KVM Networking Issue

Home » CentOS-Virt » KVM Networking Issue
CentOS-Virt 5 Comments

Hi folks,

I posted this question to the KVM list, but I thought I’d try here too–sorry if this is the wrong place to post this, can you please direct me to the correct forum or list if so, thanks!

I’m working on a network security project, using KVM installed on CentOS 6.7 through yum. I have a VM with the goal of using this as a network appliance, and two other VMs, one simulating an attack node and the other simulating a vulnerable webapp. These are all connected to the same internal private network set up in KVM. The idea with the network appliance VM is to have it act as if it’s connected to a network tap so it can see the traffic between the other two VMs. I’m not able to see the traffic currently and would appreciate your help or suggestions to see if this is possible and how I can set this up if so. I came across some information online suggesting to have the interfaces in promiscuous mode, including the virtual NIC for the private network, and I’ve tried all combinations. Thanks for any help you can offer!



5 thoughts on - KVM Networking Issue

  • From the KVM host you should be able to point tcpdump at the vnetX
    interfaces and sniff. I’ve had to do this on occasion (with a bridged network setup) when a web hosting VM was being brute forced.

    Start by determining what interface your VM is attached to.

    We have no idea the network layout of your KVM set up for VMs either. Look at the XML for your VM to determine which interface it’s tied to.

  • Hi Mike,

    Thanks for the info. I’d rather run monitoring such as tcpdump from the VM if possible and not the host as a simulation of a network appliance and with the intent eventually of giving others access to the VM and not the host. Here is the xml file for the private network:

  • Then run tcpdump on the VM. Same command or commands you’d have ran on the host node.

    I take it you’ve tried testing basic connectivity from the VMs to whatever default gateway is in place?

    I see Spanning Tree is enabled, so you might check that an interface isn’t in the blocking state. brctl showstp | egrep ‘^(em|eth|vnet)|state’

    I don’t use the NAT network mode for KVM, so hopefully someone else can chime in on that piece.

  • Thanks, Mike. When running tcpdump on the VM I’m not seeing traffic unless it’s explicitly intended for that particular VM, so no traffic between the other VMs is getting forwarded from the virtual interface to the “network appliance” VM.

    There is connectivity between the VMs on the private network and the
    “network appliance” VM which is acting as a gateway.

    Here’s the output of the brctl command:

    bridge id 8000.5254007e2f5b designated root 8000.5254007e2f5b root port 0 path cost 0
    max age 19.99 bridge max age 19.99
    hello time 1.99 bridge hello time 1.99
    forward delay 0.00 bridge forward delay 0.00
    ageing time 299.95
    hello timer 0.29 tcn timer 0.00
    topology change timer 0.00 gc timer 0.29
    hash elasticity 4 hash max 512
    mc last member count 2 mc init query count 2
    mc router 1 mc snooping 1
    mc last member timer 0.99 mc membership timer 259.96
    mc querier timer 254.96 mc query interval 124.98
    mc response interval 9.99 mc init query interval 31.24

    virbr1-nic (0)
    port id 0000 state disabled designated root 8000.5254007e2f5b path cost 100
    designated bridge 8000.5254007e2f5b message age timer 0.00
    designated port 8001 forward delay timer 0.00
    designated cost 0 hold timer 0.00
    mc router 1

    I’m not sure why virbr1-nic is showing up as disabled, and also why the vnet# interfaces don’t show up (they do show up on another host, although VMs on that host are having the same non-promiscuous issue as these VMs). I’ve tried this with and without NAT, as well as with STP
    on/off with no effect.



  • That STP output says the virbr1-nic interface is disabled — maybe your VM
    is powered off?

    You’ll need to enable IP forwarding and set rules to route the traffic for those VMs. http://www.linux-kvm.org/page/Networking#Routing_with_iptables

    The gotcha is that if you’re not doing any IP routing on the KVM node, your
    “network appliance” VM needs to have one NIC bridged to your real network and the other as part of virbr1. You could NAT it on the KVM host as well.

    Read the KVM networking documentation as it will help you determine what configuration you have and if that’s what you want.