sudo: unable to resolve host ubuntu

Sometimes after changing elastic-IP settings or stopping/starting instances on EC2, I get an irritating error like this when I execute a command with sudo:

sudo: unable to resolve host domU-12-34-ab-cd-56-78

The fix is to lookup the instance’s private dns name (via ec2-describe-instances or the AWS console ui) and update the hostname on the instance with the first segment of that DNS name (which is something that looks like ip-12-34-56-78 or domU-12-34-ab-cd-56-78). On ubuntu, this is what you need to do (assuming ip-12-34-56-78 is the new hostname):

The first line will set the hostname until you reboot; and the second line will configure the hostname to use once you do reboot.

Change the hostname in Mac OSX

When I log into the network at work my Mac’s hostname always turns to:

I have my local hostname set to:

So What I would like to do is set my Mac’s hostname to my local hostname. You can do this all from Terminal in a single line.

Run this command in Terminal:

This is also helpful if you’re in Terminal and have a really long hostname at your prompt. If you want to view your current hostname, run this command in Terminal:

Update (February 13, 2012): Some people have reported that their hostname is not updating. Please try closing your current Terminal session and starting up a new one. Then type “hostname” and you should see your changes.

This is what it looked like for me:

Apache Web Server Virtual Hosts

15.10. Virtual Hosts

Using virtual hosts, host several domains with a single web server. In this way, save the costs and administration workload for separate servers for each domain. One of the first web servers that offered this feature, Apache offers several possibilities for virtual hosts:

  • Name-based virtual hosts
  • IP-based virtual hosts
  • Operation of multiple instances of Apache on one machine

15.10.1. Name-Based Virtual Hosts

With name-based virtual hosts, one instance of Apache hosts several domains. You do not need to set up multiple IPs for a machine. This is the easiest, preferred alternative. Reasons against the use of name-based virtual hosts are covered in the Apache documentation.

Configure it directly by way of the configuration file (/etc/apache2/httpd.conf). To activate name-based virtual hosts, a suitable directive must be specified: NameVirtualHost *. * is sufficient to prompt Apache to accept all incoming requests. Subsequently, the individual hosts must be configured:

In the case of Apache 2, however, the paths of log files as shown in the above example (and in any examples further below) should be changed from /var/log/httpd to /var/log/apache2. A VirtualHost entry also must be configured for the domain originally hosted on the server ( So in this example, the original domain and one additional domain ( are hosted on the same server.

Just as in NameVirtualHost, a * is used in the VirtualHost directives. Apache uses the host field in the HTTP header to connect the request with the virtual host. The request is forwarded to the virtual host whose ServerName matches the host name specified in this field.

For the directives ErrorLog and CustomLog, the log files do not need to contain the domain name. Here, use a name of your choice.

ServerAdmin designates the e-mail address of the responsible person that can be contacted if problems arise. In the event of errors, Apache gives this address in the error messages it sends to the client.

15.10.2. IP-Based Virtual Hosts

This alternative requires the setup of multiple IPs for a machine. In this case, one instance of Apache hosts several domains, each of which is assigned a different IP. The following example shows how Apache can be configured to host the original IP ( plus two additional domains on additional IPs ( and This particular example only works on an intranet, as IPs ranging from to are not routed on the Internet. Configuring IP Aliasing

For Apache to host multiple IPs, the underlying machine must accept requests for multiple IPs. This is called multi-IP hosting. For this purpose, IP aliasing must be activated in the kernel. This is the default setting in SUSE LINUX.

Once the kernel has been configured for IP aliasing, the commands ifconfig and route can be used to set up additional IPs on the host. These commands must be executed as root. For the following example, it is assumed that the host already has its own IP (such as, which is assigned to the network device eth0.

Enter the command ifconfig to find out the IP of the host. Further IPs can be added with commands such as the following:

All these IPs will be assigned to the same physical network device (eth0). Virtual Hosts with IPs

Once IP aliasing has been set up on the system or the host has been configured with several network cards, Apache can be configured. Specify a separate VirtualHost block for every virtual server:

VirtualHost directives are only specified for the additional domains. The original domain ( is configured through its own settings (under DocumentRoot, etc.) outside the VirtualHost blocks.

15.10.3. Multiple Instances of Apache

With the above methods for providing virtual hosts, administrators of one domain can read the data of other domains. To segregate the individual domains, start several instances of Apache, each with its own settings for User, Group, and other directives in the configuration file.

In the configuration file, use the Listen directive to specify the IP handled by the respective Apache instance. For the above example, the directive for the first Apache instance would be as follows:

For the other two instances: