Using pcap or tcpdump kills the wireless network
Using pcap or tcpdump kills the wireless network
- Subject: Using pcap or tcpdump kills the wireless network
- From: "Stefan Arentz" <email@hidden>
- Date: Fri, 11 Jul 2008 15:15:21 -0400
I've been debugging some code that uses libpcap. The symptoms are that
whenever I start capturing packets from the wireless interface (en1),
all traffic on that interface stops.
I spend a lot of time in my own code to figure this out but then I
discovered that tcpdump actually showed exactly the same behaviour.
I can perfectly reproduce this as follows:
1. Open a terminal window and ping some host on your local network
2. Open a second terminal window and run: sudo tcpdump -n -i en1
As soon as the tcpdump starts, the ping stops working. It will not
receive responses anymore. When the tcpdump is killed, the ping starts
working after a couple of seconds. I can start/stop tcpdump like this
to kill/enable the network. Also, when tcpdump is running, other apps
also cannot use the network anymore. It's not just icmp, it's the
whole interface.
This is on a plain 10.5.4 system with no kexts or weird software.
I think this is a bug in the AirPort 11n drivers: when I switch from
my Time Capsule (11n) to the old AirPort Express (11g) that I use for
traveling, everything works fine and is rock solid. As soon as I
switch back to the 11n Time Capsule I see the above behaviour.
This is extremely frustrating. Not just for my code, but because many
people use their Macs to analyze networking problems using
tcpdump-like tools.
I will report this as a bug, but does anyone know of a short-term workaround?
S.
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