Package rsync contains a native Go rsync implementation.

⚠ Beware: very fresh. Might eat your data. You have been warned! ⚠

The only component currently is gokr-rsyncd, a read-only rsync daemon sender-only Go implementation of rsyncd. rsync daemon is a custom (un-standardized) network protocol, running on port 873 by default.

This project accepts contributions as time permits to merge them (best effort).

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-rsyncd
gokr-rsyncd -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

Limitations

Bandwidth

In my tests, gokr-rsyncd can easily transfer data at ≈ 2 Gbit/s. The current bottleneck is the golang.org/x/crypto/md4 implementation. With a faster implementation, we would be able to fill a 10 Gbit/s link!

Protocol related limitations

• xattrs (including acls) was introduced in rsync protocol 30, so is currently not supported.

Supported environments and privilege dropping

Supported environments:

1. systemd (Linux)
2. Docker (Linux)
3. privileged Linux
4. privileged non-Linux

In all environments, the default instructions will take care that:

• (On Linux only) The host file system is made read-only for gokr-rsyncd, to guard against accidental data exfiltration.
• gokr-rsyncd is running without privileges, as user nobody, to limit the scope of what an attacker can do when exploiting a vulnerability.

Known gaps:

• gokr-rsyncd does not guard against denial of service attacks, i.e. consuming too many resources (connections, bandwidth, CPU, …).
  • See also Per-IP rate limiting with iptables.

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-rsyncd -modulemap=pwd=/etc/tmpfiles.d

Close the editor and install the service using:

systemctl enable --now gokr-rsyncd.socket

Additional hardening recommendations:

• Restrict which IP addresses are allowed to connect to your rsync server, for example:
  • using iptables or nftables on your host system
  • using gokr-rsyncd’s built-in IP allow/deny mechanism (once implemented)
  • using systemd’s IPAddressDeny and IPAddressAllow in gokr-rsyncd.socket
• To reduce the impact of Denial Of Service attacks, you can restrict resources with systemd, see Managing Resources.
• To hide system directories not relevant to any rsync module, use systemd’s TemporaryFileSystem= and BindReadOnlyPaths= directives as described in Use TemporaryFileSystem to hide files or directories from systemd services. Note that you may need to disable ProtectSystem=strict due to a bug.

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 \
    -modulemap=pwd=/srv/rsync

Additional hardening recommendations:

• Restrict which IP addresses are allowed to connect to your rsync server, for example:
  • using iptables or nftables on your host system
  • using gokr-rsyncd’s built-in IP allow/deny mechanism (once implemented)
    • Be sure to set up Docker such that the remote IPv4 or IPv6 address is available inside the container, see https://michael.stapelberg.ch/posts/2018-12-12-docker-ipv6/

privileged Linux (including gokrazy.org)

When started as root on Linux, gokr-rsyncd will create a 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-rsyncd)/root/.

Additional hardening recommendations:

• Restrict which IP addresses are allowed to connect to your rsync server, for example:
  • using iptables or nftables on your host system
  • using gokr-rsyncd’s built-in IP allow/deny mechanism (once implemented)

privileged non-Linux (e.g. Mac)

When started as root on non-Linux (e.g. Mac), gokr-rsyncd will drop privileges using setuid(2).

unprivileged with write permission (e.g. from a shell)

To prevent accidental misconfiguration, gokr-rsyncd refuses to start when it detects that it has write permission in any configured rsync module.