gokrazy rsync
Package rsync contains a native Go rsync implementation.
This repository currently contains:
gokr-rsyncd
, a read-only rsync daemon sender-only Go implementation of rsyncd. It implements the rsync daemon network protocol (port 873/tcp by default), but can be used over SSH or locally as well.gokr-rsync
is an rsync receiver implementation that can download files via rsync (daemon protocol or SSH).
The following known improvements are not yet implemented:
- Making
gokr-rsync
also implement an rsync sender so that it can push (upload) files to a remote rsync server (daemon protocol or SSH). - 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. - Making
gokr-rsyncd
also implement an rsync receiver, so that it can accept files. - Merging
gokr-rsyncd
andgokr-rsync
into a single binary.
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
Language | URL | Note | Max Protocol | Server mode? |
---|---|---|---|---|
C | WayneD/rsync | original “tridge” implementation; I found older versions easier to study | 31 | |
C | kristapsdz/openrsync | OpenBSD, good docs | 27 | |
Go | gokrazy/rsync | → you are here ← | 27 | |
Go | jbreiding/rsync-go | rsync algorithm | ||
Go | kaiakz/rsync-os | only client/receiver | 27 | |
Go | knight42 | proxy | ||
Go | c4milo/gsync | |||
Java | APNIC-net/repositoryd | archived | ||
Java | JohannesBuchner/Jarsync | archived, internet draft RFC “The rsync Network Protocol” | ||
Java | perlundq/yajsync | |||
C++ | gilbertchen/acrosync-library | commercial | ||
Rust | sourcefrog/rsyn | client, “rsyn is rsync with no c” | 27 |
Getting started
To serve the current directory via rsync on localhost:8730
, use:
go install github.com/gokrazy/rsync/cmd/gokr-rsyncd@latest
gokr-rsyncd --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) | config required | |||||
2. anon SSH (daemon) | config required | |||||
3. SSH (command) | no config | |||||
4. SSH (daemon) | ~/.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-rsyncd --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-rsyncd --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-rsyncd
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-rsyncd
.
Example:
- Server will be started via SSH
- Client:
rsync --rsync-path=gokr-rsyncd 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
.
Example:
- Server will be started via SSH
- Client:
rsync -e ssh --rsync-path=gokr-rsyncd rsync://webserver/module/path
Limitations
Bandwidth
In my tests, gokr-rsyncd
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-rsyncd
). Implementing support for more recent protocol
versions would help here, as these include hash algorithm negotiation with more
recent choices.
Protocol related limitations
- 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-rsyncd
, to guard against data modification and data exfiltration. gokr-rsyncd
is running without privileges, as usernobody
, 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 --gokr.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
andIPAddressAllow
ingokr-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=
andBindReadOnlyPaths=
directives as described in Use TemporaryFileSystem to hide files or directories from systemd services. Note that you may need to disableProtectSystem=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 \
--gokr.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 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-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.