Can we set easy-to-remember hostnames for EC2 instances?

Before you get started, try running hostname and hostname --fqdn and take note of what the responses are.

You can edit /etc/hostname and set a hostname, which will stick around after rebooting. You can force the hostname to be "reloaded" by using hostname -F /etc/hostname to read that value into the hostname. The bash prompt will change after you logout and login.

warning / note:
Yes, it is nice to have the hostname in the bash prompt set to something more useful than ip-123-123-123-123 but I've decided to leave mine (at least for now) because it seems like a lot of things really count on having the hostname on ec2 instances set in a standard way. After editing /etc/hostname and changing the hostname to webserver a lot of the services seems to fail because the hostname would not resolve, and apache wouldn't start. Next I edited /etc/hosts and added in

127.0.0.1 webserver

as the second line. Apache would then start but complained that it couldn't find the FQDN. I confirmed that running hostname --fqdn no longer worked.

Next I consulted man hostname and learned that while you can set the hostname it appears that the FQDN is what is returned via a DNS lookup.

THE FQDN

You can't change the FQDN (as returned by hostname --fqdn) or the DNS domain name (as returned by dnsdomainname) with this command. The FQDN of the system is the name that the resolver(3) returns for the host name.

Technically: The FQDN is the name getaddrinfo(3) returns for the host name returned by gethostname(2). The DNS domain name is the part after the first dot.

Therefore it depends on the configuration (usually in /etc/host.conf) how you can change it. Usually (if the hosts file is parsed before DNS or NIS) you can change it in /etc/hosts.

I think it might be possible to set the system / fool the system into return the FQDN, something like ip-123-123-123-123.ec2.internal even though the hostname is webserver but at this point it started to seem like more trouble than it was worth, and that for me to have a nicer bash prompt might cause a lot software and configuration problems down the road and so I decided to give up.

I also learned that a lot of amazon ec2 instances use something called cloud-init:

cloud-init is the Ubuntu package that handles early initialization of a cloud instance. It is installed in the Ubuntu Cloud Images and also in the official Ubuntu images available on EC2.

Some of the things it configures are:

  • setting a default locale
  • setting hostname
  • generate ssh private keys
  • adding ssh keys to user's .ssh/authorized_keys so they can log in
  • setting up ephemeral mount points

cloud-init's behavior can be configured via user-data. User-data can be given by the user at instance launch time. This is done via the --user-data or --user-data-file argument to ec2-run-instances

I also found this which talks about how the hostname is configured with cloud-init:

On EBS instances, a shutdown and later start would end up with a different IP address.

In the case where the user has not modified /etc/hostname from its original value (seeded by metadata's 'local-hostname'), then cloud-init will again set the hostname and update /etc/hostname.

In the case where the user has modified /etc/hostname, it will remain user managed.

Additionally, if /etc/cloud/cloud.cfg contains 'preserve_hostname' value set to a True value, then /etc/hostname will not ever be touched.

The interesting takeaway is that if you don't change the hostname the cloud-init package will keep it up to date for you.

If someone else has a workaround or can address some of the issues mentioned and help reassure that nothing will break on ec2 instances because of changing the hostname I would be happy to hear it.


Edit /etc/sysconfig/network as root.

Replace 

HOSTNAME=localhost.localdomain

with

HOSTNAME=hostname.DOMAIN_NAME

Then, either reboot or run /etc/init.d/network restart The server then should report its name as a FQDN.


Another way is to simply edit ~/.bashrc and prepend PS1 with the nickname of the machine.

Edit: perhaps more correctly, machine-wide, e.g. on the AWS Linux AMI (an example) (paste this into console or add to your arbitrary install .sh):

cat << EOF | sudo tee /etc/profile.d/ps1.sh
if [ "$PS1" ]; then
  PS1="[\u@myinst1:\l \t \! \W]\\$ "
fi
EOF

Tags:

Amazon Ec2

Hostname

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