Introduction
Traceroute is a network diagnostic tool used to trace the route taken by packets from a source to a destination. It provides valuable insights into the network path and helps identify potential bottlenecks or connectivity issues. When analyzing the output of a traceroute command, you may encounter instances where the output displays asterisks (* * *) instead of IP addresses. In this article, we will explore the meaning of * * * in the traceroute output, its significance, and how to interpret it correctly.
1. Understanding Traceroute
Before diving into the meaning of * * *, let’s briefly understand how traceroute works:
Traceroute sends a series of packets with incrementing TTL (Time-To-Live) values. Each router along the path decrements the TTL of the packet by 1 before forwarding it. When the TTL becomes 0, the router discards the packet and sends an ICMP Time Exceeded message back to the source. Traceroute records the IP address of the router that generated the Time Exceeded message.
2. Significance of * * *
In the output of traceroute, asterisks (* * *) indicate that the ICMP Time Exceeded message was not received within the specified timeout for a particular TTL value. This can happen for several reasons:
2.1. Router Firewall Filtering
Some routers are configured to block or filter ICMP traffic, including Time Exceeded messages. As a result, traceroute packets may not receive a response from these routers, leading to * * * in the output.
2.2. Network Congestion
Network congestion or high traffic can cause delays in routers processing traceroute packets. If a router is overloaded, it might not respond to ICMP Time Exceeded messages in a timely manner, resulting in * * *.
2.3. ICMP Rate Limiting
To prevent abuse or flooding, routers may limit the rate at which they generate ICMP Time Exceeded messages. If the rate limit is exceeded, some packets might not receive a response, leading to * * *.
2.4. Firewall Rules or ACLs
Firewalls or Access Control Lists (ACLs) configured on routers can also affect the generation of ICMP Time Exceeded messages. If the firewall rules prevent the generation of such messages, traceroute packets may not receive responses.
3. Interpreting * * * in Traceroute Output
When you encounter * * * in the traceroute output, it’s important to consider the context:
- Middle Hops: If * * * appears in the middle of the traceroute output, it suggests that one or more routers along the path are not responding to ICMP Time Exceeded messages. This could indicate network congestion, firewall filtering, or other issues on those routers.
- Final Hop: If the last hop shows * * *, it might suggest that the destination host is not responding to the traceroute packets or is configured to block ICMP traffic.
4. Code Example: Using Traceroute in Python
To illustrate the concept of * * *, let’s use the scapy library in Python to perform a basic traceroute operation.
Install scapy library (if not already installed):
pip install scapyCode Example:
from scapy.all import *
def traceroute(destination):
ttl = 1
max_hops = 30
while ttl <= max_hops:
packet = IP(dst=destination, ttl=ttl) / ICMP()
reply = sr1(packet, verbose=0, timeout=2)
if reply is None:
print(f"{ttl}: * * *")
else:
print(f"{ttl}: {reply.src}")
ttl += 1
if reply and reply.src == destination:
break
# Replace with your destination host
destination_host = "example.com"
traceroute(destination_host)6. Troubleshooting with * * *
When encountering * * * in the traceroute output, consider the following troubleshooting steps:
6.1. Retrying Traceroute
Sometimes, a single * * * might not necessarily indicate a problem. Retrying the traceroute multiple times can help determine if the lack of response is consistent or intermittent.
6.2. Compare with Other Tools
Using alternative network diagnostic tools, such as ping or online traceroute services, can provide additional insights into the network path and confirm whether the lack of response is specific to traceroute.
6.3. Network Equipment Configuration
Check the configuration of routers along the path. Ensure that routers are not explicitly configured to block ICMP Time Exceeded messages and that firewall rules or ACLs are not preventing responses.
6.4. Firewall and Security Settings
Review the destination host’s firewall and security settings. Ensure that the host is not configured to block ICMP traffic, which could result in * * * for the final hop.
7. Advanced Traceroute Options
Traceroute offers various options to customize the behavior and improve diagnostic accuracy:
- Increasing Timeout: Adjust the timeout value for receiving responses. This might allow more time for routers to generate ICMP Time Exceeded messages.
- Specific ICMP Types: Some traceroute implementations allow you to specify a different type of ICMP message to use, which could have different effects on routers’ responses.
- TCP and UDP Traceroute: Instead of ICMP, you can perform traceroute using TCP or UDP packets, which might yield different results.
8. Limitations and Considerations
It’s important to note that the use of * * * in traceroute output does not necessarily indicate a critical issue. It is a common occurrence in complex and diverse network environments. Traceroute provides valuable information, but it may not always be able to provide a complete picture of the network path due to various factors.
9. Real-World Use Case
Imagine a scenario where a network administrator is troubleshooting connectivity issues to a remote server. By using traceroute and observing * * * at certain hops, the administrator can identify routers that are not responding to ICMP Time Exceeded messages. This information can help narrow down the problematic segment of the network path and guide further investigation.
Conclusion
The appearance of * * * in the traceroute output signifies that a router along the network path did not respond within the timeout for an ICMP Time Exceeded message. This can occur due to various factors, including firewall filtering, network congestion, rate limiting, or configuration issues. By understanding the meaning of * * * and applying appropriate troubleshooting techniques, network administrators and engineers can effectively diagnose connectivity problems, optimize network paths, and ensure reliable data transmission in complex network environments.
