Wenn der Server/Rechner keine Hostnamen auflösen kann, so liegt es daran, dass in der Datei /etc/resolv.conf keine Nameserver eingetragen sind oder diese zur Zeit alle nicht zur verfügung stehen. Sind dort Einträge vorhanden, so werden diese nacheinander durchgegangen bis ein Server von den angegebenen Nameservern eine Antwort in Form einer IP-Adresse zu dem angefragten Hostnamen erhält.

Es sollten möglichst mehrere NameServer angegeben werden. Fällt der erste Server aus, so kann auf den zweiten angegebenen Server zurückgegriffen werden. Finden Sie Nameserver, die örtlich in Ihrer Nähe liegen, damit die Antwort schnellstmöglich erfolgen kann, z.B. Nameserver der deutschen Telekom.

Die /etc/resolv.conf sieht beispielsweise folgendermaßen aus:

nameserver 194.25.0.60
nameserver 194.25.0.68

Es gibt noch eine Konfigurationsdatei /usr/include/resolv.h in der die maximale Anzahl der Nameserver, der Timeout bei fehlgeschlagenen Anfragen, usw. angegeben werden kann.

Man sollte diese Datei jedoch nur mit Bedacht anfassen bzw. ändern.

Für weitere Hilfe unter Linux man resolv.conf in der Konsole eingeben, damit die Hilfe zu diesem Thema erscheint.

Dabei bekommt man Folgendes:

DESCRIPTION
The  resolver is a set of routines in the C library that provide access
to the Internet Domain Name System (DNS).  The  resolver  configuration
file  contains  information  that  is read by the resolver routines the
first time they are invoked by a process.  The file is designed  to  be
human readable and contains a list of keywords with values that provide
various types of resolver information.

On a normally configured system this file should not be necessary.  The
only name server to be queried will be on the local machine; the domain
name is determined from the host name and the  domain  search  path  is
constructed from the domain name.

The different configuration options are:

nameserver Name server IP address
Internet  address  (in  dot  notation) of a name server that the
resolver  should  query.   Up  to  MAXNS   (currently   3,   see
<resolv.h>)  name  servers  may  be listed, one per keyword.  If
there are multiple servers, the resolver library queries them in
the  order  listed.   If  no nameserver entries are present, the
default is to use the name server on the  local  machine.   (The
algorithm  used  is to try a name server, and if the query times
out, try the next, until out of name servers, then repeat trying
all  the  name  servers  until  a  maximum number of retries are
made.)

domain Local domain name.
Most queries for names within this domain can  use  short  names
relative  to  the  local domain.  If no domain entry is present,
the domain is determined from the local host  name  returned  by
gethostname();  the  domain part is taken to be everything after
the first â.â.  Finally, if the host name  does  not  contain  a
domain part, the root domain is assumed.

search Search list for host-name lookup.
The  search  list  is  normally determined from the local domain
name; by default, it contains only the local domain name.   This
may be changed by listing the desired domain search path following
the search keyword with spaces or tabs separating the names.
Resolver  queries having fewer than ndots dots (default is 1) in
them will be attempted using each component of the  search  path
in  turn until a match is found.  For environments with multiple
subdomains please read options ndots:n below  to  avoid  man-in-
the-middle  attacks  and  unnecessary  traffic for the root-dns-
servers.  Note that this process may be slow and will generate a
lot of network traffic if the servers for the listed domains are
not local, and that queries will time out if no server is available
for one of the domains.

The search list is currently limited to six domains with a total
of 256 characters.

sortlist
Sortlist allows addresses returned by gethostbyname to be
sorted. A sortlist is specified by IP address netmask pairs.
The netmask is optional and defaults to the natural netmask of
the net. The IP address and optional network pairs are separated
by slashes. Up to 10 pairs may be specified. E.g.,
sortlist 130.155.160.0/255.255.240.0 130.155.0.0

options
Options allows certain internal resolver variables to be modified
The syntax is

options option …

where option is one of the following:

debug sets RES_DEBUG in _res.options.

ndots:n
sets a threshold for the number of dots which must appear
in a name given to res_query() (see  resolver(3)) before
an initial absolute query will be made. The default for
n is „1“, meaning that if there are any dots in a name,
the name will be tried first as an absolute name before
any search list elements are appended to it.

timeout:n
sets the amount of time the resolver will wait for a
response from a remote name server before retrying the
query via a different name server. Measured in seconds,
the default is RES_TIMEOUT (currently 5, see <resolv.h>).

attempts:n
sets the number of times the resolver will send a query
to its name servers before giving up and returning an
error to the calling application. The default is
RES_DFLRETRY (currently 2, see <resolv.h>).

rotate sets RES_ROTATE in _res.options, which causes round robin
selection of nameservers from among those listed. This
has the effect of spreading the query load among all
listed servers, rather than having all clients try the
first listed server first every time.

no-check-names
sets RES_NOCHECKNAME in _res.options, which disables the
modern BIND checking of incoming host names and mail
names for invalid characters such as underscore (_), non-
ASCII, or control characters.

inet6 sets RES_USE_INET6 in _res.options.  This has the  effect
of trying a AAAA query before an A query inside the gethostbyname()
function, and of mapping IPv4 responses in
IPv6 „tunnelled form“ if no AAAA records are found but
an A record set exists.

The domain and search keywords are mutually exclusive. If more than
one instance of these keywords is present, the last instance wins.

The search keyword of a system’s resolv.conf file can be overridden on
a per-process basis by setting the environment variable „LOCALDOMAIN“
to a space-separated list of search domains.

The options keyword of a system’s resolv.conf file can be amended on a
per-process basis by setting the environment variable „RES_OPTIONS“
to a space-separated list of resolver options as explained above under
options.

The keyword and value must appear on a single line, and the keyword
(e.g. nameserver) must start the line. The value follows the keyword,
separated by white space.

 Antworten

   
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