[CentOS] CentOS based router dropping connections

Craig Van Ham craigv at evsl.com
Thu Jul 26 14:48:22 UTC 2007

Is it dropping packets or Ethernet frames? 


Iptables may be dropping packets, check cat /proc/net/ip_conntrack | wc -w
see how many connections iptables is keeping track of. The default value
held by this entry varies heavily depending on how much memory you have. On
128 MB of RAM you will get 8192 possible entries, and at 256 MB of RAM, you
will get 16376 entries. You can read and set your settings through the
/proc/sys/net/ipv4/ip_conntrack_max setting.

Check to see if there are any errors on the Ethernet device (ifconfig) 


RX packets:29024644 errors:0 dropped:0 overruns:0 frame:0

TX packets:28064715 errors:0 dropped:0 overruns:0 carrier:0





From: centos-bounces at centos.org [mailto:centos-bounces at centos.org] On Behalf
Of Rob Lines
Sent: Thursday, July 26, 2007 8:36 AM
To: CentOS mailing list
Subject: Re: [CentOS] CentOS based router dropping connections



On 7/24/07, John R Pierce <pierce at hogranch.com> wrote:

Jesse Cantara wrote:
> So basically, what I can figure from all of the evidence at this point
> is the problem is either:
> default configuration of the network in CentOS isn't proper for what
> I'm doing (can't handle the traffic or number of connections). I get a 
> decent amount of traffic, maxing out a 10 Mbit connection all day
> long. I don't know exactly where to check to diagnose if this is the
> case though. Can anybody point me where to find things like the system 
> usage of the network (memory, any buffers, # of connections, etc)? the
> things I know to check look normal, but that's basically just
> ifconfig, and your standard /var/log/message and dmesg log files. 
> or:
> the network drop from the hosting facility is "bad" somehow, either
> the cable physically, or the way in which they are limiting me to 10
> Mbit.

check with the facility to see if that drop is 10Mbit HALF duplex, and 
if so, make sure your server's NIC is configured as such.

I had a problem like this in a  coloc many years ago, with a much older
linux version.

While not the exact same issue I had a problem similar to this between two
switches one was a Cisco 4006 and the other was a 3Com 3300 they were using
a media converter that was 10 mb over fiber and for some reason the 3com
would not negotiate properly with the media converter it was plugged into.
It kept jumping between full and half and sometimes it would try to go to
100mb.  As soon as I turnded off auto negotiate and set it to 10 mb full all
the dropped packets disappeared.  It was under similar conditions where it
would all be fine with a low load but as soon as it was running close to max
it would drop packets repeatedly and the link would seem to fail until the
load dropped off (because people thought it was down) then it would become
stable again until the traffic went back up. 

John's suggestion looks like a solid one below.  If the 'problem' server is
behaving find in your office I would really look at this as a probable


(ps Hopefuly it clears it up.  In our case the problem had been happening
for over a year and the connection fed an elementary school.  I found out
about the problem about a month into working at the place and had it fixed
within a day or two.  The previous outsourced IT dept could never track it
down because they were never there when it happened.  They would come in
after school was out and it would work fine for them without the high packet
load and they would just claim it was user error.) 


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