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Use the utility that matches the layer you need to test: ping for ICMP reachability and latency, traceroute for hop-by-hop clues, tracepath for path MTU, mtr for repeated loss and latency, nc for a service port, and iperf for throughput between two endpoints. No single command proves that an application is healthy. Run the checks in that order, record the address family and interface, and interpret results at the correct network layer.
Choose the test before you run a command
| Question | First choice | What it tells you | Important limitation |
|---|---|---|---|
| Does the host answer ICMP, and what is the round-trip time? | ping |
Packet loss and minimum, average and maximum RTT | ICMP may be filtered or deprioritized |
| Where does the route change or appear to stop? | traceroute |
Apparent hop progression | Probe filtering can hide hops |
| Could fragmentation or an unusual MTU be involved? | tracepath |
Path maximum-transmission unit (PMTU) | Router error reporting can be incomplete |
| Is loss or latency intermittent along the route? | mtr |
Repeated per-hop response statistics | Intermediate ICMP rate limiting can look like loss |
| Does a TCP or UDP service port respond? | nc |
Port-level reachability and, for TCP, a completed handshake | UDP has no handshake; an open port is not application health |
| How much capacity can two controlled endpoints sustain? | iperf |
TCP or UDP throughput | Needs a cooperating server and generates traffic |
Install the commands from your distribution’s packages if they are missing. Package names vary: ping, traceroute, tracepath and mtr are commonly supplied by separate packages; nc may be packaged as netcat; and iperf versions differ between distributions. Run command -v ping traceroute tracepath mtr nc iperf to see what is already available.
1. ping: basic reachability, latency and loss
ping sends ICMP Echo Requests and measures Echo Responses. It is a fast first signal, not a complete service test.
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ping -4 -c 4 192.0.2.10
ping -6 -c 4 2001:db8::10-c 4 limits the test to four packets; -4 and -6 force IPv4 or IPv6. The summary reports transmitted and received packets, packet loss, and minimum/average/maximum round-trip time. Save the destination name, resolved address, interface, packet count and timestamp with the output.
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How to read a timeout
A timeout can indicate filtering, a routing failure, a down host, or a host that declines ICMP. It does not prove that HTTPS, SSH or another application port is unavailable. Conversely, a successful ping proves only that ICMP replies returned; the application can still be stopped or blocked.
Use bounded tests in scripts and avoid careless continuous probing. The Linux manual cautions that ping can impose load and is unsuitable for normal operations or automated scripts when used without care.
2. traceroute: inspect hop progression
traceroute sends probes with increasing TTL values and displays the apparent path toward a destination.
traceroute example.com
traceroute -4 example.com
traceroute -6 example.comCompare the final destination response with end-to-end behavior. An intermediate router may not answer the selected probe protocol or may rate-limit its replies. Therefore * * * at one hop is not automatically a broken link. If later hops and the destination respond normally, that silent router is usually only hiding its control-plane response.
3. tracepath: discover path MTU without root
tracepath traces the route while discovering the path maximum-transmission unit. It normally does not require superuser privileges, making it useful on managed workstations and servers where you cannot run privileged diagnostics.
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tracepath example.com
tracepath -6 example.comUse it when a VPN, tunnel or unusual link produces “packet too big” messages, stalled large transfers, or symptoms that disappear when packets are small. Record the reported PMTU and whether IPv4 or IPv6 was tested. A missing router error does not necessarily mean the path has no MTU issue; network devices can omit diagnostic responses.
4. mtr: repeated path, latency and loss
mtr combines traceroute and ping by repeatedly probing routers with low TTL values and reporting response percentage and timing for each hop.
mtr example.com
mtr --report --report-cycles 10 example.com
mtr --report --no-dns example.comInteractive mode helps you watch changes; report mode creates a fixed sample suitable for incident notes. --no-dns avoids reverse-DNS delays and keeps reports focused on addresses.
Interpret downstream patterns, not one red number
A sudden increase in loss or response time that continues through every subsequent hop and the destination is evidence of a congested or faulty segment. Loss at one hop that disappears at the next hops is commonly ICMP control-plane rate limiting rather than forwarding loss. Check both the percentage and the destination row before escalating a router.
5. nc (netcat): test a service port
nc can open TCP connections, send UDP datagrams, listen on ports and perform network-daemon tests.
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nc -vz -w 3 example.com 443
nc -vz -w 3 192.0.2.10 22
nc -u -vz -w 3 192.0.2.10 53-v is verbose, -z probes without sending application data, and -w 3 limits the wait to three seconds. A successful TCP probe demonstrates reachability to that port and a completed TCP handshake, which is closer to an application check than ICMP. It does not verify TLS, authentication or the application’s response.
UDP is different: there is no handshake. A timeout can mean a closed or filtered port, or simply that the service does not send a reply to an empty datagram. Interpret UDP results with the service’s protocol and firewall policy. Use only authorized hosts and ports.
6. iperf: measure controlled throughput
iperf measures TCP or UDP throughput. One endpoint must run a server that receives or discards test traffic while the other runs the client.
# Endpoint A
iperf -s
# Endpoint B
iperf -c SERVER_IP -t 30
iperf -c SERVER_IP -u -b 100M -t 30The first client command measures TCP for 30 seconds. The UDP example requests a 100-Mbit/s sending rate for 30 seconds; UDP tests can generate substantial traffic, so choose a rate and duration that your network and change policy allow. Run tests in both directions when diagnosing an asymmetric link, and test at representative times. iperf measures the path between the two controlled endpoints, not the speed of an arbitrary public website.
A layered workflow for a real incident
- Define the endpoint. Record the hostname, resolved IPv4/IPv6 address, port, interface, VPN state and timestamp. Test the same address family consistently.
- Check ICMP. Run a bounded
pingand note loss and RTT. If it fails, continue: ICMP filtering is common. - Check the route. Use
traceroute; usemtr --report --report-cycles 10when you need repeated evidence. - Check MTU. Run
tracepathwhen tunnels, fragmentation or packet-size symptoms are present. - Check the actual port. Use
ncagainst the service’s TCP or UDP port. A failed port test is more relevant to the application than a failed ping. - Measure capacity only with permission. Use
iperfbetween endpoints you control, document the test rate and stop if it affects users.
Common failures and fixes
“command not found”
Install the distribution package that provides the utility, then rerun command -v. Do not assume a package name or option set from another distribution.
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Ping fails but a website works
ICMP is probably filtered or deprioritized. Test the website’s TCP port with nc, then perform an application-level request appropriate to that service.
Traceroute ends with asterisks
Probe replies may be filtered or rate-limited. Compare later hops and the destination; try the other address family rather than declaring the first silent hop faulty.
mtr shows loss at one router
Look for the same loss on every later hop. If downstream hops are clean, treat the isolated percentage as likely response prioritization, not forwarding loss.
TCP connects but the service still fails
The port is reachable, but TLS negotiation, credentials, protocol framing, virtual-host selection or application health may be wrong. Follow the port test with the service’s own client or health check.
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UDP appears to time out
UDP has no handshake and an empty probe may receive no response. Confirm the expected reply, server configuration and firewall rules before concluding that the port is closed.
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iperf results are unexpectedly low
Verify that the server is listening, test the reverse direction, check CPU and interface counters, and compare TCP with a deliberately chosen UDP rate. Confirm that the test is not traversing an unintended VPN, Wi-Fi segment or traffic shaper.
Large transfers stall while small pings work
Investigate PMTU with tracepath, especially across VPNs and tunnels. A successful small ICMP exchange does not prove that larger packets traverse without fragmentation problems.
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Frequently Asked Questions
Can I test a host that blocks ICMP?
Yes. Skip to the service’s protocol: use nc for a TCP port, the service’s own client for an application check, and traceroute or mtr for route clues. A blocked Echo Request does not establish that every service is blocked.
Should I use IPv4 and IPv6 separately?
Yes. DNS can return both families with different routes and failures. Force -4 or -6 where supported and record which address was tested.
Is iperf safe to run across the public internet?
Only with authorization and careful limits. It requires a cooperating server and can create sustained TCP or UDP traffic; LAN, VPN and site-to-site tests under your control are the intended use.
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