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Document that "localhost" only resolves to the loopback addresses.
prodding pb
OK phessler, sthen
Input & OK jmc

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.\"     @(#)resolver.5	5.12 (Berkeley) 5/10/91
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.Dd $Mdocdate: November 25 2023 $
.Dt RESOLV.CONF 5
.Os
.Sh NAME
.Nm resolv.conf
.Nd resolver configuration files
.Sh DESCRIPTION
The
.Nm
file specifies how the resolver routines in the C library
(which provide access to the Internet Domain Name System) should operate.
The resolver configuration file contains information that is read
by the resolver routines the first time they are invoked by a process.
If the
.Nm resolv.conf
file does not exist, only the local host file
.Pa /etc/hosts
will be consulted,
i.e. the Domain Name System will not be used to resolve hosts.
.Pp
The host "localhost" and all names underneath the localhost domain will
always resolve to the loopback addresses 127.0.0.1 or ::1.
.Pp
The file is designed to be human readable and contains a list of
keywords with values that provide various types of resolver information.
A resolv.conf file is not required for some setups, so this file is optional.
It can be created manually, and is also created as part of the
.Ox
install process
if use of the DHCP protocol is specified for any interface
or if any DNS name servers are configured.
.Pp
A keyword and its values must appear on a single line, and the keyword (e.g.\&
.Ic nameserver )
must start the line.
The value follows the keyword, separated by whitespace.
A hash mark
.Pq #
or semicolon
.Pq \&;
in the file indicates the beginning of a comment;
subsequent characters up to the end of the line are not interpreted by
the routines that read the file.
.Pp
The configuration options are:
.Bl -tag -width nameserver
.It Ic nameserver
IPv4 address (in dot notation)
or IPv6 address (in hex-and-colon notation)
of a name server that the resolver should query.
Scoped IPv6 address notation is accepted as well
(see
.Xr inet6 4
for details).
.Pp
Up to
.Dv ASR_MAXNS
(currently 5) name servers may be listed, one per line.
If there are multiple servers, the resolver library queries them in the
order listed.
If no
.Ic 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 name servers
until a maximum number of retries are performed.)
.It Ic domain
Local domain name.
Most queries for names within this domain can use short names
relative to the local domain.
If no
.Ic domain
entry is present, the domain is determined
from the local host name returned by
.Xr gethostname 3 \(en
the domain part is taken to be everything after the first dot.
Finally, if the host name does not contain a domain part, the root
domain is assumed.
.It Ic lookup
This keyword is used by the library routines
.Xr gethostbyname 3
and
.Xr gethostbyaddr 3 .
It specifies which databases should be searched, and the order to do so.
The legal space-separated values are:
.Pp
.Bl -tag -width bind -offset indent -compact
.It Cm bind
Query a domain name server.
.It Cm file
Search for entries in
.Pa /etc/hosts .
.El
.Pp
If the
.Ic lookup
keyword is not used in the system's
.Nm resolv.conf
file then the assumed order is
.Cm bind file .
Furthermore, if the system's
.Nm resolv.conf
file does not exist, then the only database used is
.Cm file .
.It Ic search
Search list for hostname lookup.
The search list is normally determined from the local domain name;
by default, it begins with the local domain name, then successive
parent domains that have at least two components in their names.
This may be changed by listing the desired domain search path following the
.Ic search
keyword with spaces or tabs separating the names.
Most resolver queries will be attempted using each component
of the search path in turn until a match is found.
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.
.Pp
The search list is currently limited to six domains
with a total of 1024 characters.
Only one
.Ic search
line should appear; if more than one is present, the last one found
overwrites any values found in earlier lines.
.It Ic sortlist
Allows addresses returned by
.Xr gethostbyname 3
to be sorted.
A
.Ic 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.
For example:
.Pp
.Dl sortlist 130.155.160.0/255.255.240.0 130.155.0.0
.It Ic family
Specify which type of Internet protocol family to prefer,
if a host is reachable using different address families.
By default IPv4 addresses are queried first,
and then IPv6 addresses.
The syntax is:
.Bd -ragged -offset indent
.Ic family Ar family Op Ar family
.Ed
.Pp
A maximum of two families can be specified, where
.Ar family
can be any of:
.Pp
.Bl -tag -width "inet4XXX" -offset indent -compact
.It Cm inet4
IPv4 queries.
.It Cm inet6
IPv6 queries.
.El
.Pp
If only one family is specified,
only that family is tried.
.It Ic options
Allows certain internal resolver variables to be modified.
The syntax is:
.Bd -ragged -offset indent
.Ic options Ar option ...
.Ed
.Pp
Where
.Ar option
is one of the following:
.Bl -tag -width insecure1
.It Cm debug
Print debugging messages,
if libc is compiled with
.Dv DEBUG .
By default on
.Ox
this option does nothing.
.It Cm edns0
Attach an OPT pseudo-RR for the EDNS0 extension,
as specified in RFC 2671.
This informs DNS servers of a client's receive buffer size,
allowing them to take advantage of a non-default receive buffer size,
and thus send larger replies.
DNS query packets with the EDNS0 extension are not compatible with
non-EDNS0 DNS servers,
so the option must be used only when all the servers listed in
.Ic nameserver
lines are able to handle the extension.
.Pp
To verify whether a server supports EDNS,
query it using the
.Xr dig 1
query option
.Li +edns=0 :
the reply indicates compliance (EDNS version 0)
and whether a UDP packet larger than 512 bytes can be used.
Note that EDNS0 can cause the server to send packets
large enough to require fragmentation.
Other factors such as packet filters may impede these,
particularly if there is a reduced MTU,
as is often the case with
.Xr pppoe 4
or with tunnels.
.It Cm inet6
On
.Ox
this option does nothing.
On some operating systems, this option enables IPv6 support in
.Xr gethostbyname 3
by setting RES_USE_INET6 in _res.options (see
.Xr res_init 3 ) .
.It Cm insecure1
Do not require IP source address on the reply packet to be equal to the
server's address.
.It Cm insecure2
Do not check if the query section of the reply packet is equal
to that of the query packet.
For testing purposes only.
.It Cm ndots : Ns Ar n
Sets a threshold for the number of dots which
must appear in a name given to
.Xr res_query 3
before an initial absolute query will be made.
The default for
.Ar 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.
.It Cm tcp
Forces the use of TCP for queries.
Normal behaviour is to query via UDP but fall back to TCP on failure.
.It Cm trust-ad
A name server indicating that it performed DNSSEC validation by setting the
Authentic Data (AD) flag in the answer can only be trusted if the
name server itself is trusted and the network path is trusted.
Generally this is not the case and the AD flag is cleared in the answer.
The
.Cm trust-ad
option lets the system administrator indicate that the name server and the
network path are trusted.
This option is automatically enabled if
.Nm resolv.conf
only lists name servers on localhost.
.El
.El
.Pp
The
.Ic domain
and
.Ic search
keywords are mutually exclusive.
If more than one instance of these keywords is present, the last instance
will override.
.Sh ENVIRONMENT
.Bl -tag -width "RES_OPTIONSXXX"
.It Ev LOCALDOMAIN
A space-separated list of search domains,
overriding the
.Ic search
keyword of a system's
.Nm resolv.conf
file.
.It Ev RES_OPTIONS
A space-separated list of resolver options,
overriding the
.Ic options
keyword of a system's
.Nm resolv.conf
file.
.El
.Sh FILES
.Bl -tag -width "/etc/resolv.confXX" -compact
.It Pa /etc/resolv.conf
.El
.Sh SEE ALSO
.Xr gethostbyname 3 ,
.Xr res_init 3 ,
.Xr hosts 5 ,
.Xr hostname 7 ,
.Xr nsd 8 ,
.Xr resolvd 8 ,
.Xr unbound 8 ,
.Xr unwind 8
.Sh HISTORY
The
.Nm
file format appeared in
.Bx 4.3 .