Backend Pool

The Backend Pool manages a set of dynamic backend virtual machines to be used by Cowrie’s proxy. The pool keeps a set of VMs running at all times, ensuring different attackers each see a “pristine” VM, while repeated connections from the same IP are served with the same VM, thus ensuring a consistent view to the attacker. Furthermore, VMs in the pool have their networking capabilities restricted by default: some attacks consist of downloading malicious software or accessing illegal content through insecure machines (such as your honeypot). Therefore, we limit any access to the Internet via a network filter, which you can configure as you see fit.

The VMs in the backend pool, and all infrastructure (snapshots, networking and filtering) are backed-up by Qemu/libvirt. We provide two example VM images (for Ubuntu Server 18.04 and OpenWRT 18.06.4) whose configurations are already set and ready to be deployed. Further below in this guide we’ll discuss how to create your own images and customise libvirt’s XML configuration files.

First of all, install the needed dependencies for the pool, as explained in the installation steps.

Provided images

To allow for a simple setup, we provide two VM images to use with the backend pool: Ubuntu 18.04 and OpenWRT. You can download them below, and then edit cowrie.cfg to match the path of the images. In the case of OpenWRT you will need two different files. Note that a separate set of configs is provided for each image in the default configuration. Choose the one you want to use and comment the other as needed.

Backend Pool initialisation

Depending on the machine that will be running the backend pool, initialisation times for VMs can vary greatly. If the pool is correctly configured, you will get the PoolServerFactory starting on 6415 message on your log.

After a while, VMs will start to boot and, when ready to be used, a message of the form Guest 0 ready for connections @ 192.168.150.68! (boot 17s) will appear for each VM. Before VMs are ready, SSH and Telnet connections from attackers will be dropped by Cowrie.

Proxy configurations

When the proxy starts, and regardless whether the backend pool runs on the same machine as the proxy or not, some configurations are sent by the proxy to the pool during runtime.

These are:

  • pool_max_vms: the number of VMs to be kept running in the pool
  • pool_vm_unused_timeout: how much time (seconds) a used VM is kept running (so that an attacker that reconnects is served the same VM.
  • apool_share_guests: what to do if no “pristine” VMs are available (i.e., all have been connected to); if set to true we serve a random one from the used, if false we throw an exception.

Backend Pool configuration

In this section we’ll discuss the [backend_pool] section of the configuration file.

The backend pool can be run in the same machine as the rest of Cowrie, or in a separate one. In the former case, you’d be running Cowrie with

[backend_pool]
pool_only = false

[proxy]
backend = pool
pool = local

If you want to deploy the backend pool in a different machine, then you’ll need to invert the configuration: the pool machine has pool_only = true (SSH and Telnet are disabled), and the proxy machine has pool = remote.

Note: The communication protocol used between the proxy and the backend pool is unencrypted. Although no sensitive data should be passed, we recommend you to only use private or local interfaces for listening when setting up listen_endpoints.

Recycling VMs

Currently, handling of virtual machines by the pool is not perfect. Sometimes, VMs reach an inconsistent state or become unreliable. To counter that, and ensure fresh VMs are ready constantly, we use the recycle_period to periodically terminate running instances, and boot new ones.

Snapshots

VMs running in the pool are based on a base image that is kept unchanged. When booting, each VM creates a snaphost that keeps track of differences between the base image and snapshot. If you want to analyse snapshots and see any changes made in the VMs, set save_snapshots to true. If set to true be mindful of space concerns, each new VM will take at least ~20MB in storage.

XML configs (advanced)

You can change libvirt’s XML configs from the default ones in share/cowrie/pool_configs. However, if you’re using one of the default images provided, then you probably don’t need to.

Guest configurations

A set of guest (VM) parameters can be defined as we explain below:

  • guest_config: the XML configuration for the guest (default_guest.xml works for x86 machines, and wrt_arm_guest.xml for ARM-based OpenWRT)
  • guest_privkey: currently unused
  • guest_tag: an identifiable name for snapshots and logging
  • guest_ssh_port / guest_telnet_port: which ports are listening for these on the VM (no relation with the ports Cowrie’s listening to)
  • guest_image_path: the base image upon which all VMs are created from
  • guest_hypervisor: the hypervisor used; if you have an older machine or the emulated architecture is different from the host one, then use software-based “qemu”; however, if you are able to, use “kvm”, it’s much faster.
  • guest_memory: memory assigned to the guest; choose a value considering the number of guests you’ll have running in total (pool_max_vms)

NATing

VMs are assigned an IP in a local network defined by libvirt. If you need to access the VMs from a different machine (i.e., running the backend pool remotely), then an external-facing IP (as defined in nat_public_ip) is needed for the proxy to connect to.

For this purpose, we provide a simple form of NAT that, for each VM request, and if enabled, starts a TCP proxy to forward data from a publicly-acessible IP to the internal libvirt interface.

Creating VM images

Creating a new type of VM involves three steps: creating a base image, installing the OS, and tweaking configs.

To create a disk image issue

sudo qemu-img create -f qcow2 image-name.qcow2 8G

(the qcow2 format is needed to ensure create snapshots, thus providing isolation between each VM instance; you can specify the size you want for the disk)

Then you’ll have to install an OS into it

virt-install --name temp-domain --memory 1024 --disk image-name.qcow2 --cdrom os-install-cd.iso --boot cdrom

(to use virt-install you need to install the virtinst package)

After install check that the VM has network connectivity. If you set the pool to use the created image and SSH does not come up, log into the VM via libvirt (e.g., using virt-manager) and try the following (might change depending on system)

# run all as root
ip link show
ip link set enp1s0 up
dhclient

In Ubuntu dhclient can be set to run with netplan, for example, to be run on startup.

Set up Telnet

Steps used in Ubuntu, can be useful in other distros

# run all as root
apt-get -y install telnetd xinetd
touch /etc/xinetd.d/telnet
printf "service telnet\n{\ndisable = no\nflags = REUSE\nsocket_type = stream\nwait = no\nuser = root\nserver = /usr/sbin/in.telnetd\nlog_on_failure += USERID\n}" > /etc/xinetd.d/telnet
printf "pts/0\npts/1\npts/2\npts/3\npts/4\npts/5\npts/6\npts/7\npts/8\npts/9" >> /etc/securetty
service xinetd start

Customising XML configs

If you want, you can customise libvirt’s XML configurations.

The main configuration for a guest is located in default_guest.xml. This defines the virtual CPU, available memory, and devices available on the guest. Most of these configurations are set by Cowrie using the guest configurations; you’ll see them in the XML as templates (“{guest_name}”). The main blocks of XML regard the disk and network interface devices.

You can include these blocks as-is in your custom configuration to allow Cowrie to manage your custom guests automatically.

<disk type='file' device='disk'>
    <driver name='qemu' type='qcow2'/>
    <source file='{disk_image}'/>
    <target dev='vda' bus='virtio'/>
    <address type='pci' domain='0x0000' bus='0x03' slot='0x00' function='0x0'/>
</disk>
<interface type='network'>
    <start mode='onboot'/>
    <mac address='{mac_address}'/>
    <source network='{network_name}'/>
    <model type='virtio'/>
    <address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
    <filterref filter='cowrie-default-filter'/>
</interface>

The other important configuration file is default_filter.xml, which handles how networking is restricted in the guest VM (aka to the attackers). This file is composed by a set of rules of the form

<rule action='accept' direction='in'>
    <tcp dstportstart='22'/>
</rule>

Each rule specifies a type of traffic (TCP, UDP…) and direction, whether to accept or drop that traffic, and the destiantion of traffic. The default filter provided allows inbound SSH and Telnet connections (without which the VM would be unusable, outbound ICMP traffic (to allow pinging) and outbound DNS querying. All other traffic is dropped as per the last rule, thus forbidding any download or tunnelling.

VERY IMPORTANT NOTE: some attacks consist of downloading malicious software or accessing illegal content through insecure machines (such as your honeypot). Our provided filter restricts networking and does its best to ensure total isolation, to the best of Qemu/libvirt (and our own) capabilities. Be very careful to protect your network and devices while allowing any more traffic!