How To Test your Firewall Configuration with Nmap and Tcpdump

Introduction #

Setting up a firewall for your infrastructure is a great way to provide security for your services. Once you’ve developed a policy you are happy with, the next step is to test your firewall rules. It is important to get a good idea of whether your firewall rules are doing what you think they are doing and to get an impression of what your infrastructure looks like to the outside world.
In this guide, we’ll go over some tools and techniques that you can use to validate your firewall rules. These are some of the same tools that malicious users may use, so you will be able to see what information they can find by making requests of your servers.

Prerequisites #

In this guide, we will assume that you have a firewall configured on at least one server. You can get started building your firewall policy by following one or more of these guides:

Iptables

Iptables Essentials: Common Firewall Rules and Commands

UFW

How To Set Up a Firewall with UFW on Ubuntu 22.04
UFW Essentials: Common Firewall Rules and Commands

FirewallD

How To Set Up a Firewall Using FirewallD on Rocky Linux 9

You can also configure DigitalOcean’s Cloud Firewalls which run as an additional, external layer to your servers on DigitalOcean infrastructure. This way, you do not have to configure a firewall on your servers themselves.
In this guide, we will call the server containing the firewall policies you wish to test the target. In addition to your target, you will also need to have access to a server to test from, located outside of the network that your firewall protects. In this guide, you will use an Ubuntu 22.04 server as your auditing machine.
Once you have a server to test from and the targets you wish to evaluate, you can continue with this guide.

Warning
You should only perform the activities outlined in this guide on infrastructure that you control, for the purpose of security auditing. The laws surrounding port scanning are uncertain in many jurisdictions. ISPs and other providers have been known to ban users who are found port scanning.

The Tools We Will Use to Test Firewall Policies #

There are quite a few different tools that you can use to test our firewall policies. Some of them have overlapping functionality. We will not cover every possible tool. Instead, we will cover some general categories of auditing tools and go over the tools we will be using in this guide.

Packet Analyzers #

Packet analyzers can be used to watch all of the network traffic that goes over an interface in great detail. Most packet analyzers have the option of operating in real time, displaying the packets as they are sent or received, or of writing packet information to a file and processing it at a later time. Packet analysis gives you the ability to see, at a granular level, what types of responses your target machines are sending back to hosts on the open network.
For the purposes of this guide, we will be using the tcpdump tool. This is a good option because it is powerful, flexible, and ubiquitous on Linux systems. You will use it to capture the raw packets as we run our tests in case we need the transcript for later analysis. Some other popular options are Wireshark (or tshark, its command line cousin) and tcpflow which can piece together entire TCP conversations in an organized fashion.

Port Scanners #

In order to generate the traffic and responses for your packet analyzer to capture, you will use a port scanner. Port scanners can be used to craft and send various types of packets to remote hosts to discover the type of traffic the server accepts. Malicious users often use this as a discovery tool to try to find vulnerable services to exploit (part of the reason to use a firewall in the first place), so you will use this to try to see what an attacker could discover.
For this guide, you will use the nmap network mapping and port scanning tool. You can use nmap to send packets of different types to try to figure out which services are on your target machine and what firewall rules protect it.

Setting Up the Auditing Machine #

Before you get started, you should make sure we have the necessary tools installed. You can get tcpdump and nmap from Ubuntu’s repositories. Run apt update to update your local package listings, then install them with apt install:

sudo apt update
sudo apt install tcpdump nmap

Next, create a directory where you can store your scan results:

mkdir ~/scan_results

To make sure that you get clean results, exit out of any sessions you might have open between your auditing system and the target system. This includes SSH sessions, any HTTP(S) connections you may have established in a web browser, etc.

Scan your Target for Open TCP Ports #

Now that we have our server and files ready, you’ll begin by scanning your target host for open TCP ports.
There are actually a few TCP scans that nmap knows how to do. The best one to usually start off with is a SYN scan, also known as a “half-open scan” because it never actually negotiates a full TCP connection. This is often used by attackers, as it does not register on some intrusion detection systems because it never completes a full handshake.

Setting Up the Packet Capture #

Before you scan, you will set up tcpdump to capture the traffic generated by the test. This will help you analyze the packets sent and received in more depth later on if needed. Create a directory within ~/scan_results so that you can keep the files related to your SYN scan together:

mkdir ~/scan_results/syn_scan

You can start a tcpdump capture and write the results to a file in your ~/scan_results/syn_scan directory with the following command:

sudo tcpdump host target_ip_addr -w ~/scan_results/syn_scan/packets

By default, tcpdump will run in the foreground. In order to run your nmap scan in the same window, you’ll need to pause the tcpdump process and then restart it in the background.
We can pause a running process by pressing CTRL-Z:

^Z
[1]+  Stopped                 sudo tcpdump host target_ip_addr -w ~/scan_results/syn_scan/packets

Now, you can restart the job in the background by typing bg:

bg

You should receive a similar line of output, this time without the “Stopped” label and with an ampersand at the end to indicate that the process will be run in the background (i.e., no longer blocking your terminal):

[1]+ sudo tcpdump host target_ip_addr -w ~/scan_results/syn_scan/packets &

The command is now running in the background, watching for any packets going between your audit and target machines. We can now run our SYN scan.

Run the SYN Scan #

With tcpdump recording your traffic to the target machine, you are ready to run nmap. You can run nmap with these flags:

-sS: This starts a SYN scan. This is technically the default scan that nmap will perform if no scan type is given, but we will include it here to be explicit.
-Pn: This tells nmap to skip the host discovery step, which would abort the test early if the host doesn’t respond to a ping. Since you know that the target is online, you can skip this.
-p-: By default, SYN scans will only try the 1000 most commonly used ports. This tells nmap to check every available port.
-T4: This sets a timing profile for nmap, telling it to speed up the test at the risk of slightly less accurate results. 0 is the slowest and 5 is the fastest. Since you’re scanning every port, you can use this as your baseline and re-check any ports later that might have been reported incorrectly.
-vv: This increases the verbosity of the output.
--reason: This tells nmap to provide the reason that a port’s state was reported a certain way.
-oN: This writes the results to a file that you can use for later analysis.

Note: To check IPv6, you will need to add the -6 flag to your commands…

Together, the command will look something like this:

sudo nmap -sS -Pn -p- -T4 -vv --reason -oN ~/scan_results/syn_scan/nmap.results target_ip_addr

Even with the timing template set at 4, the scan will likely take quite some time as it runs through 65,535 ports. Results will begin to print that look like this:

Starting Nmap 6.49BETA4 ( https://nmap.org ) at 2022-12-19 16:54 EDT
Initiating Parallel DNS resolution of 1 host. at 16:54
Completed Parallel DNS resolution of 1 host. at 16:54, 0.12s elapsed
Initiating SYN Stealth Scan at 16:54
Scanning 198.51.100.15 [65535 ports]
Discovered open port 22/tcp on 198.51.100.15
Discovered open port 80/tcp on 198.51.100.15
SYN Stealth Scan Timing: About 6.16% done; ETC: 17:02 (0:07:52 remaining)
SYN Stealth Scan Timing: About 8.60% done; ETC: 17:06 (0:10:48 remaining)

. . .

Stop the tcpdump Packet Capture #

Once the scan is complete, you can bring our tcpdump process back into the foreground and stop it.
Bring it out of the background by running fg, for “foreground”:

fg

Stop the running process by pressing Ctrl+C.

Analyzing the Results #

You should now have two files in your ~/scan_results/syn_scan directory. One called packets, generated by the tcpdump run, and one generated by nmap called nmap.results.
Let’s look at the nmap.results file first:

less ~/scan_results/syn_scan/nmap.results

~/scan_results/syn_scan/nmap.results

# Nmap 6.49BETA4 scan initiated Mon Dec 19 17:05:13 2022 as: nmap -sS -Pn -p- -T4 -vv --reason -oN /home/user/scan_results/syn_scan/nmap.results 198.51.100.15
Increasing send delay for 198.51.100.15 from 0 to 5 due to 9226 out of 23064 dropped probes since last increase.
Increasing send delay for 198.51.100.15 from 5 to 10 due to 14 out of 34 dropped probes since last increase.
Nmap scan report for 198.51.100.15
Host is up, received user-set (0.00097s latency).
Scanned at 2022-12-19 17:05:13 EDT for 2337s
Not shown: 65533 closed ports
Reason: 65533 resets
PORT   STATE SERVICE REASON
22/tcp open  ssh     syn-ack ttl 63
80/tcp open  http    syn-ack ttl 63

Read data files from: /usr/local/bin/../share/nmap
# Nmap done at Mon Dec 19 17:44:10 2022 -- 1 IP address (1 host up) scanned in 2336.85 seconds

The highlighted area above contains the main results of the scan. You can infer that port 22 and port 80 are open on the scanned host in order to allow SSH and HTTP traffic. You can also observe that 65,533 ports were closed. Another possible result would be “filtered”. Filtered means that these ports were identified as being stopped by something along the network path. It could be a firewall on the target, but it could also be filtering rules on any of the intermediate hosts between the audit and target machines.
To see the actual packet traffic that was sent to and received from the target, you can read the packets file back into tcpdump, like this:

sudo tcpdump -nn -r ~/scan_results/syn_scan/packets | less

This file contains the entire conversation that took place between the two hosts. You can filter in a number of ways.
For instance, to view only the traffic sent to the target, you can type:

sudo tcpdump -nn -r ~/scan_results/syn_scan/packets 'dst target_ip_addr' | less

Likewise, to view only the response traffic, you can change the dst to src:

sudo tcpdump -nn -r ~/scan_results/syn_scan/packets 'src target_ip_addr' | less

Open TCP ports would respond to these requests with a SYN packet. We can search directly for responses for this type with a filter like this:

sudo tcpdump -nn -r ~/scan_results/syn_scan/packets 'src target_ip_addr and tcp[tcpflags] & tcp-syn != 0' | less

This will show you only the successful SYN responses, and should match the ports that you saw in the nmap run:

reading from file packets, link-type EN10MB (Ethernet)
17:05:13.557597 IP 198.51.100.15.22 > 198.51.100.2.63872: Flags [S.], seq 2144564104, ack 4206039348, win 29200, options [mss 1460], length 0
17:05:13.558085 IP 198.51.100.15.80 > 198.51.100.2.63872: Flags [S.], seq 3550723926, ack 4206039348, win 29200, options [mss 1460], length 0

You can do more analysis of the data as you see fit. It has all been captured for asynchronous processing and analysis.

Scan your Target for Open UDP Ports #

Now that you have a good handle on how to run these tests, you can complete a similar process to scan for open UDP ports.

Setting Up the Packet Capture #

Once again, create a directory to hold your results:

mkdir ~/scan_results/udp_scan

Start a tcpdump capture again. This time, write the file to the new ~/scan_results/udp_scan directory:

sudo tcpdump host target_ip_addr -w ~/scan_results/udp_scan/packets

Pause the process and put it into the background by entering Ctrl+Z then running bg:

bg

Run the UDP Scan #

Now you are ready to run the UDP scan. Due to the nature of the UDP protocol, this scan will typically take significantly longer than the SYN scan. In fact, it could take over a day if you are scanning every port on the system. UDP is a connectionless protocol, so receiving no response could mean that the target’s port is blocked, that it was accepted, or that the packet was lost. To try to distinguish between these, nmap must retransmit additional packets to try to get a response.
Most of the flags will be the same as you used for the SYN scan. In fact, the only new flag is:

-sU: This tells nmap to perform a UDP scan.

Speeding up the UDP Test
If you are worried about the amount of time this test takes, you may only want to test a subset of your UDP ports at first. You can test only the 1000 most common ports by leaving out the -p- flag. This can shorten your scan time considerably. If you want a complete picture though, you’ll have to go back later and scan your entire port range.
Because you are scanning your own infrastructure, perhaps the best option to speed up the UDP scans is to temporarily disable ICMP rate limiting on the target system. Typically, Linux hosts limit ICMP responses to 1 per second (this is typically a good thing, but not for our auditing), which means that a full UDP scan would take over 18 hours. You can check this setting on your target machine by typing:

sudo sysctl net.ipv4.icmp_ratelimit

net.ipv4.icmp_ratelimit = 1000

The “1000” is the number of milliseconds between responses. You can temporarily disable this rate limiting on the target system by typing:

sudo sysctl -w net.ipv4.icmp_ratelimit=0

It is very important to revert this value after your test.
Running the Test
Be sure to write the results to the ~/scan_results/udp_scan directory. All together, the command should look like this:

sudo nmap -sU -Pn -p- -T4 -vv --reason -oN ~/scan_results/udp_scan/nmap.results target_ip_addr

After the scan is complete, you should revert your ICMP rate limit (if you modified it) on the target machine:

sudo sysctl -w net.ipv4.icmp_ratelimit=1000

Stop the tcpdump Packet Capture #

Bring the tcpdump process back into the foreground on your audit machine by running fg:

fg

Then, stop the packet capture with Ctrl+C.

Analyzing the Results #

Now, you can take a look at the generated files.
The resulting nmap.results file should be similar to the last result:

less ~/scan_results/udp_scan/nmap.results

~/scan_results/udp_scan/nmap.results

# Nmap 6.49BETA4 scan initiated Mon Dec 19 12:42:42 2022 as: nmap -sU -Pn -p- -T4 -vv --reason -oN /home/user/scan_results/udp_scan/nmap.results 198.51.100.15
Increasing send delay for 198.51.100.15 from 0 to 50 due to 10445 out of 26111 dropped probes since last increase.
Increasing send delay for 198.51.100.15 from 50 to 100 due to 11 out of 23 dropped probes since last increase.
Increasing send delay for 198.51.100.15 from 100 to 200 due to 3427 out of 8567 dropped probes since last increase.
Nmap scan report for 198.51.100.15
Host is up, received user-set (0.0010s latency).
Scanned at 2022-12-19 12:42:42 EDT for 9956s
Not shown: 65532 closed ports
Reason: 65532 port-unreaches
PORT    STATE         SERVICE REASON
22/udp  open|filtered ssh     no-response
80/udp  open|filtered http    no-response
443/udp open|filtered https   no-response

Read data files from: /usr/local/bin/../share/nmap
# Nmap done at Mon Dec 19 15:28:39 2022 -- 1 IP address (1 host up) scanned in 9956.97 seconds

A key difference between this result and the SYN result earlier will likely be the amount of ports marked open|filtered. This means that nmap couldn’t determine whether the lack of a response meant that a service accepted the traffic or whether it was dropped by some firewall or filtering mechanism along the delivery path.
Analyzing the tcpdump output is also significantly more difficult because there are no connection flags and because you must match ICMP responses to UDP requests.
You can see how many packets nmap had to send to the ports that were reported as open|filtered by asking to see the UDP traffic to one of the reported ports:

sudo tcpdump -nn -Q out -r ~/scan_results/udp_scan/packets 'udp and port 22'

reading from file /home/user/scan_results/udp_scan/packets, link-type EN10MB (Ethernet)
14:57:40.801956 IP 198.51.100.2.60181 > 198.51.100.15.22: UDP, length 0
14:57:41.002364 IP 198.51.100.2.60182 > 198.51.100.15.22: UDP, length 0
14:57:41.202702 IP 198.51.100.2.60183 > 198.51.100.15.22: UDP, length 0
14:57:41.403099 IP 198.51.100.2.60184 > 198.51.100.15.22: UDP, length 0
14:57:41.603431 IP 198.51.100.2.60185 > 198.51.100.15.22: UDP, length 0
14:57:41.803885 IP 198.51.100.2.60186 > 198.51.100.15.22: UDP, length 0

Compare this to the results from one of the scanned ports that was marked as “closed”:

sudo tcpdump -nn -Q out -r ~/scan_results/udp_scan/packets 'udp and port 53'

reading from file /home/user/scan_results/udp_scan/packets, link-type EN10MB (Ethernet)
13:37:24.219270 IP 198.51.100.2.60181 > 198.51.100.15.53: 0 stat [0q] (12)

You can try to manually reconstruct the process that nmap goes through by first compiling a list of all of the ports that we’re sending UDP packets to using something like this:

sudo tcpdump -nn -Q out -r ~/scan_results/udp_scan/packets "udp" | awk '{print $5;}' | awk 'BEGIN { FS = "." } ; { print $5 +0}' | sort -u | tee outgoing

Then you can see which ICMP packets we received back saying the port was unreachable:

sudo tcpdump -nn -Q in -r ~/scan_results/udp_scan/packets "icmp" |  awk '{print $10,$11}' | grep unreachable | awk '{print $1}' | sort -u | tee response

You can then take these two responses and see which UDP packets never received an ICMP response back:

comm -3 outgoing response

This should mostly match the list of ports that nmap reported (it may contain some false positives from lost return packets).

Host and Service Discovery #

You can run some additional tests on your target to see if it is possible for nmap to identify the operating system running or any of the service versions. Make a directory to hold your versioning results:

mkdir ~/scan_results/versions

Discovering the Versions of Services on the Server #

You can attempt to guess the versions of services running on the target through a process known as fingerprinting. You retrieve information from the server and compare it to known versions in our database.
A tcpdump wouldn’t be too useful in this scenario, so you can skip it. If you want to capture it anyways, follow the process you used last time.
The nmap scan you need to use is triggered by the -sV flag. Since you already did SYN and UDP scans, you can pass in the exact ports you need to look at with the -p flag. Here, you’ll look at 22 and 80 (the ports that were shown in our SYN scan):

sudo nmap -sV -Pn -p 22,80 -vv --reason -oN ~/scan_results/versions/service_versions.nmap target_ip_addr

If you view the file that results, you may get information about the service running, depending on how “chatty” the service’s response is:

less ~/scan_results/versions/service_versions.nmap

~/scan_results/versions/service_versions.nmap

# Nmap 6.49BETA4 scan initiated Mon Dec 19 15:46:12 2022 as: nmap -sV -Pn -p 22,80 -vv --reason -oN /home/user/scan_results/versions/service_versions.nmap 198.51.100.15
Nmap scan report for 198.51.100.15
Host is up, received user-set (0.0011s latency).
Scanned at 2022-12-19 15:46:13 EDT for 8s
PORT   STATE SERVICE REASON         VERSION
22/tcp open  ssh     syn-ack ttl 63 OpenSSH 6.6.1p1 Ubuntu 2ubuntu2 (Ubuntu Linux; protocol 2.0)
80/tcp open  http    syn-ack ttl 63 nginx 1.4.6 (Ubuntu)
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

Read data files from: /usr/local/bin/../share/nmap
Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
# Nmap done at Mon Dec 19 15:46:21 2022 -- 1 IP address (1 host up) scanned in 8.81 seconds

Here, you can see that the test was able to identify the SSH server version and the Linux distribution that packaged it, as well as the SSH protocol version accepted. It also recognized the version of Nginx and again identified it as matching an Ubuntu package.

Discovering the Host Operating System #

You can try to have nmap guess the host operating system based on characteristics of its software and responses as well. This works in the same way as service versioning. Once again, we will omit the tcpdump run from this test, but you can perform it if you’d like.
The flag you need to perform operating system detection is -O (the capitalized letter “O”). A full command may look something like this:

sudo nmap -O -Pn -vv --reason -oN ~/scan_results/versions/os_version.nmap target_ip_addr

If you view the output file, you might see something like this:

less ~/scan_results/versions/os_version.nmap

~/scan_results/versions/os_versions.nmap

# Nmap 6.49BETA4 scan initiated Mon Dec 19 15:53:54 2022 as: nmap -O -Pn -vv --reason -oN /home/user/scan_results/versions/os_version.nmap 198.51.100.15
Increasing send delay for 198.51.100.15 from 0 to 5 due to 65 out of 215 dropped probes since last increase.
Increasing send delay for 198.51.100.15 from 5 to 10 due to 11 out of 36 dropped probes since last increase.
Increasing send delay for 198.51.100.15 from 10 to 20 due to 11 out of 35 dropped probes since last increase.
Increasing send delay for 198.51.100.15 from 20 to 40 due to 11 out of 29 dropped probes since last increase.
Increasing send delay for 198.51.100.15 from 40 to 80 due to 11 out of 31 dropped probes since last increase.
Nmap scan report for 198.51.100.15
Host is up, received user-set (0.0012s latency).
Scanned at 2022-12-19 15:53:54 EDT for 30s
Not shown: 998 closed ports
Reason: 998 resets
PORT   STATE SERVICE REASON
22/tcp open  ssh     syn-ack ttl 63
80/tcp open  http    syn-ack ttl 63
No exact OS matches for host (If you know what OS is running on it, see https://nmap.org/submit/ ).
TCP/IP fingerprint:
OS:SCAN(V=6.49BETA4%E=4%D=8/27%OT=22%CT=1%CU=40800%PV=N%DS=2%DC=I%G=Y%TM=55
OS:DF6AF0%P=x86_64-unknown-linux-gnu)SEQ(SP=F5%GCD=1%ISR=106%TI=Z%CI=Z%TS=8
OS:)OPS(O1=M5B4ST11NW8%O2=M5B4ST11NW8%O3=M5B4NNT11NW8%O4=M5B4ST11NW8%O5=M5B
OS:4ST11NW8%O6=M5B4ST11)WIN(W1=7120%W2=7120%W3=7120%W4=7120%W5=7120%W6=7120
OS:)ECN(R=Y%DF=Y%T=40%W=7210%O=M5B4NNSNW8%CC=Y%Q=)T1(R=Y%DF=Y%T=40%S=O%A=S+
OS:%F=AS%RD=0%Q=)T2(R=N)T3(R=N)T4(R=Y%DF=Y%T=40%W=0%S=A%A=Z%F=R%O=%RD=0%Q=)
OS:T5(R=Y%DF=Y%T=40%W=0%S=Z%A=S+%F=AR%O=%RD=0%Q=)T6(R=Y%DF=Y%T=40%W=0%S=A%A
OS:=Z%F=R%O=%RD=0%Q=)T7(R=N)U1(R=Y%DF=N%T=40%IPL=164%UN=0%RIPL=G%RID=G%RIPC
OS:K=G%RUCK=G%RUD=G)U1(R=N)IE(R=N)

Uptime guess: 1.057 days (since Mon Dec 12 14:32:23 2022)
Network Distance: 2 hops
TCP Sequence Prediction: Difficulty=245 (Good luck!)
IP ID Sequence Generation: All zeros

Read data files from: /usr/local/bin/../share/nmap
OS detection performed. Please report any incorrect results at https://nmap.org/submit/ .
# Nmap done at Mon Dec 12 15:54:24 2022 -- 1 IP address (1 host up) scanned in 30.94 seconds

We can see that in this case, nmap has no guesses for the operating system based on the signature it saw. If it had received more information, it would likely show various percentages which indicate how the target machine’s signature matches the operating system signatures in its databases. You can see the fingerprint signature that nmap received from the target below the TCP/IP fingerprint: line.
Operating system identification can help an attacker determine which exploits may be useful on the system. Configuring your firewall to respond to fewer inquiries can help to hinder the accuracy of some of these detection methods.

Conclusion #

Testing your firewall and building an awareness of what your internal network looks like to an outside attacker can help minimize your risk. The information you find from probing your own infrastructure may open up a conversation about whether any of your policy decisions need to be revisited in order to increase security. It may also illuminate any gaps in your security that may have occurred due to incorrect rule ordering or forgotten test policies. It is recommended that you test your policies with the latest scanning databases regularly to improve, or at least maintain, your current level of security.
To get an idea of some policy improvements for your firewall, check out this guide.