Which client can be used to determine if a firewall is blocking traffic?

Using a traceroute is an effective method to determine if a firewall is blocking traffic because it maps the path that packets take from the source to the destination and can reveal where the connection fails. By observing the series of hops that packets take, a user can identify if the traffic is being blocked at a specific point, often indicated by a timeout or lack of response from certain intermediate devices.

This tool operates by sending packets with incremental Time-to-Live (TTL) values, allowing the user to see where the traffic stops responding. If the packets are unable to reach the destination and the traceroute displays a timeout at a certain hop, it may indicate that a firewall or similar security device is filtering or blocking the traffic at that point in the network.

While other tools listed such as sendmail, nslookup, and telnet have their purposes, they do not serve the same function as traceroute in diagnosing network paths or identifying specific blocking issues across the network infrastructure. Sendmail is primarily used for sending emails, nslookup is focused on querying DNS records, and telnet allows for basic text-based communication with devices but does not map routes or reveal blocking specifically. Thus, traceroute stands out as the best choice for diagnosing potential firewall issues.