[CentOS] Performance of CentOS as a NAT gateway

Ross S. W. Walker rwalker at medallion.com
Tue Sep 11 14:04:17 UTC 2007

Bart Schaefer wrote:
> On 9/10/07, John R Pierce <pierce at hogranch.com> wrote:
> > wireshark can process and display packet capture files from 
> tcpdump -w
> >
> > capture a few megabytes of packets on the appropriate 
> interface of the
> > firewall, then transfer them to a workstation with 
> Wireshark for analysis.
> OK, I've got some output from "tcpdump -w any" but I don't know
> precisely what I'm looking for.  (I'd be happy to take this off-list.)
>  I notice that just over 1/3 of the packets are TCP out-of-order
> segments and about 4% are duplicate ACKs.
> We also dumped eth0 and eth1 separately.  Statistics on the "any"
> output show 26Mb/s, but eth0 and eth1 independently are only 10Mb/s
> each.
> By the way, those interrupts/sec numbers in my earlier message were
> off; I chose a bad moment to look at it, when the peak had subsided.
> At peak it's more like 2500-3000 interrupts/sec, sometimes as high as
> 3500.

int/sec is fine for your hardware.

Try a tcpdump of both the external and internal interface at the same
time. Try to focus on 1 proto-typical stream of traffic from a known
host (like your own) to a known destination from connection open to
connection close.

Then open up the dump in wireshark and look at the timestamps and if
there are any resends with smaller MTUs and such.

You want to see if there is a large delay between sent packets and


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