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Filename : libc
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#!/bin/sh
#
# Script to update resolv.conf, the libc resolver configuration file,
# and to notify users of the libc resolver of changes
#
# Assumption: On entry, PWD contains the resolv.conf-type files.
#
# This script is part of the resolvconf package.
#
# Set REPORT_ABSENT_SYMLINK=no to inhibit warning message that
# /etc/resolv.conf is not a symbolic link
#
# Set TRUNCATE_NAMESERVER_LIST_AFTER_LOOPBACK_ADDRESS=no
# to allow additional nameserver addresses to be listed in
# resolv.conf after an initial loopback address 127.* or ::1.
#

set -e
PATH=/sbin:/bin

[ -x /lib/resolvconf/list-records ] || exit 1

# Default override
[ -r /etc/default/resolvconf ] && . /etc/default/resolvconf

ETC=/etc
ETCRESOLVCONF="${ETC}/resolvconf"
RESOLVCONFDIR="${ETCRESOLVCONF}/resolv.conf.d"
BASEFILE="${RESOLVCONFDIR}/base"
HEADFILE="${RESOLVCONFDIR}/head"
TAILFILE="${RESOLVCONFDIR}/tail"
DYNAMICRSLVCNFFILE="/run/resolvconf/resolv.conf"
TMPFILE="${DYNAMICRSLVCNFFILE}_new.$$"

# Set unset variables to their defaults
if [ -z "$REPORT_ABSENT_SYMLINK" ] ; then
	# '..._ALTERED_...' is the old deprecated name for this variable
	if [ "$REPORT_ALTERED_SYMLINK" ] ; then
		REPORT_ABSENT_SYMLINK="$REPORT_ALTERED_SYMLINK"
	else
		# Set to default value
		REPORT_ABSENT_SYMLINK=y
	fi
fi

if [ -z "$TRUNCATE_NAMESERVER_LIST_AFTER_LOOPBACK_ADDRESS" ] ; then
	# '..._127' is the old deprecated name for this variable
	if [ "$TRUNCATE_NAMESERVER_LIST_AFTER_127" ] ; then
		TRUNCATE_NAMESERVER_LIST_AFTER_LOOPBACK_ADDRESS="$TRUNCATE_NAMESERVER_LIST_AFTER_127"
	else
		# Set to default value
		TRUNCATE_NAMESERVER_LIST_AFTER_LOOPBACK_ADDRESS=y
	fi
fi


report_warning() { echo "$0: Warning: $*" >&2 ; }

resolv_conf_is_symlinked_to_dynamic_file() {
	[ -L ${ETC}/resolv.conf ] && [ "$(readlink -m ${ETC}/resolv.conf)" = "$DYNAMICRSLVCNFFILE" ]
}

if ! resolv_conf_is_symlinked_to_dynamic_file ; then
	case "$REPORT_ABSENT_SYMLINK" in
	  y|Y|yes|YES|Yes)
		report_warning "${ETC}/resolv.conf is not a symbolic link to $DYNAMICRSLVCNFFILE"
		;;
	esac
fi

# Args are candidate items not containing spaces
# Returns RSLT -- space-separated list of items without duplicates
#
# Stores arguments (minus duplicates) in RSLT, separated by spaces
uniquify()
{
	RSLT=""
	local D
	while [ "$1" ] ; do
		# Remove the root domain suffix
		D="${1%.}"
		for E in $RSLT ; do
			[ "$D" = "$E" ] && { shift ; continue 2 ; }
		done
		RSLT="${RSLT:+$RSLT }$D"
		shift
	done
}

# Args are candidate items not containing spaces
# Returns NSMSRVS -- space-separate list of no more than 3 items,
#                    without duplicates,
#                    truncated after loopback address if TRUNCATE_NAMESERVER_LIST_AFTER_LOOPBACK_ADDRESS set affirmatively
uniquify_nameserver_list()
{
	NMSRVRS=""
	N=0
	while [ "$1" ] ; do
		for E in $NMSRVRS ; do
			[ "$1" = "$E" ] && { shift ; continue 2 ; }
		done
		NMSRVRS="${NMSRVRS:+$NMSRVRS }$1"
		case "$TRUNCATE_NAMESERVER_LIST_AFTER_LOOPBACK_ADDRESS" in (y|Y|yes|YES|Yes) case "$1" in (127.*|::1) return 0 ;; esac ;; esac
		N=$(($N + 1))
		[ "$N" = 3 ] && return 0
		shift
	done
}

RSLVCNFFILES="$(/lib/resolvconf/list-records)"

[ -f "$BASEFILE" ] && RSLVCNFFILES="$RSLVCNFFILES
$BASEFILE"

### Compile list of nameservers ###
NMSRVRS=""
if [ "$RSLVCNFFILES" ] ; then
	uniquify_nameserver_list $(sed -n 's/^[[:space:]]*nameserver[[:space:]]\+//p' $RSLVCNFFILES)
fi

### Compile search list ###
SRCHS=""
if [ "$RSLVCNFFILES" ] ; then
	uniquify $(sed -n 's/^[[:space:]]*\(\(search\)\|\(domain\)\)[[:space:]]\+//p' $RSLVCNFFILES)
	SRCHS="$RSLT"
fi

clean_up() { rm -f "$TMPFILE" ; }
trap clean_up EXIT
clean_up

### Make the file ###
: > "$TMPFILE"
[ -f "$HEADFILE" ] && cat "$HEADFILE" >> "$TMPFILE"
for N in $NMSRVRS ; do echo "nameserver $N" >> "$TMPFILE" ; done
[ "$SRCHS" ] && echo "search $SRCHS" >> "$TMPFILE"
[ "$RSLVCNFFILES" ] && sed -e '/^[[:space:]]*$/d' -e '/^[[:space:]]*#/d' -e '/^[[:space:]]*\(\(nameserver\)\|\(search\)\|\(domain\)\)[[:space:]]/d' $RSLVCNFFILES >> "$TMPFILE" 2>/dev/null
[ -f "$TAILFILE" ] && cat "$TAILFILE" >> "$TMPFILE"

### Put the file in place ###

if [ -f "$DYNAMICRSLVCNFFILE" ] && [ "$(cat $TMPFILE)" = "$(cat $DYNAMICRSLVCNFFILE)" ] ; then
	# The file has not changed
	rm -f "$TMPFILE"
	exit 0
fi

# The file has changed
mv -f "$TMPFILE" "$DYNAMICRSLVCNFFILE"

# Only notify users of /etc/resolv.conf if /etc/resolv.conf is actually
# symlinked to the file we have just updated.
resolv_conf_is_symlinked_to_dynamic_file || exit 0

# Notify users of the resolver
if [ -d "${ETCRESOLVCONF}/update-libc.d" ] ; then
	exec run-parts "${ETCRESOLVCONF}/update-libc.d"
fi

exit 0