On Oct 5, 2013, at 10:20 AM, abhinav narain <abhinavnarai...@gmail.com> wrote:
> Hi Everyone,
> I am doing a packet capture on 480 MHz router on ethernet interface using
> pcap and see packet drops for anything greater than 50-60 Mbps.
>
> My question is following :
> Does pcap (and hence the kernel) create a new queue for all the packets in
> the promiscuous or is it collecting all packets in the same/default queue
> attached to the interface eth0 ?
I infer from "eth0" that this is Linux. (DD-WRT or some other OpenWRT-based
project?)
On Linux, libpcap creates a PF_PACKET socket for each open pcap_t for a regular
network interface such as eth0. Each PF_PACKET socket has a queue of its own,
regardless of promiscuous mode.
What queueing the *driver* for the adapter does depends on the driver.
> I am asking above question because I want to know if the router starts
> dropping packets destined to itself because of this promiscuous mode
> capture?
...
> Router used :
> http://support.netgear.com/product/WNDR3700v2
Do you have more than one machine plugged into the LAN ports of the router?
According to
http://wiki.openwrt.org/toh/netgear/wndr3700#interfaces
eth0 is for the "LAN ports (1 to 4)" and, according to
http://wiki.openwrt.org/toh/netgear/wndr3700#switch.ports.for.vlans
eth0 is port 5 on a Realtek RTL8366s:
http://realtek.info/pdf/rtl8366s_8366sr_datasheet_vpre-1.4_20071022.pdf
switch chip.
If eth0 is not in promiscuous mode, I would assume that only traffic going
between machines on the Ethernet LAN and machines on the 802.11 WLAN would
arrive on eth0, not traffic between two machines on the Ethernet LAN. I don't
know whether the eth0 port can be put into a port mirroring mode where traffic
on all the Ethernet LAN ports can be sent to the eth0 port or, if it can,
whether the driver does so if eth0 is put into promiscuous mode, so that it
sees all traffic through the switch, not just traffic to be sent to the WLAN.
If it *is* put into mirroring mode, and you have a significant amount of
traffic going between Ethernet LAN hosts, then promiscuous mode would
significantly increase the number of packets received.
If it's *not* put into mirroring mode, promiscuous mode shouldn't make any
difference.
Are you capturing in promiscuous mode? If so, do you see as many packets
dropped if you *don't* capture in promiscuous mode?
By the way, according to
http://www.dd-wrt.com/wiki/index.php/Netgear_WNDR3700
the v2 model has a Atheros AR7161 680 MHz processor; the only model they list
with a 480 MHz processor is the v3 model, with a Broadcom BCM4718A 480MHz
processor.
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