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Connections Between Computers

Applies to:
  • CrashPlan for Home
  • CrashPlan PROe


If you are unable to connect to a computer destination in the CrashPlan app, follow the troubleshooting steps in this article to identify and remedy the issue.

Note: It's possible that computers that were previously connected may experience connectivity issues after an update to the CrashPlan app, your operating system, or either computer's network environment.


Computer-to-computer backup (backing up to a computer you own or a friend's computer)


  • By default, the CrashPlan app uses port 4242 for computer-to-computer backup. In CrashPlan version 4.3 and later, if the default port is already in use by another application, then a different port will be used. The port in use by the CrashPlan service can be found by identifying ports used by the CrashPlan app.
  • When backing up to another computer, the router and firewall settings must be properly configured on each computer involved.
  • CrashPlan uses Universal Plug and Play (UPnP) and NAT Port Mapping Protocol (Nat-PMP) for computer-to-computer communication. If you previously disabled UPnP or NAT-PMP on your router, you must configure port forwarding to allow computer-to-computer backup.
  • Code42 CrashPlan only: To back up to computer destinations, the source and destination computers must belong to the same user account; you cannot back up to other people's computers.

Recommended Solution

Step 1: Manually Attempt To Reconnect

Start by manually instructing the CrashPlan app to reconnect with the computer destination by restarting the CrashPlan service. This is an easy catch-all solution that can repair a disruption in the connection. Simply enter the following command in the CrashPlan app's Command window:

  1. Open the CrashPlan app
  2. Double-click the CrashPlan logo in the upper-right corner
    The CrashPlan command-line area opens
  3. Enter this command: 
  4. Press Enter
    This closes the CrashPlan app, stops the CrashPlan service, and then restarts the CrashPlan service

​​Running the restart command

Step 2: Use Telnet To Test Your Connection

If you're still not connected to the computer destination, use a simple Telnet command to further diagnose the issue. Telnet allows text-based communication between computers. Here we use it to see if the computer being backed up (the source) can talk to the computer storing the backup (the destination).

First, find the IP address for the destination computer:

  1. You can find the address from either the source or destination computer:​
    • From the source: Go to Backup, click the destination's name​
    • From the destination: Go to Settings > Network
  2. Two IP addresses are displayed. Determine which IP address to use:
    • Internal address (displayed first): Use if the computers are on the same network
    • Public/External address: Use if the computers are on different networks or at different locations

Now, test whether or not you can connect to the IP address and computer-to-computer communication port with Telnet. By default, the CrashPlan app uses port 4242 for computer-to-computer backup. Depending on your configuration, an alternate port may be used.

  1. On the source computer, open the Command Prompt (Windows) or Terminal (OS X/Linux):
    • Windows: All Programs > Accessories > Command Prompt
      If you are using Windows Vista, 7, or 8, you must install Telnet
    • OS X: Finder > Applications > Utilities > Terminal
  2. Enter the command: telnet <IP_address_of_destination_computer> 4242
    Example: telnet 4242​

Telnet Successfully Connects

​​If Telnet successfully connects, a message similar to the following displays:

Connected to Escape character is '^]'.

If Telnet is successful, then CrashPlan is able to access the necessary port on the destination computer. However, the connection may be blocked by antivirus software on either the source or destination computer. Proceed to Step 3 for further details on troubleshooting antivirus software.

Telnet Fails To Connect

If Telnet cannot successfully connect, you may see a "connection refused" message, no response, or some other response. If Telnet fails, the connection may be blocked on either the source or destination computer, most likely by a firewall on one of the computers. Proceed to Step 3 for further details on troubleshooting firewalls.

Step 3: Check For Conflicts With Your Firewall Or Antivirus Software

If you are still unable to connect, it may be because antivirus or firewall software is blocking CrashPlan or the ports needed for computer-to-computer connections.

Antivirus Software

Antivirus software is designed to protect your system against unknown applications and unauthorized Internet activity. Consequently, you may need to add CrashPlan as an approved application in your antivirus or security software in order for the CrashPlan app to function properly. The following antivirus and security applications have been known to prevent CrashPlan connections.

Applications Known To Cause Conflicts
  • Avast 
  • AVG
  • Avira
  • BeyondTrust PowerBroker (version 6.5)
  • Charter Security Suite
  • CleanMyMac
  • Covenant Eyes
  • Intego NetBarrier
  • Intego VirusBarrier
  • Little Snitch
  • McAfee
  • One Periodic Hands Off!
  • Qustodio
CrashPlan Version Updates
When the CrashPlan app is upgraded to a later version, antivirus and security software may consider it a "new" application and block its activity. If this occurs, re-add CrashPlan as an approved application.


Firewalls are designed to prevent unauthorized network activity on your computer. Many operating systems come with a built-in firewall:

Your computer may have other software firewalls installed. If you are using a firewall, you may need to add CrashPlan as an approved application on the destination computer, or allow inbound TCP connections on port 4242 or an alternate port.

Diagnose Security Software Conflicts

To determine if your antivirus software or firewall is blocking access to CrashPlan Central:

  1. Temporarily disable all firewall and antivirus software on the source and destination computer.
    See External Resources for links to commonly used firewalls and antivirus software
  2. Open the CrashPlan app.
    • If you still do not have a connection to the destination computer, repeat Step 1: Manually Attempt To Reconnect.
    • If you are able to connect, re-enable any software you disabled one-at-a-time to determine which application is blocking CrashPlan.
  3. If CrashPlan connects to the destination computer with the antivirus or firewall applications disabled, then either the CrashPlan app or TCP port is being blocked. The steps below use the default port. Depending on your configuration, an alternative port may be used. Once you determine the source of the conflict:
    • If antivirus software is preventing a connection, create an exception or rule for CrashPlan. If possible, include a rule to allow inbound connections (on the destination computer) or outbound connections (on the source computer) for TCP port 4242.
    • If the firewall on the destination computer is preventing the connection, create a rule to allow inbound connections on TCP port 4242.
      If both computers are backing up to each other, allow inbound and outbound connections for TCP port 4242 from both computers
  4. ​After adding antivirus software or firewall exceptions, open the CrashPlan app to check your connection status. If you are still unable to connect, repeat Step 1: Manually Attempt To Reconnect.

See External Resources below for links to instructions for commonly used antivirus applications, as well as Windows and OS X firewalls.

Re-enable your firewall and antivirus software when you are done testing your connection.

Step 4: Configure Port Forwarding

If you are still unable to connect to the computer destination after creating exceptions for the CrashPlan app and TCP port, you may need to configure port forwarding. Port forwarding is a way to direct traffic on the Internet. When two computers want to talk to each other, they need to know where to send information. IP addresses allow computers to send information to the right place.

NAT Traversal
When connecting between computers, the CrashPlan app usually calls ports in the 1,024 - 14500 and 49,000 - 52,000 ranges for NAT traversal, but may call ports outside of those ranges as well. We recommend allowing all CrashPlan connections in your firewall.

Computers On Different Networks

Enable port forwarding on the destination computer's router to tell it to send incoming connections on the TCP port to the destination computer's IP address. The default TCP port for computer-to-computer backup is 4242. Depending on your configuration, an alternative port may be used. For assistance finding the alternate port, please contact our Customer Champions​.

Port forwarding is configured differently for each router manufacturer. You can review your manufacturer's documentation for details, or you can find a guide for your router at or HowToGeek.

Computers On The Same Network

If multiple computers are running CrashPlan on a single network, you may need to assign a unique listener port to each computer. Typically, if this is needed, you will observe that one computer connects properly, but the other does not.

You can try any port between 1024 and 5999, with the exception of ports 4242 and 4243, which are defaults used by the CrashPlan app to communicate to the CrashPlan service. Therefore, do not select those ports.

For example, you could change the listener port on one of the computers to 4252. To change the listener port:

  1. Open the CrashPlan app​
  2. Go to Settings > General > Inbound backup from other computers > Configure
  3. Update the Listen port
  4. Configure port forwarding for the port
    Review your manufacturer's documentation for details, or find a guide for your router at or HowToGeek.
  5. If you still do not have a connection to the destination computer, repeat Step 1: Manually Attempt To Reconnect
Optional: Check Port Availability

If you want to verify that another application on your computer isn't using a particular port, you can run a simple command. For example, to check whether or not port 4252 is in use:

  1. Open the Command Prompt (Windows) or Terminal (OS X/Linux):
    • Windows: All Programs > Accessories > Command Prompt
    • OS X: Finder > Applications > Utilities > Terminal
  2. Enter the command:
    • Windows: netstat -ano | findstr “4252"
    • OS X/Linux: netstat -an | grep 4252

If the port is not in use, then the command will not return a result, and you can use it as a listener port for the computer that is unable to connect.

Still Having Trouble?

If the connection issue persists, please contact our Customer Champion​ team for further assistance.

Include the results of the troubleshooting steps above to help our team identify the issue as quickly as possible.

Include the following information in your request to help our Customer Champions diagnose the issue faster:

  • Product name helps us direct your request to the team that can best assist you
  • Operating system and version(s) of relevant system(s)
  • Detailed description of the question or problem
  • Screen shots of relevant settings in the app, console, or error messages
  • Computer ID (GUID) of affected device or devices (where's my computer ID?)
  • Logs can help us diagnose your issue more efficiently (how to find and send logs)
  • Username (or email address) of affected user account
  • Version of software you are running