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Q. I get following error message for my postfix email server:

Mail Delivery Failed With DNS Service Unavailable Error and Solution

How do I fix this problem?
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How do I find out my DHCP server address?

Q. In Windows XP / 2000 / Vista ipconfig /all command will show me DNS server and DHCP server ip address. But, how to know DHCP server address in Linux?

A. The DHCP protocol allows a host to contact a central server which maintains a list of IP addresses which may be assigned on one or more subnets. A DHCP client may request an address from this pool, and then use it on a temporary basis for communication on network.

dhclient.leases file

Under Linux you use command dhclient to obtain and managing dhcp tasks. In order to keep track of leases across system reboots and server restarts, dhclient keeps a list of leases it has been assigned in the dhclient.leases file. On startup, after reading the dhclient.conf file, dhclient reads the dhclient.leases file to refresh its memory about what leases it has been assigned.

When a new lease is acquired, it is appended to the end of the dhclient.leases file. In order to prevent the file from becoming arbitrarily large, from time to time dhclient creates a new dhclient.leases file from its in-core lease database. The old version of the dhclient.leases file is retained under the name dhclient.leases~ until the next time dhclient rewrites the database.

Find out DHCP server address

Usually dhclient.leases file is located at /var/lib/dhcp3/dhclient.leases, type the following command:
less /var/lib/dhcp3/dhclient.leases
OR
cat /var/lib/dhcp3/dhclient.leases
Alternatively, you can just use grep command to get DHCP server address, enter:
grep dhcp-server-identifier /var/lib/dhcp3/dhclient.leases
Output:

lease {
  interface "ra0";
  fixed-address 192.168.1.106;
  option subnet-mask 255.255.255.0;
  option dhcp-lease-time 86400;
  option routers 192.168.1.1;
  option dhcp-message-type 5;
  option dhcp-server-identifier 192.168.1.1;
  option domain-name-servers 208.67.222.222,208.67.220.220;
  option dhcp-renewal-time 43200;
  option dhcp-rebinding-time 75600;
  option host-name "vivek-desktop";
  renew 0 2007/12/9 05:17:36;
  rebind 0 2007/12/9 15:06:37;
  expire 0 2007/12/9 18:06:37;
}
lease {
  interface "ra0";
  fixed-address 192.168.1.106;
  option subnet-mask 255.255.255.0;
  option routers 192.168.1.1;
  option dhcp-lease-time 86400;
  option dhcp-message-type 5;
  option domain-name-servers 208.67.222.222,208.67.220.220;
  option dhcp-server-identifier 192.168.1.1;
  option dhcp-renewal-time 43200;
  option dhcp-rebinding-time 75600;
  option host-name "vivek-desktop";
  renew 0 2007/12/9 06:11:22;
  rebind 0 2007/12/9 16:13:50;
  expire 0 2007/12/9 19:13:50;
}

A note about RHEL / CentOS / Fedora Linux user

Redhat and friends uses /var/lib/dhcp/dhclient.leases file:
less /var/lib/dhcp/dhclient.leases

Q. How do I configure lighttpd web server as virtual host for serving multiple web site from a single public IP address (name-based virtual hosting)?

A. Virtual hosting is a method that servers such as webservers use to host more than one domain name on the same server, sometimes on the same IP address.

There are two basic methods of accomplishing virtual hosting .
(A) name-based virtual hosting : You use multiple host names for the same webserver IP address. For example domain nixcraft.com and theos.in uses same IP address called 65.111.211.111.

(B) ) IIP address / ip-based virtual hosting

How do I configure Lighttpd for name-based virtual hosting?

Let us say your setup is as follows:

  • Public IP address: 72.12.5.10
  • Domain names: domain1.com and domain2.net
  • Default Document Root: /home/lighttpd/default/http
  • Default Document Root for domain1.com: /home/lighttpd/domain1.com/http
  • Default Document Root for domain2.net: /home/lighttpd/domain2.net/http

First, create required directories:
# mkdir -p /home/lighttpd/default/http
Next, open lighttpd.conf file:
# vi /etc/lighttpd/lighttpd.conf
Setup default document root:
server.document-root = "/home/lighttpd/default/http/"
Setup public IP address:
server.port = 80
server.bind = "72.12.5.10"

At the bottom of the file, add:
include "domain1.com.conf"
include "domain2.net.conf"

Save and close the file.

Create domain1.com virtual host configuration

Create required directories:
# mkdir -p /home/lighttpd/domain1.com/http
# mkdir -p /home/lighttpd/domain1.com/logs

Open /etc/lighttpd/domain1.com.conf file:
# vi /etc/lighttpd/domain1.com.conf
Add following configuration directive:

$HTTP["host"] =~ "domain1\.com" {
            server.document-root = "/home/lighttpd/domain1.com/http"
            accesslog.filename         = "/home/lighttpd/domain1.com/logs/access.log"
}

Save and close the file.

Create domain2.net virtual host configuration

Create required directories:
# mkdir -p /home/lighttpd/domain2.net/http
# mkdir -p /home/lighttpd/domain2.net/logs

Open /etc/lighttpd/domain2.net.conf file:
# vi /etc/lighttpd/domain2.net.conf
Add following configuration directive:

$HTTP["host"] =~ "domain2\.net" {
            server.document-root = "/home/lighttpd/domain2.net/http"
            accesslog.filename         = "/home/lighttpd/domain2.net/logs/access.log"
}

Save and close the file.

Restart the lighttpd web server:
# /etc/init.d/lighttpd restart

Make sure document root is owned by your web server user such as www-data or lighttpd:
# chown -R lighttpd:lighttpd /home/lighttpd/

Q. Under Windows Server 2003 I can use active directory domain tools to get information about current domain and hostname. Can you tell me command to list current domain name and hostname under Red hat enterprise Linux 5?

A. Both Linux / UNIX comes with the following utilities to display hostname / domain name:

a) hostname - show or set the system’s host name

b) domainname - show or set the system’s NIS/YP domain name

c) dnsdomainname - show the system’s DNS domain name

d) nisdomainname - show or set system’s NIS/YP domain name

e) ypdomainname - show or set the system’s NIS/YP domain name

For example, hostname is the program that is used to either set or display the current host, domain or node name of the system. These names are used by many of the networking programs to identify the machine.
$ hostname
Output

sun521nixcraft.com

The domain name is also used by NIS/YP or Internet DNS:
$ dnsdomainname
Output:

nixcraft.com

Q. How do I check my domain zone file for error and validity the same under BIND DNS Server version 9?

A. You need to use named- checkzone command as zone file validity checking tool. It is useful for checking zone files before configuring them into a name server.

named-checkzone command syntax

named-checkzone {zonename} {filename}
Where,

  • zonename : The domain name of the zone being checked.
  • filename : The name of the zone file.

How do I check my zone file configuration for errors?

Use named-checkzone to check the zone files as follows:
named-checkzone nixcraft.com /var/named/zone.nixcraft.com
Output:

OK

If you see "OK" as result, the zone file is properly configured and you can restart bind name server.

I'm writing a wrapper bash shell script that will get the last argument (a domain name) from the command line into a shell variable called $_domain. I need to find all other parameters before last parameter in $@ and stored in a shell variable called $allargs. So that I can pass them as follows:

/path/to/real/binary "$allargs" "$_domain"

How do I do this using bash shell under Unix like operating systems?
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