rsync to/from tar archive

Question

Is it possible to rsync a real filesystem (remote) with a tar archive (local)? If so, how?

Problem is I need to correctly backup user/group/permission settings and, while I have root access on remote I would like to avoid running as root on the local machine.

My first (and foremost) usage case would be to update a remote embedded target (ARM9) from a .tar produced by Buildroot. I do not have the "real thing" on disk (I can produce a copy while being root) and I would like to avoid transferring the whole rootfs to update a few files.

Answer 1

I haven't actually tried this, but it should work.

Using 'archivemount' (source from:)

http://www.cybernoia.de/software/archivemount/

and a 'libarchive' included in many distros (suse, redhat, etc)...

Or a pre-built one from:

https://rpmfind.net/linux/rpm2html/search.php?query=archivemount

You can mount a tar-archive using the fusermount facility in linux.

From there, you should be able to use rsync directly to the final system.

I wrote a simple passthrough batchfile to test rsync's passthrough:

#!/bin/bash
# ussh -- use root@ssh to target system
exec ssh  root@"$@"

then, as a test, used rsync to pass dir 'test1' to 'ishtar', calling it /tmp/test2 on the target:

RSYNC_RSH=$PWD/Ussh rsync -uva /tmp/test1/ ishtar:/tmp/test2

It will ask you for the password of the target sys's root logon, or you could setup the target system to accept a root login via a certificate so no password would be needed.

This would seem to be the most efficient way to do what you want (you might need to modify the rsync options to not copy dir times and things like that), but is this the type of thing you were looking for?

-Astara

Answer 2

The right answer seems to be to unpack the tar archive using fakeroot (to avoid becoming root) and then use rsync. Possibly repack the archive, if needed.

Unfortunately things are not this easy because bad interaction between ssh and fakeroot. I will detail what I did to help whoever will search.

Theory is straightforward:

1. create a temp directory
2. unpack tar archive into it
3. rysnc as needed
4. if something changed locally repack int a new tar archive
5. cleanup

In order to preserve all user/group/permissions steps 2..4 must be done under fakeroot.

Catch is rsync uses ssh for communication (and I want it to!) and thus, being "fakerooted" it tries to open root credentials (in /root/.ssh/), failing badly. The following set of options work for me.

#!/bin/bash

target=myHost
here=$(pwd)

# 1. create a temp directory
cd /tmp
mkdir TMPfs
cd TMPfs

fakeroot bash <<- EOF
    # 2. unpack tar archive into it
    tar xf $here/archive.tar
    # 3. rysnc as needed (ssh options are *the* relevant thing)
    rsync -av -e "ssh -i $HOME/.ssh/id_rsa -oUserKnownHostsFile=$HOME/.ssh/known_hosts" . root@$target:/
    # 4. if something changed locally repack int a new tar archive (not needed here)
EOF

# 5. cleanup
rm -rf *
cd ..
rmdir TMPfs

I still get the error "Could not create directory '/root/.ssh'." but it appears to be benign (no files are created/used there).