lcsync(1)

NAME
lcsync - IPv6 multicast file sync tool

SYNOPSIS

Serve a a single file:
    lcshare [OPTION...] FILENAME
Serve all files below a directory:
    lcshare [OPTION...] DIRECTORY
Sync remote file(s) with local:
    lcsync [OPTION...] REMOTEFILENAME LOCALFILENAME
Sync two local files (Path required. Can be ./):
    lcsync ./LOCALFILE1 ./LOCALFILE2

DESCRIPTION
lcsync is a pair of tools to sync files over IPv6 multicast or the local filesystem. It splits the file into blocks, hashes them, and compares them in order to efficiently transfer a minimal amount of data.

lcshare is the server-side component which indexes files and listens for MLD multicast group joins and sends the file and directory block data.

lcsync is the client-side component which joins multicast groups to receive files and directories.

lcshare (server) Options:

--loopback
    enable multicast loopback (sending host will receive sent data)

lcsync (client) Options:

-a, --archive
    set archive options [-g -o -p -r -t]

-n, --dry-run
    don't copy any data

-g, --group
    set group on destination

-o, --owner
    set owner on destination

-p, --perms
    set permissions on destination

--remote
    source path is remote

-t, --times
    set modification times on destination

General Options:

-b, --batch
    Batch mode. No prompting. NB: if you do not specify a keyfile for encryption, encryption will be disabled.

--bwlimit INTEGER
    Set send rate limit (bps). An SI prefix of T, G, M or K may be added (eg. --bwlimit 10M)

--hex
    print file hashes in hex

-i, --interface INTERFACE
    Set network interface to use (default: all)

--keyfile keyfile
    Read symmetric key from keyfile, which must be the path to a file containing a 128 byte random key. This can be created with a command like:

dd if=/dev/random of=keyfile count=1 bs=128

--loglevel INTEGER
    set loglevel

-r, --recursive
    recurse into directories

-q, --quiet
    shhh - we're hunting wabbits

-v, --verbose
    increase verbosity

-V, --version
    display version and exit

To sync remote files, each file is split into blocks and a merkle tree is built by hashing the blocks using BLAKE3. On the sending/server side, this tree is sent on Librecast Channel (IPv6 multicast group) that is formed from the hash of the filename. The receiver/client joins this channel, and receives the tree. If the client already has some data to compare, it builds a merkle tree of the destination file and uses this to quickly compare which blocks differ. It builds a bitmap with this information, and then joins the Channel(s) for the block(s) required which are sent by the server

Forward Error Correction (FEC) is enabled by default using RaptorQ (RFC 6330) from the Librecast LCRQ library.

Symmetric encryption is provided using the XSalsa20 stream cipher from libsodium with Poly1305 MAC authentication tags. A keyfile can be provided, or a key can be derived from a user-supplied password.

There is no unicast communication with the server. There are no requests sent, and the server can sit behind a firewall which is completely closed to inbound TCP and UDP traffic. Instead, the server listens on a raw socket for Multicast Listener Discovery (MLD2) reports. It compares any MLD multicast group JOINs against the index it built on startup and finds matches for file (tree) and blocks. In this way, the server only sends data when at least one client is subscribed. If more clients want to download the data, the server need take no further action. Thus, the load on the server does not change at all, regardless of whether there is one client or a billion.

ENVIRONMENT

LCSYNC_PASSWORD
    the value of LCSYNC_PASSWORD will be used to derive the symmetric encryption key used for encryption and decryption.

BUGS
If you find one, email bugs@librecast.net

SEE ALSO
rsync(1), ipv6(7), lcrq(7)

AUTHOR
lcsync was written by Brett Sheffield <bacs@librecast.net> and released under the terms of the GPL-2 or (at your option) GPL-3.

https://librecast.net/lcsync.html