wrenn/envd-rs

envd (Rust)

Wrenn guest agent daemon — runs as PID 1 inside Cloud Hypervisor microVMs. Provides process management, filesystem operations, file transfer, port forwarding, and VM lifecycle control over Connect RPC and HTTP.

Rust rewrite of envd/ (Go). Drop-in replacement — same wire protocol, same endpoints, same CLI flags.

Prerequisites

• Rust 1.88+ (required by connectrpc 0.3.3)
• protoc (protobuf compiler, for proto codegen at build time)
• musl-tools (for static linking)

# Ubuntu/Debian
sudo apt install musl-tools protobuf-compiler

# Rust musl target
rustup target add x86_64-unknown-linux-musl

Building

Static binary (production — what goes into the rootfs)

cd envd-rs
ENVD_COMMIT=$(git rev-parse --short HEAD) \
  cargo build --release --target x86_64-unknown-linux-musl

Output: target/x86_64-unknown-linux-musl/release/envd

Verify static linking:

file target/x86_64-unknown-linux-musl/release/envd
# should say: "statically linked"

ldd target/x86_64-unknown-linux-musl/release/envd
# should say: "not a dynamic executable"

Debug binary (dev machine, dynamically linked)

cd envd-rs
cargo build

Run locally (outside a VM):

./target/debug/envd --port 49983

Via Makefile (from repo root)

make build-envd        # static musl release build
make build-envd-go     # Go version (for comparison)

CLI Flags

--port <PORT>          Listen port [default: 49983]
--version              Print version and exit
--commit               Print git commit and exit
--cmd <CMD>            Spawn a process at startup (e.g. --cmd "/bin/bash")
--cgroup-root <PATH>   Cgroup v2 root [default: /sys/fs/cgroup]

Endpoints

HTTP

Method	Path	Description
GET	/health	Health check, triggers post-restore
GET	/metrics	System metrics (CPU, memory, disk)
GET	/envs	Current environment variables
POST	/init	Host agent init (token, env, mounts)
POST	/snapshot/prepare	Quiesce before Cloud Hypervisor snapshot
GET	/files	Download file (gzip, range support)
POST	/files	Upload file(s) via multipart

Connect RPC (same port)

Service	RPCs
Process	List, Start, Connect, Update, StreamInput, SendInput, SendSignal, CloseStdin
Filesystem	Stat, MakeDir, Move, ListDir, Remove, WatchDir, CreateWatcher, GetWatcherEvents, RemoveWatcher

Architecture

42 files, ~4200 LOC Rust
Binary: ~4 MB (stripped, LTO, musl static)

src/
├── main.rs              # Entry point, CLI, server setup
├── state.rs             # Shared AppState
├── config.rs            # Constants
├── conntracker.rs       # TCP connection tracking for snapshot/restore
├── execcontext.rs       # Default user/workdir/env
├── logging.rs           # tracing-subscriber (JSON or pretty)
├── util.rs              # AtomicMax
├── auth/                # Token, signing, middleware
├── crypto/              # SHA-256, SHA-512, HMAC
├── host/                # System metrics
├── http/                # Axum handlers (health, init, snapshot, files, encoding)
├── permissions/         # Path resolution, user lookup, chown
├── rpc/                 # Connect RPC services
│   ├── pb.rs            # Generated proto types
│   ├── process_*.rs     # Process service + handler (PTY, pipe, broadcast)
│   ├── filesystem_*.rs  # Filesystem service (stat, list, watch, mkdir, move, remove)
│   └── entry.rs         # EntryInfo builder
├── port/                # Port subsystem
│   ├── conn.rs          # /proc/net/tcp parser
│   ├── scanner.rs       # Periodic TCP port scanner
│   ├── forwarder.rs     # socat-based port forwarding
│   └── subsystem.rs     # Lifecycle (start/stop/restart)
└── cgroups/             # Cgroup v2 manager (pty/user/socat groups)

Updating the rootfs

After building the static binary, copy it into the rootfs:

bash scripts/update-debug-rootfs.sh [rootfs_path]

Or manually:

sudo mount -o loop /var/lib/wrenn/images/minimal.ext4 /mnt
sudo cp target/x86_64-unknown-linux-musl/release/envd /mnt/usr/bin/envd
sudo umount /mnt