This is the third post in a series exploring Delay-Tolerant Networking (DTN) and resilient communication stacks built for amateur radio, space payloads, and emergency networks.

Interfacing with the BPA over gRPC

Once you have a Bundle Processing Agent (like Hardy) running in the background, how do your user-facing programs talk to it?

Instead of forcing applications to implement complex routing tables, database storage, and transmission retry loops, Hardy exposes a lightweight gRPC API. User applications connect as simple clients. They can send bundles, register to receive bundles sent to a specific Endpoint Identifier EID, or query the local store.

To make this interaction simple on Linux systems, I built dtn-hdy-utils, a set of command-line utilities for Hardy.

The Core Utilities

  • dtnping: The DTN equivalent of ICMP ping. It registers as a lightweight client, sends a bundle to a destination EID, and measures the Round-Trip Time (RTT) and paths taken. But contrary to bp ping from the Hardy project, dtnping is not its own BPA. It leverages your existing Hardy instance.
  • dtnsend & dtnprint: The fundamental send/receive pipes. You can pipe any raw text or binary payload from stdin into dtnsend and have dtnprint listen and dump incoming data directly to stdout.
  • dtnquery: Inspects local bundle storage offline (SQLite/PostgreSQL databases) to check the state of the bundle queues and analyze network health.
  • dtnfiles: Receive BPv7 bundles and save them as files in a directory. It automatically adjust the filename extension for some well-known file types.
  • hdy-stats: Send a text bundle with some statistics of usage per source EID for last 24h / last week / last month.
  • dtntrigger: A versatile CLI to run anything on bundle reception that I will examplify right after.

Automation via dtntrigger

The most powerful utility in the stack is dtntrigger. It subscribes to a specific DTN service endpoint and blocks. Whenever a new bundle is received:

  1. It writes the bundle payload to a temporary file.
  2. It executes a pre-configured script or executable, passing the sender EID and the temporary file path as arguments.

By combining dtntrigger with simple scripts, you can build full offline application gateways. Here are three examples available in the dtn-hdy-utils/examples directory:

1. Matrix Notification Bridge (dtn-matrix.py)

If a disaster zones triggers an alert, how do we forward it to the internet once a gateway node gets connectivity? The dtn-matrix.py script acts as a bridge.

  • It intercepts incoming bundles targeting a matrix endpoint.
  • It logs into a specified Matrix home server and posts the message directly to a chat room.
  • Security: It fully supports End-to-End Encryption (E2EE) using matrix-nio[e2e], managing cryptographic keys locally on the gateway node.

2. Fortune teller (dtn-fortune.sh)

This script acts as a classic fortune teller.

  • A remote station sends a bundle containing their preferred language (e.g. fr or en) to dtn://<node>/fortune.
  • dtntrigger receives it and starts a script leveraging the well known fortune cli.
  • The script reads the request, generates a random quote using the local fortune database, and triggers dtnsend to transmit the reply bundle back to the original sender’s EID.

For the fun of it, a list of fortunes related to DTN has been generated.

3. Local Desktop Alerts (dtn-notify.sh)

A lightweight utility for home stations or field laptops. It subscribes to a notification endpoint and triggers standard system desktop alerts (using notify-send) showing the sender’s EID and the message text whenever a bundle arrives.

Beyond Text: Chat and File Sharing

Using these tools, you are not limited to simple command-line pings. Because the Bundle Protocol handles arbitrary payloads:

  • Chat: You can pipe live chat applications. Text messages are encapsulated into DTN bundles and routed opportunistically to the destination. More to come in the next post.
  • File Exchange: Large binary payloads—such as high-resolution images, PDF documents, or weather charts—can be packaged and sent over the DTN link, with the protocol ensuring the files are stored and forwarded hop-by-hop across the mesh.
  • Sensors Value: By leveraging SenML RFC, one can send regularly timestamped sensors’ values, like temperature or pressure. I am currently receiving my weather station data like this. I planned to try it on our satelitte payload too.

Next steps

Actually I have already a few more utilities but I will keep those for later posts. Anyway with those at hand, DTN have started to be usefull in my daily routine.

In the next post, we will take a step away from the desktop and look at how we can carry these delay-tolerant conversations in our pockets using a native Android messenger application.