gokrazy rsync

This repository contains a native Go rsync implementation: the gokr-rsync
command implements an rsync client and server, which can send or receive files
(all directions supported). Daemon mode is supported, meaning you can deploy
gokr-rsync
behind SSH (anonymous or authorized), as command or daemon, or
listening directly on the network (on port 873/tcp by default).
The following known improvements are not yet implemented:
- Making
gokr-rsync
chroot (and/or Linux mount namespaces when available?)
into the destination directory to reduce chances of accidental file system
manipulation in case of bugs.
This project accepts contributions as time permits to merge them (best effort).
How do I know this project won’t eat my data?
This rsync implementation is very fresh. It was started in 2021 and doesn’t have
many users yet.
With that warning out of the way, the rsync protocol uses MD4 checksums over
file contents, so at least your file contents should never be able to be
corrupted.
There is enough other functionality (delta transfers, file metadata, special
files like symlinks or devices, directory structures, etc.) in the rsync
protocol that provides opportunities for bugs to hide.
I recommend you carefully check that your transfers work, and please do report
any issues you run into!
Existing rsync implementation survey
Getting started
To serve the current directory via rsync on localhost:8730
, use:
go install github.com/gokrazy/rsync/cmd/gokr-rsync@latest
gokr-rsync --daemon --gokr.listen=localhost:8730 --gokr.modulemap=pwd=$PWD
You can then copy the contents of the current directory with clients such as
rsync(1)
:
% rsync -v --archive --port 8730 rsync://localhost/pwd/ quine
receiving file list ... done
created directory quine
./
.git/
[…]
.github/workflows/main.yml
LICENSE
Makefile
README.md
cmd/gokr-rsyncd/rsyncd.go
doc.go
go.mod
go.sum
internal/rsyncd/connection.go
internal/rsyncd/rsyncd.go
interop_test.go
sent 1,234 bytes received 5,678 bytes 13,824.00 bytes/sec
total size is 666 speedup is 0.10
…or openrsync(1)
, shown doing a
differential update:
% openrsync -v --archive --port 8730 rsync://localhost/pwd/ quine
socket.c:109: warning: connect refused: ::1, localhost
Transfer starting: 369 files
.git/index (1.1 KB, 100.0% downloaded)
Transfer complete: 5.5 KB sent, 1.2 KB read, 666 B file size
Usage / Setup
setup |
encrypted |
authenticated |
private files? |
privileges |
protocol version |
config required |
1. rsync daemon protocol (TCP port 873) |
❌ no |
⚠ rsync (insecure) |
❌ only world-readable |
✔ dropped + namespace |
✔ negotiated |
config required |
2. anon SSH (daemon) |
✔ yes |
✔ rsync |
❌ only world-readable |
✔ dropped + namespace |
✔ negotiated |
config required |
3. SSH (command) |
✔ yes |
✔ SSH |
✔ yes |
⚠ full user |
⚠ assumed |
no config |
4. SSH (daemon) |
✔ yes |
✔ SSH (+ rsync) |
✔ yes |
⚠ full user |
✔ negotiated |
~/.config/gokr-rsyncd.toml required |
Regarding protocol version “assumed”: the flags to send over the network are
computed before starting SSH and hence the remote rsync process. You might
need to specify --protocol=27
explicitly on the client. Once the connection is
established, both sides do negotiate the protocol, though.
Setup 1: rsync daemon protocol (TCP port 873)
Serving rsync daemon protocol on TCP port 873 is only safe where the network
layer ensures trusted communication, e.g. in a local network (LAN), or when
using Tailscale or similar. In untrusted networks,
attackers can eavesdrop on file transfers and possibly even modify file
contents.
Prefer setup 2 instead.
Example:
- Server:
gokr-rsync --daemon --gokr.modulemap=module=/srv/rsync-module
- Client:
rsync rsync://webserver/module/path
Setup 2: anon SSH (daemon)
This setup is well suited for serving world-readable files without
authentication.
Example:
- Server:
gokr-rsync --daemon --gokr.modulemap=module=/srv/rsync-module --gokr.anonssh_listen=:22873
- Client:
rsync -e ssh rsync://webserver/module/path
Setup 3: SSH (command)
This setup is well suited for interactive one-off transfers or regular backups,
and uses SSH for both encryption and authentication.
Note that because gokr-rsync
is invoked with user privileges (not root
privileges), it cannot do namespacing
and hence retains more privileges. When serving public data, it is generally
preferable to use setup 2 instead.
Note that rsync(1)
assumes the server process understands all flags that it
sends, i.e. is running the same version on client and server, or at least a
compatible-enough version. You can either specify --protocol=27
on the client,
or use setup 4, which negotiates the protocol version, side-stepping possible
compatibility gaps between rsync clients and gokr-rsync
.
Example:
- Server will be started via SSH
- Client:
rsync --rsync-path=gokr-rsync webserver:path
Setup 4: SSH (daemon)
This setup is more reliable than setup 3 because the rsync protocol version will
be negotiated between client and server. This setup is slightly inconvenient
because it requires a config file to be present on the server in
~/.config/gokr-rsyncd.toml
.
Note that this mode of operation is only implemented by the original “trigde”
rsync, not in openrsync. Apple started shipping openrsync with macOS 15 Sequoia,
so you might need to explicitly start /usr/libexec/rsync/rsync.samba on Macs.
Example:
- Server will be started via SSH
- Client:
rsync -e ssh --rsync-path=gokr-rsync rsync://webserver/module/path
Limitations
Bandwidth
In my tests, gokr-rsync
can easily transfer data at > 6 Gbit/s. The current
bottleneck is the MD4 algorithm itself (not sure whether in the “tridge” rsync
client, or in gokr-rsync
). Implementing support for more recent protocol
versions would help here, as these include hash algorithm negotiation with more
recent choices.
- xattrs (including acls) was introduced in rsync protocol 30, so is currently
not supported.
Supported environments and privilege dropping
Supported environments:
- systemd (Linux)
- Docker (Linux)
- privileged Linux
- privileged non-Linux
In all environments, the default instructions will take care that:
- (On Linux only) Only configured rsync modules from the host file system are
mounted read-only into a Linux mount namespace for
gokr-rsync
, to guard
against data modification and data exfiltration.
gokr-rsync
is running without privileges, as user nobody
, to limit the
scope of what an attacker can do when exploiting a vulnerability.
Known gaps:
gokr-rsync
does not guard against denial of service attacks, i.e. consuming
too many resources (connections, bandwidth, CPU, …).
systemd (unprivileged)
We provide a gokr-rsyncd.socket
and gokr-rsyncd.service
file for systemd. These
files enables most of systemd’s security features. You can check by running
systemd-analyze security gokr-rsyncd.service
, which should result in an
exposure level of “0.2 SAFE” as of systemd 249 (September 2021).
First, configure your server flags by creating a systemd service override file:
systemctl edit gokr-rsyncd.service
In the opened editor, change the file to:
[Service]
ExecStart=
ExecStart=/usr/bin/gokr-rsync --daemon --gokr.modulemap=pwd=/etc/tmpfiles.d
Close the editor and install the service using:
systemctl enable --now gokr-rsyncd.socket
Additional hardening recommendations:
Docker (unprivileged)
We provide a Dockerfile
for
gokr-rsyncd
.
docker run \
--read-only \
-p 127.0.0.1:8730:8730 \
-v /etc/tmpfiles.d:/srv/rsync:ro \
stapelberg/gokrazy-rsync:latest \
--gokr.modulemap=pwd=/srv/rsync
Additional hardening recommendations:
- Restrict which IP addresses are allowed to connect to your rsync server, for example:
privileged Linux (including gokrazy.org)
When started as root
on Linux, gokr-rsync
will create a Linux mount
namespace, mount all configured
rsync modules read-only into the namespace, then change into the namespace using
chroot(2)
and drop privileges using
setuid(2)
.
Tip: you can verify which file system objects the daemon process can see by
using ls -l /proc/$(pidof gokr-rsync)/root/
.
Additional hardening recommendations:
- Restrict which IP addresses are allowed to connect to your rsync server, for example:
privileged non-Linux (e.g. Mac)
When started as root
on non-Linux (e.g. Mac), gokr-rsync
will drop
privileges using setuid(2)
.
unprivileged with write permission (e.g. from a shell)
To prevent accidental misconfiguration, gokr-rsync
refuses to start when it
detects that it has write permission in any configured rsync module.