question, but I have an iOS app communicating via UDP with a networked PC. Both are using some UDP socket classes that I wrote in C. Here are my symptoms:

When the PC app sends out broadcast traffic, Wireshark on the PC itself sees it, as does the iOS app. A third PC on the same network also sees the packets via Wireshark.
When the iOS app sends out broadcast traffic, the PC running the app doesn't even see the packets in Wireshark. However, the third PC does.

Additional information:

The PC in question is running Windows 7.
It has an ethernet port that is being used for a camera.
It has a wireless network card in it that is both joined to a network and is also using a tricky little app called Connectify to separately create its own wireless network. It is across the Connectify network that I am trying to send data. However, I have tried sending the data over the other non-Connectify wireless network, and it still fails (though the third PC on the network still sees the traffic). UPDATE Disregard the last sentence. It is false. If I change wireless networks, it works.

What I've tried:

I first assumed this was a routing table issue, so I added persistent routes with low metric numbers for any entries relating to that interface. The routing table is here:

IPv4 Route Table
===========================================================================
Active Routes:
Network Destination        Netmask          Gateway       Interface  Metric
          0.0.0.0          0.0.0.0        10.1.11.1      10.1.11.169     25
        10.1.11.0    255.255.255.0         On-link       10.1.11.169    281
      10.1.11.169  255.255.255.255         On-link       10.1.11.169    281
      10.1.11.255  255.255.255.255         On-link       10.1.11.169    281
        127.0.0.0        255.0.0.0         On-link         127.0.0.1    306
        127.0.0.1  255.255.255.255         On-link         127.0.0.1    306
  127.255.255.255  255.255.255.255         On-link         127.0.0.1    306
    192.168.190.0    255.255.255.0         On-link     192.168.190.1      6
    192.168.190.1  255.255.255.255         On-link     192.168.190.1      6
  192.168.190.255  255.255.255.255         On-link     192.168.190.1      6
        224.0.0.0        240.0.0.0         On-link         127.0.0.1    306
        224.0.0.0        240.0.0.0         On-link       10.1.11.169    281
        224.0.0.0        240.0.0.0         On-link     192.168.190.1    257
  255.255.255.255  255.255.255.255         On-link     192.168.190.1      6
===========================================================================
Persistent Routes:
  Network Address          Netmask  Gateway Address  Metric
  255.255.255.255  255.255.255.255    192.168.190.1       5
    192.168.190.0    255.255.255.0    192.168.190.1       5
    192.168.190.1  255.255.255.255    192.168.190.1       5
  192.168.190.255  255.255.255.255    192.168.190.1       5
===========================================================================

I then assumed that there was something strange going on with this Connectify program, so I tried broadcasting over the 10.1.11 network (after undoing my persistent routes). As noted above, this succeeds.

Any ideas? I'm stumped. I suppose it could be a limitation of the network card, but then why can it see its own broadcast packets going out? Could it be that the packet sizes being output by iOS are too large for the PC NIC?

If PC2 is seeing UDP broadcasts from the iOS device but not PC1, the problem would appear to lie in PC2. Evidently the iOS device is generating valid UDP traffic and hence this probably isn't a network library issue). Connectify seems the most likely culprit. In that situation, retagging your question or asking the Connectify devs for advice would seem like the most sensible thing to try. — Rook
just a thought... do you have a firewall on the PC ? (sometimes bundled along the antivirus). Firewalls tend to be suspicious about external brodcast. Try disabling it if you have any. — Offirmo

Offirmo Offirmo · Accepted Answer · 2012-06-28T17:12:23

Your problem seems to be related to connectify, but not necessarily.

I would suggest :

be careful about limitations of broadcast, cf. UDP broadcast packets across subnets
check if the receiving app and the sending app are both on the same subnet
check your firewall settings if you have any : http://support.connectify.me/entries/20313068-firewall-security-settings (or temporary disable it to check)

Just thoughts.

UDP Broadcast Traffic Not Received

1 Answers