Understanding the 'tracepath' Command (with examples)

The tracepath command is a versatile network tool used to trace the path to a network host and discover the Maximum Transmission Unit (MTU) size along this path. Unlike its more well-known sibling traceroute, tracepath does not require superuser rights and can be particularly useful in network diagnostics, troubleshooting, and educational settings for seeing how data travels across the network. It helps users identify issues related to connectivity and routing by displaying each hop a packet takes to reach its destination.

Use case 1: Trace the path to a host with a preferred initial destination port

Code:

tracepath -p 33434 host

Motivation:

In some network environments, the default settings used by tracepath may be blocked by firewalls or other network policies, especially if non-standard ports are used for security reasons. In such cases, the -p option allows the user to specify an initial destination port that works within the existing network configuration.

Explanation:

• -p 33434: This argument specifies that the initial probe should use destination port 33434. This is useful when the default starting port needs to be overridden due to network restrictions or firewall configurations.

• host: Represents the target hostname or IP address you want to trace. This is the endpoint toward which the packets are sent.

Example Output:

1?: [LOCALHOST]                      pmtu 1500
1:  192.168.1.1                      0.507ms 
2:  10.0.0.1                         1.125ms
3:  172.16.0.1                       2.345ms
4:  no reply
5:  example.com                      12.231ms reached

Use case 2: Specify the initial destination port

Code:

tracepath -p destination_port host

Motivation:

Network environments with strict firewall settings often restrict traffic on specific ports. Specifying the destination port helps users circumvent these restrictions and allows for successful tracepath execution. This flexibility ensures that the command can reach its intended target without interruption due to security measures.

Explanation:

• -p destination_port: By selecting a specific destination port, users can tailor the command to work within networks that might block the default ports used by tracepath.

• host: The target host to which the user wants to trace the route is specified here. This can be either a domain name or an IP address.

Example Output:

1?: [LOCALHOST]                      pmtu 1400
1:  192.168.100.1                    0.798ms 
2:  no reply
3:  198.51.100.5                     3.987ms reached

Use case 3: Print both hostnames and numerical IP addresses

Code:

tracepath -b host

Motivation:

For network administrators and IT professionals, having both the hostname and IP address of each hop is crucial for diagnostics and comprehensive understanding of the network topology. This information can help in quickly identifying potential bottlenecks or misconfigurations.

Explanation:

• -b: This option directs tracepath to provide both the domain name (if resolved) and the corresponding IP address for each hop, offering more detailed insight than IP addresses alone.

• host: The hostname or IP address of the destination you are tracing.

Example Output:

1?: [LOCALHOST]                      pmtu 1500
1:  router.local (192.168.1.1)       0.321ms 
2:  isp-gateway.example.com (203.0.113.1) 2.876ms
3:  core-router.example.net (198.51.100.4) 4.238ms reached

Use case 4: Specify a maximum TTL (number of hops)

Code:

tracepath -m max_hops host

Motivation:

In long-distance or complex network routes, specifying a maximum Time To Live (TTL) can be extremely beneficial to avoid long waits and to focus only on a particular segment of the network. TTL effectively controls how far you want to trace the path, by limiting the number of hops.

Explanation:

• -m max_hops: This parameter sets the upper limit on how many hops tracepath should probe. This helps in focusing on a specific path section or reducing unnecessary data collection.

• host: Here, you enter the destination you need to trace route to.

Example Output:

1?: [LOCALHOST]                      pmtu 1500
1:  192.0.2.2                        1.118ms 
2:  198.51.100.3                     4.324ms reached

Use case 5: Specify the initial packet length

Code:

tracepath -l packet_length host

Motivation:

Different networks may require the manipulation of packet sizes for successful data transmission. By setting a custom packet length, users can test how networks handle specific packet sizes, which is beneficial for diagnosing issues related to packet fragmentation or MTU mismatches.

Explanation:

• -l packet_length: This sets the initial packet size which defaults to 65535 for IPv4 and 128000 for IPv6, but can be modified for diagnostic purposes.

• host: This specifies the destination host or IP address.

Example Output:

1?: [LOCALHOST]                      pmtu 1500
1:  203.0.113.2                      0.456ms 
2:  no reply 
3:  203.0.113.34                     7.562ms reached

Use case 6: Use only IPv6 addresses

Code:

tracepath -6 host

Motivation:

With the growing adoption of IPv6, it’s essential to diagnose and troubleshoot IPv6 networks specifically. The -6 flag ensures that tracepath uses IPv6 addresses, making it valuable for scenarios where IPv6 connectivity and performance are under evaluation.

Explanation:

• -6: This flag restricts tracepath to utilize only IPv6 addresses, ensuring that the command works exclusively over IPv6 networks.

• host: The target host which needs to be checked for its IPv6 route.

Example Output:

1?: [LOCALHOST]                      pmtu 1492
1: 2001:db8::1                       0.673ms 
2: 2001:db8::2                       8.214ms reached

Conclusion:

The tracepath command offers a rich set of options for diagnosing and troubleshooting network paths, uncovering not just the path but also possible MTU issues. By varying parameters such as ports, packet lengths, and the number of hops, users can tailor the command to fit the specific needs of their network environment, whether working within IPv4 or IPv6. Through these examples, users can develop a deeper understanding of network routing and connectivity, and optimize performance while pinpointing potential issues efficiently.