Category Archives: Technology

Lazy Linux: 10 Essential tricks for admins

Lazy Linux: 10 Essential tricks for admins is an awesome list of cool things you can do with Linux. I learned about trick #3 (collaboration with screen) from an admin at the datacenter where one of my servers is hosted. I remember being thrilled sitting there watching him do stuff on my server while sharing the keyboard to type messages back and forth in vi (think Remote Desktop or VNC, but on the console). Trick #5 (SSH back door) is something I’ve been using for years at work for remote diagnostics. It is an invaluable trick for getting around firewalls. Very cool stuff!

Being Greedy With Bash

Last night at my C/Unix class the professor quickly glossed over an interesting shell scripting technique that allows you to strip stuff off the beginning or end of a variable. I forgot about it until I saw the technique used again while editing a shell script at work today.

I didn’t know what the technique was called but I remembered the professor saying something about “greedy clobbering” and, since I cannot search Google for special characters, I Googled “Bash greedy” and luckily found 10 Steps to Beautiful Shell Scripts, which just so happened to contain the technique I was looking for (#5).

There are basically four versions of this technique:

${var#pattern}
Search var from left-to-right and return everything after the first occurrence of pattern

${var##pattern}
Search var from left-to-right and return everything after the last occurrence of pattern (be greedy)

${var%pattern}
Search var from right-to-left and return everything after the first occurrence of pattern

${var%%pattern}
Search var from right-to-left and return everything after the last occurrence of pattern (be greedy)

Here’s how it works. Let’s say you have a variable that contains the path to a file:

FILE=/home/raam/bin/myscript.sh

Now let’s say you wanted to extract the myscript.sh part from that variable. You could do some funky stuff with awk but there is a much easier solution built into Bash:

SCRIPTNAME=${FILE##*/}

Now $SCRIPTNAME will contain myscript.sh!

The ##*/ tells the shell to search left-to-right for everything before and including the slash (*/), be greedy while doing it so that all the slashes will be found (##), and then return whatever is left over (in this case, myscript.sh is the only thing remaining after the last slash).

AFAIK, this is a Bash-specific feature, but I’m not entirely certain and I wasn’t sure where I could look to find out. It’s amazing how four characters can do so much work so easily. The more I learn about what I can do with Bash, the more I wonder how I ever lived without all this knowledge!

Bounce-back Spam (Backscatter)

I really hate bounce-back spam! (I call it bounce-back spam, but the official name for it is Backscatter.) I’ve read, and been told by sysadmins, that there is not much that can be done about it. The Wikipedia page on bounce messages has a little section that explains why:

Excluding MDAs, all MTAs forward mails to another MTA. This next MTA is free to reject the mail with an SMTP error message like user unknown, over quota, etc. At this point the sending MTA has to inform the originator, or as RFC 5321 puts it:

If an SMTP server has accepted the task of relaying the mail and later finds that the destination is incorrect or that the mail cannot be delivered for some other reason, then it MUST construct an “undeliverable mail” notification message and send it to the originator of the undeliverable mail (as indicated by the reverse-path).

This rule is essential for SMTP: as the name says, it’s a simple protocol, it cannot reliably work if mail silently vanishes in black holes, so bounces are required to spot and fix problems.

Today, however, most email is spam, which usually utilizes forged Return-Paths. It is then often impossible for the MTA to inform the originator, and sending a bounce to the forged Return-Path would hit an innocent third party. This inherent flaw in today’s SMTP (without the deprecated source routes) is addressed by various proposals, most directly by BATV and SPF.

It looks like I’ll have to just deal with it. (I could set up filters and such, but then I might miss a real bounce-back and not know that my message didn’t go through!) I’m just grateful it comes in waves of a few hours every few weeks instead of non-stop! Has anyone else had to deal with this? If so, what did you do about it?

Backscatter Spam

I subconsciously converted a problem into a shell script

I have been writing a lot of shell scripts lately as part of the C/Unix class that I’m taking at Harvard Extension. My familiarity with how the Unix shell and the underlying system works has grown exponentially. When I came across a problem earlier today, I subconsciously turned the problem into a shell script without even thinking about it!

The problem: “How can I check to make sure my program is running every 30 minutes and restart it if it’s not?”

Answer:

I’m sure there are many better ways to solve this problem, but the fact that I instantly translated the problem into shell scripting code (and that it worked as expected on my first try) astonished me. I can see how good programmers who write in a particular language, and know the in’s and out’s the like the back of their hand can turn problems into code seamlessly (or know exactly where to look to find answers if they’re unsure).

It’s really amazing how easily you can solve simple problems when you have a deeper understanding of how the system works.

That’s all. I just wanted to share my excitement. :)

Using ‘rsync –exclude-from’ to Exclude Files Containing Spaces

A few months ago I wrote a post about escaping filename or directory spaces for rsync. Well that wasn’t the end of rsync giving me problems with spaces.

When I used the --exclude-from rsync option to specify a list of exclusions, I figured using single or double-quotes around files/directories that contain spaces would be enough to escape them. However, after swashing through hundreds and hundreds of lines from rsync’s output, I discovered the excluded directories were still being synced!

When using --exclude-from, files and directories should not contain any single or double quotes, only a backslash:

/afs/*
/automount/*
/Users/raam/Documents/Virtual\ Machines/*

Yahoo DNS Issues Cause Problems in the United States

Yahoo! appears to be inaccessible to people in the US. Visiting yahoo.com redirects to www.yahoo.com and fails to load. I confirmed it was at least somewhat limited to the US by trying the connection from a shell account on a server in Europe.

Using dig (a Unix DNS lookup utility), we can see from within the United States that there is a problem with DNS. There is no A record with an IP address listed in the ANSWERS section:

;; QUESTION SECTION:
;www.yahoo.com. IN A

;; ANSWER SECTION:
www.yahoo.com. 129 IN CNAME www.wa1.b.yahoo.com.

And from the server in Europe:

;; QUESTION SECTION:
;www.yahoo.com. IN A

;; ANSWER SECTION:
www.yahoo.com. 272 IN CNAME www.wa1.b.yahoo.com.
www.wa1.b.yahoo.com. 33 IN CNAME www-real.wa1.b.yahoo.com.
www-real.wa1.b.yahoo.com. 33 IN A 209.191.93.52

;; AUTHORITY SECTION:
wa1.b.yahoo.com. 273 IN NS yf2.yahoo.com.
wa1.b.yahoo.com. 273 IN NS yf1.yahoo.com.

If you try connecting directly to the missing IP address, you should at least be able to get the main Yahoo page: http://209.191.93.52. You might also try temporarily adding an entry to your /etc/hosts or C:\Windows\system32\drivers\etc\hosts if you want to continue being able to use the FQDN.

UPDATE: As of 15:50 EST, Yahoo appears to be working again. The outage appeared to start around 15:11 EST, so that’s a good 40 minutes of downtime.

Mounting HFS+ with Write Access in Debian

When I decided to reformat and install my Mac Mini with the latest testing version of Debian (lenny, at the time of this writing) I discovered that I couldn’t mount my HFS+ OS X backup drive with write access:

erin:/# mount -t hfsplus /dev/sda /osx-backup
[ 630.769804] hfs: write access to a journaled filesystem is not supported, use the force option at your own risk, mounting read-only.

This warning puzzled me because I was able to mount fine before the reinstall and, since the external drive is to be used as the bootable backup for my MBP, anything with “at your own risk” was unacceptable.

I had already erased my previous Linux installation so I had no way of checking what might have previously given me write access to the HFS+ drive. A quick apt-cache search hfs revealed a bunch of packages related to the HFS filesystem. I installed the two that looked relevant to what I was trying to do:

hfsplus - Tools to access HFS+ formatted volumes
hfsutils - Tools for reading and writing Macintosh volumes

No dice. I still couldn’t get write access without that warning. I tried loading the hfsplus module and then adding it to /etc/modules to see if that would make a difference. As I expected, it didn’t. I was almost ready to give up but there was another HFS package in the list that, even though it seemed unrelated to what was trying to do, seemed worth a shot:

hfsprogs - mkfs and fsck for HFS and HFS+ file systems

It worked! I have no idea how or why (and I’m not interested enough to figure it out), but after installing the hfsprogs package I was able to mount my HFS+ partition with write access.

Update:

As Massimiliano and Matthias have confirmed in the comments below, the following solution seems to work with Ubuntu 8.04:

From Linux, after installing the tools suggested before, you must run:
mount -o force /dev/sdx /mnt/blabla

Otherwise, in my fstab, I have an entry like this:
UUID=489276e8-7f9b-3ae6-8c73-69b99ccaab9c /media/Leopard hfsplus defaults,force 0 0


Understanding the Linux Load Averages

I have been using Linux for several years now and although I have looked at the load averages from time to time (either using top or uptime), I never really understood what they meant. All I knew was that the three different numbers stood for averages over three different time spans (1, 5, and 15 minutes) and that under normal operation the numbers should stay under 1.00 (which I now know is only true for single-core CPUs).

Earlier this week at work I needed to figure out why a box was running slow. I was put in charge of determining the cause, whether it be excessive heat, low system resources, or something else. Here’s what I saw for load averages when I ran the top command on the box:

load average: 2.86, 3.00, 2.89

I knew that looked high, but I had no idea how to explain what “normal” was and why. I quickly realized that I needed a better understanding of what I was looking at before I could confidently explain what was going on. A quick Google search turned up this very detailed article about Linux load averages, including a look at some of the C functions that actually do the calculations (this was particularly interesting to me because I’m currently learning C).

To keep this post shorter than the aforementioned article, I’ll simply quote the two sentences that gave me a clear-as-day explanation of how to read Linux load averages:

The point of perfect utilization, meaning that the CPUs are always busy and, yet, no process ever waits for one, is the average matching the number of CPUs. If there are four CPUs on a machine and the reported one-minute load average is 4.00, the machine has been utilizing its processors perfectly for the last 60 seconds.

The machine I was checking at work was a single-core Celeron machine. This meant with a continuous load of almost 3.00 the CPU was being stressed much higher than it should be. Theoretically, a dual-core machine would drop this load to around 1.50 and a quad-core would drop it to 0.75.

There is a lot more behind truly understanding the Linux load averages, but the most important thing to understand is that they do not represent CPU usage. Rather they represent the load on the CPU by processes waiting for their chance to use the CPU. If you still can’t get your brain away from thinking in terms of percentages, consider 1.00 to be 100% load for single-core CPU’s, 2.00 to be 100% load for dual-core CPUs, and so on.

Update: John Gilmartin had some insightful feedback and shared a link to Understanding Load Averages where there’s a nice graphical description for how load averages work.

Creating a Bootable OS X Backup on Linux: Impossible?

I’ve had plans for a while now to set up a backup system using a Debian Linux server and rsync to back up my MacBook Pro laptop. At first glance, it seemed like it would be pretty straight forward. I’ve been able to make a bootable copy of my entire MBP using nothing but rsync (thanks to some very helpful directions by Mike Bombich, the creator of the popular, and free, Carbon Copy Cloner software). And by bootable copy I mean I could literally plug in the USB drive and boot my MBP from the drive (hold down the Alt/Option key while booting). Restoring a backup is as simple as running the rsync command again, but in the reverse direction. I know this solution works because I used it when I upgraded to a 320GB hard drive.

To start, I needed to create a big enough partition on the external USB drive using Disk Utility (formatted with Mac OS Extended (Journaled)). I then made a bootable copy of my MBP with one rsync command:

sudo rsync -aNHAXx --protect-args --fileflags --force-change \
--rsync-path="/usr/local/bin/rsync" / /Volumes/OSXBackup

But my dream backup system was more unattended. I wanted something that would periodically (a couple times a day) run that rsync command over SSH (in the background) and magically keep an up-to-date bootable copy of my MBP on a remote server.

I love Linux and I jump at any opportunity to use it for something new, especially in a heterogeneous network environment. So when I decided to set up a backup server, I naturally wanted to make use my existing Debian Linux machine (which just so happens to be running on an older G4 Mac Mini).

So, after making a bootable copy of my MBP using the local method mentioned above, I plugged the drive into my Linux machine, created a mount point (/osx-backup), and added an entry to /etc/fstab to make sure it was mounted on boot (note the filesystem type is hfsplus):

/dev/sda /osx-backup hfsplus rw,user,auto 0 0

All that’s left to do now is to run the same rsync command as earlier but this time specifying the remote path in the destination (root@myserver.example.com:/osx-backup/). This causes rsync to tunnel through SSH and run the sync. Unfortunately, this is where things started to fall apart.

OS X uses certain file metadata which must be copied for the backup to be complete (again, we’re talking about a true bootable copy that looks no different than the original). Several of the flags used in the rsync command above are required to maintain this metadata and unfortunately Linux doesn’t support all the necessary system calls to set this data. In particular, here are the necessary flags that don’t work when rsyncing an OS X partition to Linux:

-X (rsync: rsync_xal_set: lsetxattr() failed: Operation not supported (95))
-A (recv_acl_access: value out of range: 8000)
–fileflags (on remote machine: –fileflags: unknown option)
–force-change (on remote machine: –force-change: unknown option)
-N (on remote machine: -svlHogDtNpXrxe.iL: unknown option)

According to the man page for rsync on my MBP, the -N flag is used to preserve create times (crtimes) and the --fileflags option requires chflags system call. When I compiled the newer rsync 3.0.3 on my MBP, I had to apply two patches to the source that were relevant to preserving Mac OS X metadata:

patch -p1 <patches/fileflags.diff
patch -p1 <patches/crtimes.diff

I thought that maybe if I downloaded the source to my Linux server, applied those same patches, and then recompiled rsync, that it would be able to use those options. Unfortunately, those patches require system-level function calls (such as chflags) that simply don’t exist in Linux (the patched source wouldn’t even compile).

So I tried removing all unsupported flags even though I knew lots of OS X metadata would be lost. After the sync finished, I tried booting from the backup drive to see if everything worked. It booted into OS X, but when I logged into my account lots of configuration was gone and several things didn’t work. My Dock and Desktop were both reset and accessing my Documents directory gave me a “permission denied” error. Obviously that metadata is necessary for a viable bootable backup.

So, where to from here? Well, I obviously cannot use Linux to create a bootable backup of my OS X machine using rsync. I read of other possibilities (like mounting my Linux drive as an NFS share on the Mac and then using rsync on the Mac to sync to the NFS share) but they seemed like a lot more work than I was looking for. I liked the rsync solution because it could easily be tunneled over SSH (secure) and it was simple (one command). I can still use the rsync solution, but the backup server will need to be OS X. I’ll be setting that up soon, so look for another post with those details.