Category Archives: Sysadmin

Upgrading a VM from macOS 10.12 Sierra to macOS 10.13 High Sierra in VirtualBox

For testing purposes, I have a VM in VirtualBox currently runnning macOS 10.12 Sierra. Now that macOS 10.13 High Sierra is in Beta, I wanted to upgrade my VM to this new release. However, this proved to be difficult with the usual ways. This blog post will describe how to upgrade a Sierra VM to High Sierra.

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How to run rsync on remote host with sudo

Sometimes I want to transfer files including ownership. This is not possible as normal user as the chown(2) system call requires special privileges, that is: uid == 0. However, I do not want to open ssh access for root, but go with the usual way to elevate my privileges: sudo.

I will go through common solutions presented on the web and explain why these do not work at all without significant modifications on the remote host and then present a working solution using X11-Forwarding that is less invasive.

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Should we distrust Comodo after issuing a rogue SSL certificate for Windows Live?

About a year ago, I wrote an article why I no longer trust StartSSL. Back then, I said I switched to a paid certificate issued by Comodo under the PositiveSSL brand instead. A reader now brought a recent issue with a Comodo certificate erroneously issued for Microsoft’s Windows Live to my attention and asked whether I would still prefer them over StartSSL.

Arno wrote this comment (link):

Do you still trust Commodo to be more trustworthy than StartCom just because they asked for money to handle revocations? Think twice – a guy from Finland managed to get a valid certificate from Commodo for “”, (Microsoft Live in Finland), just because he was able to register “” as his e-mail-address:

I started to type my answer as a comment as well, but soon I realized my explanation just became too long to be a comment, so I turned it into an article on its own.
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Backup with duply to Amazon S3: BackendException: No connection to backend

I stumbled across this problem during setting up duplicity backups to a S3 bucket. As it took me quite a while to resolve this, I wanted to document this problem and its solution here. I just hope someone else with the same problem may find this blog post.

I tried to set up duply, a frontend for the backup tool duplicity, to back up to Amazon S3 storage.

The challenge appeared to be that I wanted to do this with the version available in Debian wheezy. The problem described here is probably already fixed in duplicity >= 0.7.0. These are the versions I used:

i duplicity	wheezy-backports	0.6.24-1~bpo70
i duply         stable
i python-boto   wheezy-backports	2.25.0-1~bpo7


I added S3 as a target to the duply configuration as documented on various places on the web. However, I always ran into this error message:

$ duply donkey-s3-test status
Start duply v1.5.5.5, time is 2015-03-12 00:03:40.
Using profile '/etc/duply/donkey-s3-test'.
Using installed duplicity version 0.6.24, python 2.7.3, gpg 1.4.12 (Home: ~/.gnupg), awk 'GNU Awk 4.0.1', bash '4.2.37(1)-release (x86_64-pc-linux-gnu)'.
Signing disabled. Not GPG_KEY entries in config.
Test - Encryption with passphrase (OK)
Test - Decryption with passphrase (OK)
Test - Compare (OK)
Cleanup - Delete '/tmp/duply.10622.1426115020_*'(OK)

--- Start running command STATUS at 00:03:40.984 ---
BackendException: No connection to backend
00:03:41.301 Task 'STATUS' failed with exit code '23'.
--- Finished state FAILED 'code 23' at 00:03:41.301 - Runtime 00:00:00.316 ---

Similar occurrences of this bug are also tracked here:


The exception above is highly unspecific and returning such a generic error message is bad style in my opinion. It took me quite a while to find the solution. To make it short, with this snippet from my /etc/duply/donkey-s3-test/conf file I got this to work:

# XXX: workaround for S3 with boto to s3-eu-central-1
export S3_USE_SIGV4="True"

Using a shell export in the configuration file is clearly a hack, but it works. In fact, you can also export it to the environment before running duply or set it in the configuration file of the boto library. However, with the former, you do not have to change anything on the duply invocation.

Why does this solve the problem?

I found out that the problem was not reproducible for some people because it only appears in specific regions. I use Frankfurt, EU (eu-central-1) as my Amazon S3 region. According to the documentation, only the newest API V4 is supported in this region:

Any new regions after January 30, 2014 will support only Signature Version 4 and therefore all requests to those regions must be made with Signature Version 4.

The region Frankfurt, EU was introduced after this date. This means this new region only accepts requests with “Signature Version 4” and not any prior version. Meanwhile other regions continue to accept the old API requests.

This kind of setup is complete madness for me. Especially for open source projects with developers all around the globe, this just means that some developers could not reproduce the problem. Who would assume your endpoint region matters?

In fact, the duplicity manual page has a whole section on how European endpoints are different from other locations. Unfortunately, the recommended --s3-use-new-style --s3-european-buckets does not solve this problem. I could not even observe any difference in behavior with these flags.

Apparently, the boto library used by duplicity for access to Amazon S3 supports the new “Signature Version 4” for API requests, but it is not enabled by default. By exporting this environment variable S3_USE_SIGV4=True the library is forced to use “Signature Version 4”.

The specification of the target protocol for duplicity is another peculiarity. Make sure you use s3:// and specify an explicit endpoint region in the URL, as I could not get it work with s3+http:// and also always with the hostname for your region.

Further Investigations

Unfortunately, the duplicity option --s3-use-rrs which is supposed to put the files into the cheaper Reduced Redundancy Storage (RRS) does not seem to do anything and all uploaded files get the standard storage class. Probably I have to maintain my own installation of the latest versions of duplicity and boto to get all the features to work.

Depending on where you are in the world, YMMV.