Posts tagged Linux

Getting snmpd to work through IPv6

I have Cacti installed so that I have pretty graphs (CPU/load/memory/disk/network) for all my servers (VPS) in one place. I just enabled full IPv6 on one of them and Cacti stopped working. The problem wasn’t Cacti though, it was the server it was now trying to contact through IPv6.

As I only need SNMP for Cacti, the snmpd.conf file on all my servers is very simple, just:

rocommunity public

However for IPv6 that isn’t enough. On Ubuntu 14.04 (what the server is running) snmpd will only work on IPv4 this way. I quickly found out you can use an agentAddress directive in the snmpd.conf file to enable the daemon to listen on IPv6, but more changes are required if you want to actually return data through IPv6. As it turns out the “rocommunity” directive is apparently also IPv4 only.

The final solution was this:

agentAddress udp:161,udp6:161
rocommunity public
rocommunity6 public

Warning: above configuration will cause snmpd to listen on all network interfaces. Be sure to lock down access to UDP port 161 in your firewall to only authorized hosts. And make sure your firewall supports IPv6.

Keep WordPress & phpMyAdmin up-to-date with a single command II

A few years ago I wrote about keeping WordPress & phpMyAdmin up-to-date with a single command. While those methods still work it is time to make a few changes (and revive this blog while I’m at it, it has been too long).

First off, using Subversion to track stable versions might keep WordPress itself up-to-date (unless something breaks due to subtle changes in the repository as I found out with WordPress 4.1), it won’t do anything for your themes or plugins.

If you like to keep everything in WordPress up-to-date through the command line (instead of FTP, which I don’t even have installed) there now fortunately is a tool called wp-cli. Updating all plugins for example is as simple as:

wp plugin --all update

There is loads more it can do, see the wp-cli site for documentation of all commands and features.

Second, while the method of using git to update phpMyAdmin in my previous article wasn’t inaccurate, it was highly inefficient. The commands would first download the whole phpMyAdmin repository (including all branches and commit history) and then switch to tracking the stable branch. This was slow and wasted a lot of space.

So here are the updated commands:

git clone --depth=1 -b STABLE
git pull -q origin STABLE
composer update --no-dev

With this we tell git that we want only the latest commit (depth flag) in the stable branch (b flag). This executes must faster and occupies only about 70 MB of space instead of nearly 500 MB. I recommend calling the git pull command from a cron job every day, so that your phpMyAdmin is always up-to-date.

Mac OS X Time Machine backup on a Debian server

December 2015: I just wanted to note that the solution below stopped working for me after a while. However I recently purchased a Synology NAS and configuring as a Time Machine backup destination is fairly easy.

I just got a MacBook Air and had already read about how you can setup your own network based Time Machine backup on a Linux server. The problem is that there is some outdated information floating around, which caused me some headaches getting it to work with errors such as “The network backup disk does not support the required AFP features” and “backup failed with error 45” (in Console).

First: you need version 2.2.x of the “netatalk” package. Debian 6.0 (Squeeze) comes with 2.1 by default, which won’t work (with Mac OS X 10.8 “Mountain Lion” at least). Second, lots of how-to information includes instructions on creating a /etc/avahi/services/afpd.service file. With netatalk 2.2, this file is redundant: you do not need to create it.

Back to netatalk. Really old information will talk about needing to compile it from source to have it support the required authentication protocol. That is no longer relevant with the most recent version. If you are still running Debian 6.0 (still the stable version at time of writing) you can get netatalk 2.2 from Debian 7.0 (Wheezy) by doing the following (all as root): first add the following line to /etc/apt/sources.list

deb-src wheezy main contrib non-free

Then run the following commands:

aptitude update
apt-get build-dep netatalk
apt-get -b source netatalk
dpkg -i netatalk_2.2.2-1_amd64.deb

Change the filename accordingly if the version number has slightly changed. With netatalk 2.2 installed, follow other guides on the net (such as this one or this one) to configure a Time Machine AFP share on your Debian server.

Hiding MySQL/PostgreSQL password from the “ps” command

If you want to run a database tool for MySQL from the command line it would seem you cannot avoid including the password using a –password argument. The problem is that the whole command line while the command is being run will be visible to any other user of the same server using the “ps” command.

The solution is to specify the password in a “option” file (as MySQL calls it). This is a file named .my.cnf (note the dot at the beginning) in your account’s root folder (ie: /home/myname/ or /root/ for the root user) with the following contents:


Of course replace the “password” with your real password. You should als make this file only readable by yourself with the following command:

chmod 600 .my.cnf

That ensures that nobody else can open the file (well except root if you are not an admin, but the root user would have access to everything anyway).

Now whenever you run a MySQL tool (mysql, mysqldump, etc) the password from the option file will be automatically used. If your username matches the MySQL username you also don’t need to specify a username. This can be very useful especially as root, when you need to do daily backups using mysqldump for example.

This tricks works in a similar way for PostgreSQL: put a file named .pgpass with the following contents:


You can use an asterisk (*) to match “any” value (wildcard), for example for port or database.

Arrays in Linux shell scripting

For a script I want to build for maintaining WordPress themes & plugins a bit easier I wanted to basically loop through a list of locations of WordPress installations on a single server. This basically means I need to be able to create an “array”.

I knew that you could use arrays in a shell script as I was using it in another script to automatically make a backup of all databases in a MySQL or PostgreSQL database, however that script gets the list from the output of an application: how do you define them yourself?

First thing to know is that array support differs by which shell you use. I prefer simply “/bin/sh”, but that only has extremely basic array support. Bash (and others) have better support. Fortunately the array support in “/bin/sh” is sufficient in my case.

Anyway, on to the solution. In “/bin/sh” you define an array as a list of strings separated by spaces (as such, I’m not sure if you can include strings with spaces) and use a very simple “for” loop to iterate through them:


ARRAY="Hello World"

for WORD in $ARRAY
  echo $WORD

This script will display each word from the array on a different line. Note that there is no way to refer to a specific member of the array (you can’t use an index).

In Bash you can use arrays in a more advanced way:


ARRAY=("Hello" "World")
ARRAY[2]="In Bash"

for ((i = 0; i < $LENGTH; i++))
  echo ${ARRAY[i]}

Now as you can see you can easily define a list of strings (with spaces if you want), refer to specific members of the array using an index and get the length of the array.

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