Linux uses a resolver library to obtain the IP address corresponding to a host name. The /etc/host.conf file specifies how names are resolved. The entries in the etc/host.conf file tell the resolver library what services to use, and in what order, to resolve names. Edit the host.conf file vi /etc/host.conf and add the following lines:

# Lookup names via DNS first then fall back to /etc/hosts.
order bind,hosts
# We have machines with multiple IP addresses.
multi on
# Check for IP address spoofing.
nospoof on


The order option indicates the order of services. The sample entry specifies that the resolver library should first consult the name server to resolve a name and then check the /etc/hosts file. It is recommended to set the resolver library to first check the name server, bind and then the hosts file (hosts) for better performance and security on all your servers. Of course you must have the DNS/BIND software installed or this configuration will not work.

The multi option determines whether a host in the /etc/hosts file can have multiple IP addresses i.e.multiple interface ethN. Hosts that have more than one IP address are said to be multiomed, because the presence of multiple IP addresses implies that host has several network interfaces. As an example, a Gateway Server will always have multiple IP address and must have this option set to ON.

The nospoof option indicates to take care of not permitting spoofing on this machine. IP-Spoofing is a security exploit that works by tricking computers in a trust relationship that you are someone that you really aren't. In this type of attack, a machine is set up to look like a legitimate server and then issue connections and other types of network activities to legitimate end systems, other servers or large data repository systems. This option must be set ON for all types of servers.

sample file for vlager is shown below:

# /etc/host.conf
# We have named running, but no NIS (yet)
order bind hosts
# Allow multiple addrs
multi on
# Guard against spoof attempts
nospoof on
# Trim local domain (not really necessary).
trim vbrew.com.

Answered by Sharad Singh , an ibibo Master, at 6:11 PM on September 12, 2008

The /etc/host.conf tells the older Linux standard library resolver functions which services to use, and in what order. Options in host.conf must appear on separate lines. Fields may be separated by white space (spaces or tabs). A hash sign (#) introduces a comment that extends to the next newline. The following options are available:

order :This option determines the order in which the resolving services are tried.
Valid :options are bind for querying the name server, hosts for lookups in /etc/hosts, and nis for NIS lookups. Any or all of them may be specified. The order in which they appear on the line determines the order in which the respective services are tried.
multi :multi takes on or off as options. This determines if a host in /etc/hosts is allowed to have several IP addresses, which is usually referred to as being “multihomed.” The default is off. This flag has no effect on DNS or NIS queries.

nospoof : DNS allows you to find the hostname belonging to an IP address by using the inaddr. arpa domain. Attempts by name servers to supply a false hostname are called spoofing. To guard against this, the resolver can be configured to check whether the original IP address is in fact associated with the obtained hostname. If not, the name is rejected and an error is returned. This behavior is turned on by setting nospoof on.

alert :This option takes on or off as arguments. If it is turned on, any spoof attempts will cause the resolver to log a message to the syslog facility.
trim This option takes an argument specifying a domain name that will be removed from hostnames before lookup. This is useful for hosts entries, for which you might only want to specify hostnames without a local domain. If you specify your local domain name here, it will be removed from a lookup of a host with the local domain name appended, thus allowing the lookup in /etc/hosts to succeed. The domain name you add must end with the (.) character (e.g., :linux.org.au.) if trim is to work correctly. trim options accumulate; you can consider your host as being local to several domains.

Answered by Shobhit , an ibibo Master, at 3:43 PM on September 01, 2008

Options in host.conf must appear on separate lines. Fields may be separated by white space (spaces or tabs). A hash sign (#) introduces a comment that extends to the next newline. The following options are available:

order

This option determines the order in which the resolving services are tried. Valid options are bind for querying the name server, hosts for lookups in /etc/hosts, and nis for NIS lookups. Any or all of them may be specified. The order in which they appear on the line determines the order in which the respective services are tried.

multi

multi takes on or off as options. This determines if a host in /etc/hosts is allowed to have several IP addresses, which is usually referred to as being "multi-homed." The default is off. This flag has no effect on DNS or NIS queries.

nospoof

As we'll explain in the section "Reverse Lookups", DNS allows you to find the hostname belonging to an IP address by using the in-addr.arpa domain. Attempts by name servers to supply a false hostname are called spoofing. To guard against this, the resolver can be configured to check whether the original IP address is in fact associated with the obtained hostname. If not, the name is rejected and an error is returned. This behavior is turned on by setting nospoof on.

alert

This option takes on or off as arguments. If it is turned on, any spoof attempts will cause the resolver to log a message to the syslog facility.

trim

This option takes an argument specifying a domain name that will be removed from hostnames before lookup. This is useful for hosts entries, for which you might only want to specify hostnames without a local domain. If you specify your local domain name here, it will be removed from a lookup of a host with the local domain name appended, thus allowing the lookup in /etc/hosts to succeed. The domain name you add must end with the (.) character (e.g., :linux.org.au.) if trim is to work correctly.

trim options accumulate; you can consider your host as being local to several domains.

Example-
Sample host.conf File

# /etc/host.conf
# We have named running, but no NIS (yet)
order bind,hosts
# Allow multiple addrs
multi on
# Guard against spoof attempts
nospoof on
# Trim local domain (not really necessary).
trim vbrew.com.

For more detail-
http://oreilly.com/ca talog/linag2/book/ch06.html

Answered by Poonam Singh , an ibibo Master, at 7:39 AM on September 01, 2008

The /etc/host.conf tells the older Linux standard library resolver functions which services to use, and in what order. Options in host.conf must appear on separate lines. Fields may be separated by white space (spaces or tabs). A hash sign (#) introduces a comment that extends to the next newline. The following options are available:

order :This option determines the order in which the resolving services are tried.
Valid :options are bind for querying the name server, hosts for lookups in /etc/hosts, and nis for NIS lookups. Any or all of them may be specified. The order in which they appear on the line determines the order in which the respective services are tried.
multi :multi takes on or off as options. This determines if a host in /etc/hosts is allowed to have several IP addresses, which is usually referred to as being “multihomed.” The default is off. This flag has no effect on DNS or NIS queries.

nospoof : DNS allows you to find the hostname belonging to an IP address by using the inaddr. arpa domain. Attempts by name servers to supply a false hostname are called spoofing. To guard against this, the resolver can be configured to check whether the original IP address is in fact associated with the obtained hostname. If not, the name is rejected and an error is returned. This behavior is turned on by setting nospoof on.

alert :This option takes on or off as arguments. If it is turned on, any spoof attempts will cause the resolver to log a message to the syslog facility.
trim This option takes an argument specifying a domain name that will be removed from hostnames before lookup. This is useful for hosts entries, for which you might only want to specify hostnames without a local domain. If you specify your local domain name here, it will be removed from a lookup of a host with the local domain name appended, thus allowing the lookup in /etc/hosts to succeed. The domain name you add must end with the (.) character (e.g., :linux.org.au.) if trim is to work correctly. trim options accumulate; you can consider your host as being local to several domains.

Answered by Anju Devi , an ibibo Master, at 7:42 PM on August 26, 2008

Here we explain host.conf file with option and examples-

The /etc/host.conf tells the older Linux standard library resolver functions which services to use, and in what order.

Options in host.conf must appear on separate lines. Fields may be separated by white space (spaces or tabs). A hash sign (#) introduces a comment that extends to the next newline. The following options are available:

order

This option determines the order in which the resolving services are tried. Valid options are bind for querying the name server, hosts for lookups in /etc/hosts, and nis for NIS lookups. Any or all of them may be specified. The order in which they appear on the line determines the order in which the respective services are tried.

multi

multi takes on or off as options. This determines if a host in /etc/hosts is allowed to have several IP addresses, which is usually referred to as being "multi-homed." The default is off. This flag has no effect on DNS or NIS queries.

nospoof

As we'll explain in the section "Reverse Lookups", DNS allows you to find the hostname belonging to an IP address by using the in-addr.arpa domain. Attempts by name servers to supply a false hostname are called spoofing. To guard against this, the resolver can be configured to check whether the original IP address is in fact associated with the obtained hostname. If not, the name is rejected and an error is returned. This behavior is turned on by setting nospoof on.

alert

This option takes on or off as arguments. If it is turned on, any spoof attempts will cause the resolver to log a message to the syslog facility.

trim

This option takes an argument specifying a domain name that will be removed from hostnames before lookup. This is useful for hosts entries, for which you might only want to specify hostnames without a local domain. If you specify your local domain name here, it will be removed from a lookup of a host with the local domain name appended, thus allowing the lookup in /etc/hosts to succeed. The domain name you add must end with the (.) character (e.g., :linux.org.au.) if trim is to work correctly.

trim options accumulate; you can consider your host as being local to several domains.

Example-
Sample host.conf File

# /etc/host.conf
# We have named running, but no NIS (yet)
order bind,hosts
# Allow multiple addrs
multi on
# Guard against spoof attempts
nospoof on
# Trim local domain (not really necessary).
trim vbrew.com.

For more detail-
http://oreilly.com/ca talog/linag2/book/ch06.html

Answered by Nagendra , an ibibo Master, at 8:44 AM on August 19, 2008

hi dear..

NAME
host.conf - resolver configuration file

DESCRIPTION
The file /etc/host.conf contains configuration information specific to the resolver library. It should contain one configuration keyword per line, followed by appropriate configuration information. The keywords recognized are order ", " trim ", " multi ", " nospoof ", " spoof ", and " reorder. These keywords are described below.

order
This keyword specifies how host lookups are to be performed. It should be followed by one or more lookup methods, separated by commas. Valid methods are bind ", " hosts ", and " nis.
trim
This keyword may be listed more than once. Each time it should be followed by a list of domains, separated by colons (':'), semicolons (';') or commas (','), with the leading dot. When set, the resolv+ library will automatically trim the given domain name from the end of any hostname resolved via DNS. This is intended for use with local hosts and domains. (Related note: trim will not affect hostnames gathered via NIS or the hosts file. Care should be taken to ensure that the first hostname for each entry in the hosts file is fully qualified or non-qualified, as appropriate for the local installation.)

multi
Valid values are on " and " off. If set to on, the resolv+ library will return all valid addresses for a host that appears in the /etc/hosts file, instead of only the first. This is off by default, as it may cause a substantial performance loss at sites with large hosts files.
nospoof
Valid values are on " and " off. If set to on, the resolv+ library will attempt to prevent hostname spoofing to enhance the security of rlogin " and " rsh . It works as follows: after performing a host address lookup, resolv+ will perform a hostname lookup for that address. If the two hostnames do not match, the query will fail. The default value is off.

For more detail Plzzz refer:
http://kerneltrap.org/man/l inux/man5/host.conf.5

Answered by Dan !!!!!! , an ibibo Specialist, at 11:28 PM on August 18, 2008

The /etc/host.conf tells the older Linux standard library resolver functions which services to use, and in what order. Options in host.conf must appear on separate lines. Fields may be separated by white space (spaces or tabs). A hash sign (#) introduces a comment that extends to the next newline. The following options are available:

order :This option determines the order in which the resolving services are tried.
Valid :options are bind for querying the name server, hosts for lookups in /etc/hosts, and nis for NIS lookups. Any or all of them may be specified. The order in which they appear on the line determines the order in which the respective services are tried.
multi :multi takes on or off as options. This determines if a host in /etc/hosts is allowed to have several IP addresses, which is usually referred to as being “multihomed.” The default is off. This flag has no effect on DNS or NIS queries.

nospoof : DNS allows you to find the hostname belonging to an IP address by using the inaddr. arpa domain. Attempts by name servers to supply a false hostname are called spoofing. To guard against this, the resolver can be configured to check whether the original IP address is in fact associated with the obtained hostname. If not, the name is rejected and an error is returned. This behavior is turned on by setting nospoof on.

alert :This option takes on or off as arguments. If it is turned on, any spoof attempts will cause the resolver to log a message to the syslog facility.
trim This option takes an argument specifying a domain name that will be removed from hostnames before lookup. This is useful for hosts entries, for which you might only want to specify hostnames without a local domain. If you specify your local domain name here, it will be removed from a lookup of a host with the local domain name appended, thus allowing the lookup in /etc/hosts to succeed. The domain name you add must end with the (.) character (e.g., :linux.org.au.) if trim is to work correctly. trim options accumulate; you can consider your host as being local to several domains.

Answered by Ashish Yadav , an ibibo Wizard, at 7:03 PM on August 18, 2008

well, After you have subnetted your network, you should prepare for some simple sort of hostname resolution using the /etc/hosts file. If you are not going to use DNS or NIS for address resolution, you have to put all hosts in the hosts file.
Even if you want to run DNS or NIS during normal operation, you should have some subset of all hostnames in /etc/hosts. You should have some sort of name resolution, even when no network interfaces are running, for example, during boot time. This is not only a matter of convenience, but it allows you to use symbolic hostnames in your network rc scripts. Thus, when changing IP addresses, you only have to copy an updated hosts file to all machines and reboot, rather than edit a large number of rc files separately. Usually you put all local hostnames and addresses in hosts, adding those of any gateways and NIS servers used.
You should make sure your resolver only uses information from the hosts file during initial testing. Sample files that come with your DNS or NIS software may produce strange results. To make all applications use /etc/hosts exclusively when looking up the IP address of a host, you have to edit the /etc/host.conf file. Comment out any lines that begin with the keyword order by preceding them with a hash sign, and insert the line:

The hosts file contains one entry per line, consisting of an IP address, a hostname, and an optional list of aliases for the hostname. The fields are separated by spaces or tabs, and the address field must begin in the first column. Anything following a hash sign (#) is regarded as a comment and is ignored.
Hostnames can be either fully qualified or relative to the local domain. For vale, you would usually enter the fully qualified name, vale.vbrew.com, and vale by itself in the hosts file, so that it is known by both its official name and the shorter local name.

http://oreilly.com/ca talog/9781565924000/toc.html

Answered by Romi , an ibibo Master, at 2:05 PM on August 18, 2008

The purpose of host.conf file is to provide a name resolution mechanism so users do not need to remember IP addresses.

Network packets that are sent over the physical network transport layer communicate not via IP addresses but rather using the Media Access Control address, or MAC address. IP addresses are currently 32 bits in length and are typically presented as four decimal numbers that are separated by a dot (or period) for example, 168.192.1.1.

MAC addresses use 48 bits (or 6 bytes) and are typically represented as two-digit hexadecimal numbers separated by colons: 40:8e:0a:12:34:56.

Every network interface must have a MAC address. Associated with a MAC address may be one or more IP addresses. There is no relationship between an IP address and a MAC address; all such assignments are arbitrary or discretionary in nature. At the most basic level, all network communications take place using MAC addressing. Since MAC addresses must be globally unique and generally remain fixed for any particular interface, the assignment of an IP address makes sense from a network management perspective. More than one IP address can be assigned per MAC address. One address must be the primary IP address this is the address that will be returned in the Address Resolution Protocol (ARP) reply.

When a user or a process wants to communicate with another machine, the protocol implementation ensures that the “machine name” or “host name” is resolved to an IP address in a manner that is controlled by the TCP/IP configuration control files. The file /etc/hosts is one such file.

When the IP address of the destination interface has been determined, a protocol called ARP/RARP is used to identify the MAC address of the target interface. ARP is a broadcast-oriented method that uses User Datagram Protocol (UDP) to send a request to all interfaces on the local network segment using the all 1s MAC address. Network interfaces are programmed to respond to two MAC addresses only; their own unique address and the address ff:ff:ff:ff:ff:ff. The reply packet from an ARP request will contain the MAC address and the primary IP address for each interface.

The /etc/hosts file is foundational to all UNIX/Linux TCP/IP installations and as a minimum will contain the localhost and local network interface IP addresses and the primary names by which they are known within the local machine. This file helps to prime the pump so a basic level of name resolution can exist before any other method of name resolution becomes available.

Answered by Hem@nt .... , an ibibo Master, at 1:06 PM on August 18, 2008

The /etc/host.conf tells the older Linux standard library resolver functions which
services to use, and in what order.
Options in host.conf must appear on separate lines. Fields may be separated by white
space (spaces or tabs). A hash sign (#) introduces a comment that extends to the next
newline. The following options are available:
order
This option determines the order in which the resolving services are tried. Valid
options are bind for querying the name server, hosts for lookups in /etc/hosts,
and nis for NIS lookups. Any or all of them may be specified. The order in which
they appear on the line determines the order in which the respective services are
tried.
multi
multi takes on or off as options. This determines if a host in /etc/hosts is
allowed to have several IP addresses, which is usually referred to as being “multihomed.”
The default is off. This flag has no effect on DNS or NIS queries.
nospoof
DNS allows you to find the hostname belonging to an IP address by using the inaddr.
arpa domain. Attempts by name servers to supply a false hostname are called
spoofing. To guard against this, the resolver can be configured to check whether
the original IP address is in fact associated with the obtained hostname. If not, the
name is rejected and an error is returned. This behavior is turned on by setting
nospoof on.
alert
This option takes on or off as arguments. If it is turned on, any spoof attempts will
cause the resolver to log a message to the syslog facility.
trim
This option takes an argument specifying a domain name that will be removed
from hostnames before lookup. This is useful for hosts entries, for which you
might only want to specify hostnames without a local domain. If you specify your
local domain name here, it will be removed from a lookup of a host with the local
domain name appended, thus allowing the lookup in /etc/hosts to succeed. The
domain name you add must end with the (.) character (e.g., :linux.org.au.) if
trim is to work correctly. trim options accumulate; you can consider your host as being local to several domains.

Answered by Sumit Goel , an ibibo Citizen, at 11:49 AM on August 18, 2008