Tag Archives: netbook

Redirecting mail for the local root user

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postfix is Ubuntu’s default mail transfer agent (MTA) and can be configured to deliver mail using a relay host that requires SMTP authentication.

Get the necessary packages with the following command:

user@ubuntu:~$ sudo apt-get install postfix bsd-mailx

Begin to configure your postfix installation by choosing satellite system as the general type of configuration. Enter the local machine name as the mail name (eg mycomputer.edafe.org) and the SMTP server address of your email service provider as the SMTP relay host (eg smtp.relayhost.com).

Edit the file /etc/postfix/main.cf and add the following:

smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options =
relay_domains =

Create the file /etc/postfix/sasl_passwd and make the following entries:

smtp.relayhost.com user:password

Substitute smtp.relayhost.com with the address of the SMTP relay host and user:password with your login details.

Continue by executing the following three commands:

user@ubuntu:~$ sudo chown root.root /etc/postfix/sasl_passwd
user@ubuntu:~$ sudo chmod 600 /etc/postfix/sasl_passwd
user@ubuntu:~$ sudo postmap hash:/etc/postfix/sasl_passwd

Instruct postfix to reload its settings with the following command:

user@ubuntu:~$ sudo /etc/init.d/postfix reload

Making changes to the alias table

The aliases table provides a system-wide mechanism to redirect mail for local recipients.

Edit the file /etc/aliases to contain the following entries:

postmaster: root
root: user@yourdomain.com

Substitute user@yourdomain.com with the email address that you would like mail for the root user to be redirected to.

Finally, update /etc/aliases.db using the following command:

user@ubuntu:~$ sudo newaliases

Mail for the local root user from now on will automatically be forwarded to user@yourdomain.com , using smtp.relayhost.com as the relay host.
www.postfix.org, help.ubuntu.com

Monitoring hard disks with smartmontools

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SMART stands for Self-Monitoring, Analysis and Reporting Technology and is built into most modern hard disks. The smartd daemon is part of smartmontools and monitors a disk’s SMART data for any signs of hardware problems.

SMART is available with Parallel and Serial ATA disks, drives appearing as either /dev/hd* or /dev/sd*, respectively. Use the following command to obtain relevant information for your system:

user@ubuntu:~$ df -hl

If required, start by configuring postfix to redirect mail for the local root user.

Get the necessary packages with the following command:

user@ubuntu:~$ sudo apt-get install smartmontools bsd-mailx

Configuring smartd

Edit the file /etc/smartd.conf and comment out any lines beginning with DEVICESCAN.

If you are using a netbook or a laptop, add the following line for the smartd daemon to monitor the device /dev/sda:

/dev/sda -a -d ata -n standby -o on -S on -m root -M daily -M test

If you are using a desktop or a server, add the following line for the smartd daemon to monitor the device /dev/hda:

/dev/hda -a -d ata -n never -o on -S on -s (L/../../7/04|S/../.././02) -m root -M daily -M test

See man smartd.conf for more information on how to tailor the operation of smartd to your needs.

Starting smartd

Edit the file /etc/default/smartmontools and uncomment the line containing start_smartd=yes.

Restart the smartd daemon with the following command:

user@ubuntu:~$ sudo /etc/init.d/smartmontools restart

Verify that the local root user has received a test message from the smartd daemon.

From now on, the smartd daemon will monitor the disk and, in the event of impending disk failure, alert the local root user by email.

Public/private key authentication with SSH

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SSH is a protocol that enables secure logins to your computer over a network. SSH supports the use of public/private key pairs for user authentication. Private keys are kept locally, while public keys are stored on the remote machine.

On the Local Machine

Use the command ssh-keygen -t dsa to generate a key pair for the local user. Use an appropriate passphrase to secure your private key (don’t be tempted to use an empty passphrase).
Set the permissions for the private key file with the following command:

user@ubuntu:~$ chmod 600 ~/.ssh/id_dsa

On the Remote Machine

Get the necessary packages with the following command:

user@ubuntu:~$ sudo apt-get install ssh

Copy the public key file ~/.ssh/id_dsa.pub from the local to the remote machine.

On the remote machine, move and rename the file with the following command:

user@ubuntu:~$ mv id_dsa.pub .ssh/authorized_keys2

Set the permissions for the file with the following command:

user@ubuntu:~$ chmod 600 ~/.ssh/authorized_keys2

Add the user user to the group ssh:

user@ubuntu:~$ sudo adduser user ssh

Get the file sshd_config and move it to /etc/ssh/:

user@ubuntu:~# sudo wget "http://edafe.org/wp-content/uploads/2006/09/sshd_config"
user@ubuntu:~# sudo mv sshd_config /etc/ssh/

The downloaded file contains the following changes from the Ubuntu default configuration:

LogLevel VERBOSE

AllowGroups ssh
LoginGraceTime 20
PermitRootLogin no
MaxAuthTries 1

RSAAuthentication no

PasswordAuthentication no

X11 Forwarding no
TCPKeepAlive yes
ClientAliveInterval 15
ClientAliveCountMax 3

MaxStartups 3

UsePAM no

Setting these options makes root logins impossible. Only users belonging to the group ssh may establish a connection. Access to the remote machine is strictly tied to the private key and the passphrase used to encrypt it.

Next, restart the SSH server on the remote machine with the following command:

user@ubuntu:~$ sudo /etc/init.d/ssh restart

You should now be able to log into the remote machine using the private key stored on the local machine:

user@ubuntu:~$ ssh remote.machine
Enter passphrase for key '/home/user/.ssh/id_dsa':

The book SSH The Secure Shell by Daniel Barrett, Richard Silverman and Robert Byrnes is useful as a reference text and has information on other clever stuff you can do with SSH.
www.ibm.com/developerworks/linux/