SEED Labs – Linux Firewall Exploration Lab 1Linux Firewall.docx

SEED Labs – Linux Firewall Exploration Lab 1
Linux Firewall Exploration Lab
Copyright © 2006 - 2016 Wenliang Du, Syracuse University.
The development of this document was partially funded by the
National Science Foundation under Award
No. 1303306 and 1318814. This work is licensed under a
Creative Commons Attribution-NonCommercial-
ShareAlike 4.0 International License. A human-readable
summary of (and not a substitute for) the license is
the following: You are free to copy and redistribute the material
in any medium or format. You must give
appropriate credit. If you remix, transform, or build upon the
material, you must distribute your contributions
under the same license as the original. You may not use the
material for commercial purposes.
1 Overview
The learning objective of this lab is for students to gain the
insights on how firewalls work by playing
with firewall software and implement a simplified packet
filtering firewall. Firewalls have several types; in
this lab, we focus on packet filter. Packet filters inspect
packets, and decide whether to drop or forward
a packet based on firewall rules. Packet filters are usually
stateless; they filter each packet based only on
the information contained in that packet, without paying
attention to whether a packet is part of an existing
stream of traffic. Packet filters often use a combination of a
packet’s source and destination address, its

protocol, and, for TCP and UDP traffic, port numbers. In this
lab, students will play with this type of
firewall, and also through the implementation of some of the
key functionalities, they can understand how
firewalls work. Moreover, students will learn how to use SSH
tunnels to bypass firewalls. This lab covers
the following topics:
• Firewall
• Netfilter
• Loadable kernel module
• SSH tunnel
Readings and related topics. Detailed coverage of Firewalls can
be found in Chapter 14 of the SEED
book, Computer Security: A Hands-on Approach, by Wenliang
Du. A related lab is the Firewall Bypassing
lab, which shows how to use VPN to bypass firewalls.
Lab environment. This lab has been tested on our pre-built
Ubuntu 16.04 VM, which can be downloaded
from the SEED website.
2 Lab Tasks
2.1 Task 1: Using Firewall
Linux has a tool called iptables, which is essentially a firewall.
It has a nice front end program called
ufw. In this task, the objective is to use ufw to set up some
firewall policies, and observe the behaviors of
your system after the policies become effective. You need to set
up at least two VMs, one called Machine A,
and other called Machine B. You run the firewall on your
Machine A. Basically, we use ufw as a personal
firewall. Optionally, if you have more VMs, you can set up a

firewall at your router, so it can protect
a network, instead of just one single computer. After you set up
the two VMs, you should perform the
following tasks:
• Prevent A from doing telnet to Machine B.
• Prevent B from doing telnet to Machine A.
• Prevent A from visiting an external web site. You can choose
any web site that you like to block, but
keep in mind, some web servers have multiple IP addresses.
You can find the manual of ufw by typing "man ufw" or search
it online. It is pretty straightforward to
use. Please remember that the firewall is not enabled by default,
so you should run a command to specifically
enable it. We list some commonly used commands in Appendix
A.
Before starting the task, go to the default policy file
/etc/default/ufw. find the following entry,
and change the rule from DROP to ACCEPT; otherwise, all the
incoming traffic will be dropped by default.
# Set the default input policy to ACCEPT, DROP, or REJECT.
Please note that if
# you change this you will most likely want to adjust your rules.
DEFAULT_INPUT_POLICY="DROP"
2.2 Task 2: Implementing a Simple Firewall

The firewall you used in the previous task is a packet filtering
type of firewall. The main part of this type of
firewall is the filtering part, which inspects each incoming and
outgoing packets, and enforces the firewall
policies set by the administrator. Since the packet processing is
done within the kernel, the filtering must
also be done within the kernel. Therefore, it seems that
implementing such a firewall requires us to modify
the Linux kernel. In the past, this had to be done by modifying
and rebuilding the kernel. The modern
Linux operating systems provide several new mechanisms to
facilitate the manipulation of packets without
rebuilding the kernel image. These two mechanisms are
Loadable Kernel Module (LKM) and Netfilter.
LKM allows us to add a new module to the kernel at the
runtime. This new module enables us to extend
the functionalities of the kernel, without rebuilding the kernel
or even rebooting the computer. The packet
filtering part of a firewall can be implemented as an LKM.
However, this is not enough. In order for the
filtering module to block incoming/outgoing packets, the
module must be inserted into the packet processing
path. This cannot be easily done in the past before the Netfilter
was introduced into the Linux.
Netfilter is designed to facilitate the manipulation of packets by
authorized users. Netfilter
achieves this goal by implementing a number of hooks in the
Linux kernel. These hooks are inserted into
various places, including the packet incoming and outgoing
paths. If we want to manipulate the incoming
packets, we simply need to connect our own programs (within
LKM) to the corresponding hooks. Once an
incoming packet arrives, our program will be invoked. Our
program can decide whether this packet should

be blocked or not; moreover, we can also modify the packets in
the program.
In this task, you need to use LKM and Netfilter to implement
the packet filtering module. This
module will fetch the firewall policies from a data structure,
and use the policies to decide whether packets
should be blocked or not. To make your life easier, so you can
focus on the filtering part, the core of
firewalls, we allow you to hardcode your firewall policies in the
program. You should support at least five
different rules, including the ones specified in the previous
task. Guidelines on how to use Netfilter can
be found in Section 3. In addition, Chapter 14 (§14.4) of the
SEED book provides more detailed explanation
on Netfilter.
Note for Ubuntu 16.04 VM: The code in the SEED book was
developed in Ubuntu 12.04. It needs
to be changed slightly to work in Ubuntu 16.04. The change is
in the definition of the callback func-
tion telnetFilter(), because the prototype of Netfilter’s callback
function has been changed in
Ubuntu 16.04. See the difference in the following:
// In Ubuntu 12.04
unsigned int telnetFilter(unsigned int hooknum, struct sk_buff
*skb,
const struct net_device *in, const struct net_device *out,
int (*okfn)(struct sk_buff *))

// In Ubuntu 16.04
unsigned int telnetFilter(void *priv, struct sk_buff *skb,
const struct nf_hook_state *state)
2.3 Task 3: Evading Egress Filtering
Many companies and schools enforce egress filtering, which
blocks users inside of their networks from
reaching out to certain web sites or Internet services. They do
allow users to access other web sites. In many
cases, this type of firewalls inspect the destination IP address
and port number in the outgoing packets. If
a packet matches the restrictions, it will be dropped. They
usually do not conduct deep packet inspections
(i.e., looking into the data part of packets) due to the
performance reason. In this task, we show how such
egress filtering can be bypassed using the tunnel mechanism.
There are many ways to establish tunnels; in
this task, we only focus on SSH tunnels.
You need two VMs A and B for this task (three will be better).
Machine A is running behind a firewall
(i.e., inside the company or school’s network), and Machine B
is outside of the firewall. Typically, there is a
dedicated machine that runs the firewall, but in this task, for the
sake of convenience, you can run the firewall
on Machine A. You can use the firewall program that you
implemented in the previous task, or directly use
ufw. You need to set up the following two firewall rules:
• Block all the outgoing traffic to external telnet servers. In
reality, the servers that are blocked are usu-
ally game servers or other types of servers that may affect the
productivity of employees. In this task,
we use the telnet server for demonstration purposes. You can

run the telnet server on Machine B. If
you have a third VM, Machine C, you can run the telnet server
on Machine C.
• Block all the outgoing traffic to www.facebook.com, so
employees (or school kids) are not dis-
tracted during their work/school hours. Social network sites are
commonly blocked by companies
and schools. After you set up the firewall, launch your Firefox
browser, and try to connect to Face-
book, and report what happens. If you have already visited
Facebook before using this browser,
you need to clear all the caches using Firefox’s menu: Edit ->
Preferences -> Privacy
& Security (left pane) -> Clear History (Button on right);
otherwise, the
cached pages may be displayed. If everything is set up properly,
you should not be able to see Face-
book pages. It should be noted that Facebook has many IP
addresses, it can change over the time.
Remember to check whether the address is still the same by
using ping or dig command. If the
address has changed, you need to update your firewall rules.
You can also choose web sites with static
IP addresses, instead of using Facebook. For example, most
universities’ web servers use static IP
addresses (e.g. www.syr.edu); for demonstration purposes, you
can try block these IPs, instead of
Facebook.
In addition to set up the firewall rules, you also need the
following commands to manage the firewall:
$ sudo ufw enable // this will enable the firewall.
$ sudo ufw disable // this will disable the firewall.
$ sudo ufw status numbered // this will display the firewall

rules.
$ sudo ufw delete 3 // this will delete the 3rd rule.
8000 22
Telnet
client
Machine: home
Machine: work
ssh client ssh server
Machine: apollo
22
23
Telnet
server
Firewall
Figure 1: SSH Tunnel Example
Task 3.a: Telnet to Machine B through the firewall To bypass
the firewall, we can establish an SSH
tunnel between Machine A and B, so all the telnet traffic will go
through this tunnel (encrypted), evading the
inspection. Figure 1 illustrates how the tunnel works. The
following command establishes an SSH tunnel

between the localhost (port 8000) and machine B (using the
default port 22); when packets come out of B’s
end, it will be forwarded to Machine C’s port 23 (telnet port). If
you only have two VMs, you can use
one VM for both Machine B and Machine C.
$ ssh -L 8000:Machine_C_IP:23 [email protected]_B_IP
// We can now telnet to Machine C via the tunnel:
$ telnet localhost 8000
When we telnet to localhost’s port 8000, SSH will transfer all
our TCP packets from one end
of the tunnel on localhost:8000 to the other end of the tunnel on
Machine B; from there, the packets
will be forwarded to Machine C:23. Replies from Machine C
will take a reverse path, and eventually
reach our telnet client. Essentially, we are able to telnet to
Machine C. Please describe your observation and
explain how you are able to bypass the egress filtering. You
should use Wireshark to see what exactly is
happening on the wire.
Task 3.b: Connect to Facebook using SSH Tunnel. To achieve
this goal, we can use the approach
similar to that in Task 3.a, i.e., establishing a tunnel between
your localhost:port and Machine B, and ask B
to forward packets to Facebook. To do that, you can use the
following command to set up the tunnel: "ssh
-L 8000:FacebookIP:80 ...". We will not use this approach, and
instead, we use a more generic
approach, called dynamic port forwarding, instead of a static
one like that in Task 3.a. To do that, we only
specify the local port number, not the final destination. When
Machine B receives a packet from the tunnel,
it will dynamically decide where it should forward the packet to

based on the destination information of the
packet.
$ ssh -D 9000 -C [email protected]_B
Figure 2: Configure the SOCKS Proxy
Similar to the telnet program, which connects localhost:9000,
we need to ask Firefox to connect
to localhost:9000 every time it needs to connect to a web server,
so the traffic can go through our SSH
tunnel. To achieve that, we can tell Firefox to use
localhost:9000 as its proxy. To support dynamic port
forwarding, we need a special type of proxy called SOCKS
proxy, which is supported by most browsers. To
set up the proxy in Firefox, go to the menu bar, click Edit ->
Preferences, scroll down to Network
Proxy, and click the Settings button. Then follow Figure 2.
After the setup is done, please do the
following:
1. Run Firefox and go visit the Facebook page. Can you see the
Facebook page? Please describe your
observation.
2. After you get the facebook page, break the SSH tunnel, clear
the Firefox cache, and try the connection
again. Please describe your observation.
3. Establish the SSH tunnel again and connect to Facebook.
Describe your observation.

4. Please explain what you have observed, especially on why the
SSH tunnel can help bypass the egress
filtering. You should use Wireshark to see what exactly is
happening on the wire. Please describe
your observations and explain them using the packets that you
have captured.
2.4 Task 4: Evading Ingress Filtering
Machine A runs a web server behind a firewall; so only the
machines in the internal network can access
this web server. You are working from home and needs to
access this internal web server. You do not have
VPN, but you have SSH, which is considered as a poor man’s
VPN. You do have an account on Machine A
(or another internal machine behind the firewall), but the
firewall also blocks incoming SSH connection, so
you cannot SSH into any machine on the internal network.
Otherwise, you can use the same technique from
Task 3 to access the web server. The firewall, however, does
not block outgoing SSH connection, i.e., if you
want to connect to an outside SSH server, you can still do that.
The objective of this task is to be able to access the web server
on Machine A from outside. We will use
two machines to emulate the setup. Machine A is the internal
computer, running the protected web server;
Machine B is the outside machine at home. On Machine A, we
block Machine B from accessing its port 80
(web server) and 22 (SSH server). You need to set up a reverse
SSH tunnel on Machine A, so once you get
home, you can still access the protected web server on A from

home.
3 Guidelines
3.1 Loadable Kernel Module
The following is a simple loadable kernel module. It prints out
"Hello World!" when the module is
loaded; when the module is removed from the kernel, it prints
out "Bye-bye World!". The messages
are not printed out on the screen; they are actually printed into
the /var/log/syslog file. You can use
dmesg | tail -10 to read the last 10 lines of message.
#include <linux/module.h>
#include <linux/kernel.h>
int init_module(void)
{
printk(KERN_INFO "Hello World!n");
return 0;
}
void cleanup_module(void)
{
printk(KERN_INFO "Bye-bye World!.n");
}
We now need to create Makefile, which includes the following
contents (the above program is named
hello.c). Then just type make, and the above program will be
compiled into a loadable kernel module.

obj-m += hello.o
all:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD)
modules
clean:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
Once the module is built by typing make, you can use the
following commands to load the module, list
all modules, and remove the module:
$ sudo insmod mymod.ko (inserting a module)
$ lsmod (list all modules)
$ sudo rmmod mymod.ko (remove the module)
Also, you can use modinfo mymod.ko to show information
about a Linux Kernel module.
3.2 A Simple Program that Uses Netfilter
Using Netfilter is quite straightforward. All we need to do is to
hook our functions (in the kernel
module) to the corresponding Netfilter hooks. Here we show an
example:
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/netfilter.h>
#include <linux/netfilter_ipv4.h>

/* This is the structure we shall use to register our function */
static struct nf_hook_ops nfho;
/* This is the hook function itself */
unsigned int hook_func(void *priv, struct sk_buff *skb,
const struct nf_hook_state *state)
{
/* This is where you can inspect the packet contained in
the structure pointed by skb, and decide whether to accept
or drop it. You can even modify the packet */
// In this example, we simply drop all packets
return NF_DROP; /* Drop ALL packets */
}
/* Initialization routine */
int init_module()
{ /* Fill in our hook structure */
nfho.hook = hook_func; /* Handler function */
nfho.hooknum = NF_INET_PRE_ROUTING; /* First hook for
IPv4 */
nfho.pf = PF_INET;
nfho.priority = NF_IP_PRI_FIRST; /* Make our function first
*/
nf_register_hook(&nfho);
return 0;
}
/* Cleanup routine */

void cleanup_module()
{
nf_unregister_hook(&nfho);
}
4 Submission and Demonstration
Students need to submit a detailed lab report to describe what
they have done, what they have observed,
and explanation. Reports should include the evidences to
support the observations. Evidences include
packet traces, screendumps, etc. Students also need to answer
all the questions in the lab description. For
the programming tasks, students should list the important code
snippets followed by explanation. Simply
attaching code without any explanation is not enough.
A Firewall Lab Cheat Sheet
Header Files. You may need to take a look at several header
files, including the skbuff.h, ip.h,
icmp.h, tcp.h, udp.h, and netfilter.h. They are stored in the
following folder:
/lib/modules/$(uname -r)/build/include/linux/
IP Header. The following code shows how you can get the IP

header, and its source/destination IP ad-
dresses.
struct iphdr *ip_header = (struct iphdr
*)skb_network_header(skb);
unsigned int src_ip = (unsigned int)ip_header->saddr;
unsigned int dest_ip = (unsigned int)ip_header->daddr;
TCP/UDP Header. The following code shows how you can get
the UDP header, and its source/destination
port numbers. It should be noted that we use the ntohs()
function to convert the unsigned short integer
from the network byte order to the host byte order. This is
because in the 80x86 architecture, the host byte
order is the Least Significant Byte first, whereas the network
byte order, as used on the Internet, is Most
Significant Byte first. If you want to put a short integer into a
packet, you should use htons(), which is
reverse to ntohs().
struct udphdr *udp_header = (struct udphdr
*)skb_transport_header(skb);
src_port = (unsigned int)ntohs(udp_header->source);
dest_port = (unsigned int)ntohs(udp_header->dest);
IP Addresses in different formats. You may find the following
library functions useful when you convert
IP addresses from one format to another (e.g. from a string
"128.230.5.3" to its corresponding integer
in the network byte order or the host byte order.
int inet_aton(const char *cp, struct in_addr *inp);
in_addr_t inet_addr(const char *cp);
in_addr_t inet_network(const char *cp);
char *inet_ntoa(struct in_addr in);
struct in_addr inet_makeaddr(int net, int host);

in_addr_t inet_lnaof(struct in_addr in);
in_addr_t inet_netof(struct in_addr in);
Using ufw. The default firewall configuration tool for Ubuntu is
ufw, which is developed to ease iptables
firewall configuration. By default UFW is disabled, so you need
to enable it first.
$ sudo ufw enable // Enable the firewall
$ sudo ufw disable // Disable the firewall
$ sudo ufw status numbered // Display the firewall rules
$ sudo ufw delete 2 // Delete the 2nd rule

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