Ubuntu Manpage: lcsync - IPv6 multicast file sync tool

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

AUTHOR