# breakmancer
Drop a breakpoint into any shell.
Need to debug a build script, but don't have a way to SSH into the
server to poke around in the build environment—or you do, but you need
a way to pause the script at exactly the right point? `breakmancer`
allows you to do just that!
Functionally, this is a reverse shell for people who are *authorized*
to jump into a remote server. Unlike the malicious kind of reverse
shell, this one aims to preserve the security of the target.
## Requirements
- A development computer that is capable of listening on a public IP
address (or an intranet IP if it's on the same network as the
computer you're debugging.)
- A target computer where you're trying to debug a script and that you
have permission to shell into. This computer must be able to reach
out to the development computer over TCP.
## Usage
In this example, the development computer is reachable on port 12345
at the domain name `home.timmc.org`. The target is a GitHub Actions
workflow.
1. **Start controller session locally:** On my laptop I run `breakmancer start
home.timmc.org:12345`. It prints out instructions and a command line and
waits for a connection.
2. **Set up the breakpoint:** I add the breakpoint command that was printed
out above to my GitHub workflow file. It might look like this:
```
name: Breakmancer Demo
on: [push]
jobs:
demo:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v4
- name: Do some stuff
run: |
date > something.txt
- name: Invoke breakpoint
run: |
# If I want to use the hosted version:
curl -sS https://codeberg.org/timmc/breakmancer/raw/branch/master/fetch-and-run.sh \
| bash -s -- break --auth dual-pub -c home.timmc.org:12345 -i 4TXapNw9DLbnjhFx7
# Or if I want to use a copy I've temporarily checked into git:
chmod +x ./breakmancer
./breakmancer break --auth dual-pub -c home.timmc.org:12345 -i 4TXapNw9DLbnjhFx7
```
3. **Run:** I push the branch, the GitHub Action runs, and breakmancer reaches
back to my laptop.
4. **Verify:** The breakpoint prints out a verification string like
`xWeBv1Qw2BnxEBuQ` and instructs me to paste it into the controller
when prompted.
5. **Use the shell:** A shell opens, and I can run commands on the breakpoint
from the controller session:
```
Connection from [::ffff:52.234.38.67]:61440
Session ready. You can enter single-line commands. Use `exit` to exit.
>> hostname
[out] fv-az1756-422
[exit: 0]
>> pwd
[out] /home/runner/work/manual-testing/manual-testing
[exit: 0]
>>
```
6. **End:** When I'm done, I use `exit` (or ^C or ^D) to exit and allow
the workflow to continue; at the new prompt I can choose to wait for
a new connection or end the program entirely.
7. **Audit log:** Looking at the GitHub workflow output, I see a log of
what commands were run:
```
[2025-04-28T00:23:52Z] Breakpoint will attempt to connect to controller at 'localhost:12345', expecting controller identity of '4TXapNw9DLbnjhFx7'.
Connecting to controller at 12.34.56.78:12345
[2025-04-28T00:23:52Z] Waiting for first command...
[2025-04-28T00:23:57Z] Running command: hostname
[2025-04-28T00:23:59Z] Running command: pwd
Controller asked for normal execution to resume; exiting breakpoint.
```
Tip: Once the controller has exited, the secret and command line given
to the breakpoint side will no longer work and will need to be
replaced. That can be annoying. So until you're sure that you're done,
you may want to leave the controller running.
## Build, test, and release
- Review changelog to ensure nothing important has been omitted
- Update version in `Cargo.toml`
- Update the changelog version (add a new release header below the `Unreleased` header)
- Update the version in `fetch-and-run.sh`
- `cargo clippy --all-targets && cargo test && cargo doc --no-deps`
- `cargo build --release`
- Do any manual testing
- Commit version change
- `cargo publish`
- `git tag -a vX.Y.Z -m X.Y.Z`
- `git push && git push --tags`
- Cut a new release on Codeberg:
- Name the release X.Y.Z (no `v` prefix)
- Attach `./target/release/breakmancer` to the release
- Hide auto-generated archives
- Test the release by running `./fetch-and-run.sh`
## Manual testing
Some interesting things to enter:
- `(yes "out" | head -n20)& (yes " err" | head -n20 >&2)` to show interleaving of stdout and stderr
- `echo start; sleep 10; echo stop` to experiment with blocking commands
- `while true; do date; sleep 0.001; done` for a rapid output stream (not as fast as `yes`)
## Limitations
- As breakmancer does not create a PTY, buffering can occur in some
pipelines. This creates two problems:
1. All of the output happens at once. In the command `for x in
{1..5}; do date; sleep 1; done | head` we see nothing for 5
seconds, and then 5 lines of output all at once. (The output
reveals that each `date` invocation did indeed occur 1 second
apart.)
2. If such a streaming command never terminates, the output is
never sent. The command `while true; do date; sleep 1; done |
head` will hang forever with no output.
In those examples, dropping `head` from the pipeline (or piping to a
different binary such as `cat`) allows the output to stream
normally.
- See TODO.md for more bugs and wanted features.
## License
Copyright 2024-2025 Tim McCormack
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License as
published by the Free Software Foundation, either version 3 of the
License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Affero General Public License for more details.