Networking Pivoting via SSH – Scanning with Nessus Professional behind a Firewall or NAT.

In this post I’m going to be covering the process to scan a network behind a Firewall or NAT using Networking Pivoting via SSH without being limited to proxychains, specific ports and protocols. Essentially this will use SSH tunneling, virtual tap adapters, some routing and masquarding in IPtables. The beauty of this method is the prerequisites are very low, for the most part no additional packages or standalone tools are required, we can use what is shipped with most Linux builds.

There are many use cases for this, scanning an internal network without being on prem, cloud environments, various pentesting scenarios, which can often be the stumbling point once a shell has been landed. Traditionally this type of task would have been done with the use of proxy chains, through some form of shell access via a netcat listener, Metasploit or SSH dynamic port forward, which I have previous walked through here. However this is an extremely slow method and rely’s on being able to tunnel through a single port with proxy chains, I have never had any luck scanning with more complex tools like Nessus in this way. Full SYN scans (-sT) with nmap great, Nessus not so much.

Lets take the following scenario and set the pivot up:

Networking Pivoting via SSH

We can use tunctl or ip tuntap, the difference being that ip tuntap is part of the iptools suite and therefore general supported on most Linux operating systems. Tunctl can usually be downloaded from your repo of choice ie with Ubuntu its part of the apt repository. In this example we will be working with Kali as the scanning system and a Ubuntu server as the pivot point, which has SSH accessible. (It is worth mentioning at this point it doesn’t matter which end the SSH connection is initiated from).

First we need to create a virtual tunnel and therefore need to create two virtual interfaces at both ends of the tunnel. For this we are going to use a tap interface. For reference a tap interface operates at layer 2 and a tun interface operates at layer 3.

Using tunctl: First we will need to install tunctl with apt install uml-utilities

# apt install uml-utilities

Create the virtual tap interface with the following command:

# tunctl -t tap0

Using ip tuntap: First verify your ip tools version installed supports tuntap, type ‘ip’ you will see if the command is available:

# ip

Create the virtual tap interface with the following command:

# ip tuntap add dev tap0 mod tap

Once this is setup assign it an ip address and raise the interface, assign a different address for each end of the tunnel:

So on the scanner:

# ip a a 10.100.100.100/24 dev tap0 && ip link set tap0 up

On the pivot server:

# ip a a 10.100.100.101/24 dev tap0 && ip link set tap0 up

On each end of the tunnel we will also need to make sure our SSH config will allow us to tunnel. Lets modify our /etc/ssh/sshd_config file by adding ‘ PermitTunnel=yes ‘ to the end and restart the service. More about this option can be found in SSH man page here.

Now for the magic, lets bring the tunnel up by establishing an SSH session, this will need to be done with a privileged account:

ssh -o Tunnel=ethernet -w 0:0 root@11.1.1.11

Lets cover off these options:

  • -o = allows us to specify options
  • Tunnel=ethernet = is our option for the tunnel
  • -w 0:0 = specifies the next available interface for the tunnel, and corresponds to each side of the tunnel.

Next lets take a moment to verify our tunnel is up with a couple of quick checks:

First verify the link is up with ethtool:

# ethtool tap0

You will notice the link is up, try this without the connection you will find the link is down.

Second verify you can ping the other end of the tunnel:

# ping 10.100.100.101

Again disconnect your SSH connection and watch icmp response drop.

Next in order to get our traffic to our destination servers/subnet we are going to need some routes adding to Kali to tell the system where to send the traffic, ie the other end of the tunnel, so, something similar to this where 192.168.1.0/24 being the network you are targeting:

# ip route add 192.168.1.0/24 via 10.100.100.101

# ip route add 192.168.2.0/24 via 10.100.100.101

Finally we need to setup some iptables rules and turn our pivot point into a router by enabling IPv4 forwarding:

# echo 1 > /proc/sys/net/ipv4/ip_forward

# iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

# iptables -t nat -A POSTROUTING -o tap0 -j MASQUERADE

# iptables -A INPUT -i eth0 -m state –state RELATED,ESTABLISHED -j ACCEPT

# iptables -A INPUT -i tap0 -m state –state RELATED,ESTABLISHED -j ACCEPT

# iptables -A FORWARD -j ACCEPT

At this point the pivot should be up and running, test this by doing some basic checks with a known host on your target network.

Happy pivoting testers!

Facebooktwitterpinterestlinkedinmail

VLAN tagging in Kali Linux 2.0

Connecting to Trunked Ports!

Just a quick post on how to configure VLAN tagging in Kali Linux 2.0. If we have a trunk port presented to us, how do we utilise it?

To setup vlan tagging in Kali Linux 2.0 is pretty straight forward, to set the scene and demonstrate this further we need a lab. The below lab is our ‘test.local’ environment set up in GNS3. There are 3 vlans, 10 20 and 30. 10 and 20 are routable, vlan 30 is isolated from 10 and 20. In the lab we have a ‘router on stick’ configured  at R4, fa0/1 is sub interfaced with vlans 10 and 20. utilising DHCP, vlan 10 for servers and vlan 20 for clients. All devices in vlan 30 are statically assigned IP addresses and not routable to the 10 and 20 vlan networks.  In all switches there are a variety of 802.1q trunked and access ports.

The idea of the lab is that vlan 30 can’t talk to vlan 10 or 20. However as a trunked port is presented to the Kali vm, it will be able to communicate to all vlans.

This is how it looks:

VLAN tagging in Kali Linux 2.0 - switch configuration

Lets look at how we would configure Kali to test all hosts in the different vlans, first via CLI and then via GUI in Network Manager:

As you can see with the current trunk connection we can’t access any of the networks, however a quick Wireshark does reveal we can see traffic and the different vlans…

First lets open up ‘/etc/network/interfaces’ in nano and add our interfaces. The idea is very similar to a cisco router we are essentially sub interfacing our network connection in the interfaces file:

An alternative configuration for the interfaces file which I have used as well is the following, both work just as well:

Save our file  restart the networking service and then simply bring up the sub-interfaces with ‘ifconfig XXX up’ where XXX is our subinterface so:

‘ifconfig vlan10 up’ or ‘ifconfig eth0.10 up’ as below:

We can now access all of our vlans in question.

Further to this, if we set off a ping to each network and Wireshark the trunk connection we can see our tagged packets. Hooray.

The Gui is even easier, lets configure it via the Network Manager. We will do this via opening up our network connections/Network Manager, simple click on the ‘+’ sign, select ‘VLAN’ fill in the details on the vlan tab as well as the ‘IPv4Settings’ tab:

And there you have it.

I hope this helps someone!Facebooktwitterpinterestlinkedinmail

Greetings!

Greetings! Welcome to my InfoSec Site!

Here I will be mostly be talking to myself about Information Security Topics, News, and mini guides. Feel free to join in and comment!

I am also going to be sharing some of the latest Labs and Projects that I am currently working on, short tutorials that I have been through and had to learn along the way. This will be both Windows and Linux based. I will also be sharing some Security techniques, not only in an effort to raise awareness for Information Security, but also to use as my own quick lookup when I need to remember something!

I was inspired to create this site after listening to Defensive Security Podcast and subsequently reading an article titled How to build a successful information security career on their site.Facebooktwitterpinterestlinkedinmail