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2 __NAME__
3
4
5 hostname - host name resolution description
6 __DESCRIPTION__
7
8
9 Hostnames are domains. A domain is a hierarchical, dot-sep-
10 arated list of subdomains. For example, the machine
11 ``monet'', in the ``Berkeley'' subdomain
12 of the ``EDU'' sub- domain of the Internet Domain
13 Name System would be repre- sented as
14
15
16 monet.Berkeley.EDU
17
18
19 (with no trailing dot).
20
21
22 Hostnames are often used with network client and server pro-
23 grams, which must generally translate the name to an address
24 for use. (This task is usually performed by the library
25 routine gethostbyname(3).) The default method for
26 resolving hostnames by the Internet name resolver is to
27 follow RFC 1535's security recommendations. Actions can be
28 taken by the administrator to override these recommendations
29 and to have the resolver behave the same as earlier, non-RFC
30 1535 resolvers.
31
32
33 The default method (using RFC 1535 guidelines)
34 follows:
35
36
37 If the name consists of a single component, i.e. contains no
38 dot, and if the environment variable
39 ``HOSTALIASES'' is set to the name of a file, that
40 file is searched for a string matching the input hostname.
41 The file should consist of lines made up of two strings
42 separated by white-space, the first of which is the hostname
43 alias, and the second of which is the complete hostname to
44 be substituted for that alias. If a case-insensitive match
45 is found between the hostname to be resolved and the first
46 field of a line in the file, the substituted name is looked
47 up with no further pro- cessing.
48
49
50 If there is at least one dot in the name, then the name is
51 first tried ``as-is''. The number of dots to cause this
52 action is configurable by setting the threshold using the
53 ``ndots'' option in /etc/resolv.conf
54 (default: 1). If the name ends with a dot, the trailing dot
55 is removed, and the remaining name is looked up (regardless
56 of the setting of the ndots option), without
57 further processing.
58
59
60 If the input name does not end with a trailing dot, it is
61 looked up by searching through a list of domains until a
62 match is found. If neither the search option in the
63 /etc/resolv.conf file or the
64 ``LOCALDOMAIN'' environment variable is used, then
65 the search list of domains contains only the full domain
66 specified by the domain option (in
67 /etc/resolv.conf) or the domain used in the local
68 hostname (see hostname(1) and
69 resolver(5)). For example, if the
70 ``domain'' option is set to
71 CS.Berkeley.EDU, then only CS.Berkeley.EDU
72 will be in the search list, and this will be the only domain
73 appended to the partial hostname. For exam- ple, if
74 ``lithium'' is the name to be resolved, this would
75 make lithium.CS.Berkeley.EDU the only name to be
76 tried using the search list.
77
78
79 If the search option is used in
80 /etc/resolv.conf or the environment variable
81 ``LOCALDOMAIN'' is set by the user, then the search
82 list will include what is set by these meth- ods. For
83 example, if the ``search'' option
84 contained
85
86
87 CS.Berkeley.EDU CChem.Berkeley.EDU
88 Berkeley.EDU
89
90
91 then the partial hostname (e.g., ``lithium'') will
92 be tried with ''each'' domain name appended (in the same
93 order speci- fied); the resulting hostnames that would be
94 tried are:
95
96
97 lithium.CS.Berkeley.EDU
98 lithium.CChem.Berkeley.EDU
99 lithium.Berkeley.EDU
100 The environment variable ``LOCALDOMAIN'' overrides the ``search'' and ``domain'' options, and if both search and domain options are present in the resolver configuration file, then only the ''last'' one listed is used (see resolver(5)).
101
102
103 If the name was not previously tried ``as-is'' (i.e., it
104 fell below the ``ndots'' threshold or did not
105 contain a dot), then the name as originally provided is
106 attempted.
107 __ENVIRONMENT__
108 LOCALDOMAIN
109 Affects domains appended to partial host- names.
110
111
112 HOSTALIASES
113 Name of file containing (host alias, full hostname) pairs.
114
115
116 __FILES__
117 /etc/resolv.conf
118 See resolve(5).
119
120
121 __SEE ALSO__
122
123
124 gethostbyname(3), resolver(5),
125 mailaddr(7), named(8).
126
127
2 perry 128 4th Berkeley !DistributionFebruary 16, 1994 1
1 perry 129 ----
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