Server Fault is a question and answer site for system and network administrators. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I create an open ad-hoc wlan by using iwconfig (I have the same issue with wpa_supplicant as well). there are 4 nodes on the network as seen on the figure below. The nodes run ubuntu 12.04 and debian squeeze, and have 3.7.1, 3.5 and 3.2 kernels. I use two different usb dongle brands (TP link and ZCN) that all have AR9271 chipset and ath9k_htc driver (here is lsusb output and ethtool output).

The problem I am experiencing is that two nodes ( and which have TP link usb wifi dongles can ping any node on the network, and vice-versa. However, the other nodes ( and that have ZCN wifi dongle cannot ping each other, but they have no problem communicating with TP-link wifi modules. tcpdump shows that and cannot see their arp-request, e.g.

20:37:52.470305 ARP, Request who-has tell, length 28
20:37:53.463713 ARP, Request who-has tell, length 28
20:37:54.463622 ARP, Request who-has tell, length 28
20:37:55.472868 ARP, Request who-has tell, length 28
20:37:56.463439 ARP, Request who-has tell, length 28
20:37:57.463469 ARP, Request who-has tell, length 28

but they are able to see and get reply from TP-link's modules.

20:39:23.634459 ARP, Request who-has tell, length 28
20:39:23.634551 ARP, Reply is-at 64:70:02:18:d4:6a (oui Unknown), length 28
20:39:23.636687 IP > ICMP echo request, id 572, seq 1, length 64
20:39:23.636809 IP > ICMP echo reply, id 572, seq 1, length 64
20:39:24.635497 IP > ICMP echo request, id 572, seq 2, length 64
20:39:24.635558 IP > ICMP echo reply, id 572, seq 2, length 64
20:39:28.651946 ARP, Request who-has tell, length 28
20:39:28.654021 ARP, Reply is-at 00:19:70:94:7c:8b (oui Unknown), length 28

My question is that what could be the reason that and cannot see the arp-request that they send each other? How can I find out the problem?

If I add couple more nodes with ZCN wifi dongle on the network, these nodes are also not able to talk with each other, but they are fine with TP-link. Or if I swap the wifi modules, the nodes with ZCN have always problem but TP-link modules are fine. enter image description here

here is the /etc/network/interfaces, ifconfig, iwconfig, ip a, ip r, route outputs

EDIT: I was suspecting if the problem is arp_filter related but /proc/sys/net/ipv4/conf/*/arp_filter is 0 on the all subdomains(*). If I add arp info of and manually on these nodes, tcpdump and wireshark does not show that they send ping to each other. If I ping the broadcast address ( in my case), and are able hear it.

EDIT2: Here is pcap files from (ZCN module), (ZCN module), and (TP-link module that does not have problem). here is the ping outputs from I captured the packages simultaneously. The link also includes ifconfig; iwconfig; and uname- a outputs for each node.

share|improve this question
Can you make a network capture of the ARP traffic on both and machines at the same time? Use tcp dump and share it as a pcap file. – Mircea Vutcovici Apr 1 '13 at 15:30
Thank you Mircea Vutcovici, please see the EDIT2 for the pcap files. Please let me know if you would like to have more information. – johan Apr 3 '13 at 8:38
Well, you can try using static ARP and see how/if it changes the connectivity problem. – poige Apr 3 '13 at 8:44
Could you post a dump of the traffic from a wireless sniffer tool like kismet? This will include the 802.11 headers in case there's something odd about them. – Flup Apr 3 '13 at 8:47
given the issues you are having with the ZCN dongles, and your requirement to have the clients all talk directly to each other on the network I would just throw them out and replace with the TPLink dongles that actually work on your network. Or it could be a driver issue with the ZCN adapters - try another. – August Apr 5 '13 at 15:19
up vote 1 down vote accepted

I had the same problem recently. I figured out that AR9271 chipset's have problem on onboard transmitter antenna. If you use an external antenna, then you will not have a problem. And this problem only occurs on ad-hoc mode.

The reason you don't experience problem with the TP-link should be that these modules uses external antenna which overcomes the chipset's problem, and ZCN modules should not have an external antenna.

share|improve this answer

This could be related to the "hidden node problem" if .6 and .7 aren't in direct radio contact, but without knowing the distances involved it's impossible to say.

Also either or both of the chipsets could have a buggy ad-hoc mode, it's not used much these days and wouldn't be surprising.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.