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HOST

Section: User Commands (1)
Index

NAME

host - look up host names using domain server

SYNOPSIS

host [-l] [-v] [-w] [-r] [-d] [-t querytype] [-a] host [ server ]

DESCRIPTION

Hostlooks for information about Internet hosts. It gets this informationfrom a set of interconnected servers that are spread across thecountry. By default, it simply converts between host names andInternet addresses. However with the -t or -a options, it can be usedto find all of the information about this host that is maintainedby the domain server.

The arguments can be either host names or host numbers. The programfirst attempts to interpret them as host numbers. If this fails,it will treat them as host names. A host number consists offirst decimal numbers separated by dots, e.g. 128.6.4.194A host nameconsists of names separated by dots, e.g. topaz.rutgers.edu. Unless the name ends in a dot, the local domainis automatically tacked on the end. Thus a Rutgers user can say"host topaz", and it will actually look up "topaz.rutgers.edu".If this fails, the name is tried unchanged (in this case, "topaz").This same convention is used for mail and other network utilities.The actual suffix to tack on the end is obtainedby looking at the results of a "hostname" call, and using everythingstarting at the first dot. (See below for a description ofhow to customize the host name lookup.)

The first argument is the host name you want to look up.If this is a number, an "inverse query" is done, i.e. the domainsystem looks in a separate set of databases used to convert numbersto names.

The second argument is optional. Itallows you to specify a particular server to query. If you don'tspecify this argument, the default server (normally the local machine)is used.

If a name is specified, you may see output of three different kinds.Here is an example that shows all of them:

   % host sun4

   sun4.rutgers.edu is a nickname for ATHOS.RUTGERS.EDU

   ATHOS.RUTGERS.EDU has address 128.6.5.46

   ATHOS.RUTGERS.EDU has address 128.6.4.4

   ATHOS.RUTGERS.EDU mail is handled by ARAMIS.RUTGERS.EDU
The user has typed the command "host sun4". The first line indicatesthat the name "sun4.rutgers.edu" is actually a nickname. The officialhost name is "ATHOS.RUTGERS.EDU'. The next two lines show theaddress. If a system has more than one network interface, therewill be a separate address for each. The last line indicatesthat ATHOS.RUTGERS.EDU does not receive its own mail. Mail forit is taken by ARAMIS.RUTGERS.EDU. There may be more than onesuch line, since some systems have more than one other systemthat will handle mail for them. Technically, every system thatcan receive mail is supposed to have an entry of this kind. Ifthe system receives its own mail, there should be an entrythe mentions the system itself, for example"XXX mail is handled by XXX". However many systems that receivetheir own mail do not bother to mention that fact. If a systemhas a "mail is handled by" entry, but no address, this indicatesthat it is not really part of the Internet, but a system that ison the network will forward mail to it. Systems on Usenet, Bitnet,and a number of other networks have entries of this kind.

There are a number of options that can be used before thehost name. Most of these options are meaningful only to thestaff who have to maintain the domain database.

The option -w causes host to wait forever for a response. Normallyit will time out after around a minute.

The option -v causes printout to be in a "verbose" format. Thisis the official domain master file format, which is documented in the man page for "named". Without this option, output still followsthis format in general terms, but some attempt is made to make itmore intelligible to normal users. Without -v,"a", "mx", and "cname" recordsare written out as "has address", "mail is handled by", and"is a nickname for", and TTL and class fields are not shown.

The option -r causes recursion to be turned off in the request.This means that the name server will return only data it has inits own database. It will not ask other servers for more information.

The option -d turns on debugging. Network transactions are shownin detail.

The option -t allows you to specify a particular type of informationto be looked up. The arguments are defined in the man page for"named". Currently supported types are a, ns, md, mf, cname,soa, mb, mg, mr, null, wks, ptr, hinfo, minfo, mx, uinfo,uid, gid, unspec, and the wildcard, which may be writtenas either "any" or "*". Types must be given in lower case.Note that the default is to look first for "a", and then "mx", exceptthat if the verbose option is turned on, the default is only "a".

The option -a (for "all") is equivalent to "-v -t any".

The option -l causes a listing of a complete domain. E.g.

host -l rutgers.edu
will give a listing of all hosts in the rutgers.edu domain. The -toption is used to filter what information is presented, as you would expect. The default is address information, which alsoinclude PTR and NS records. The command

host -l -v -t any rutgers.edu
will give a complete download of the zone data for rutgers.edu,in the official master file format. (However the SOA record islisted twice, for arcane reasons.) NOTE: -l is implemented bydoing a complete zone transfer and then filtering out the informationthe you have asked for. This command should be used only if itis absolutely necessary.

CUSTOMIZING HOST NAME LOOKUP

In general, if the name supplied by the user does nothave any dots in it, a default domain is appended to the end.This domain can be defined in /etc/resolv.conf, but is normally derivedby taking the local hostname after its first dot. The user can overridethis, and specify a different default domain, using the environmentvariableLOCALDOMAIN.In addition, the user can supply his own abbreviations for host names.They should be in a file consisting of one line per abbreviation.Each line contains an abbreviation, a space, and then the fullhost name. This file must be pointed to by an environment variableHOSTALIASES,which is the name of the file.

BUGS

Unexpected effects can happen when you type a name that is notpart of the local domain. Please always keep in mind thefact that the local domain name is tacked onto the end of everyname, unless it ends in a dot. Only if this fails is the nameused unchanged.

The -l option only tries the first name server listed for thedomain that you have requested. If this server is dead, youmay need to specify a server manually. E.g. to get a listingof foo.edu, you could try "host -t ns foo.edu" to get a listof all the name servers for foo.edu, and then try "host -l foo.edu xxx"for all xxx on the list of name servers, until you find one thatworks.

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