Design Document

This tool is designed to allow requests to Tor onion service to be directed to multiple back-end Tor instances, thereby increasing availability and reliability. The design involves collating the set of introduction points created by one or more independent Tor onion service instances into a single ‘master’ descriptor.


This tool is designed to allow requests to Tor onion service to be directed to multiple back-end Tor instances, thereby increasing availability and reliability. The design involves collating the set of introduction points created by one or more independent Tor onion service instances into a single ‘master’ onion service descriptor.

The master descriptor is signed by the onion service permanent key and published to the HSDir system as normal.

Clients who wish to access the onion service would then retrieve the master service descriptor and try to connect to introduction points from the descriptor in a random order. If a client successfully establishes an introduction circuit, they can begin communicating with one of the onion services instances with the normal onion service protocol defined in rend-spec.txt


A load-balancing node running an individual onion service.

Introduction Point

A Tor relay chosen by an onion service instance as a medium-term meeting-place for initial client connections.

Master Descriptor

An onion service descriptor published with the desired onion address containing introduction points for each instance.

Management Server

Server running Onionbalance which collates introduction points and publishes a master descriptor.

Metadata Channel

A direct connection from an instance to a management server which can be used for instance descriptor upload and transfer of other data.

Retrieving Introduction Point Data

The core functionality of the Onionbalance service is the collation of introduction point data from multiple onion service instances by the management server.

In its basic mode of operation, the introduction point information is transferred from the onion service instances to the management server via the HSDir system. Each instance runs an onion service with an instance specific permanent key. The instance publishes a descriptor to the DHT at regularly intervals or when its introduction point set changes.

On initial startup the management server will load the previously published master descriptor from the DHT if it exists. The master descriptor is used to prepopulate the introduction point set. The management server regularly polls the HSDir system for a descriptor for each of its instances. Currently polling occurs every 10 minutes. This polling period can be tuned for onion services with shorter or longer lasting introduction points.

When the management server receives a new descriptor from the HSDir system, it should before a number of checks to ensure that it is valid:

  • Confirm that the descriptor has a valid signature and that the public key matches the instance that was requested.

  • Confirm that the descriptor timestamp is equal or newer than the previously received descriptor for that onion service instance. This reduces the ability of a HSDir to replay older descriptors for an instance which may contain expired introduction points.

  • Confirm that the descriptor timestamp is not more than 4 hours in the past. An older descriptor indicates that the instance may no longer be online and publishing descriptors. The instance should not be included in the master descriptor.

It should be possible for two or more independent management servers to publish descriptors for a single onion service. The servers would publish independent descriptors which will replace each other on the HSDir system.. Any difference in introduction point selection between descriptors should not impact the end user.


  • A malicious HSDir could replay old instance descriptors in an attempt to include expired introduction points in the master descriptor. When an attacker does not control all of the responsible HSDirs this attack can be mitigated by not accepting descriptors with a timestamp older than the most recently retrieved descriptor.

  • The management server may also retrieve an old instance descriptor as a result of churn in the DHT. The management server may attempt to fetch the instance descriptor from a different set of HSDirs than the instance published to.

  • An onion service instance may rapidly rotate its introduction point circuits when subjected to a Denial of Service attack. An introduction point circuit is closed by the onion service when it has received max_introductions for that circuit. During DoS this circuit rotating may occur faster than the management server polls the HSDir system for new descriptors. As a result clients may retrieve master descriptors which contain no currently valid introduction points.

  • It is trivial for a HSDir to determine that a onion service is using Onionbalance. Onionbalance will try poll for instance descriptors on a regular basis. A HSDir which connects to onion services published to it would find that a backend instance is serving the same content as the master service. This allows a HSDir to trivially determine the onion addresses for a service’s backend instances.

Onionbalance allows for scaling across multiple onion service instances with no additional software or Tor modifications necessary on the onion service instance. Onionbalance does not hide that a service is using Onionbalance. It also does not significantly protect a service from introduction point denial of service or actively malicious HSDirs.

Choice of Introduction Points

Tor onion service descriptors can include a maximum of 10 introduction points. Onionbalance should select introduction points so as to uniformly distribute load across the available backend instances.

Onionbalance will upload multiple distinct descriptors if you have configured more than 10 instances.

  • 1 instance - 3 IPs

  • 2 instance - 6 IPs (3 IPs from each instance)

  • 3 instance - 9 IPs (3 IPs from each instance)

  • 4 instance - 10 IPs (3 IPs from one instance, 2 from each other instance)

  • 5 instance - 10 IPs (2 IPs from each instance)

  • 6-10 instances - 10 IPs (selection from all instances)

  • 11 or more instances - 10 IPs (distinct descriptors - selection from all instances)

Always attempting to choose 3 introduction points per descriptor may make it more difficult for a passive observer to confirm that a service is running Onionbalance. However behavioral characteristics such as the rate of introduction point rotation may still allow a passive observer to distinguish an Onionbalance service from a standard Tor onion service. Selecting a smaller set of introduction points may impact on performance or reliability of the service.

  • 1 instance - 3 IPs

  • 2 instances - 3 IPs (2 IPs from one instance, 1 IP from the other instance)

  • 3 instances - 3 IPs (1 IP from each instance)

  • more than 3 instances - Select the maximum set of introduction points as outlined previously.

It may be advantageous to select introduction points in a non-random manner. The longest-lived introduction points published by a backend instance are likely to be stable. Conversely selecting more recently created introduction points may more evenly distribute client introductions across an instances introduction point circuits. Further investigation of these options should indicate if there is significant advantages to any of these approaches.

Generation and Publication of Master Descriptor

The management server should generate a onion service descriptor containing the selected introduction points. This master descriptor is then signed by the actual onion service permanent key. The signed master descriptor should be published to the responsible HSDirs as normal.

Clients who wish to access the onion service would then retrieve the ‘master’ service descriptor and begin connect to introduction points at random from the introduction point list. After successful introduction the client will have created an onion service circuit to one of the available onion services instances and can then begin communicating as normally along that circuit.