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DNS configuration on RHEL

DNS configuration on RHEL

Passive Capture on RHEL - Configuring DNS

This section provides a brief description of the configuration files for Domain Name Service (DNS), the service that is used to convert between host names and IP addresses. The file /etc/nsswitch.conf controls the use of various system database files and name services.

When DNS is enabled by the nsswitch.conf file, the file /etc/resolv.conf controls how the lookups are performed. The changes that you make to these system files take after effect you save your changes to the files.

You can also use the redhat-config-network graphical utility to configure DNS. It is available when the redhat-config-network package is installed. This package and the commands it provides are not available with a minimal RHEL installation.

For more information, go to:

Disable DNS

To disable DNS lookups:

  1. Edit file /etc/nsswitch.conf.
  2. Place a pound sign (#) at the beginning of the line that reads hosts: files dns.
  3. After editing, that line must look like the following code: 
    
#hosts: files dns

Enabling DNS lookups

To enable DNS lookups:

  1. Edit /etc/nsswitch.conf.
  2. Remove any pound signs (#) at the beginning of the line that reads hosts: files dns. After editing, that line looks like: 
    
hosts: files dns

Specifying the DNS domain and servers with the /etc/resolv.conf file

To specify the DNS domain and servers:

  1. Edit the file /etc/resolv.conf.
  2. When DHCP is enabled for a network interface, the file /etc/resolv.conf is automatically generated by the DHCP client program with the DNS servers specified by the DHCP server.
    Note: You normally edit /etc/resolv.conf only when using fixed (static) IP addresses.
  3. The /etc/resolv.conf file should specify the domain name suffix to use when a hostname is not fully qualified.
  4. You must also specify the name of at least one DNS server to use for hostname and IP resolution. Below is an example of /etc/resolv.conf for the domain machines mycompany.com with two DNS servers: 
    
search machines.mycompany.com
nameserver 172.16.0.5
nameserver 172.16.0.6

Passive capture on RHEL - Configuring network interfaces

The network configuration files in the /etc/sysconfig directory are read and processed during system initialization. To apply changes, eboot the machine by using the command shutdown -r.

Instead of rebooting the machine, you can also bring the machine into single-user mode by using the command shutdown now. At the command prompt, enter the command exit to leave single-user mode and reenter multi-user mode, which enables networking and starts the network interfaces.

Configuring NTP for passive capture on RHEL

You can configure an NTP daemon to synchronize the time of the machine with one or more NTP servers.

First, install the NTP package, which is not included with a minimal installation. After you install the NTP package, select NTP servers, create configuration files, and then enable and start the service.

Table 1. Configuring NTP
Configuration overview Configuration steps
Install the NTP package If you have not done so already, install the NTP package from your Linux distribution.
Select NTP servers

To synchronize your workstation's time, the NTP daemon on your workstation contacts one or more NTP servers specified in the configuration file /etc/ntp.conf.

If an NTP server is not available on the local network, do one of the following steps:

  • Select a public NTP server (visit http://www.ntp.org/ and click Public Time Server Lists). If you select a public NTP server, read the Rules of Engagement (click Rules of Engagement on the main page of the NTP site).
  • Use the NTP time server pool (visit http://www.pool.ntp.org/ and click How do I use pool.ntp.org).

As user root on the workstation, verify that the machine can contact the selected NTP servers. Use the ntpdate command with the query -q option. For example, to query an NTP server whose IP address is 1.2.3.4, use the following command:

ntpdate -q 1.2.3.4

The output must look like the following, which displays the contacted server and the time difference between the local workstation and the server.

server 1.2.3.4, stratum 2, offset 150.695779, delay 0.03366
17 Nov 10:27:09 ntpdate[21597]: step time server 1.2.3.4 offset 150.695779 sec

If the query fails, following output is likely to come:

server 1.2.3.4, stratum 0, offset 0.000000, delay 0.00000
17 Nov 10:29:04 ntpdate[21599]: no server suitable for synchronization found
Create configuration files

Perform the following steps as user root on the machine:

  1. Create file /etc/ntp/step-tickers. You can back up any existing version of the file. The file contains the host names or IP addresses of the NTP servers to contact during startup to initially set the time. If you use the NTP server pool, you must use host names, which requires DNS.

    For example, if the two NTP servers to use are 1.2.3.4 and 5.6.7.8, then use the following commands to create the file:

    echo 1.2.3.4 > /etc/ntp/step-tickers
echo 5.6.7.8 >> /etc/ntp/step-tickers
  2. Create file /etc/ntp.conf with the following commands. You can back up any existing version of the file.
    echo restrict default ignore > /etc/ntp.conf
echo restrict 127.0.0.1 >> /etc/ntp.conf
echo driftfile /var/lib/ntp/drift >> /etc/ntp.conf
  3. Add entries by using the restrict and server keywords for each NTP server. The following example adds entries for the hypothetical 1.2.3.4 and 5.6.7.8 NTP servers. The mask, nomodify, notrap, and noquery options prevent the server from modifying the NTP service on the Passive Capture host machine.
    nullecho
restrict 1.2.3.4 mask 255.255.255.255 nomodify notrap noquery >> /etc/ntp.con
echo server 1.2.3.4 >> /etc/ntp.conf
echo restrict 5.6.7.8 mask 255.255.255.255 nomodify notrap noquery >> \
/etc/ntp.con
echo server 5.6.7.8 >> /etc/ntp.conf
Enable and start the NTP service

Perform the following steps as user root on the machine:

  1. Configure the service to start at boot time by using the following command:
    chkconfig ntpd on
  2. Start the service immediately by using the following command:
    service ntpd start
  3. Verify that the service started and contacted a server by using the following command:
    ntpq -np
  4. View log messages for the NTP daemon in file /var/log/messages.

Install NTP package and select NTP servers

To synchronize your workstation's time, the NTP daemon on your workstation contacts one or more NTP servers specified in the configuration file /etc/ntp.conf.

If you have not done so already, install the NTP package for your Linux distribution.

  1. If an NTP server is not available on the local network, do one of the following steps:
    • Select a public NTP server (visit http://www.ntp.org/ and click Public Time Server Lists). If you select a public NTP server, read the Rules of Engagement (click Rules of Engagement on the main page of the NTP site).
    • Use the NTP time server pool (visit http://www.pool.ntp.org/ and click How do I use pool.ntp.org).
  2. As user root on the workstation, verify that the machine can contact the selected NTP servers. Use the ntpdate command with the query -q option. For example, to query an NTP server whose IP address is 1.2.3.4, use the following command:
    ntpdate -q 1.2.3.4

    The output must look like the following, which displays the contacted server and the time difference between the local workstation and the server.

    server 1.2.3.4, stratum 2, offset 150.695779, delay 0.03366
17 Nov 10:27:09 ntpdate[21597]: step time server 1.2.3.4 offset 150.695779 sec

    If the query fails, following output is likely to come:

    server 1.2.3.4, stratum 0, offset 0.000000, delay 0.00000
17 Nov 10:29:04 ntpdate[21599]: no server suitable for synchronization found

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