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

lcsync                                             2023-07-31                                          lcsync(1)