Remote backups via rsync between RHEL 3 and Mac OS XEdit

*Note: these notes were made with older versions of RHEL and Mac OS X. See "Remote backups via rsync" for links to other articles using other operating system versions.*

These are notes I made while setting up an rsync-based backup mechanism between my remote server (running Red Hat Enterprise Linux) and my local machine (running Mac OS X).

Client set-up

Key generation

ssh-keygen -t dsa -f ~/.ssh/id_dsa_rsync
chmod 400 ~/.ssh/id_dsa_rsync*

Backup storage

sudo mkdir /var/root/backups
sudo mkdir /var/root/backups/hostname
sudo chmod 700 /var/root/backups
sudo chmod 700 /var/root/backups/hostname

Remote set-up

Edit /etc/ssh/sshd_config, changing:

PermitRootLogin no


PermitRootLogin forced-commands-only

Instruct sshd to re-read the configuration file by sending it a SIGHUP signal:

sudo kill -s SIGHUP pid_of_sshd_process

Then we set things up in root’s home directory:

sudo mkdir /root/.ssh
sudo chmod 700 /root/.ssh

Copy the public key to the remote host by executing a command like this on the local machine:

scp ~/.ssh/

Then on the remote host:

sudo cat /home/remoteuser/ >> /root/.ssh/authorized_keys

It is necessary to customize the authorized_keys file so that it resembles the following:

command="rsync --server --daemon .",no-port-forwarding,no-agent-forwarding,no-X11-forwarding,no-pty ssh-dss AAAACkiWA==

That is, we prepend command="rsync --server --daemon .",no-port-forwarding,no-agent-forwarding,no-X11-forwarding,no-pty immediately prior to the public key that was previously added to the file.

The /etc/rsyncd.conf file should have the following content added:

max connections = 1

    path = /
    list = false

Performing the backup

Options to be passed to rsync:

  • -a: archive, equivalent to -rlptgoD
  • -r: recursive
  • -l: copy symlinks as symlinks
  • -p: preserve permissions
  • -t: preserve times
  • -g: preserve group
  • -o: preserve owner
  • -D: preserve devices
  • -v: verbose
  • -z: compress
  • -x: don’t cross filesystem boundaries
  • --numeric-ids: don’t map uid/gid values by user/group name
  • --delete: delete files that don’t exist on the sending side
  • --progress: show progress during transfer
sudo rsync -e "ssh -i /absolute_path_to/id_dsa_rsync" \
           -avzx --numeric-ids --delete --progress \

In addition, during testing the following additional switch can be applied:

  • -n: dry run mode

My first test produced this error message:

protocol version mismatch - is your shell clean?

As instructed by the rsync man page I tested to see if any extraneous output was breaking things:

ssh echo -n 2>/dev/null > out.dat

This test required me to temporarily set PermitRootLogin to yes. If all is well, out.dat should be a zero-byte file; it was.

So I ran rsync --version on the remote host:

rsync  version 2.5.7  protocol version 26

And the local machine:

rsync  version 2.6.3  protocol version 28

Given that the protocol versions are different, that leaves me with three options:

  1. Build a custom version of rsync on the server that uses the newer protocol
  2. Build a custom version on the local machine that uses the older protocol
  3. A combination of both, build new versions on the server and the local machine that use the latest protocol
  4. And another option, discovered too late, the --protocol=NUM command line switch

Building rsync

Download the latest source (2.6.8) from and extract/build it:

tar xzvf rsync-2.6.8.tar.gz
cd rsync-2.6.8
sudo make install

The procedure is the same for both the local Mac OS X machine and the remote Red Hat Enterprise Linux machine. The custom rsync binary is installed into /usr/local/bin/. rsync --version now outputs:

rsync  version 2.6.8  protocol version 29

To ensure that the remote server uses the custom build I modify the command section of the authorized_keys file to read:

command="/usr/local/bin/rsync --server --daemon ."

Likewise, in the local invocation I must explicitly provide a full path:

sudo /usr/local/bin/rsync -e "ssh -i /absolute_path_to/id_dsa_rsync" \
                          -avzx --numeric-ids --delete --progress \

Unfortunately, even using identical versions of rsync on both machines I still get the same "protocol version mismatch" error.

Troubleshooting attempts

I’ve tried the following troubleshooting methods:

  • Perform the out.dat test both with and without redirection of stderr to /dev/null
  • Perform the test with the command (echo -n) with and without a corresponding command directive in the authorized_keys file

All tests produce zero bytes of output, as required.

I also tried running the backup with the following variations:

  • Omit the no-port-forwarding specification in the authorized_keys file
  • Omit the no-pty specification
  • Omit all specifications

So looks like I am going to have to reduce this to a simpler test case and then try ramping the complexity up from there.

mkdir /tmp/test
/usr/local/bin/rsync -e ssh -avzxn --numeric-ids --delete --progress \

That works. So try with root directory:

/usr/local/bin/rsync -e ssh -avzxn --numeric-ids --delete --progress \

Works (although insufficient privileges lead to permissions errors).

One of the differences is that the non-root user is identified by a key that is already in memory and managed by ssh-agent. See if removing it from the agent has any effect:

ssh-add -d ~/.ssh/id_dsa
/usr/local/bin/rsync -e ssh -avzxn --numeric-ids --delete --progress \

Still works. Try running with root privileges (still same user):

sudo /usr/local/bin/rsync -e "ssh -i /full_path_to_identity/id_dsa" \
                          -avzxn --numeric-ids --delete --progress \

Works. Now the only difference is that one invocation connects as root and the other does not. Swap root for non_root_user in the invocation above and we’re back into "protocol mismatch" territory again…

Try removing the command restriction entirely from the authorized_keys file, setting PermitRootLogin to yes and sending the SIGHUP signal to the sshd daemon again.

Works. So there must be something wrong with the command specification. Restore PermitRootLogin and send SIGHUP again. I suspect that rsync is not behaving as documented on

I created a script and placed that in the forced command specification, then attempted to connect:


echo "$SSH_ORIGINAL_COMMAND" > /tmp/debug.out

This revealed that the actual command being sent by rsync was:

rsync --server --sender -vnlogDtprxz --numeric-ids . /

There are several problems with this:

  • The passed command as stated in the rsync man page is incorrect
  • There is no trailing period as stated in the man page; the trailing entry is /
  • The --sender option is not documented in the man page or anywhere that I can find, not even by /usr/local/bin/rsync --help (update: the current CVS version of the man page mentions the switch)
  • The full path to the custom build of rsync is not being passed through

After updating my command definition to reflect the actually-passed parameters, things seem to work (in "dry run" mode).

Byte-ordering issues

Real ("non-dry" runs) all fail with messages like this:

Invalid file index: -1610612736 (count=9406) [sender]
rsync error: protocol incompatibility (code 2) at sender.c(169) [sender=2.6.8]
rsync: writefd_unbuffered failed to write 4092 bytes [generator]:
Broken pipe (32)rsync: connection unexpectedly closed (195901 bytes received so far) [receiver]
rsync error: error in rsync protocol data stream (code 12) at io.c(463) [receiver=2.6.8]
rsync error: error in rsync protocol data stream (code 12) at io.c(1119) [generator=2.6.8]

This mailing list post by the creator of rsync suggests that this might be a byte-ordering issue:

This number is exactly 0x60000000, so another possibility that comes to mind is that the byte-order messed up somehow. Look in byteorder.h and make sure that CAREFUL_ALIGNMENT is being defined on any system that uses most-significant-byte-first ordering.

Now, the i386 is not a big-endian system, but I thought I’d try setting CAREFUL_ALIGNMENT anyway and rebuilding. No effect:

Invalid file index: 524288 (count=161486) [sender]

Other errors:

Invalid file index: 524288 (count=161442) [sender]
Invalid file index: -1610612736 (count=161454) [sender]

Note that 524288 is exactly 0x80000.

I tried pulling down the latest source from CVS but got the same problem. Will try downgrading to previously installed versions using the --protocol switch:

  • Change command specification to refer to /usr/bin/rsync
  • Invoke local copy of /usr/bin/rsync with the --protocol=26 switch

No longer see "Invalid file index" errors, but the backup appears to stall after a number of files have been copied (no error message) and then crashes:

Exception:  EXC_BAD_ACCESS (0x0001)
Codes:      KERN_INVALID_ADDRESS (0x0001) at 0x3fffd0d6

Thread 0 Crashed:
0   <<00000000>> 	0xffff0ac4 __memcpy + 804 (cpu_capabilities.h:228)
1   rsync 	0x0000a5a7 0x1000 + 38311
2   rsync 	0x000041c8 0x1000 + 12744
3   rsync 	0x00004ea8 0x1000 + 16040
4   rsync 	0x0000899e 0x1000 + 31134
5   rsync 	0x000092ad 0x1000 + 33453
6   rsync 	0x0000a215 0x1000 + 37397
7   rsync 	0x00001afe 0x1000 + 2814
8   rsync 	0x00001a19 0x1000 + 2585

Thread 0 crashed with X86 Thread State (32-bit):
  eax: 0xffff07a0    ebx: 0x7fffffff ecx: 0x7ffffffc edx: 0x00000003
  edi: 0x80031383    esi: 0x3fffd0d6 ebp: 0xbfffcff8 esp: 0xbfffcff0
   ss: 0x0000001f    efl: 0x00010202 eip: 0xffff0ac4  cs: 0x00000017
   ds: 0x0000001f     es: 0x0000001f  fs: 0x00000000  gs: 0x00000037

Trying with 2.6.9pre1

On both the remote server and the local machine:

tar xzvf rsync-2.6.9pre1.tar.gz
cd rsync-2.6.9pre1
sudo make check
sudo make install

Seems to work, so something changed between 2.6.9pre1 and the version from the CVS that I tried a few days ago.


Configuring a cron job

For now I’ll be running this manually.

Avoiding password prompts

The private key file used to log in to the remote server is protected by a password. It would be possible to use a technique like that mentioned in "Setting up multiple repository access methods with Subversion" but given that the key allows the holder to read any file at all on the server for the time being I prefer to be prompted for a password.