NAME

       lcsync - IPv6 multicast file sync tool

SYNOPSIS

       Serve a a single file:
           lcsync FILENAME

       Serve all files below a directory:
           lcsync DIRECTORY

       Sync remote file with local:
           lcsync REMOTEFILENAME ./LOCALFILENAME

       Sync two local files (Path required. Can be ./):
           lcsync ./LOCALFILE1 ./LOCALFILE2

DESCRIPTION

       lcsync  is a tool 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.

       Options:

       -a, --archive
              set archive options [presently only -p]

       -b, --batch
              Batch mode. No prompting.

       --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

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

       --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

       -p, --perms
              set file permissions on destination

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

       -v, --verbose
              increase verbosity

       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