The Usual Tech Ramblings

Configuration Management (Part III): Managing your configuration

This one has been sitting idle for a couple of weeks. Lethargy kicked in, and work was consuming too much of my brain power, but here it is, I’m working on getting it out the door. In this episode, we’ll be running through the basics of svn, and getting it working with the files we have in place, adding new files, reverting, and planning for big upgrades… Note, this is more of an SVN primer than anything. If you know how to tag, branch, revert, have a good general knowledge of SVN, this one can be skipped.

Modifying a File

The machine I’m working on has Bind installed. First lets edit the named.conf.local file, and add reference to a new zone file.

cat >> /etc/bind/named.conf.local

zone "" {
    type master;
    file "/etc/bind/";

Hit CTRL-D and it’ll quit the cat command, and append the data to the file (note I used »). We can verify this by simply doing a quick cat again:

cat /etc/bind/named.conf.local

Now we’ve verified we have content added to the file, lets see what SVN thinks about our new directory changes.

cd /etc
svn diff

Now we see a handful of lines that look like this:

Index: bind/named.conf.local
— bind/named.conf.local       (revision 5)
+++ bind/named.conf.local       (working copy)
@@ -6,3 +6,9 @@
// organization
//include "/etc/bind/zones.rfc1918";
+zone "" {
+    type master;
+    file "/etc/bind/";

This can be sent to a file, and used as a patch on another system. The format is a predefined layout that tells another system all the information it needs to update another file. For example, lines starting with a – are removed from a file, those with a + are added to the file. This shows that the difference between my SVN backup, and my live copy is the changes I’d just made. Now as I want to keep those, I’m going to check them in.

svn commit -m "Adding zone reference" named.conf.local

The –m argument allows you to add a message. If you need to add a longer message, such as a multi-line entry, omit that argument, and the text, and SVN should prompt in your default editor. Adding a New File

Now we’ve added our reference, lets create the zone file quickly. I’m going to cheat in this case for speed, and copy Debian’s db.local file to the new name.

cp /etc/bind/db.local /etc/bind/

Now bind is going to be happy, however, we now have a new file to add to svn. We can see this by using the status command.

svn status
? bind/
svn add bind/
svn commit -m "Adding zone file for for Bind" bind/

The ? shows the new file, modified files will show M, deleted D, and A will show added files with no commits.

Reverting a change

You know we’ve all done it. Edited a file, it didn’t work out, but you forgot what you changed. That’s what version control does, keeps previous versions, saves our bacon when we need it. We’re going to go extreme here, we’re going to trash the main Bind configuration with some random data. Ignoring the syntax if you don’t know what it does, I’m basically taking the random entropy source, and dumping it into the named.conf file, not a good thing, that’s for sure.

 dd bs=1024 count=1 if=/dev/urandom of=/etc/bind/named.conf
  1+0 records in
  1+0 records out
  1024 bytes (1.0 kB) copied, 0.000929448 s, 1.1 MB/s
head /etc/bind/named.conf
  #lots of random stuff#
svn status
M    bind/named.conf

So using my above, rather destructive code, I’ve overwritten /etc/named.conf with 1MB of random data. That’s okay, we’re going to call on the revert command.

svn revert bind/named.conf
Reverted ‘bind/named.conf’
head bind/named.conf

And we’re back to how we were. That’s great, it means Bind will be happy again.

What happens if it’s a change we committed? What if we made a change, committed it, and discovered it was an issue a week later, but we don’t remember exactly what it was? First, lets look at the log, and see when the file was changed, revision, and the (hopefully) useful log entry we added…

svn log bind/named.conf.local
r7 | root | 2010-08-09 21:38:36 -0500 (Mon, 09 Aug 2010) | 1 line

Adding zone reference
r5 | root | 2010-06-30 20:06:27 -0500 (Wed, 30 Jun 2010) | 2 lines


There, we see there were 2 changes committed to this file. Part of the initial commit (yes I created a bad log entry there), and r7, which is the one we just made. So lets say we want to get back to r5, unfortunately svn doesn’t have a “rollback” to a specific version, you have to merge backwards the changes. This isn’t too difficult to do, we’ll use the example from the modifying files at the top to roll back…

cd /etc/bind
svn log named.conf.local
svn merge –revision 7:5 named.conf.local
svn status
svn diff named.conf.local
svn commit -m "Removing from Bind" named.conf.local

In this example, we looked at the log, and see the entry was r7 for when we committed the entry for the zone entry, and r5 was the last change before that. So the command merges from 7 down to 5. svn status will now show that the file has changed, and diff will show the inverse of the top second. Commit just solidifies the work.

Handling Major Upgrades

Major upgrades are difficult, we want a point in time restore. What happens if the upgrade goes badly, and we need to roll back a couple of packages, but the config files don’t get reverted? Whilst SVN will handle that using the above methods, we’re going to want to keep a way of finding those changes easily. This is where tags come in handy. They’re a complete copy of an existing branch. We’re going to make a folder for tags, and make a full copy of the current config.

svn mkdir –parents -m "Creating tags folder" file:///srv/svn/thebe_config/tags
svn copy -m "Backup for Thebe Debian 5.0.5 on 08/09/2010" \
  file:///srv/svn/thebe_config/trunk \

You’ll see the standard SVN commit confirmation, now we can checkout the results using a command we used in part II.

svnlook tree /srv/svn/thebe_config | more

If you skim through the results, you’ll now see a tags folder, with a subfolder 20100809deb505, and a complete copy of Trunk.

Now we’ve done that, and we do the upgrade, we can go back at any point, and see what was changed…

svn diff –old=file:///srv/svn/thebe_config/tags/20100809deb505 \

Finishing up

There is one thing I didn’t do above, but thought I’d point it out at the bottom to emphasis it a little bit. Some files need special permissions, and we want to retain those too. You might remember in the previous edition, we introduced asvn, which was a wrapper script around the svn command. Once you’ve added your files to svn, use the asvn script to update svn with the info flags for the file, and check that in too. You can see how to use the command in part II.

Now we have a good basis for storing and archiving our configurations, next time we’ll look at monitoring them for changes that aren’t checked in.