CentOS6 NAT router/firewall behind a 120Mbps cable modem connection seems to be capping throughput at 30Mbps after recent updates and security "hardening".

Prior to updates and hardening I was getting 90Mbps.

I've checked CPU and network usage and neither of those seems to be a limiting factor. tc does not show any traffic shaping going on and I don't know how to troubleshoot this further.


I have a CentOS 6 system running as a NAT router/firewall behind a Comcast cable modem, which is also running as a NAT router

                              1000     100
                              eth1     eth0

The double NAT is a legacy from the CentOS system having previously served as a router/firewall behind a Time-Warner cable modem that ran in bridge mode. When I moved into Comcast territory I intended to switch the modem to bridge mode but never got around to it, and the double NAT never caused a problem. I was getting 90Mbps throughput with no issues.

In preparing to convert to bridged mode on the Comcast modem I decided to "harden" the CentOS system by disabling some unneeded services and doing "yum update", which I hadn't done in a while. After hardening I did a speed test and was surprised to find throughput down to 30Mbps.

I tried connecting my primary desktop system directly to the modem like this

                          eth1     eth0

Running speedtest.net verified that my Comcast connection is capable of 120Mbps, so something I changed on the CentOS system has resulted in capping throughput at 30Mbps. Every time I do a speed test from the LAN (behind the CentOS system) I get a value within 1-2% of 30Mbps, so it almost feels like something is artificially capping throughput.

I thought maybe traffic shaping got enabled somehow, but tc seems to indicate it's not active

[jhg@perseus ~]$ sudo tc -s qdisc
qdisc pfifo_fast 0: dev eth0 root refcnt 2 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
 Sent 64159459406 bytes 44745482 pkt (dropped 0, overlimits 0 requeues 0)
 rate 0bit 0pps backlog 0b 0p requeues 0
qdisc pfifo_fast 0: dev eth1 root refcnt 2 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
 Sent 2871293442 bytes 26151570 pkt (dropped 0, overlimits 0 requeues 0)
 rate 0bit 0pps backlog 0b 0p requeues 0

The "hardening" consisted of

  1. removing some unneeded packages
  2. shutting down unneeded services
  3. setting up iptables to filter all incoming traffic except for one non-standard port for ssh
  4. installing and configuring tripwire

Removed packages:

redis                    dovecot
redhat-lsb-compat        ipa-client
redhat-lsb               nfs-utils-lib
redhat-lsb-printing      nfs-utils
foomatic                 subversion
foomatic-db              spamassassin
foomatic-db-ppds         certmonger
cups                     yp-tools
mysql-server             ypbind
mysql                    rpcbind

Currently enabled services:

abrt-ccpp            cpuspeed               kdump            nmb       
abrt-oops            crond                  lvm2-monitor     ntpd      
abrtd                dhcpd                  mcelogd          postfix   
acpid                dkms_autoinstaller     mdmonitor        rsyslog   
atd                  haldaemon              messagebus       smb       
auditd               ip6tables              named            sshd      
autofs               iptables               netfs            sysstat   
blk-availability     irqbalance             network          udev-post 

My question is: What should I do next to figure out why my CentOS 6 router seems to be artificially capping throughput at 30Mbps?

  • Scientific method... What did you do to "harden" the system? What hardware is in use? Is it healthy?
    – ewwhite
    Jan 3, 2017 at 6:37
  • @ewwhite see the updates to the post.
    – Ex Umbris
    Jan 3, 2017 at 6:56
  • What happens if you disable the iptables inspection?
    – ewwhite
    Jan 3, 2017 at 7:03
  • I set the default policies to ACCEPT on all three chains (input, forward, output) and flushed all rules... no change, still 30Mbps
    – Ex Umbris
    Jan 3, 2017 at 7:10
  • Downvoter care to comment?
    – Ex Umbris
    Jan 3, 2017 at 7:18

2 Answers 2


So, the problem here turned out to be a hardware issue. Things were working fine a month ago, and one does not expect failed hardware to still "work" in a degraded mode, but that's what was happening.

The troubleshooting step that revealed the issue was to actually look at the ethernet port lights on the back of the cable modem. Instead of the green "1Gbps" light it was orange, signifying "100Mbps". In that mode, it appears the modem supports throughput only up to 30Mbps or so.

I know the modem (Arris TG-852G) has GBEthernet ports, so something was preventing the Centos from talking to the modem at 1Gbps. Using ethtool I saw this:

Settings for eth1:
        Supported ports: [ TP MII ]
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Half 1000baseT/Full
        Supported pause frame use: No
        Supports auto-negotiation: Yes
        Advertised link modes:  10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Half 1000baseT/Full
        Advertised pause frame use: Symmetric Receive-only
        Advertised auto-negotiation: Yes
        Link partner advertised link modes:  10baseT/Half 10baseT/Full
                                             100baseT/Half 100baseT/Full
        Link partner advertised pause frame use: Symmetric Receive-only
        Link partner advertised auto-negotiation: Yes
        Speed: 100Mb/s
        Duplex: Full
        Port: MII
        PHYAD: 0
        Transceiver: internal
        Auto-negotiation: on
        Supports Wake-on: pumbg
        Wake-on: g
        Current message level: 0x00000033 (51)
                               drv probe ifdown ifup
        Link detected: yes

which essentially said (from the Centos adapter's viewpoint) "I can support GBEthernet, and am advertising GBEthernet, but the peer doesn't support GBEthernet -- so I'm connected at 100Mbps instead".

I tried various fixes suggested in several online fora (including here) such as using a different cable, turning off auto-negotiation, advertising only 1GB speed, or setting the speed to 1GB manually. Turning off auto-neg and trying several different Cat6 cables had no effect, and the other two prevented a connection from being established at all.

I concluded it had to be the adapter itself and ordered a new adapter. When it was installed it immediately connected at 1Gbps. Problem solved.

The moral of the story is, of course, that even though hardware failures in devices without moving parts are rare these days, they're still possible and should be eliminated before blaming the software.

  • 1
    According to this output, the peer is literally not even offering GBE, so no amount of your configuration would override that. If it were e.g. a bad cable, you'd see a lower negotiated link speed, but you'd still see GBE in the advertised speeds from your peer. Given that it was resolved by changing your network card, that means something at a very low level inside or near the [G]MII was misbehaving. This is deep enough into "weird" territory that I'll do as thousands of net.engs have done before me and say, "I blame the firmware."
    – BMDan
    Jan 13, 2017 at 21:14

What I would do here is revert the changes individually and run a speed test after each or revert all the changes. Benchmark an unmodified version of CentOS (baseline) and then apply each change individually and run the speed test after each change.

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