Linux DNS timeout, attempts.---DNS超时，重试的配置
Linux具有的独特优势，很多问题你都可以动手自己动手解决，任何问题先试试 man 一下。对于Linux DNS 超时 timeout，重试 attempts 机制及如何设置可以 man resolv.conf 阅读查看来解决：
查看后发现超时timeout 默认是5秒，上限是30秒。重试attempts默认2次，上限是5次。当然准确值需要查看
RESOLV.CONF(5) Linux Programmer's Manual RESOLV.CONF(5)
NAME
resolv.conf - resolver configuration file
SYNOPSIS
/etc/resolv.conf
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 read‐
able and contains a list of keywords with values that provide various types of resolver information.
If this file doesn't exist the only name server to be queried will be on the local machine; the domain name is determined from the hostname 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
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 hostname returned by gethostname(2); the domain part is taken to be everything after the first '.'. Finally, if
the hostname 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
This option allows addresses returned by gethostbyname(3) 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. Here is an example:
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(3) (see resolver(3)) before an initial abso‐
lute 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. The value for this option is silently capped to 15.
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
capped to 30.
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
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 hostnames and mail names for invalid char‐
acters 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(3) func‐
tion, and of mapping IPv4 responses in IPv6 "tunneled form" if no AAAA records are found but an A record set exists.
Some programs behave strangely when this option is turned on.
ip6-bytestring (since glibc 2.3.4)
sets RES_USE_BSTRING in _res.options. This causes reverse IPv6 lookups to be made using the bit-label format described in RFC 2673;
if this option is not set, then nibble format is used.
ip6-dotint/no-ip6-dotint (since glibc 2.3.4)
Clear/set RES_NOIP6DOTINT in _res.options. When this option is clear (ip6-dotint), reverse IPv6 lookups are made in the (depre‐
cated) ip6.int zone; when this option is set (no-ip6-dotint), reverse IPv6 lookups are made in the ip6.arpa zone by default. This
option is set by default.
edns0 (since glibc 2.6)
sets RES_USE_EDNSO in _res.options. This enables support for the DNS extensions described in RFC 2671.
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, sepa‐
rated by white space.
FILES
/etc/resolv.conf,
SEE ALSO
gethostbyname(3), resolver(3), hostname(7), named(8)
Name Server Operations Guide for BIND
COLOPHON
This page is part of release 3.23 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be
found at http://www.kernel.org/doc/man-pages/.
4th Berkeley Distribution 2009-03-01 RESOLV.CONF(5)
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