Sentry Nodes: The Good, The Bad and the Ugly

Overview #

Sentry nodes have become a widely adopted strategy for blockchain validators, mainly due to their ability to bolster security. However, as with most tech solutions, they’re not without their drawbacks and potential pitfalls. Below, we’ll explore benefits, as well as issues with adopting a Sentry node architecture.

What is a Sentry node? #

At its core, a Sentry node is a simple blockchain node placed between your validator node and the public internet. It does not need to be a full node, it can also be a light client.

Rather than advertising your validator’s IP address to the entire network (and possible attackers), you configure the sentry nodes as the “public-facing” endpoints. Your validator node then connects only to these intermediary sentry nodes, staying safely tucked behind an additional layer of security, in a private network cluster.

Think of sentry nodes like bodyguards who interface with the outside world – they handle inbound and outbound requests so that your validator doesn’t have to.

Sentry nodes rely exclusively on the p2p layer of the blockchain network to “shield” the validators they are relaying for. They utilize the gossip layer of the network to transmit consensus messages to the validators. The validators themselves only have the sentry nodes as their peers, and use them for synchronizing the network state.

The Good #

1. Stronger Security #

• DDoS Mitigation
  Because the validator node’s IP isn’t advertised, it’s significantly harder for malicious actors to target it with a Distributed Denial of Service (DDoS) attack. Any attempt at overwhelming your validator will likely be directed at the sentry nodes instead – shielding the core of your cluster.

• Isolation from Direct Attacks
  By keeping the validator node private, you reduce the risk of exploits that may stem from zero-day vulnerabilities or misconfigured clients. Your validator can operate in a tightly controlled environment with minimal external exposure.

2. Network Stability and Redundancy #

• Minimized Downtime
  Validators play a critical role in block production. If a sentry node is overwhelmed or fails, you can operate multiple sentries to ensure your validator stays connected and online.

• Load Balancing
  Distributing traffic across multiple sentry nodes helps avoid bottlenecks. If one sentry becomes a target of heavy traffic, another can step in to manage the load, maintaining stable performance.

3. Flexibility and Scalability #

• Scaling for Multiple Chains
  Many validators choose to run nodes for multiple blockchains. You can scale up the number of sentry nodes without drastically changing your underlying validator configuration - great for future growth.

• Customizable Configurations
  You can place sentry nodes in different geographic locations, on various cloud providers, or even on-premise. This variety not only bolsters redundancy but also allows you to optimize for latency and performance.

The Bad #

1. Increased Complexity #

• More Infrastructure Components
  Implementing a Sentry node architecture isn’t as straightforward as running a single validator node. You’ll need to configure and maintain multiple machines, handle firewall rules, handle traffic forwarding, and manage secure communications between sentries and your validator.

• Higher Operational Costs
  Running extra nodes means increased hosting fees and hardware expenses. Even if you’re using cloud-based solutions, each sentry node has its own associated costs in terms of storage, bandwidth, and compute resources.

2. Configuration Overhead #

• Firewall and Networking
  Properly setting up sentry nodes requires robust firewall rules that only allow connections between your validator and the sentry nodes. Any misconfiguration could inadvertently expose your validator or block necessary communication.

• Monitoring and Maintenance
  Each additional node needs monitoring. If a sentry goes down, you’ll need to identify the root cause quickly - whether it’s a hardware failure, a networking issue, or a targeted attack.

The Ugly #

1. Misconfigurations Can Hurt More Than They Help #

• Inadvertent Exposure
  If you accidentally expose your validator IP address while trying to configure your sentry nodes, attackers can target your validator directly. This negates the core advantage of having a sentry architecture in the first place. This assumes your validator node is reachable and doesn’t have connection rules of its own.

• Single Points of Failure
  Over-reliance on a single cloud provider or region can create a hidden single point of failure. If that provider experiences an outage or a large-scale attack, all of your sentries (and therefore your validator) could be affected simultaneously.

2. Potential for Over-Engineering #

• Overcomplicating Simple Needs
  For smaller operators or those just starting out, setting up multiple sentry nodes and complex firewalls might be overkill. A simpler configuration, well-secured and closely monitored, can sometimes be just as effective (especially when resources are limited).

• Performance Degradation
  If you route all traffic through geographically distant sentry nodes or over-complicate your network path, you might introduce unnecessary latency or lose important peer connections. This can lead to slower block confirmations or missed blocks in extreme cases.

Why It’s Still Worth the Effort #

Despite its challenges, the Sentry node architecture is widely considered best practice for professional validators in blockchain ecosystems like Gno. The extra layers of security, the ability to mitigate DDoS attacks, and the flexible, scalable nature of this setup far outweigh the potential challenges – especially for high-stakes or enterprise-level validators who can’t afford significant downtime or reputational damage.

When implemented correctly, sentry nodes are a powerful tool that strikes a balance between accessibility and security, ensuring you can continue participating in block production without constantly worrying about external attacks or single points of failure.

Configuring a Sentry node architecture in config.toml #

While firewall rules and infrastructure setup are crucial parts of implementing a Sentry node architecture, your config.toml files for both the validator and sentry nodes also play a critical role. Below is a simplified overview of how you might configure these files. Note that specific parameter names and best practices can vary depending on the network topology you aim to have.

NOTE: these example commands assume the default p2p port of 26656. Please change the port number if it’s something different for your setup

On the Validator node #

1. Hide the Validator’s External IP You generally want your validator to avoid advertising its own IP. This can be achieved by leaving external_address blank or set to a private LAN IP if your infrastructure allows.

2. Listen on all interfaces
   Generally, since the validator machine is in a private network pocket, you’d want to set the p2p.laddr to be bound to all interfaces (0.0.0.0).

gnoland config set p2p.laddr 0.0.0.0:0

3. Connect only to your Sentry nodes
   Set the persistent_peers to reference only your sentry nodes. This ensures the validator does not accept connections from any unknown peers.

# Make sure the external address is not set, or is a private IP if needed
gnoland config set p2p.external_address "" # or  "tcp://<private_ip>:26656" 

# Use the sentries as the persistent peers
gnoland config set p2p.persistent_peers "sentryNodeID1@<sentry_ip1>:26656,sentryNodeID2@<sentry_ip2>:26656"

# Optionally, you can reduce the max inbound/outbound peers if you only want to connect to sentries.
# Make sure that for inbound / outbound limits, you respect the dialing direction (sentry -> validator or vice-versa)
gnoland config set p2p.max_num_outbound_peers 2 # if you have 2 sentries

# NOT recommended, but is an option if you want tighter control:
gnoland config set p2p.max_num_inbound_peers 0 # if no sentries are gonna dial the validator, but the other way around

4. Disable peer exchange The Validator nodes are valid participants in the network, but use the Sentry nodes as relays. Make sure they don’t participate in the peer exchange process with other nodes.

gnoland config set p2p.pex false

On the Sentry node #

1. Publicly advertise the Sentry Unlike the validator, each sentry node should have a reachable external address so that it can connect with other peers.

gnoland config set p2p.external_address "tcp://<sentry_public_ip>:26656"

2. Connect to the Validators Your sentry node will list the validators it’s guarding as a persistent peers. NOTE: in some kube environments, it’s impossible to know the IP of the validator node pod. In this case, it is strongly encouraged to use DNS for the validator location.

gnoland config set p2p.persistent_peers "validatorNodeID@<validator_private_ip>:26656"

3. Connect to other Sentries (if any) If you’re running multiple Sentry node clusters in your setup, make sure that sentries have each other in their persistent peer lists.

gnoland config set p2p.persistent_peers "...,sentryNodeID2@<sentry_ip2>:26656"

4. Make the validator peers private By setting the peer IDs of the validator nodes as private in the Sentry configuration, their information is not shared with other nodes in the peer discovery process.

gnoland config set p2p.private_peer_ids = "validatorNodeID"

5. Enable peer exchange The Sentry nodes are valid participants in the network. Make sure they participate in the peer exchange process with other nodes.

gnoland config set p2p.pex true

6. Connect with the rest of the network You can add official bootnodes nodes or well-known peers to ensure your sentry node participates in gossip and block propagation.

gnoland config set p2p.seeds "<seed_id_1>@<seed_ip_1>:26656,<seed_id_2>@<seed_ip_2>:26656"

Conclusion #

In conclusion, while Sentry node architectures do add extra complexity to your setup, the trade-off in security, redundancy, and overall resilience is typically well worth it – especially for those operating on high-stakes networks.

By carefully configuring your sentry and validator nodes, monitoring performance, and planning for redundancy, you’ll be far better equipped to withstand malicious attacks and unexpected downtimes. Sentry nodes essentially serve as a dedicated layer of protection, allowing you to confidently focus on producing blocks and growing your operation without constantly worrying about exposing your validator to the open internet.