Troubleshoot Nmap Scans

Introduction

In this lab, you will learn how to troubleshoot Nmap scans using packet tracing and debugging techniques. The lab guides you through running Nmap scans with the --packet-trace option to observe network packets, enabling debug level 2 with -d2, and combining tracing and debugging.

You will also learn how to save trace output to a file using -oN and analyze packet details within the Xfce terminal to identify scan issues. The lab utilizes commands such as nmap --packet-trace 192.168.1.1, nmap -d2 127.0.0.1, and nmap --packet-trace -oN trace.txt 127.0.0.1 to provide hands-on experience in diagnosing Nmap scan problems.

Skills Graph

%%%%{init: {'theme':'neutral'}}%%%% flowchart RL nmap(("Nmap")) -.-> nmap/NmapGroup(["Nmap"]) nmap/NmapGroup -.-> nmap/output_formats("Output Formats") nmap/NmapGroup -.-> nmap/save_output("Save Output to File") nmap/NmapGroup -.-> nmap/port_scanning("Port Scanning Methods") nmap/NmapGroup -.-> nmap/target_specification("Target Specification") nmap/NmapGroup -.-> nmap/verbosity("Verbosity Levels") nmap/NmapGroup -.-> nmap/service_detection("Service Detection") nmap/NmapGroup -.-> nmap/stealth_scanning("Stealth and Covert Scanning") subgraph Lab Skills nmap/output_formats -.-> lab-547117{{"Troubleshoot Nmap Scans"}} nmap/save_output -.-> lab-547117{{"Troubleshoot Nmap Scans"}} nmap/port_scanning -.-> lab-547117{{"Troubleshoot Nmap Scans"}} nmap/target_specification -.-> lab-547117{{"Troubleshoot Nmap Scans"}} nmap/verbosity -.-> lab-547117{{"Troubleshoot Nmap Scans"}} nmap/service_detection -.-> lab-547117{{"Troubleshoot Nmap Scans"}} nmap/stealth_scanning -.-> lab-547117{{"Troubleshoot Nmap Scans"}} end

Run scan with packet trace using nmap --packet-trace 192.168.1.1

In this step, we will use the --packet-trace option with Nmap to observe the packets being sent and received during a scan. This is a powerful debugging tool that allows you to understand exactly what Nmap is doing at the network level.

Before we begin, let's briefly discuss what packet tracing is. Packet tracing involves capturing and analyzing network packets to understand the communication between two or more devices. Nmap's --packet-trace option provides a simplified way to view these packets directly from within Nmap.

To perform a scan with packet tracing, follow these steps:

Open a terminal: If you don't already have one open, launch the Xfce terminal.
Execute the Nmap command: In the terminal, type the following command and press Enter:
```
sudo nmap --packet-trace 192.168.1.1
```
- sudo: This command requires root privileges to capture network packets.
- nmap: This is the command-line Nmap scanner.
- --packet-trace: This option tells Nmap to print a trace of the packets it sends and receives.
- 192.168.1.1: This is the target IP address. If this IP address is not available on your network, you can replace it with the IP address of a device that is accessible from your LabEx VM. A common alternative is 127.0.0.1 (localhost).
Observe the output: Nmap will now perform a scan of the target IP address and print detailed information about each packet sent and received. The output will show the type of packet (e.g., SYN, ACK), the source and destination IP addresses and ports, and other relevant information.

The output will look similar to this (the exact output will vary depending on your network and the target host):
```
Starting Nmap 7.80 ( https://nmap.org ) at 2023-10-27 10:00 UTC
SENT (0.0034s) to 192.168.1.1: TCP 192.168.1.100:54321 > 192.168.1.1:80 S ttl=64 id=12345 win=65535
RECV (0.0045s) from 192.168.1.1: TCP 192.168.1.1:80 > 192.168.1.100:54321 SA ttl=64 id=67890 win=14600
SENT (0.0045s) to 192.168.1.1: TCP 192.168.1.100:54321 > 192.168.1.1:80 A ttl=64 id=12346 win=65535
...
Nmap done: 1 IP address (1 host up) scanned in 0.123 seconds
```
Each line represents a packet. SENT indicates a packet sent by Nmap, and RECV indicates a packet received by Nmap. The information following SENT or RECV describes the packet's protocol (TCP), source and destination IP addresses and ports, flags (S for SYN, A for ACK, SA for SYN-ACK), TTL (Time To Live), ID, and window size.
Interrupt the scan: The scan will continue for a while. You can interrupt it by pressing Ctrl+C.

This step has demonstrated how to use the --packet-trace option to gain insight into Nmap's network activity. In the following steps, we will explore other debugging options and techniques.

Enable debug level 2 with nmap -d2 127.0.0.1

In this step, we will enable debug level 2 in Nmap. Debugging output provides detailed information about Nmap's internal operations, which can be helpful for understanding how Nmap works and troubleshooting issues.

Nmap's debugging levels range from 1 to 9. Higher levels provide more detailed output. For this step, we will use debug level 2, which provides a good balance between detail and readability.

To enable debug level 2, follow these steps:

Open a terminal: If you don't already have one open, launch the Xfce terminal.
Execute the Nmap command: In the terminal, type the following command and press Enter:
```
sudo nmap -d2 127.0.0.1
```
- sudo: This command might require root privileges depending on the scan type.
- nmap: This is the command-line Nmap scanner.
- -d2: This option sets the debug level to 2.
- 127.0.0.1: This is the target IP address, which is the loopback address (localhost).
Observe the output: Nmap will now perform a scan of localhost and print debugging information to the terminal. The output will include details about Nmap's configuration, the probes it is sending, and the responses it is receiving.

The output will look similar to this (the exact output will vary):
```
Starting Nmap 7.80 ( https://nmap.org ) at 2023-10-27 10:05 UTC
--------------- Timing report ---------------
  hostgroups: min 1, max 1024
  rtt-timeouts: init 1250, min 100, max 10000
  max-scan-delay: 0 ms, brtt 1250 ms, rttvar 625 ms
  parallelism: min 1, max 256
---------------------------------------------
...
Nmap done: 1 IP address (1 host up) scanned in 0.123 seconds
```
The output shows various internal parameters and decisions made by Nmap during the scan. This level of detail can be useful for understanding how Nmap is operating and for diagnosing problems.
Interrupt the scan: The scan will continue for a while. You can interrupt it by pressing Ctrl+C.

This step has demonstrated how to enable debug level 2 in Nmap. In the next step, we will combine packet tracing and debugging output for even more detailed analysis.

Combine trace and debug with nmap --packet-trace -d 192.168.1.1

In this step, we will combine the --packet-trace and -d options to get a comprehensive view of Nmap's operation. This will provide both packet-level details and internal debugging information, which can be very useful for advanced troubleshooting and understanding Nmap's behavior.

To combine packet tracing and debugging, follow these steps:

Open a terminal: If you don't already have one open, launch the Xfce terminal.
Execute the Nmap command: In the terminal, type the following command and press Enter:
```
sudo nmap --packet-trace -d 192.168.1.1
```
- sudo: This command requires root privileges to capture network packets.
- nmap: This is the command-line Nmap scanner.
- --packet-trace: This option tells Nmap to print a trace of the packets it sends and receives.
- -d: This option enables debugging output. By default, it sets the debug level to 1. You can use -d2, -d3, etc., to increase the debug level. In this case, we are using the default level 1.
- 192.168.1.1: This is the target IP address. If this IP address is not available on your network, you can replace it with the IP address of a device that is accessible from your LabEx VM. A common alternative is 127.0.0.1 (localhost).
Observe the output: Nmap will now perform a scan of the target IP address and print both packet traces and debugging information to the terminal. The output will be a mix of packet details (as seen in step 1) and internal Nmap messages (as seen in step 2).

The output will look similar to this (the exact output will vary depending on your network and the target host):
```
Starting Nmap 7.80 ( https://nmap.org ) at 2023-10-27 10:10 UTC
SENT (0.0034s) to 192.168.1.1: TCP 192.168.1.100:54321 > 192.168.1.1:80 S ttl=64 id=12345 win=65535
RECV (0.0045s) from 192.168.1.1: TCP 192.168.1.1:80 > 192.168.1.100:54321 SA ttl=64 id=67890 win=14600
NSE: Using Lua engine for script processing.
SENT (0.0045s) to 192.168.1.1: TCP 192.168.1.100:54321 > 192.168.1.1:80 A ttl=64 id=12346 win=65535
...
Nmap done: 1 IP address (1 host up) scanned in 0.123 seconds
```
You will see the SENT and RECV lines interleaved with debugging messages like NSE: Using Lua engine for script processing..
Interrupt the scan: The scan will continue for a while. You can interrupt it by pressing Ctrl+C.

This step has demonstrated how to combine packet tracing and debugging output in Nmap. This combination provides a powerful tool for understanding and troubleshooting Nmap scans.

Save trace output with nmap --packet-trace -oN trace.txt 127.0.0.1

In this step, we will save the packet trace output to a file for later analysis. This is useful when you want to examine the packet details more closely or share the output with others. Nmap provides several options for saving scan results in different formats. Here, we'll use the normal output format (-oN) to save the packet trace.

To save the packet trace output to a file, follow these steps:

Open a terminal: If you don't already have one open, launch the Xfce terminal.
Execute the Nmap command: In the terminal, type the following command and press Enter:
```
sudo nmap --packet-trace -oN trace.txt 127.0.0.1
```
- sudo: This command might require root privileges depending on the scan type.
- nmap: This is the command-line Nmap scanner.
- --packet-trace: This option tells Nmap to print a trace of the packets it sends and receives.
- -oN trace.txt: This option specifies that the output should be saved in normal format to the file trace.txt. The file will be created in your current directory (~/project).
- 127.0.0.1: This is the target IP address, which is the loopback address (localhost).

Observe the output: Nmap will perform a scan of localhost and print the packet trace to the terminal, as well as save it to the file trace.txt.

The terminal output will be similar to this:

Starting Nmap 7.80 ( https://nmap.org ) at 2023-10-27 10:15 UTC
SENT (0.0034s) to 127.0.0.1: TCP 127.0.0.1:54321 > 127.0.0.1:80 S ttl=64 id=12345 win=65535
RECV (0.0045s) from 127.0.0.1: TCP 127.0.0.1:80 > 127.0.0.1:54321 SA ttl=64 id=67890 win=14600
SENT (0.0045s) to 127.0.0.1: TCP 127.0.0.1:54321 > 127.0.0.1:80 A ttl=64 id=12346 win=65535
...
Nmap done: 1 IP address (1 host up) scanned in 0.123 seconds

Verify the file creation: After the scan is complete, verify that the file trace.txt has been created in your ~/project directory. You can use the ls command to check:
```
ls ~/project
```
You should see trace.txt in the output.
View the file content: You can view the content of the trace.txt file using the cat command or a text editor like nano:
```
cat ~/project/trace.txt
```
or
```
nano ~/project/trace.txt
```
The file will contain the same packet trace output that was printed to the terminal.

This step has demonstrated how to save Nmap's packet trace output to a file. In the next step, we will analyze the packet details in the Xfce terminal.

Analyze packet details in Xfce terminal

In this step, we will analyze the packet details captured by Nmap's --packet-trace option. Understanding the packet trace output can help you diagnose network issues, understand how Nmap probes work, and identify potential security vulnerabilities. We'll use the grep command in the Xfce terminal to filter and examine specific packets.

To analyze the packet details, follow these steps:

Open a terminal: If you don't already have one open, launch the Xfce terminal.
Review the trace.txt file: In the previous step, you saved the packet trace output to the file trace.txt in your ~/project directory. If you haven't done that step, please go back and complete it first.
Use grep to filter packets: The grep command is a powerful tool for searching text files. We can use it to filter the trace.txt file and find packets that match specific criteria.

For example, to find all packets sent to port 80, you can use the following command:
```
grep " > 127.0.0.1:80" ~/project/trace.txt
```
- grep: This is the command-line utility for searching text.
- " > 127.0.0.1:80": This is the search pattern. It looks for lines that contain " > 127.0.0.1:80", which indicates packets sent to port 80 on the localhost (127.0.0.1). The space before > is important to avoid matching source ports.
- ~/project/trace.txt: This is the path to the file containing the packet trace output.
The output will show all lines in the trace.txt file that match the search pattern. For example:
```
SENT (0.0034s) to 127.0.0.1: TCP 127.0.0.1:54321 > 127.0.0.1:80 S ttl=64 id=12345 win=65535
RECV (0.0045s) from 127.0.0.1: TCP 127.0.0.1:80 > 127.0.0.1:54321 SA ttl=64 id=67890 win=14600
SENT (0.0045s) to 127.0.0.1: TCP 127.0.0.1:54321 > 127.0.0.1:80 A ttl=64 id=12346 win=65535
```
Analyze the packet details: Examine the output to understand the packet details. The output shows the direction of the packet (SENT or RECV), the source and destination IP addresses and ports, the protocol (TCP), and other flags and options.
- SENT: Indicates a packet sent by Nmap.
- RECV: Indicates a packet received by Nmap.
- TCP 127.0.0.1:54321 > 127.0.0.1:80: Shows the source IP address and port (127.0.0.1:54321) and the destination IP address and port (127.0.0.1:80).
- S: Indicates the SYN flag is set, which is used to initiate a TCP connection.
- SA: Indicates the SYN-ACK flags are set, which is a response to a SYN packet.
- A: Indicates the ACK flag is set, which is used to acknowledge received data.
- ttl: Time to live.
- id: IP identification number.
- win: Window size.
Experiment with different grep patterns: Try different grep patterns to filter the packet trace output based on your specific needs. For example:
- To find all SYN packets:
```
grep "S ttl=" ~/project/trace.txt
```
- To find all packets received from port 80:
```
grep "from 127.0.0.1:80" ~/project/trace.txt
```
- To find all packets with a specific IP ID:
```
grep "id=12345" ~/project/trace.txt
```

By using grep to filter and analyze the packet trace output, you can gain a deeper understanding of Nmap's behavior and the network traffic it generates.

Identify scan issues in Xfce terminal

In this step, we will use the packet trace and debug output from Nmap to identify potential scan issues. This involves looking for anomalies in the packet exchange, such as retransmissions, unexpected responses, or lack of responses. By analyzing these issues, you can troubleshoot scan problems and improve the accuracy and reliability of your Nmap scans.

To identify scan issues, follow these steps:

Open a terminal: If you don't already have one open, launch the Xfce terminal.
Review the trace.txt file: In the previous steps, you saved the packet trace output to the file trace.txt in your ~/project directory. Ensure this file exists and contains the packet trace information.
Look for retransmissions: Retransmissions occur when a packet is not acknowledged by the target host, and Nmap resends the packet. This can indicate network congestion, packet loss, or a firewall blocking the traffic. Use grep to search for retransmissions:
```
grep "Retransmission" ~/project/trace.txt
```
If you see lines containing "Retransmission", it indicates that Nmap had to resend packets. This could be a sign of network issues or a firewall.
Analyze TCP flags: Examine the TCP flags in the packet trace to understand the state of the TCP connection. Look for unexpected flag combinations or missing flags. For example, a SYN packet without a corresponding SYN-ACK response could indicate a firewall blocking the connection.

You can use grep to filter for specific TCP flags. For example, to find SYN packets:
```
grep "S ttl=" ~/project/trace.txt
```
And to find SYN-ACK packets:
```
grep "SA ttl=" ~/project/trace.txt
```
Compare the number of SYN packets sent with the number of SYN-ACK packets received. If there are significantly fewer SYN-ACK packets than SYN packets, it could indicate a problem.
Check for unexpected responses: Sometimes, the target host may send unexpected responses, such as ICMP error messages. These responses can provide valuable information about why a scan is failing.

To look for ICMP packets, use the following command:
```
grep "ICMP" ~/project/trace.txt
```
Analyze the ICMP messages to understand the nature of the error. For example, "Destination Unreachable" indicates that the target host is not reachable.
Look for missing responses: If Nmap sends a probe and does not receive a response, it could indicate that the target host is down, a firewall is blocking the traffic, or the probe was lost in transit.

Examine the packet trace to identify probes that did not receive a response. For example, if you sent a SYN packet to port 80 but did not receive a SYN-ACK packet, it could indicate that port 80 is closed or filtered.
Consider Debug Output: While the previous steps focused on the packet trace, remember that the debug output (from -d or -d2) can also provide valuable clues. It often contains information about Nmap's internal decision-making process, which can help you understand why it's behaving in a certain way. Unfortunately, we didn't save the debug output to a file in previous steps, but in a real-world scenario, you would analyze that output alongside the packet trace.

By carefully analyzing the packet trace and debug output, you can identify potential scan issues and take steps to resolve them. This will improve the accuracy and reliability of your Nmap scans.

Summary

In this lab, we explored Nmap's debugging capabilities by using the --packet-trace option to observe the network packets exchanged during a scan. This involved executing the command sudo nmap --packet-trace 192.168.1.1 (or 127.0.0.1 as an alternative) in the Xfce terminal and analyzing the detailed output, which included packet types, source and destination IP addresses and ports, and other relevant information.

The key takeaway is understanding how --packet-trace provides a simplified way to view network packets directly from within Nmap, enabling a deeper understanding of Nmap's network-level operations and facilitating the identification of potential scan issues.