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Encryption and key management 

This section follows "Encryption" in the "Overview and Concepts" section.

The user's password is used to derive two 192-bit keys (the "L" and "R" keys) via PBKDF2-SHA512, with hard-coded parameters for repeatable output.

  • The L-key is used to log in to the Auth Role server in place of the real password; the server stores only a bcrypt(sha512) hash of this L-key.
  • The R-key never leaves the client, and is used to encrypt secret keys stored within the user's profile on the server.

This means that one password can be used for all client-side account operations, while preventing servers from uncovering client-only secrets.

When Comet sets up a Storage Vault for the first time, it generates two high-entropy random keys (the 256-bit "A" and 128-bit "E" keys). All user data in the Storage Vault is stored encrypted with the A-key using AES-256 in CTR mode, and authenticated using Poly1305 in AEAD (encrypt-then-MAC) mode.

The permanent A-key is stored inside the Storage Vault, encrypted with the E-key. The E-key is then encrypted with the R-key and stored in the user's profile on the Auth Role server. When a backup is performed, the client uses its password to derive the private R-key, to decrypt the E-key from the vault, to decrypt the A-key for data storage. This extra level of indirection enables some key rotation scenarios, as a new E-key can be generated without needing to re-encrypt all the data in the Storage Vault.

If the Storage Vault is for a Storage Role bucket, a high-entropy random 128-bit PSK is used to gate access to the bucket. The Storage Role server stores only a bcrypt(sha512) hash of this PSK. The client encrypts this PSK with the R-key and stores it in the user's profile on the Auth Role server.

Compatibility events 

This section follows "Backward Compatibility" in the "Overview and Concepts" section.

The following compatibility events have occurred:

Version Details Upgrade Compatible Downgrade Compatible
18.6.2 A server-side encryption format changed for bucket access keys. Yes Partial. Accessing the bucket with Comet 18.5.5 (or later 18.5.x) will convert the key format to the backward-compatible format
17.9.3 A metadata format was enforced. Yes, but pre-existing Storage Vaults will not take advantage of the new features No. Storage Vaults created with newer versions of Comet cannot be used in old versions of Comet
17.6.1 A compression format was added. Yes Partial. If the new compression format was used, old versions of Comet will not be able to restore data
2.8.2 Beta The encryption format changed. Yes. Accounts are automatically upgraded upon login No. Old versions of Comet will not understand secrets in the new format
2.3.0 Beta The compression format changed. Yes. New versions of Comet can still read old backed-up data No. Old versions of Comet cannot read newly backed-up data
2.2.0 Beta The license server address changed. Yes No. Old versions of Comet Server can no longer be used
2.0.0 Beta The Storage Vault format changed. Partial. Storage Vaults must be recreated No. Old versions of Comet are unable to use new Storage Vaults
1.7.0 Beta The Storage Vault format changed. Yes. Comet will automatically upgrade Local Copy and Comet Server types, but S3 and SFTP types must be upgraded manually No. Old versions of Comet are unable to use new Storage Vaults
1.0.0 Beta Initial compatibility milestone Yes No
0.?.? Beta No compatibility guarantees made in either direction. n/a n/a

Migrating user data 

It is possible to migrate user data to balance your storage requirements.

Migrating server-side user data to a different volume 

You can migrate user data in different ways.

Off-line server migration, without Spanning 

  1. Stop Comet Server
  2. Move files to the new volume
  3. Update disk path in Comet Server's configuration file
  4. Start Comet Server

Gradual, on-line server migration, without Spanning 

  1. Use rsync/robocopy/rclone/... to synchronize the current drive contents to the new drive.
  2. Repeat step #1 until there is very little data change in a single sync run
  3. Stop Comet Server
  4. Perform one more sync pass
  5. Update disk path in Comet Server's configuration file
  6. Start Comet Server
  7. Delete all content from the old disk volume

Gradual, on-line server migration with Spanning 

If multiple volumes are Spanned together in the same Storage Role Comet Server, then you can move files freely. Comet Server will instantly recognize the changes, because it looks in all attached volumes when looking for a chunk.

  1. Configure Comet Server to Span between both the old and new volumes

    • Newly uploaded data will be written in a balanced way to both volumes.
  2. Live migrate data from old volume to the new volume

    • You can move data between the two volumes on-the-fly while Comet is running. You can even move data for an in-use Storage Vault. This is a safe operation.
  3. Stop Comet Server; move any remaining data that was written during step #2; disable the Spanning configuration; and then restart Comet Server

Migrating user data to a different Comet Server 

  1. Create a new bucket on a different Comet Server

    • You can either manually create the bucket, or Request a bucket on a new target server
  2. Copy the file content to the new server

  3. Edit the user's profile in the Auth Role Comet Server to change the address, bucket, and bucket-key that it points to

    • You should first ensure that this user is not running any backup jobs to the original server.
    • You must take care to preserve the Encryption Key settings. The key absolutely must not change.
  4. Remove the file content from the original server
    • You should first ensure that this user is not running any restore jobs from the original server.

Migrating user data between Storage Vault types 

All Storage Vault types (e.g. Comet Server, Local Copy, SFTP, Amazon S3 etc) use the same on-disk layout. It is possible to follow the above steps for "Migrating user data to a different Comet Server" even when either the old- or new- targets are not a Comet Server.

For more information, please see the "Comet Backup Usage" > "Seed Load" section.

Multiple Comet Server instances 

It is possible to run multiple Comet Server instances on the same machine or IP, by using a load balancer or a frontend proxy software. All Comet Server communication is performed over HTTP / HTTPS / Websockets, so applications such as nginx, Apache, HAProxy, Traefik or Caddy are all suitable for this purpose.

If you choose to do this, take care that the frontend proxy does not introduce additional buffering or timeouts that could interrupt the connection between Comet Backup and Comet Server.

For instance with nginx, the following configuration could be used as an example:

proxy_connect_timeout 3000;
proxy_send_timeout    3000;
proxy_read_timeout    3000;
client_body_timeout   3000;
proxy_buffering off;

HAProxy introduces additional network timeouts that may prevent live-connection websockets from staying online. Because an HTTPS handshake involves multiple network roundtrips, accidental disconnections may decrease performance. You can adjust HAProxy timeouts to prevent disconnecting long-running connections:

timeout connect 30s
timeout client 120s
timeout http-keep-alive 120s
timeout http-request 120s

Using Comet Backup behind a network proxy 

Comet Backup can be used behind an HTTP or SOCKS proxy.

In Comet 17.11.x, proxy settings are controlled by an environment variable named HTTP_PROXY.

Multiple programs use this configuration method, so the environment variable may already be present for other software on the machine.

On Windows, the environment variable should be set in the "System variables" section to ensure that any settings also apply to background services.

On Linux, environment variables can be set system-wide (e.g. in /etc/environment or /etc/profile.d/, or for the root user running Comet Backup (e.g. in /root/.profile), or in your startup script for Comet Backup (e.g. in /etc/rc.local).

The HTTP_PROXY environment variable should be set to a string of the form or or socks5://

A future version of Comet will provide GUI settings to configure the network proxy.

Windows Event Log 

Comet Backup logs all job messages to the Windows Event Log. The log content is identical to the job log content seen in the Comet Server log browser, or the Comet Backup history table, or the Comet Server API. This should allow you to check for errors and/or ensure that jobs are running on time, by monitoring the Windows Event Log.

However, please note that this only covers client-side jobs, that do actually run. e.g. because "Missed Backup" job entries are generated server-side, they won't appear in the client's event log. It is not feasible to use the Windows Event Log as a complete monitoring solution for your customer base.

The Comet Backup installer also logs some events, that can be used as a proxy for detecting software installation or upgrades.

Event IDs 

Source Event ID Description Available since
backup-service any Messages about installing and starting the Elevator service, the Pre-Logon Service (prior to 18.6.0), and the Delegate service (18.6.0 and later) -
backup-tool.exe 50 Backup job started 18.3.8 or later
backup-tool.exe 51 Backup job finished 18.3.8 or later
backup-tool.exe 52 Backup job log message. The Event Log entry severity corresponds to the Comet log entry severity (Info/Warning/Error) 18.3.8 or later
backup-tool.exe 53 Comet Backup installer has registered the backup-tool.exe Event Log source 18.3.8 or later
backup-tool.exe 54 Comet Backup uninstaller has de-registered the backup-tool.exe Event Log source 18.3.8 or later
backup-tool.exe 55 Message from the Comet Delegate service 18.6.0 or later

Data validation 

Comet stores files inside the Storage Vault data location as opaque, encrypted, compressed files. The filenames are the SHA256 hash of the file content. Comet automatically verifies file integrity during backup and restore operations.

However, for additional peace-of-mind, you can verify the integrity of the files on disk at any time, by comparing the filename to their SHA256 hash.

A future version of Comet will add built-in functionality to verify file integrity in this way.

Example data validation commands 

The following equivalent commands read all files in the current directory, take the SHA256 hash, and compare it to the filename.

These commands exclude the config file, as these are known to be safe for other reasons.

These commands do not exclude any other temporary files (e.g. /tmp/ subdirectory, or "~"-named files) that may be used by some storage location types for temporary uploaded data. Such temporary files will almost certainly cause a hash mismatch, but do not interfere with normal backup or restore operations.

On Linux, you can use the following command:

find . ! -name 'config' -type f -exec sha256sum '{}' \; | awk '{ sub("^.*/", "", $2) ; if ($1 == $2) { print $2,"ok" } else { print "[!!!]",$2,"MISMATCH",$1 } }'

On Windows, you can use the following Powershell (4.0 or later) command:

Get-ChildItem -Recurse -File | Where-Object { $_.Name -ne "config" } | ForEach-Object {
    $h = (Get-FileHash -Path $_.FullName -Algorithm SHA256)
    if ($_.Name -eq $h.Hash) { echo "$($_.Name) ok"; } else { echo "[!!!] $($_.Name) MISMATCH $($h.Hash)"; }

Cloud storage 

Taking hash values of files in this way requires fully reading the file from the storage location. If the storage location is on cloud storage, this is equivalent to fully downloading the entire contents of the storage location. This may result in significant network traffic. In this case, we recommend relying on Comet's normal verification that happens automatically during backup- and restore- operations.


Error "local error: tls: record overflow" 

This message means the connection was corrupted over the network, and Comet aborted the connection.

This can happen because of random network conditions. Retrying the operation should fix the issue.

If the issue keeps happening repeatedly, this message indicates that something is interfering with packets in your network.

  • Failing NIC
  • Bad NIC driver or driver configuration
  • Failing RAM, on either the endpoint machine or any of the intermediate routers
  • Outdated firewall or proxy, performing incorrect SSL interception

For more information, please see the record_overflow section in IETF RFC 5246.

"Paused" state on Windows service 

Comet Server on Windows consists of two parts: cometd.exe and cometd-service.exe. The latter binary is registered as a service with Windows, and is responsible for ensuring that the former binary stays running.

If Comet Server encounters an error and closes, the service binary will restart it. If the Comet Server is repeatedly unable to start - if it closes immediately when launched, several hundred times consecutively - then the service manager will assume the error is permanent and abandon restarting the process. This condition is displayed as the "Paused" state.

You can resolve this issue by resolving the underlying issue with the service. In Comet Server Service Manager, use the Service menu > Diagnostic > "Run in popup" option to determine the underlying issue. Alternatively, you can read Comet Server's log files from the disk.

Forgotten administrator password 

If you are locked out of the Comet Server web interface, you can change your administrator password by editing the cometd.cfg file.

  1. Stop the server, and edit the cometd.cfg file
  2. Find the AdminUsers section for the administrator user in question
  3. Set "PasswordFormat" to 0
  4. Set "Password" to e.g. "admin"
  5. Save the file and restart the server.

You should now be able to log in with the reset password. The password will be hashed and/or encrypted after first use.

Storage server "" in use by accounts but not managed by Constellation 

A user account has a bucket on a Storage Role Comet Server at the address, but this address was not selected for management by Constellation. Aside from the standard warnings about managing Constellation, the is a special case.

If Comet Server is configured to listen on all network interfaces, the server will be accessible on both as well as LAN, WAN, or DNS addresses. If you log in to either the Comet Backup application or the Comet Server web interface at, and request a new Storage Vault from a Comet Server configured as "Local" (or $self$ in cometd.cfg), the resulting Storage Vault will be configured using as the remote network address.

However, has a different meaning depending on where it is found. The connection will fail if the Comet Backup client is not running on the same machine. There may also be unintended consequences if the account is replicated to another server.

To avoid this problem, either

  • always use the external DNS name for your Comet Server when requesting new Storage Vaults, or
  • change the Request destination to use "Remote" with an external DNS name instead of "Local" (or $self$ in cometd.cfg).

A similar caveat applies to the software downloads, which can include embedded server address details.

Microsoft SQL Server backup encountered a VDI error 

You should ensure that the necessary VDI .dll files are registered and are the correct version for your SQL Server installation. You can use Microsoft SQL Server Backup Simulator to check the status of the VDI .dll files.

"Access is denied" backing up files and folders on Windows 

An "Access Denied" error message means that the Windows user account running the backup job does not have access to read the file content.

Comet 18.6.0 and later 

Comet automatically creates a service account with all necessary permissions to read local files. If you are experiencing "Access Denied" errors on Comet 18.6.0 or later, you may be trying to back up a network path that has been mounted as a directory. Please see the "Accessing Windows network shares and UNC paths" section below for more information.

If you are experiencing "Access Denied" errors on Comet 18.6.0 or later, and you are certain that you are not backing up a mounted network path, please contact support.

Comet 18.5.x and earlier 

In Comet 18.5.x and earlier:

  • Comet normally runs as the logged-on user session
  • If the Pre-logon service is enabled, Comet runs as a background session for the same user account
  • If the "take filesystem snapshot" option is enabled, Comet runs as LOCAL SYSTEM

You may be able to resolve this issue by:

  1. enabling the "take filesystem snapshot" option in the Protected Item settings, which switches to run the backup as the LOCAL SYSTEM user; or

  2. changing the permissions of the file, to give access rights to the specific Windows user account running the backup job; or

  3. using lusrmgr.msc to add the specific Windows user account to the "Backup Operators" group, which will allow Comet to bypass filesystem permissions (requires Comet 18.3.14 or later); or

  4. excluding the content from the backup job. This may be appropriate for some temporary directories or cache files; or

  5. upgrading to Comet 18.6.0 or later.

Antivirus detects Comet Backup as a virus or malware 

Comet Backup is a safe application. Any such detection is a "false positive".

When Comet Backup is rebranded, it might seem like a new, unknown program. An unknown program that installs system services, accesses files on the disk and uploads them to the network, might be considered to be malware if it was installed without consent. Unfortunately it's understandable for an Antivirus product to detect this.

In this situation, there are some actions you can take:

  • Please ensure your Antivirus product is fully up-to-date.
  • Please contact Comet Support with a screenshot of the error message. In some situations, it may be possible for our developers to resolve the issue.
  • Choose to "allow" or "white-list" the file in the Antivirus software. This may send a signal to the Antivirus software vendor that the software is safe (e.g. ESET LiveGrid, Windows Defender Automatic Sample Submission, Kaspersky KSN, etc).
  • Enable Authenticode signing on Windows. This may provide additional "reputation" to the software installer.

Avast "FileRepMalware" 

You will receive the "FileRepMalware" error message for any file that was

  • downloaded from the internet; and
  • does not have an Authenticode certificate; and
  • has not been seen yet by many Avast users.

Many custom-branded Comet Backup installers do fall into this category.

You can resolve this issue by purchasing and installing an Authenticode certificate.

Missing files from Storage Vault 

Please take care to ensure that files do not go missing from the Storage Vault. The loss of any file within the Storage Vault compromises the integrity of your backup data. You are likely to be in a data-loss situation.

More specifically:

If there are missing files from the snapshots subdirectory, Comet will not know that there is a backup snapshot available to be restored. There is no solution for this, other than ensuring all the files are present. This is unfortunate, but should only have a limited impact.

If there are missing files from the packindex subdirectory, some operations will be slower until Comet runs an optimization pass during the next retention pass. This is not a significant problem.

If there are missing files from the locks subdirectory, Comet may perform a dangerous operation, such as deleting in-use data during a retention pass. This is potentially a significant problem.

If there are missing files from the index subdirectory, Comet will re-upload some data that it could have otherwise deduplicated. This is unfortunate, but should only have a limited impact. Excess data will be cleaned up by a future retention pass.

If there are missing files from the keys subdirectory, Comet will be entirely unable to access the Storage Vault. There is no solution for this, other than ensuring all the files are present. This could be a significant problem. However, because the files in this directory do not change often, it's very likely that their replication is up-to-date.

If there are missing files from the data subdirectory, Comet will be unable to restore some data. When a retention pass next runs, Comet should detect this problem, and alert you to which backup snapshots are unrecoverable. You can then delete the corresponding files from the snapshots subdirectory and continue on with what is left of the Storage Vault.

Network connectivity errors 

Comet Backup uploads files to Comet Server (or to a cloud storage provider) over the internet. Occasionally, you may see errors such as the following:

  • Couldn't save data chunk:
  • HTTP/1.x transport connection broken
  • net/http: request canceled (Client.Timeout exceeded while awaiting headers)
  • wsarecv
  • wsasend
  • An existing connection was forcibly closed by the remote host
  • dial tcp: lookup [...]: no such host
  • connectex: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond.

Comet Backup retries the upload several times, but eventually gives up. After a failed data chunk upload, you may see several more messages of the form Couldn't save data chunk: context canceled while Comet terminates the other upload threads.

Network errors have many possible causes:

  • Customer's PC
  • Customer's network
  • Customer's ISP
  • Internet-wide outages between customer's ISP and your ISP
  • Your ISP
  • Your network
  • Your Comet Server hardware
  • Comet Server software

To troubleshoot these issues, please check:

  • Does the backup succeed if it is retried?

    • Many network errors are temporary and will only occur rarely. In addition, a repeated second backup job will often run faster because many of the existing data chunks have already been uploaded. (Any unused data chunks in the Storage Vault will be automatically cleaned up by the next retention pass.)
  • Does the error message always happen at a certain time of day?

    • It may be possible to reschedule the backup to avoid times of heavy internet congestion.
  • Are there any corresponding messages for around the same time in your Comet Server logs?
    • This is important to determine the cause of some failures.
    • Some relevant Comet Server log messages take the form Error saving upload stream or Blocking re-upload of preexisting file

Accessing Windows network shares and UNC paths 

This section applies to both Comet Server and Comet Backup.

Comet can back up Windows network paths, and back up to Windows network storage. However, because Comet runs as a service user, there are some issues with authentication to be aware of.

Mapped network drives 

On Windows, each logged-on user session has its own set of mapped network drives. The service user account is unlikely to have any mapped drives. If you see error messages like WARNING Missing: 'Z:\', this is probably the reason. You can work around this by using a UNC path instead.

  • In Comet Backup, when choosing items in a Files and Folders Protected Item, you can use the "Options" button > "Add " to browse inside a UNC path. Note that this browsing occurs as your logged-in Windows user, not as the service user, and may have different file access as a result. All backup jobs run as the service user.
  • In Comet Server, when configuring Local Path storage in the First Use Wizard, you can browse to a UNC path directly.

A future version of Comet will automatically convert mapped network drives to their UNC path equivalent.


If the UNC share requires authentication, the service user account is probably not logged-in to the UNC share. If you see error messages like WARNING Lstat: CreateFile \\?\UNC\...: Access is denied., this is probably the reason.

A future version of Comet will add simple built-in options for setting UNC credentials.

Several workarounds are available, for both Comet Backup and Comet Server. Ranked in order of preference:

If you are using Comet Backup to back up data from a network device, you can also work around this issue by installing Comet Backup directly on the network device, instead of backing it up over the network. This will also significantly improve performance, as less data needs to be transferred over the LAN.

If you are using Comet Server to store data on a network device, you may be able to install Comet Server on the network device. If the network device is a NAS box (e.g. Synology / QNAP), Comet Server can be installed on Linux x86_64 NAS boxes.

If you are storing data on a network share, you can also work around this issue by switching from Windows network shares (SMB) to a network protocol that has built-in credential support. For instance, a S3-compatible server (e.g. the free Minio server) or an SFTP server.

In Comet Backup, you can work around this issue by adding net use \\HOST\SHARE /user:USERNAME PASSWORD as a "Before" command to the backup job.

  • If you are storing data on a UNC path, you can add this "Before" command on the Storage Vault instead of on the Protected Item. This will ensure it is run for all backup jobs going to that Storage Vault.

You can work around this issue in Comet Backup or in Comet Server by changing the Windows Service to use a different user account.

  • For Comet Server, this is the Comet Server service.
  • For Comet Backup 18.6.0 and later, this is the Comet Backup (delegate service) service.
  • For Comet Backup 18.5.x and earlier, this is the Comet Backup (dispatcher service) after you have enabled the Pre-Logon Service; and this change only affects scheduled, non-VSS backups only. Changing the Comet Backup (elevator service) will affect VSS backup jobs, but would also prevent remote software updates from working.
  • If you are using Comet on a Windows Server machine that is acting as the Domain Controller, you must choose a domain account.

Out of memory 

Comet Backup needs RAM to run. The main cause for this is to hold deduplication indexes; therefore the amount of RAM used is proportional to the size of the Storage Vault.

You might see these error messages:

  • runtime: VirtualAlloc of 1048576 bytes failed with errno=1455 on Windows
  • 0x5AF ERROR_COMMITMENT_LIMIT: The paging file is too small for this operation to complete. on Windows
  • fatal error: out of memory on all platforms

On Linux, when the system is out of memory (OOM), the kernel "OOM Killer" subsystem will immediately terminate a process of its choosing, to free up memory. If you see an error message like signal: killed in Comet on Linux, this means the process was terminated by a user or a subsystem, that might possibly be the OOM Killer. You can check for this in dmesg or kern.log.

You can reduce Comet Backup's RAM usage by trying to limit how much data is in each Storage Vault. For instance, instead of having multiple devices backing up into a single Storage Vault, create multiple Storage Vaults for each device. This will reduce the deduplication efficiency, but it will also reduce the necessary memory usage.

A future version of Comet may add options to trade-off memory usage in other ways. For instance, by using more temporary files on disk instead of more memory; or, by using more network bandwidth instead of more memory.

HTTP 500 in Comet Backup logs 

If you see an HTTP 500 error message in the Comet Backup logs, this means the server encountered an error.

If you see this while performing an operation to

  • Comet Server storage, then you should check the Comet Server log file for around the same timestamp, to see if there was a corresponding error on the server.
  • Cloud storage, then the cloud storage provider experienced an error at their end.
    • The error message may contain more detail; or
    • You can contact the cloud provider for more information; or
    • The operation may succeed if you retry it a short time later.