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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteTest TCP port 21 from the computer that cannot connect: in Windows PowerShell run Test-NetConnection -ComputerName ftp.example.com -Port 21; on Linux or macOS run nc -vz ftp.example.com 21. A successful TCP handshake proves that the FTP control endpoint is reachable from that network. It does not prove that credentials, directory listings, uploads, or downloads will work, because FTP uses a separate data connection.
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What port 21 actually tells you
The Internet Assigned Numbers Authority assigns TCP port 21 to FTP’s control service. The control connection carries the server banner, login exchange, directory commands, and requests such as PASV or EPSV. File contents and directory listings travel over a second data connection.
That split explains a common symptom: you can reach a server, see a welcome banner, and even log in, yet a directory listing hangs or a transfer fails. Opening only TCP 21 allows the control conversation; the server’s data port and the path to it must also be permitted. In passive mode, the server chooses a port from its configured passive range and tells the client which port and address to use.
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A port test is therefore a reachability test from one vantage point, not proof of a complete FTP configuration and not proof of which device is blocking traffic. To locate the fault, test the name and address, compare networks, inspect the data channel, and check server and firewall logs.
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Run a TCP/21 test from the affected client
Windows PowerShell
- Open PowerShell on the computer that is failing to connect.
- Run
Test-NetConnection -ComputerName ftp.example.com -Port 21, replacing the hostname with the FTP server name. - Read the
TcpTestSucceededfield.Truemeans the TCP handshake completed;Falsemeans it did not.
PowerShell’s -Port parameter specifies the remote TCP port used for the connectivity test. Save the complete output, including the resolved remote address, because an address difference can explain why two users see different results.
You can test a known address directly with Test-NetConnection -ComputerName 203.0.113.25 -Port 21. Use an address only when you have verified that it belongs to the intended server; bypassing DNS can otherwise send the test to the wrong host.
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Linux and macOS
- Open a terminal on the affected machine.
- Run
nc -vz ftp.example.com 21. - Repeat with the server’s verified IP address if DNS is in doubt:
nc -vz 203.0.113.25 21.
The exact wording differs between netcat implementations, but a message indicating that the connection succeeded means the TCP handshake completed. A refusal or timeout is the important evidence to record, not just a generic “FTP failed” message.
Resolve DNS before deciding that a port is blocked
First establish what address the hostname returns on the failing client. On Windows, use Resolve-DnsName ftp.example.com. On Linux or macOS, use dig ftp.example.com or nslookup ftp.example.com. Note any A (IPv4) and AAAA (IPv6) answers.
- If the name resolves to an unexpected address, correct the DNS record, split-horizon DNS zone, hosts-file entry, or VPN DNS policy before testing firewalls.
- If both IPv4 and IPv6 are published, a client may try IPv6 first. Test each address explicitly. A working IPv4 path does not prove that the IPv6 listener, route, or firewall rule works.
- If the hostname resolves only on an internal DNS server, a test from the public internet may be aimed at a different service or may fail before TCP begins.
Record the name, resolved address, address family, and network used for each test. This makes a later comparison meaningful.
Interpret the TCP result without overdiagnosing it
| Observed result | What it establishes | What it does not establish | Next check |
|---|---|---|---|
TcpTestSucceeded: True or netcat reports success |
A TCP handshake to port 21 completed from that client and address. | It does not establish valid credentials, a usable data channel, or correct FTP application behavior. | Connect with a verbose FTP client and test a passive listing or transfer. |
| Connection refused immediately | The destination or an intermediate device actively rejected the attempt, or no process is listening at the destination. | It does not identify whether the rejection came from the host firewall, an edge device, or an absent service. | Check the listener, host-firewall rules, security groups, NAT, and perimeter logs. |
| Connection times out | No completing handshake was observed before the probe timed out. | It does not distinguish filtering from a routing failure, powered-off host, wrong address, or an unreachable service. | Compare another network and address family; inspect routes and firewall logs. |
| Name cannot be resolved | The client did not obtain a usable address for the hostname. | It says nothing about TCP/21 itself. | Fix DNS or test the verified IP separately. |
Do not describe a timeout as definitive proof that “the firewall is blocking port 21.” It is consistent with filtering, but the probe alone cannot identify the blocking device.
Confirm the control channel with an FTP client
After a successful port probe, connect with the FTP client used by the application and enable its verbose, debug, or transcript mode. The exact menu label varies by client; look for an option that displays server replies and connection details.
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- Connect to the same hostname and port 21 tested above.
- Confirm that a server banner appears. A banner demonstrates that an FTP service, rather than merely an open TCP socket, is speaking on the control channel.
- Attempt authentication and record the numeric FTP replies. A rejected password is an application or account issue, not evidence that port 21 is blocked.
- Request a directory listing. Make sure the client is using passive mode for the first data-channel test unless your server is deliberately configured for active mode.
- Try a small download and upload, if your account is authorized, and save the transcript when the operation fails.
A banner and successful login establish control-channel operation only. If login succeeds but LIST, MLSD, downloads, or uploads stall, move to passive-port and advertised-address checks rather than repeatedly changing credentials.
Test the passive data connection
Understand PASV and EPSV
In passive FTP, the client sends PASV or EPSV over the control connection. The server responds with a data address and port, and the client opens a new outbound TCP connection to that endpoint. The port is normally selected from a configured passive range, not port 21.
Ask the server administrator for the configured range and one port that is currently available for testing. Do not guess a random port: a successful probe to an unrelated service would be misleading.
Probe one advertised data port
- Start a verbose FTP session and issue a passive directory-listing command.
- Copy the address and port shown in the
PASVorEPSVreply. With EPSV, the reply normally supplies a port while the client retains the server address. - From the same client network, probe that port with PowerShell, for example
Test-NetConnection -ComputerName 203.0.113.25 -Port 50000, or withnc -vz 203.0.113.25 50000on Linux or macOS. Replace50000with the actual advertised port. - Repeat a listing or transfer while watching the client transcript and server logs.
A successful port-21 test combined with a failed probe to the advertised data port points to the passive range, a firewall rule, a cloud security-group rule, NAT, or an incorrect address in the server’s reply. Allow the complete configured range where necessary, but restrict it to the required source and destination rather than opening an arbitrary broad range.
Check the FTP server and its local firewall
Verify that a listener exists
On the server, verify that the FTP daemon is bound to the expected interface and TCP 21. On Linux, administrators commonly inspect this with ss -ltnp and filter for :21. On Windows, Get-NetTCPConnection -LocalPort 21 -State Listen or netstat -ano | findstr :21 shows whether a process is listening; map a returned process ID to the service before changing anything.
- A listener bound only to
127.0.0.1accepts local connections but not remote clients. - A listener on a private address must be reachable through the intended routing and NAT path; a public DNS address cannot reach it automatically.
- If no listener exists, start the FTP service or correct its configured port. Restarting a client cannot fix an absent server listener.
Review host-firewall policy
Check inbound TCP 21 on the server’s operating-system firewall and review its logs for the test time and source address. A rule may allow a management subnet while rejecting internet or VPN clients. Apply the same scrutiny to the passive data range; allowing port 21 alone is insufficient for listings and transfers.
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IIS-specific checks
For FTP hosted in IIS, Microsoft’s firewall guidance gives an inbound rule example for local TCP port 21 and explains that separate data-channel handling is required. In IIS Manager, verify the FTP site binding, the listening IP, and the passive data-port range in the server’s FTP firewall-support settings. The external address advertised to clients must be the address they can actually reach. Also check Windows Defender Firewall and any upstream firewall for both the control port and that range.
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A server can listen correctly and still be unreachable because traffic is stopped before it reaches the host.
- Cloud security groups or network ACLs: confirm inbound TCP 21 from the client source range and the passive range. Check whether an egress policy on the client subnet blocks outbound FTP.
- Router or firewall NAT: verify that public TCP 21 forwards to the intended private address and that passive ports forward to the same FTP host. A control-port forward without matching data-port forwards produces login-without-transfer symptoms.
- Perimeter inspection: inspect accept, reject, and drop logs at the test timestamp. Some devices reject immediately; others silently drop and create a timeout.
- Address advertisement: ensure the PASV response does not contain a private address when clients are outside the NAT. Conversely, internal clients may need an internal address or split DNS.
Make one controlled change at a time, repeat the same probe, and record the result. Avoid disabling an entire firewall as a diagnostic shortcut; a narrow temporary rule with a defined source, destination, and expiration is safer.
Use multiple networks to locate the blocking side
Repeat the identical hostname and IP tests from a second vantage point, such as another internet connection or a mobile hotspot, subject to your organization’s policy.
| Comparison | Likely focus |
|---|---|
| Works from one network, fails from another | Client-network egress filtering, ISP policy, VPN routing, source allow-lists, or different DNS answers. |
| Fails from every tested network | Server listener, host firewall, cloud security group, NAT/forwarding, perimeter firewall, wrong address, or an unavailable host. |
| Port 21 works everywhere, transfers fail everywhere | Passive range, advertised address, data-port forwarding, or server-side data-channel policy. |
| Hostname fails but direct verified IP works | DNS, split-horizon records, IPv6 preference, or a certificate/name policy in the client rather than TCP reachability. |
| IPv4 works and IPv6 fails | Missing IPv6 listener, route, firewall rule, or incorrect AAAA record. |
These comparisons narrow the fault domain; they do not replace logs. A source network can also have a different route or DNS answer, so record both for every test.
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Common symptoms and targeted fixes
“The FTP client cannot connect”
Run the raw TCP/21 test first. If DNS fails, correct name resolution. If TCP is refused, verify the listener and rejection rules. If it times out, compare another network and inspect routing, NAT, security-group, and perimeter logs. Only after TCP succeeds should you troubleshoot account credentials or FTP client settings.
“Login works, but the directory listing hangs”
This is usually a data-channel problem, not a blocked control port. Capture the PASV/EPSV response, test the advertised port, and verify that the passive range is allowed through host and upstream firewalls. Correct the server’s advertised public or internal address and matching NAT rules.
“Downloads fail but the listing appears”
A listing proves that at least one data connection worked; it does not guarantee every passive port is reachable. Confirm that the entire configured range is permitted and that the server has enough ports for concurrent sessions. Review logs for a different advertised port during the failed download.
“It works on the office network but not at home”
Compare DNS answers and test results from both networks. The office may have an allow-list or internal route, while the home ISP, VPN, or endpoint firewall may restrict outbound FTP. If the server permits only the office’s source addresses, add an appropriate secure access path rather than broadly opening the service.
“The client reports a TLS error”
Encryption changes the application negotiation, not the initial fact that explicit FTP commonly begins on TCP 21. First establish TCP reachability, then troubleshoot certificate trust, TLS mode, and the client’s hostname policy. Implicit FTPS commonly uses a different configured port; test the port specified by that service instead of assuming 21.
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What to collect before escalating
- Exact hostname, verified IP address, and whether the attempt used IPv4 or IPv6.
- Date and time in UTC, client network, public source address if known, and whether a VPN or proxy was active.
- Full PowerShell or netcat output, including success, refusal, or timeout behavior.
- FTP client transcript showing the banner, login result, PASV/EPSV response, and the command that failed.
- Server listener output and the FTP service’s logs for the same timestamp.
- Host-firewall, cloud security-group, NAT, and perimeter-firewall decisions for TCP 21 and the advertised passive port.
Redact passwords, private keys, session tokens, and sensitive filenames before sharing logs. The source address and timestamps are usually essential for matching a firewall event.
A repeatable decision procedure
- Resolve the hostname and record A and AAAA answers.
- Test TCP 21 by hostname, then by each verified address.
- Classify the result as success, refusal, timeout, or DNS failure without claiming to know the blocking device.
- Repeat from a second network when possible.
- If TCP 21 succeeds, use a verbose FTP client to confirm banner, authentication, and a passive listing.
- Capture the PASV/EPSV address and port, then test that data endpoint from the same client.
- Check the server listener, local firewall, passive-range configuration, security groups, NAT, and perimeter logs.
- Change the narrowest rule or configuration that the evidence identifies, retest, and document the result.
This sequence separates DNS and routing problems from control-port filtering, then separates control-channel success from passive data-channel failure. It prevents repeated password changes from obscuring a network problem.
Security and operational cautions
FTP transmits credentials and data without encryption unless a TLS mode is explicitly configured. A reachable port is not automatically a safe service. Restrict source networks where possible, use encrypted FTP when supported, keep the passive range deliberately sized, monitor authentication and firewall logs, and remove temporary diagnostic rules after testing.
Do not expose a large untracked passive range merely to make one transfer work. Document the exact control port, passive range, advertised addresses, NAT mappings, and allowed source networks so a future port test has an unambiguous expected result.
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