Servers

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DNS
Domain Name System (DNS) converts the name of a Web site to an IP address .This step is important, because the IP address of a Web site's server, not the Web site's name, is used in routing traffic over the Internet. DNS resolution maps a fully qualified domain name (FQDN), such as www.google.co.in, to an IP address. This is also known as a forward lookup. The reverse is also true: By performing a reverse lookup, DNS can determining the fully qualified domain name associated with an IP address.

nslookup and host commands
nslookup www.google.co.in host www.google.co.in

BIND
BIND is an acronym for the Berkeley Internet Name Domain project, which is a group that maintains the DNS-related software suite that runs under Linux. The most well known program in BIND is named, the daemon that responds to DNS queries from remote machines.

How DNS Servers Find Out Your Site Information
There are 13 root authoritative DNS servers (super duper authorities) that all DNS servers query first. These root servers know all the authoritative DNS servers for all the main domains - .com, .net, and the rest. This layer of servers keep track of all the DNS servers that Web site systems administrators have assigned for their sub domains. For example, when you register your domain my-site.com, you are actually inserting a record on the .com DNS servers that point to the authoritative DNS servers you assigned for your domain.

A DNS Caching Name Server
Most servers don’t ask authoritative servers for DNS directly, they usually ask a caching DNS server to do it on their behalf. These servers, through a process called recursion, sequentially query the authoritative servers at the root, main domain and sub domain levels to get eventually

get the specific information requested. The most frequently requested information is then stored (or cached) to reduce the lookup overhead of subsequent queries.

Downloading and Installing the BIND Packages
bind-9.2.2.P3-9.i386.rpm

Important File Locations
The locations of the BIND configuration files vary by Linux distribution RedHat BIND is installed using Linux's chroot feature to not only run named as user named, but also to limit the files named can see. When installed, named is fooled into thinking that the directory /var/named/chroot is actually the root or / directory. Therefore, named files normally found in the /etc directory are found in /var/named/chroot/etc directory instead, and those you'd expect to find in /var/named are actually located in /var/named/chroot/var/named. The advantage of the chroot feature is that if a hacker enters your system via a BIND exploit, the hacker's access to the rest of your system is isolated to the files under the chroot directory and nothing else. This type of security is also known as a chroot jail.

You can determine whether you have the chroot add-on RPM by using this command, which returns the name of the RPM.

[root@localhost etc]# rpm -q bind-chroot bind-chroot-9.2.3-13

Configuring Your Name server
Assume your ISP assigned you is 192.168.10.1 with a subnet mask of 255.255.255.0

Configuring resolv.conf
You'll have to make your DNS server refer to itself for all DNS queries by configuring the /etc/resolv.conf file to reference localhost only. nameserver 127.0.0.1

NFS
Network File System (NFS) is a distributed file system protocol originally developed by Sun Microsystems in 1984, allowing a user on a client computer to access files over a network in a manner similar to how local storage is accessed. NFS, like many other protocols, builds on the Open Network Computing Remote Procedure Call system. The Network File System is an open standard defined in RFCs, allowing anyone to implement the protocol. Some of the most notable benefits that NFS can provide are:
 



Local workstations use less disk space because commonly used data can be stored on a single machine and still remain accessible to others over the network. There is no need for users to have separate home directories on every network machine. Home directories could be set up on the NFS server and made available throughout the network. Storage devices such as floppy disks, CDROM drives, and Zip® drives can be used by other machines on the network. This may reduce the number of removable media drives throughout the network.

Configuring NFS Server in RHEL 6
Editing /etc/exports <export> <host1>(<options>) <host2>(<options>)...

Journaling
A journaling file system is a fault-resilient file system in which data integrity is ensured because updates to directories and bitmaps are constantly written to a serial log on disk before the original disk log is updated. In the event of a system failure, a full journaling filesystem ensures that the data on the disk has been restored to its pre-crash configuration. It also recovers unsaved

data and stores it in the location where it would have gone if the computer had not crashed, making it an important feature for mission-critical applications.

SAMBA SERVER
Samba is a strong network service for file and print sharing that works on the majority of operating systems available today.

Samba allows file and print sharing between computers running Windows and computers running Unix

DHCP
What is DHCP? DHCP is engaged in assigning unique dynamic IP addresses and the corresponding subnet masks and default gateways to TCP/IP running computers within a particular server network. Using DHCP, a computer can have a different IP address every time it connects to the network it belongs to, without the intervention of a UNIX administrator. Through this DHCP functionality every new computer added to a network is automatically assigned a unique IP address. DHCP servers greatly simplify the configuration of networks and are built in the majority of the wireless access points and wired Ethernet routers.

How does the DHCP work? In a network, a DHCP server manages a pool of IP addresses, as well as default gateway details, DNS details and other information for the clients’ network configuration. When a new computer is introduced into a DHCP server-enabled network, it will send a query to the DHCP server requesting all the necessary information. When the query reaches the DHCP server, it will grant the new computer a new IP address and a lease - a time frame for which the computer can use this IP address, as well as other configuration details. The whole process takes place immediately after the new computer boots, and to be successful, it has to be completed before initiating IP based communication with other hosts in the network.

DHCP allocation methods Depending on its configuration, the DHCP server can work in 3 ways: Dynamic allocation When the DHCP server is configured to use dynamic allocation, this means that it uses a lease policy. This way, when an assigned IP address from the available pool is no longer used, it will be transferred back to the pool, making it available for someone else to use. The advantage of this method is that the IP addresses are used to their maximum - as soon as they are no longer used by the client, they are instantly made available to others. The disadvantage of this method is that a client will always have a random IP address.

Automatic allocation The automatic allocation method resembles very much the dynamic allocation method - as soon as a client connects, the DHCP server provides him with an IP address from the IP address pool. However, when automatic allocation is used, the DHCP server keeps a database of previous IP grants, and tries to give the client the same IP address he used the last time, if available.
Static allocation

The static allocation method is very popular in modern ISP networks, which do not use dial-up methods. With the static allocation, the DHCP sever keeps a database with all clients' LAN MAC addresses and gives them an IP address only if their MAC address is in the database. This way, the clients can be sure that they will be getting the same IP address every time. A DHCP server can be set to work using a combination of the allocation methods. For example, in a public WiFi network, all of the known hosts and permanent clients can use the static allocation, whereas for guests, the dynamic allocation is used. This way, known hosts can always use the same IP address and the IP address pool is equally available to everyone.

DHCP Configuration

Subnet declaration
subnet 192.168.1.0 netmask 255.255.255.0 { option routers 192.168.1.254; option subnet-mask 255.255.255.0; option domain-search "example.com"; option domain-name-servers 192.168.1.1;

option time-offset -18000; range 192.168.1.10 192.168.1.100; }

# Eastern Standard Time

To configure a DHCP server that leases a dynamic IP address to a system within a subnet, modify Example 12.2, “Range parameter” with your values. It declares a default lease time, maximum lease time, and network configuration values for the clients. This example assigns IP addresses in the range 192.168.1.10 and 192.168.1.100 to client systems.

Range parameter
default-lease-time 600; max-lease-time 7200; option subnet-mask 255.255.255.0; option broadcast-address 192.168.1.255; option routers 192.168.1.254; option domain-name-servers 192.168.1.1, 192.168.1.2; option domain-search "example.com"; subnet 192.168.1.0 netmask 255.255.255.0 { range 192.168.1.10 192.168.1.100; }

To assign an IP address to a client based on the MAC address of the network interface card, use the hardware ethernet parameter within a host declaration. As demonstrated in Example 12.3, “Static IP address using DHCP”, the host apex declaration specifies that the network interface card with the MAC address 00:A0:78:8E:9E:AA always receives the IP address 192.168.1.4. Note that the optional parameter host-name can also be used to assign a host name to the client.

Static IP address using DHCP
host apex { option host-name "apex.example.com"; hardware ethernet 00:A0:78:8E:9E:AA; fixed-address 192.168.1.4; }

All subnets that share the same physical network should be declared within a shared-network declaration as shown in Example 12.4, “Shared-network declaration”. Parameters within the sharednetwork, but outside the enclosed subnet declarations, are considered to be global parameters. The name of the shared-network must be a descriptive title for the network, such as using the title 'testlab' to describe all the subnets in a test lab environment.

Shared-network declaration
shared-network name { option domain-search "test.redhat.com"; option domain-name-servers ns1.redhat.com, ns2.redhat.com; option routers 192.168.0.254; more parameters for EXAMPLE shared-network subnet 192.168.1.0 netmask 255.255.252.0 { parameters for subnet range 192.168.1.1 192.168.1.254; } subnet 192.168.2.0 netmask 255.255.252.0 { parameters for subnet range 192.168.2.1 192.168.2.254; } }

“Group declaration”, the group declaration is used to apply global parameters to a group of declarations. For example, shared networks, subnets, and hosts can be grouped.

Group declaration
group { option routers 192.168.1.254; option subnet-mask 255.255.255.0; option domain-search "example.com"; option domain-name-servers 192.168.1.1; option time-offset -18000; # Eastern Standard Time host apex { option host-name "apex.example.com"; hardware ethernet 00:A0:78:8E:9E:AA; fixed-address 192.168.1.4; } host raleigh { option host-name "raleigh.example.com"; hardware ethernet 00:A1:DD:74:C3:F2;

fixed-address 192.168.1.6; } }

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