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Cannot access the internet. CentOS guest isn't receiving incoming traffic.

Private network is fully accessible.

Scratching my head here... I don't have this problem on Debian/Ubuntu/Windows! :(


  • Hyper-V host (
  • Freshly built CEntOS guest (
  • Guest resides on Internal Network with Internet Connection Sharing(ICS) enabled
  • Address is assigned by DHCP server on the host
  • Address range:
  • DNS is set to my ISP's public nameservers (

Additional Configuration

  • iptables is stopped
  • SELinux is disabled

Some info that may be useful


eth0      Link encap:Ethernet  HWaddr 00:15:5D:C8:34:5A
          inet addr:  Bcast:  Mask:
          inet6 addr: fe80::215:5dff:fec8:345a/64 Scope:Link
          RX packets:111 errors:0 dropped:0 overruns:0 frame:0
          TX packets:18 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:12748 (12.4 KiB)  TX bytes:1788 (1.7 KiB)

eth1      Link encap:Ethernet  HWaddr 00:15:5D:C8:34:5C
          inet addr:  Bcast:  Mask:
          inet6 addr: fe80::215:5dff:fec8:345c/64 Scope:Link
          RX packets:147 errors:0 dropped:0 overruns:0 frame:0
          TX packets:117 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:14087 (13.7 KiB)  TX bytes:14587 (14.2 KiB)
          Interrupt:9 Base address:0x2000

netstat -rn

Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface   U         0 0          0 eth0     U         0 0          0 eth0         UG        0 0          0 eth0

cat /etc/resolv.conf

; generated by /sbin/dhclient-script

cat /etc/sysconfig/network-scripts/ifcfg-eth0


cat /etc/sysconfig/network


cat /etc/udev/rules.d/70-persistent-net-rules

# net device ()
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="00:15:5d:c8:34:5a", ATTR{type}=="1", KERNEL=="eth*", NAME="eth0"

# PCI device 0x1011:0x0009 (tulip)
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="00:15:5d:c8:34:5c", ATTR{type}=="1", KERNEL=="eth*", NAME="eth1"

Questions I've already looked at to answer my own (in vain)

Unable to ping or access internet

Connect to the internet with CENTOS 5

Centos internet not working

My CentOS 5.7 server can't access the internet

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Painfully obvious question, but when you say incoming traffic, I assume you have started the original connection rather than a new incoming connection coming in? – Matthew Ife Mar 12 '14 at 18:58
Example: I try yum update, dns requests go out, nothing comes back, yum keeps retrying in vain. So... yes, I think. – Vasili Syrakis Mar 12 '14 at 20:36
How did you configure the Hyper-V virtual switch? – Michael Hampton Mar 12 '14 at 22:23

Can machine with IP address forward traffic from network to the internet?

If this machine is Linux, it has to have allow forwarding, try to see /etc/sysctl.conf file.

On routing machine can be denied forwarding by iptables (try exec iptables -L -n and check rules in the FORWARD chain).

You can try to install tshark (or tcpdump or tethereal) utility even on the new host even on the routing machine and watch packets. Then you can on the new machine exec ping, for example, and you will see, if router can forward packet to the next hop correctly.

If you have another router in the network: know this another router, that your first router (with one of IP address routing this network? Try to check it by running command ping on this next router (preferably border router). If this ping don't work, you have to set static route, somethink like this: ip route add via x.x.x.x, where x.x.x.x is IP address of router, which have as another address

I'm very sory about my English, but I believe, that this post can help to you...

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The Hyper-V host is and can forward traffic to and from the internet. Each machine on the network can communicate with each other without a problem. What do I look for inside the sysctl.conf file? – Vasili Syrakis Mar 12 '14 at 8:07
You have to have in this file somethink like net.ipv4.ip_forward=1 (it's depend on the version of kernel). – Jan Marek Mar 12 '14 at 8:43
I edited my sysctl.conf and it did not seem to change anything. I checked on wireshark and it looks like the guest can send traffic outgoing, but nothing comes back. iptables is disabled. – Vasili Syrakis Mar 12 '14 at 11:12

I think your problem lies with your VM Guest configuration in Hyper-V, and not the OS itself. I'm no expert - but i vaguely recall similar problems a long time ago that were related to which type of virtualized network interface I had selected in my HyperV configuration.

The solution for me was to remove the network adapter configured by default, and add a 'Legacy Network' adapter. After that it seemed to work.

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up vote 0 down vote accepted


Okay, so I found out what it was.

I do not expect anyone to ever have guessed this!

I have two network adapters:

  • 1x Ethernet Network Adapter -
  • 1x Wireless Adapter -

The Hyper-V Internal network was running on the Wireless Adapter with Internet Connection Sharing (ICS) enabled.

Packets were successfully going out, but upon coming back in they would go to my Ethernet card and they would have no clue where to go from there, hence, my VMs did not receive anything back.

I've now split my network in two, so I have my Wireless card on and my Ethernet card on

This configuration now works.


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