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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's Internet Protocol (IP) implementation, and serves the function of translating human-friendly hostnames into numeric protocol addresses, called IP 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 the Domain Name System (DNS), but many systems implement name service switches, e.g., nsswitch.conf for Linux and Unix, to provide customization. Unlike remote DNS resolvers, the hosts file is under the direct control of the local computer's administrator
The hosts file contains lines of text consisting of an IP address in the first text field followed by one or more host names. 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 an octothorpe (#) in the first position of such lines. Entirely blank lines in the file are ignored. For example, a typical hosts file may contain the following:
127.0.0.1 localhost # The following lines are desirable for IPv6 capable hosts ::1 ip6-localhost ip6-loopback fe00::0 ip6-localnet ff00::0 ip6-mcastprefix ff02::1 ip6-allnodes ff02::2 ip6-allrouters ff02::3 ip6-allhosts
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 and loopback), and that a host name may be mapped to both IPv4 and IPv6 IP addresses, as shown on the first and second lines respectively.
Location in the file system
The location of the hosts file in the file system hierarchy varies by operating system. It is usually named hosts, without an extension.
|Unix, Unix-like, POSIX||/etc/hosts|
|95, 98, ME||%WinDir%|
|NT, 2000, XP, 2003, Vista, 2008, 7, 2012, 8, 10||%SystemRoot%\System32\drivers\etc\hosts|
|Windows Mobile, Windows Phone||Registry key under HKEY_LOCAL_MACHINE\Comm\Tcpip\Hosts|
|Apple Macintosh||9 and earlier||Preferences or System folder|
|Mac OS X 10.0–10.1.5||(Added through NetInfo or niload)|
|Mac OS X 10.2 and newer||/etc/hosts (a symbolic link to /private/etc/hosts)|
|OS/2 & eComStation||"bootdrive":\mptn\etc\|
|Symbian||Symbian OS 6.1–9.0||C:\system\data\hosts|
|Symbian OS 9.1+||C:\private\10000882\hosts|
|Android||/etc/hosts (a symbolic link to /system/etc/hosts)|
|iOS||iOS 2.0 and newer||/etc/hosts (a symbolic link to /private/etc/hosts)|
|RISC OS||3.7, 5||!Boot.Resources.!Internet.files.Hosts|
|later boot sequence||!Boot.Choices.Hardware.Disabled.Internet.Files.Hosts|
In its function of resolving host names, the hosts file may be used to define any hostname or domain name for use in the local system.
- Redirecting local domains
- Some web service and intranet developers and administrators define locally defined domains in a LAN for various purposes, such as accessing the company's internal resources or to test local websites in development.
- Internet resource blocking
- Entries in the hosts file may be used to block online advertising, or the domains of known malicious resources and servers that contain spyware, adware, and other malware. 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 the local machine. 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.
Fravia described these files variously as "scrolls", "precious", and "powerful" in his anti-advertisement pages, where this usage of hosts was first published.
- Software piracy
- Some pirated versions of software rely on a modified hosts file to prevent software from contacting the activation servers of the publisher, although activation servers sometimes appear in general purpose hosts files.
Common security issues
The hosts file may present an attack vector for malicious software. The file may be modified, for example, by adware, computer viruses, or trojan horse software to redirect traffic from the intended destination to sites hosting malicious or unwanted content. The widespread computer worm Mydoom.B blocked users from visiting sites about computer security and antivirus software and also affected access from the compromised computer to the Microsoft Windows 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.