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A timeout, a traceroute asterisk, or a TCP retransmission can point to a delivery problem—but none proves that your virtual dedicated server (VDS) is dropping packets, let alone where. To locate a suspected drop, capture the same traffic at two controlled points and compare its direction, timestamps, and TCP sequence and acknowledgment behavior. A single capture shows only what was visible at that capture point.
What counts as evidence of packet loss?
Start with the symptom you actually observed: an application timeout, a slow transfer, a retransmission, or a probe that received no reply. These are different observations. Call the issue suspected packet loss until evidence shows where a packet stops being visible.
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Linux packet captures can show endpoints, ports, TCP flags, sequence and acknowledgment numbers, and packet lengths. But a capture establishes visibility only at the interface where it was taken, subject to its filter and capture conditions. It cannot show what happened farther along the path. See the tcpdump(8) manual.
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What each diagnostic can—and cannot—tell you
| Evidence | What it can establish | What it cannot establish alone |
|---|---|---|
| tcpdump at one interface | Whether matching packets were visible at that interface during the capture, plus packet details useful for identifying the flow and TCP progression. | What happened beyond that capture point. |
| Captures at two controlled points | Whether matching packets are visible at one point and absent at the other, narrowing the interval where visibility changes. | The exact device or owner responsible inside an unobserved interval. |
| traceroute | Which probes received hop responses and their reported round-trip behavior. | That a non-responding hop is dropping ordinary transit traffic; ICMP replies may be filtered or throttled. |
ss |
Socket state and timer details, including retransmission-related information. | The network location responsible for retransmissions. |
| TCP behavior | That TCP is retransmitting to support reliable, ordered delivery. | A packet’s exact route or the segment where it disappeared. |
The traceroute(8) manual explains that probes can elicit ICMP time-exceeded replies from gateways, while missing replies may reflect filtering or rate limiting. The ss(8) manual documents socket and timer information; the tcp(7) manual describes Linux TCP behavior and retransmissions. Neither a retransmission nor a missing hop reply locates a fault by itself.
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A guest-visible workflow for narrowing the fault
- Describe the symptom and time window. Record whether you saw an application timeout, retransmission, slow transfer, or probe timeout, and when it occurred. Do not label it a packet drop yet.
- Identify the affected flow. Record source and destination addresses, address family, protocol, port, direction, and the time window. Check that your eventual capture filter includes the traffic you need.
- Inspect the socket. For a Linux TCP connection, run
ss -tinand record relevant socket state and retransmission-related timer information. This documents a symptom; it does not identify where along the path it arose. - Probe the actual destination. For example,
traceroute -T -p 443 example.comsends TCP SYN probes to port 443. Replace the destination and port with values appropriate to the affected service. Note which hops respond and whether later hops respond after an asterisk; a missing response alone is not proof of forwarding loss. Consult the traceroute(8) manual for probe options and caveats. - Capture the relevant traffic on the VDS. An illustrative command is
tcpdump -ni eth0 -s 0 -tttt 'host 203.0.113.10 and tcp port 443'. Replaceeth0, the host address, and the protocol or port with values for your environment. The filter must match the affected flow; retain timestamps and capture during the symptom. The tcpdump(8) manual documents capture filters and packet output. Capture privileges may be required by your system configuration. - Compare a second controlled observation point, if available. Capture the same flow at the destination host or an adjacent network device. Align timestamps, then compare direction, endpoints, ports, TCP sequence and acknowledgment behavior, and packet presence. If a packet is visible at the first point but not the second, the evidence narrows the interval between them; it does not identify a particular router without more observations.
- Escalate an unobservable interval. If you can capture only inside the guest and the suspected boundary is outside it, request provider-side observations for the relevant flow and time window. Guest-visible manuals do not establish access to a provider’s hypervisor, virtual switch, or host-side packet telemetry.
How to make the comparison trustworthy
Matching captures are useful only when they refer to the same traffic and compatible times. Before interpreting a difference, check these points:
- Observation point: Document the interface or device where each capture was taken.
- Direction: Distinguish packets sent from the VDS from packets arriving at it; compare the same direction at both points.
- Flow and protocol: Match addresses, ports, protocol, and address family. A filter that excludes the relevant packets can make traffic appear absent.
- Time: Preserve timestamps and account for clock differences between capture points.
- Packet progression: Use flow identity and TCP sequence and acknowledgment behavior to determine whether observations correspond to the same exchange.
- Visibility boundary: State plainly which parts of the path you can observe and which require provider or remote-host evidence.
Keep captures scoped to the relevant host, protocol, and port, and run them during the symptom. The example commands are illustrative rather than tested against a particular VDS; verify installed tool versions, options, interface names, and permissions on your Linux distribution.
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What to send your provider
If the evidence points to a segment you cannot observe, a useful escalation is specific and reproducible. Include the flow endpoints, address family, protocol and port, direction, affected time window and time zone, the capture points you controlled, and the relevant socket or probe observations. Ask for provider-side evidence for that interval rather than asserting that a named device is dropping packets. A paired observation can narrow the boundary; only evidence from inside the remaining interval can identify what happened there.
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