Archive for 'Sysadmin'

So we took on some vendors to help us out. I wanted to provide these individuals authenticated, read-only access to our git repos so they could stay current with the project, but not commit code directly (they’ll have their own repo). Google yielded these excellent results pages.

When read altogether, I had a few options.

  1. Run the git-daemon. The repo would be publicy available over the internet via the git (git://) protocol.  This provides read-only access, but it provides it to the whole world.  Not a solution.
  2. Run a GitHub-like server, like Gitosis or Gitolite.  This was the sledgehammer solution.  Fine-grained access control, plus all the other bells and whistles. But I was not interested in installing and understanding a whole git-hosting server application.
  3. SSH via git-shell.  Each user gets a regular SSH account on the server, but each user gets a git-shell instead of a regular login shell like bash. The git-shell restricts the user to git operations.  Write permissions are restricted using standard Linux permissions on the repo itself.

So #3 was what I needed. I created an SSH login for each user that needed access to the repo. I set the login shell for each user to /usr/bin/git-shell. I put each user in a group that had read-only file system permissions to the repo.  Testing it out worked well.  Users could git clone and pull, but pushing failed and attempting to SSH directly failed.

One last note: as the man page mentions, you can create a directory called git-shell-commands in home directories of git-shell usersgit-shell users will be able to run any command in this directory.  If there is a help program in the directory, it is run when a git-shell user logs in.  More details on git-shell-commands here, including the location of sample git-shell commands on your server.

MySQL Tip Of The Day: ~/.my.cnf

Sometimes, I want to pop onto a database server, check the status of something, and then logout. So, for example, if I want to check on the number query cache free blocks, I run this long command:

% mysqladmin -u admin -p extended | grep -i qcache

Then I type in the password. Well, I grew tired of typing in the extra options, plus the password. Turns out, MySQL will look for the configuration file .my.cnf in your home directory after it looks in /etc/my.cnf (it looks in a few other places as well). So I put this in my ~/.my.cnf:


And now I can simply run:

% mysqladmin extended | grep -i qcache

and it works right away.  Note that the password is stored in the clear.

HTTP Keep-Alive

Like most people, I did not know much about HTTP Keep-Alive headers other than that they could be very bad if used incorrectly. So I’ve kept them off, which is the default. But I ran across this blog post which explains the HTTP Keep-Alive, including its benefits and potential pitfalls pretty clearly.

It’s all pretty simple really. There is an overhead to opening and closing TCP connections. To alleviate this, Apache can agree to provide persistent connections by sending HTTP Keep-Alive headers. Then the browser can open a single connection to download multiple resources. But Apache won’t know when the browser is done downloading, so it simply keeps the connection open according to a Keep-Alive timeout, which is set to 15 seconds by default. The problem is the machine can only keep so many simultaneous requests open due to physical limitations (e.g. RAM, CPU, etc.) And 15 seconds is a long time.

To allow browsers to gain some parallelism on downloading files, without keeping persistent connections open too long, the Keep-Alive timeout value should be set to something very low, e.g. 2 seconds.

I’ve done this for static content only. Why only static content? It doesn’t really make much sense for the main page source itself since that’s the page the user wants to view.

I’ve mentioned before that by serving all static content on dedicated subdomains, we indirectly get the benefit of being able to optimize just those subdomains. So far, this meant:

  1. disabling .htaccess files
  2. setting a far-future Expires: header
  3. avoiding setting cookies on the subdomain

Now we can add to the list: enabling HTTP Keep-Alive headers. The VirtualHost block might look like this now:

    DocumentRoot    /var/www/vhosts/
    KeepAlive On
    KeepAliveTimeout 2
        AllowOverride None
        ExpiresActive On
        ExpiresByType text/css "access plus 1 year"
        ExpiresByType application/x-javascript "access plus 1 year"
        ExpiresByType image/jpeg "access plus 1 year"
        ExpiresByType image/gif "access plus 1 year"
        ExpiresByType image/png "access plus 1 year"

Note the following applies to Windows Vista, but is probably easier on MacOS/Linux.

Is your hosts file becoming monstrous?  Do you have an alias or shortcut to your hosts file because you edit it so often?  Tired of manually adding every subdomain and domain you work on?

I was too when I thought there must be a better way.  And there was.

The general idea is this: by installing a local DNS nameserver in BIND, we can set up local development domains that look like regular domains on the internet. For real domains, we’ll just forward the requests on to a real nameserver.  This gives us a couple more benefits: 1) we can use the local nameserver as a caching nameserver to speed up DNS queries (in theory, I have not actually done this), and 2) we can choose to use any DNS service we wish, i.e. OpenDNS, or Google DNS.

Here are the steps.

  1. Follow these instructions on installing and configuring BIND and configuring a zone for your local domain.
    1. I installed BIND to C:\Windows\system32\dns.
    2. Here is my named.conf in its entirety.
      options {
          directory ";c:\windows\system32\dns\zones";
          allow-transfer { none; };
          forward only;
          forwarders {
              //; // OpenDNS
    ; // Google DNS
          query-source address * port 53;
      logging {
          channel queries_log {
              file "c:\windows\system32\dns\var\queries.log";
              print-severity yes;
              print-time yes;
          category queries { queries_log ; };
      zone "work.local" IN {
          type master;
          file "work.local.txt";
      key "rndc-key" {
          algorithm hmac-md5;
          secret "xxxxxxxxxxxxxxxxxxxxxxxx";
      controls {
          inet port 953
              allow {; } keys { "rndc-key"; };
    3. I created a zone file for my development domain work.local following this zone file example. Here is the zone file in its entirety.  Note the CNAME wildcard record.
      $TTL 86400
      @	IN SOA (
      			86400 )
      @		NS
      	IN A
      ns1	IN A
      www	IN A
      *	IN CNAME	www
  2. Start or restart the BIND service.
  3. Configure you network connection to use as your primary nameserver, instead of DHCP.  My IPv4 properties look like this:

    Set DNS nameserver to

  4. Flush the Windows DNS cache by running:
    C:\> ipconfig /flushdns
  5. Test BIND by pinging  If you have errors, you can uncomment the logging block in named.conf.
  6. Once that is working, create a VirtualHost in Apache for your development domain.  Thanks to VirtualDocumentRoot, we can map any number of subdomains to project roots.  Here is my VirtualHost block.
        ServerAlias *.work.local
        VirtualDocumentRoot "C:/_work/%1"
            Options Indexes FollowSymLinks Includes ExecCGI
            AllowOverride All
            Order allow,deny
            Allow from all
  7. Start or restart Apache.
  8. Create a directory in C:\_work, for example, C:\_work\awesomeapp.  Create a test index.html file in that directory.
  9. You should now be able to go to in your browser and see your index.html file!

Now, you should be able to repeat step 8 for any new website you create!  No editing of hosts files, no bouncing the webserver!  Just create the project directory and it’s immediately available.

One other important note: Firefox has its own DNS cache independent of the OS.  For sanity, restarting Firefox resets its DNS cache. You can also permanently disable DNS caching in Firefox.

Securely Running A Command As Root

As much as I wish we deployed builds from our continuous integration server, all but one of our products is deployed with good ol’ `svn up`.  Developers generally have access to only one web server, so I needed an rsync command to propagate new code to the rest of the web servers.  I wanted normal user accounts to be able to run it at any time in any directory with one command.  Then developers would be instructed to run this command after updating any files.

So I whipped up an shell script that called rsync with some predefined options and targets.  Unfortunately, in order to preserve ownership and permissions in the destination, rsync needed to be run as root.

At first, I looked at the setuid bit. By changing the ownership of the rsync shell script and running `chmod u+s` on the script, setting the setuid, any user could execute it and it would run as root. Well, it turns out that the kernel will not honor setuid on shell scripts for security reasons. But what if I wrote a C program instead of a shell script? That actually worked, and ran with root privileges, but it still did not rsync as root for some reason. So that was out.

The second solution was to insert sudo before the rsync command in the script. I modified /etc/sudoers to allow the users group to run rsync under sudo. That worked perfectly. So if I put this script in /usr/local/bin, I would be done. But I had already written this magnificent (two-line) C program.  Why not make it even more secure (sudo does not work on shell scripts either)?  Instead of allowing all users to run rsync under sudo, I could limit them to running only my C program under sudo, instead of rsync in general. Then, in my script, I could replace rsync with my C program. So that’s what I did. I again modified /etc/sudoers and my shell script, threw both the script and C executable in /usr/local/bin and I was done.

I named the final command `zipsync`. Here is the shell script for that, anonymized a bit.

cd /var/www/vhosts

# repeat for each web server
sudo zipsync.bin \
   -av --delete \
   --exclude=".svn" \
   --exclude="logs" \
   --exclude="tmp" \
   --exclude="cache" \
   --exclude="*.swp" \

cd -

And the C program, zipsync.bin.

int main(int argc, char** argv)
   *argv = "rsync";
   return execvp(*argv, argv);