Thecomputer filehosts is an operating system file that mapshostnames toIP addresses. It is aplain text file. Originally a file named HOSTS.TXT was manually maintained and made available via file sharing byStanford Research Institute for theARPANET membership, containing the hostnames and address of hosts as contributed for inclusion by member organizations. TheDomain Name System, first described in 1983 and implemented in 1984,^[1] automated the publication process and provided instantaneous and dynamic hostname resolution in the rapidly growing network. In modern operating systems, the hosts file remains an alternativename resolution mechanism, configurable often as part of facilities such as theName Service Switch as either the primary method or as a fallback method.

The hosts file is one of several system facilities that assists in addressing network nodes in a computer network. It is a common part of an operating system'sInternet Protocol (IP) implementation, and serves the function of translating human-friendly hostnames into numeric protocol addresses, calledIP addresses, that identify and locate a host in an IP network.

In some operating systems, the contents of the hosts file is used preferentially to other name resolution methods, such as theDomain Name System (DNS), but many systems implementname service switches, e.g.,nsswitch.conf forLinux andUnix, to provide customization. Unlike remote DNS resolvers, the hosts file is under the direct control of the local computer's administrator.^[2]

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The hosts file contains lines of text consisting of anIP address in the first text field followed by one or more host names.^[3] The host names map to the IPs, not vice versa. Each field is separated by white space – tabs are often preferred for historical reasons, but spaces are also used. Comment lines may be included; they are indicated by anoctothorpe (#) in the first position of such lines. Entirely blank lines in the file are ignored.^[4] For example, a typical hosts file may contain the following:

127.0.0.1  localhost loopback::1        localhost

This example only contains entries for the loopback addresses of the system and their host names, a typical default content of the hosts file. The example illustrates that an IP address may have multiple host names (localhost andloopback), and that a host name may be mapped to bothIPv4 andIPv6 IP addresses, as shown on the first and second lines respectively.

The location of the hosts file in thefile system hierarchy varies by operating system. It is usually namedhosts, without an extension.

TheARPANET, the predecessor of theInternet, had no distributed host name database. Each network node maintained its own map of the network nodes as needed and assigned them names that were memorable to the users of the system. There was no method for ensuring that all references to a given node in a network were using the same name, nor was there a way to read the hosts file of another computer to automatically obtain a copy.

The small size of the ARPANET kept the administrative overhead small to maintain an accurate hosts file. Network nodes typically had one address and could have many names. As local areaTCP/IP computer networks gained popularity, however, the maintenance of hosts files became a larger burden on system administrators as networks and network nodes were being added to the system with increasing frequency.

Standardization efforts, such as the format specification of the fileHOSTS.TXT in RFC 952, and distribution protocols, e.g., the hostname server described in RFC 953, helped with these problems, but the centralized and monolithic nature of hosts files eventually necessitated the creation of the distributedDomain Name System (DNS).

On some old systems a file named networks is present that is similar to a hosts file, containing names of networks.^[13]

In its function of resolving host names, the hosts file may be used to define anyhostname ordomain name for use in the local system.

Redirecting local domains

Some web service andintranet developers and administrators define locally defined domains in aLAN for various purposes, such as accessing the company's internal resources or to test local websites in development.^[14]

Internet resource blocking

Entries in the hosts file may be used to blockonline advertising, or the domains of known malicious resources and servers that containspyware,adware, and othermalware. This may be achieved by adding entries for those sites to redirect requests to another address that does not exist or to a harmless destination such as thelocal machine.^[15] Commercial software applications may be used to populate the hosts file with entries of known undesirable Internet resources automatically. In addition, user-created hosts files which block nuisance servers are publicly available.^[16][17]

Fravia described these files variously as "scrolls", "precious", and "powerful" in his anti-advertisement pages, where this usage of hosts was first published.^[17]

Software piracy

Somepirated versions of software rely on a modified hosts file to prevent software fromcontacting the activation servers of the publisher, although activation servers sometimes appear in general purpose hosts files.^[18]

The hosts file may present an attack vector for malicious software. The file may be modified, for example, byadware,computer viruses, ortrojan horse software to redirect traffic from the intended destination to sites hosting malicious or unwanted content.^[19] The widespreadcomputer wormMydoom.B blocked users from visiting sites aboutcomputer security andantivirus software and also affected access from the compromised computer to the MicrosoftWindows Update website.

In some cases, malware has modified the library responsible for loading the hosts file in order to redirect it to a file it is able to control freely.^[20]

• DNSBL, a DNS-based blackhole list
• Content-control software
• Ad filtering

• Windows
  • Beginners Guide on Editing Hosts File
  • Microsoft TCP/IP Host Name Resolution Order
  • List of Microsoft domains that Windows won't redirect with a host file
• Mac
  • How to block unwanted domains with a HOSTS file on Mac OS X 10.2 or later
• Linux
  • How to block unwanted advertisements with /etc/hosts file on Linux

• Domain Name System
• Configuration files

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