Category: Linux

In the past, I’ve had trouble getting my Ubuntu machine to resolve the .local hostnames at work. I didn’t know why Ubuntu had this problem while other machines did not.

When I did a DNS lookup, it failed, and ping of host.something.local failed. Yet ping of the hostname without the .something.local extension worked. Odd. I googled various terms, but nothing useful came up. I tried watching the DNS lookup with tcpdump, but it didn’t capture anything.

Eventually, I thought of using ‘strace ping host.something.local’ to see what was happening, and it turns out that DNS was never being queried — it was talking to something called avahi.

I googled “avahi”, and was reminded that hostname resolution is configured in /etc/nsswitch.conf. In the case of Ubuntu, it’s configured to send *.local requests to Avahi (mdns4_minimal), and no further — i.e. if Avahi doesn’t resolve it, it doesn’t try DNS.

In my case, I want corporate DNS to resolve .local addresses. So I changed my /etc/nsswitch.conf from this:

hosts: files mdns4_minimal [NOTFOUND=return] wins dns mdns4

to this:

hosts: files wins dns mdns4_minimal mdns4

And now my Ubuntu development machine can communicate with our internal .local machines without having to resort to using IP addresses, short names, or having to place the mapping in /etc/hosts.

I needed to customize a custom Linux VM image with a static IP address in vCloud Director. Here’s the script I came up with. It doesn’t work unless VMWare tools is installed and running in the VM (kernel upgrades tend to break it). The VM should be stopped, then paste the script into the “Guest OS Customization” script area. Boot the VM with the “Power On and Force Recustomization” option.

 

#!/bin/bash
HOSTNAME=YourHostNameHere
IPADDR=StaticIpAddressGoesHere
NETMASK=255.255.255.0
(
echo "------------------------------------------------------------------------------"
date
if [ x$1 == x"precustomization" ]; then
  set -e
  echo "Running Pre-Customization tasks..."
  if [ x$HOSTNAME != x ]; then
    echo "Configuring hostname as $HOSTNAME"
    sed -i "/HOSTNAME/d" /etc/sysconfig/network
    echo "HOSTNAME=$HOSTNAME" >> /etc/sysconfig/network 
    hostname $HOSTNAME
    hostname
  else
    # Set it so it can be used below
    HOSTNAME=`hostname`
  fi
  if [ x$IPADDR != x ]; then
    echo "Configuring network IP=$IPADDR NETMASK=$NETMASK"
    rm /etc/udev/rule.d/70-persistent-net.rules
    sed -i "/IPADDR/d" /etc/sysconfig/network-scripts/ifcfg-eth0
    sed -i "/NETMASK/d" /etc/sysconfig/network-scripts/ifcfg-eth0
    sed -i "/HWADDR/d" /etc/sysconfig/network-scripts/ifcfg-eth0
    sed -i "/BOOTPROTO/d" /etc/sysconfig/network-scripts/ifcfg-eth0
    sed -i "/DHCPHOSTNAME/d" /etc/sysconfig/network-scripts/ifcfg-eth0
    cat >> /etc/sysconfig/network-scripts/ifcfg-eth0 <<ENDCFG
BOOTPROTO=static
IPADDR=$IPADDR
NETMASK=$NETMASK
DHCPHOSTNAME=$HOSTNAME
ENDCFG
  fi
  echo "Done with Pre-Customization"
elif [ x$1 == x"postcustomization" ]; then
  echo "Running Post-Customization tasks..."
fi
) | tee -a /root/customize.log

 

Update: It turns out that having VMWare Tools functioning properly is enough for vCloud Director to configure the network settings all by itself, without the help of a script.

When I want to debug a shell/bash shell script, I add the following to the script:


set -x
export PS4='+(${BASH_SOURCE}:${LINENO}): '


Other times, I export PS4 in my interactive shell, and run the script with “bash -x /path/to/script”. More bash script debugging tips are here: http://wiki.bash-hackers.org/scripting/debuggingtips

When I want to see what commands are being run on a Linux machine by a higher-level user interface, I turn to startmon. It shows me every process created, along with its arguments. This can be useful for figuring out how a CD/DVD-creation program calls wodim/cdrecord, or how it mounts a drive.

When I don’t have source code to a compiled program, and I want to see what system calls it is making, and check into why it might be failing to run, I use strace.

When I want to find out which programs are preventing me from unmounting a DVD or a USB flash drive, I use ‘lsof‘, which is available for nearly every UNIX-like operating system.

When I want to know what programs are consuming the most disk I/O and making things go slow, I use ‘top’ to see the top-memory consumers, and whether they have a high number of page faults. I use ‘iotop’ as well.

After having had Ubuntu 12.04 installed for months, I finally moved my old Fedora services over to it, including Postfix. Here’s how I configured it.

I configured /etc/aliases so that local email destined for ‘root’ (including regularly scheduled system cron jobs) is delivered to my local account, instead of being sent to root@comcast.net (they don’t appreciate getting my SPAM):

root: myusername@localhost

I added the following to /etc/postfix/main.cf. Notice that I appended a “.NOT” to the relayhost. I did this initially so that I could send test email messages, and make sure message delivery was correct, before allowing email to go to my ISP. More on this later.

alias\_maps = hash:/etc/aliases
alias\_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = $myhostname, localhost.$mydomain, localhost, robinson-lin
relayhost = [smtp.comcast.net.NOT]:587
smtp\_sasl\_auth\_enable = yes
smtp\_sasl\_password\_maps = hash:/etc/postfix/sasl\_passwd
smtp\_sasl\_security\_options =
sender\_canonical\_maps = hash:/etc/postfix/sender\_canonical
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox\_command = /usr/bin/procmail -a "$EXTENSION"
mailbox\_size\_limit = 0
recipient\_delimiter = +
inet\_interfaces = loopback-only

I added the following to /etc/postfix/sasl\_passwd:

smtp.comcast.net MyUsername:MyPassword

I added the following to /etc/postfix/sender\_canonical:

jaredrob MyUsername@comcast.net

I ran the following commands:

  chmod o-r /etc/postfix/sasl_passwd
  postfix check
  postmap /etc/postfix/sasl\_passwd
  postmap /etc/postfix/sender\_canonical
  postalias /etc/aliases

Next, I tested my email configuration by sending test email messages (using ‘mutt’ or ‘mail’)…

1. From my local linux account to my local linux account, to make sure local delivery worked and didn’t get sent to my ISP.
2. From the local ‘root’ account to my local linux account.
3. From the local ‘root’ account to my local ‘root’ account, to make sure it was forwarded to my local user account (remember /etc/aliases?)
4. From my local account to an external account (gmail, etc.)
   1. I didn’t expect it to be delivered at this point. Remember the “.NOT” I added to my relay host? This way, I was able to see where outbound email was destined using “mailq”. This saved me grief because I found misconfigurations that would have lost the email.  I used ‘postsuper -d ALL’ to delete all test messages from the queue.
   2. When I knew delivery was destined for the correct machines, I removed the “.NOT” from main.cf, and ran “service postfix reload” (as root).
   3. Next, I sent email to an external email address, and made sure it was delivered

Then I configured and ran ‘fetchmail’ briefly (hitting CTRL-C after a few were downloaded), and made sure messages were delivered to my local account properly. I’ve been pleased using Ubuntu with postfix, fetchmail and procmail and archivemail. I often log in remotely using SSH and use mutt to read my email. I also use Google Mail. I’ve set up rules to forward some email to my ISP account, which get downloaded using fetchmal via cron job.

Why do I bother saving my email to my local machine? I like redundancy and backups, I like the speed of using a command line email reader, and I have ten years of email archived on my local machine. It’s been very useful to be able to search through that email for old software license keys, website passwords, email addresses and such.