Most ProFTPD connection failures on a Linux VPS are caused by the FTP data channel, not the login connection. In passive mode, the server must advertise its public address and accept a second connection on a predictable port. Set MasqueradeAddress to the VPS public IP, define a fixed PassivePorts range, allow TCP port 21 and that range through both local and provider firewalls, restart ProFTPD, and verify the advertised address in the client response and server logs.
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Understand what is failing: control versus data
FTP uses more than one TCP connection. The control connection carries authentication and commands, normally through TCP port 21. File transfers and directory listings use a separate data connection. A client can therefore authenticate successfully and still fail when it lists a directory, uploads a file, or starts a download.
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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 minuteOn a VPS behind NAT, a private address can be present on the server’s network interface while clients reach the machine through a public address. In passive mode, the client asks the server to open a listening data port. The server replies with an address and port, usually in a 227 Entering Passive Mode response. If that reply contains a LAN address such as 192.168.x.x, 10.x.x.x, or 172.16.x.x–172.31.x.x, an Internet client cannot normally connect to it.
Passive mode is usually the practical choice for VPS hosting because the client initiates both connections. Active mode reverses the direction of the data connection and is often blocked by client-side routers or firewalls. Changing clients from passive to active can be a diagnostic experiment, but it does not repair an incorrectly advertised passive address.
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Use this decision path before changing configuration
- Separate login from transfer. If the client cannot reach port 21, investigate service status, the control-port firewall rule, listening addresses, and provider filtering. If login works but directory listing or transfers time out, focus on passive addressing and the passive-port firewall rules.
- Inspect the PASV reply. Enable protocol logging in the FTP client or use a client that displays server responses. Confirm that the address in the
227reply is the VPS public IPv4 address clients should use. A private or stale address indicates a ProFTPD configuration problem. - Check the advertised port. The port returned in the same reply must fall inside the range you intend to permit. A port outside that range usually means another configuration file is being loaded, the service was not restarted, or no fixed range is configured.
- Check both firewalls. A local UFW, iptables, or firewalld rule can allow a port while a cloud-provider firewall blocks it, and the reverse is also possible. Both layers must allow TCP 21 and every port in the passive range.
- Read the ProFTPD log after a fresh attempt. Restart the service, make one directory-listing attempt, and inspect the configured system or transfer log. A line showing the configured masquerade address confirms that ProFTPD loaded that value; it does not by itself prove that an upstream firewall allows the data connection.
Correct the public address with MasqueradeAddress
Find the address clients should use
Use the public IPv4 address assigned to the VPS or exposed by its provider NAT. Do not substitute a private interface address. The value should be the literal numeric address, not a DNS name. A DNS name can resolve differently over time or resolve to an address that is not the address through which this VPS accepts FTP data connections.
For illustration only, a configuration might contain:
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Locate the active configuration
ProFTPD installations commonly include a main configuration file and additional files from an included directory. Edit the file that is actually loaded by your service, then search the configuration directory for repeated directives. For example:
grep -Rni --include='*.conf' 'MasqueradeAddress' /etc/proftpdThe exact directory differs by distribution and package. If the search returns more than one active entry, remove or disable the unintended one so there is a single authoritative value. Duplicate-entry behavior can vary by configuration structure, so treat the final loaded value as the one that matters.
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Restart and validate
Restart ProFTPD using the service manager on your distribution, then make a new passive-mode connection. A configuration edit without a restart leaves the running daemon unchanged. If the service refuses to restart, inspect the service-manager error and ProFTPD’s syntax diagnostics before testing the firewall; a daemon that did not start cannot accept either control or data connections.
Pin a passive-port range
Why a fixed range matters
PassivePorts tells ProFTPD which local TCP ports it may use for passive data connections. There is no universal default range. If the directive is absent, the operating system chooses a port, which makes it difficult to create a narrow, reliable firewall policy. A fixed range lets you permit exactly the ports ProFTPD may advertise.
Use non-privileged ports (1024 or higher). An example is:
PassivePorts 50000 51000The example provides 1,001 possible port numbers. Choose a range appropriate for your expected concurrency and any other services on the VPS; there is no single correct size. Each simultaneous passive data connection needs an available port. A range that is too small can exhaust available ports under parallel transfers. Some third-party guidance reports that an undersized range may fall back to a kernel-assigned port; that behavior is not guaranteed across versions, so do not rely on it as a design feature. Size and open the range instead.
Keep the range consistent
Use the same start and end values in ProFTPD and every firewall layer. If you later change either endpoint, update the host firewall and provider firewall in the same maintenance window. Confirm the port shown in each new 227 response belongs to the configured interval.
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Open the control and passive ports in the VPS firewall
Permit TCP port 21 and the complete passive range. The following examples assume the illustrative range 50000-51000; substitute your configured values. Review existing rules before applying changes so you do not remove unrelated access controls.
UFW
sudo ufw allow 21/tcp
sudo ufw allow 50000:51000/tcp
sudo ufw status numberedEnsure UFW is enabled and that the displayed rules use TCP. A rule for UDP does not permit FTP data connections.
iptables
sudo iptables -A INPUT -p tcp --dport 21 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 50000:51000 -j ACCEPT
sudo iptables -L INPUT -n --line-numbersiptables rules may not survive a reboot unless your distribution saves and restores them. Use the persistence mechanism supplied by your distribution rather than assuming that an in-memory rule is permanent.
firewalld
sudo firewall-cmd --permanent --add-port=21/tcp
sudo firewall-cmd --permanent --add-port=50000-51000/tcp
sudo firewall-cmd --reload
sudo firewall-cmd --list-portsDo not mix a temporary rule with a permanent rule and assume they are equivalent. With firewalld, the reload step applies permanent changes to the active configuration.
Allow the same ports in the provider firewall
Many VPS platforms place a security group, network ACL, or provider firewall in front of the virtual machine. Add inbound TCP rules for port 21 and the complete passive range there as well. If the provider firewall supports source restrictions, limit the source to the networks that need FTP access; otherwise, the service is exposed to the Internet and must be protected with strong accounts and other access controls appropriate to your deployment.
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When diagnosing a timeout, verify the provider rule first if the local firewall clearly shows an allow rule. A packet must pass every filtering layer between the client and ProFTPD. Opening a port only in the guest operating system cannot override a provider-level deny.
Restart ProFTPD and verify the loaded settings
After changing MasqueradeAddress or PassivePorts, restart the daemon through your operating system’s service manager. Then perform a fresh login and directory listing. Do not use a previously established client session: it may retain an old control connection or cached directory state.
Check the configured ProFTPD log for a line that records the masquerade address. The exact wording and file location depend on the package and logging directives installed on your VPS. Search the active log after generating a new passive request, and confirm that the address shown is the public address you configured. If the log still shows an old address, look for an included configuration file, a duplicate directive, or a restart that did not actually complete.
A useful verification sequence is:
- Confirm the service reports a successful restart.
- Connect with passive mode enabled.
- Capture the
227response and record its address and port. - Check that the address is public and the port is inside
PassivePorts. - Check the ProFTPD log for the corresponding request and masquerade value.
- Check both firewall layers for an allow rule covering that port.
Diagnose symptoms by where they stop
Cannot connect to port 21
- Confirm ProFTPD is running and listening on the expected interface.
- Verify the local firewall allows TCP 21.
- Verify the provider firewall or security group allows TCP 21.
- Check that the client is using the VPS public address and the correct protocol.
- Review service and system logs for a failed start, bind error, or immediate connection refusal.
If the client reports a refusal, a daemon or listening-address problem is more likely than a passive-port problem. If it reports a timeout, a filtering layer or unreachable address is more likely.
Login succeeds, but directory listing times out
This is the classic passive-data failure. Inspect the 227 response first. A private address points to MasqueradeAddress; a public address with an unreachable port points to the passive range or firewall. Confirm that the client is actually in passive mode and that the provider firewall permits the returned port.
Downloads work, but uploads fail (or the reverse)
Both operations use passive data connections, so a one-direction failure can indicate a client-specific mode, account permission, storage issue, or a transient port exhaustion problem rather than a different NAT setting. Compare the PASV response and server log for a successful and failed operation. If the failed operation uses a port outside the permitted range, correct the loaded configuration and restart.
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The client receives a private address in PASV
Set MasqueradeAddress to the VPS public IP as a numeric value, search for duplicate entries, restart ProFTPD, and capture a new response. Do not merely add another firewall rule for the private address: an Internet client still cannot route to that address.
The client receives the right address but a port outside the range
Check whether ProFTPD loaded the file you edited and whether the service was restarted after the edit. Search included configuration files for another PassivePorts directive. If no fixed range is active, the kernel may select an arbitrary port, which will not match a narrow firewall rule.
Connections fail only after several simultaneous transfers
The passive range may be too small for the client’s concurrency. Increase the range, open the enlarged range in both firewalls, restart ProFTPD if required by your configuration process, and retest with the same number of parallel transfers. Also check for provider connection-rate limits or host resource exhaustion; those are separate from address masquerading.
Idle sessions disconnect unexpectedly
ProFTPD’s TimeoutIdle default is 600 seconds. A value of 0 disables that idle timeout. Verify the value for your installed version and configuration before changing it, and distinguish an idle-control disconnect from a passive data timeout. Extending an idle timeout does not fix a blocked passive port.
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Know the relevant ProFTPD defaults
| Directive or setting | What is established | Operational implication |
|---|---|---|
MasqueradeAddress |
Set it to the public address clients must reach; use a numeric address rather than a DNS name. | Controls the address advertised in passive replies when the VPS has NAT or a private interface address. |
PassivePorts |
No fixed default is established; without it, the kernel chooses a port. | Define and firewall an explicit non-privileged range for predictable passive connections. |
MaxClients |
No default is established. | Do not assume a particular client limit when diagnosing concurrency; inspect your installed configuration. |
TimeoutIdle |
Defaults to 600 seconds; 0 disables the idle timeout. |
Relevant to long pauses between commands, not a substitute for fixing passive data connectivity. |
Defaults can vary with the ProFTPD version, distribution package, included files, and local overrides. Treat the values above as a starting point and verify the effective configuration on the VPS.
Use a safe configuration change workflow
- Back up the active configuration. Preserve the file before editing so you can restore a known-good state.
- Record the current public address and intended passive range. Keep the values together; changing one without the other creates a predictable mismatch.
- Edit the active ProFTPD configuration. Add one
MasqueradeAddressand onePassivePortsdefinition in the appropriate scope. - Search included files. Look for duplicate directives and confirm that the file you changed is included by the service.
- Apply matching firewall rules. Add TCP 21 and the entire passive range to the VPS firewall and provider firewall.
- Restart and check status. Stop here if the service fails to start; fix syntax or include errors first.
- Test with one passive listing, then one upload and download. Capture the PASV response for each test.
- Test expected concurrency. If users or automation open multiple transfers, repeat with that workload and verify that the range remains sufficient.
Common fixes that do not solve the underlying problem
- Opening only port 21: this permits login but not the separate passive data connection.
- Opening a random large port range: it may hide a missing
PassivePortssetting and unnecessarily increase exposure. Configure a deliberate range and allow that range. - Using the private VPS address in the client: this works only when the client is on a network that can route to that private address. Internet users need the reachable public address.
- Putting a DNS name in
MasqueradeAddress: use the public numeric address required by the configuration guidance, then update it deliberately if the provider changes the address. - Changing to active mode as a permanent fix: active mode shifts the inbound connection problem to the client and often fails behind consumer NAT.
- Editing a file without restarting: the running daemon continues using its old values.
- Checking only the guest firewall: a provider firewall can still drop the connection before it reaches the VPS.
- Assuming a login proves FTP is healthy: it proves only that the control channel worked; directory listings and transfers still require the data channel.
When to stop changing ProFTPD settings
Once the PASV reply contains the correct public address, the port is inside the configured range, both firewall layers allow it, and the log confirms the loaded masquerade value, further edits to those directives are unlikely to help. At that point investigate issues outside this core NAT pattern: the FTP account’s filesystem permissions, available disk space, client-side filtering, provider-level connection limits, or a service that is not listening on the interface the provider exposes.
Keep a timestamped record of the client response, selected passive port, service restart result, and firewall status for one failed and one successful attempt. Those four observations identify whether the failure is addressing, port selection, filtering, or an application-level problem without repeatedly changing unrelated settings.
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