#!/bin/bash
# Need bash because we use ${foo//bar/baz}
#
# Example script illustrating how to write a dynamic BIND named
# options file containing an up-to-date fowarders list.
#
# To use the dynamically generated file, include it from named.conf.
# 
# This example was prepared for BIND version 8.  For use with BIND 9
# the script should first be modified so that it generates only the
# forwarders{} substatement, not the whole options{} statement.
# Much safer. See http://bugs.debian.org/483098 for the request that
# such a file be included in the bind9 package.
#
# Resolvconf may run this script even if named is not running.
# If a bind package is installed then we go ahead and update
# the named configuration in case named is started later.
#
# Assumption: On entry, PWD contains the resolv.conf-type files.
#
# Written by Thomas Hood <jdthood@gmail.com>
#
# Licensed under the GNU GPL.  See /usr/share/doc/resolvconf/copyright.

set -e
PATH=/sbin:/bin

[ -x /usr/sbin/named ] || exit 0
[ -x /lib/resolvconf/list-records ] || exit 1
[ -f /etc/bind/named.conf.options ] || exit 0

# Defaults
NAMED_RUN_DIR=/var/run/bind

# Defaults overrides
[ -r /etc/default/resolvconf ] && . /etc/default/resolvconf

[ "$NAMED_RUN_DIR" ] || exit 0
[ -d "$NAMED_RUN_DIR" ] || mkdir --parents --mode=0755 "$NAMED_RUN_DIR"

DYNAMIC_OPTS_FILE=named.options

# Stores arguments (minus duplicates) in RSLT, separated by spaces
# Doesn't work properly if an argument itself contains whitespace
uniquify()
{
	RSLT=""
	while [ "$1" ] ; do
		for E in $RSLT ; do
			[ "$1" = "$E" ] && { shift ; continue 2 ; }
		done
		RSLT="${RSLT:+$RSLT }$1"
		shift
	done
}

# Get list of records, excluding all those for the loopback interface
RSLVCNFFILES="$(/lib/resolvconf/list-records | sed -e '/^lo$/d' -e '/^lo[.]/d')"
 
### Compile semicolon-separated list nameservers ###
NMSRVRS=""
if [ "$RSLVCNFFILES" ] ; then
	uniquify $(sed -n -e 's/^[[:space:]]*nameserver[[:space:]]\+//p' $RSLVCNFFILES)
	[ "$RSLT" ] && NMSRVRS="${RSLT// /; }; "
fi

# N.B.: After changing directory we no longer have access to the resolv.conf-type files
cd "$NAMED_RUN_DIR"

TMP_FILE="${DYNAMIC_OPTS_FILE}_new.$$"
clean_up() { rm -f "${NAMED_RUN_DIR}/$TMP_FILE" ; }
trap clean_up EXIT
rm -f "$TMP_FILE"

# We want to process named.conf.options such that the new forwarders 
# statement gets inserted but nothing else is corrupted in the process.
# We want to do this using only commands available in /bin and /sbin, i.e.,
# with sh, sed and/or grep.   Sed can be made to work -- with difficulty.
# Even so, the following script does not work properly if comment
# delimiters of one style of commenting appear inside another kind of
# comment.  (Named supports C, C++ and sh comment styles.)
#
# First, we do our best to delete all and only comments.
# Then we delete any existing forwarders statement, taking into account
# the fact that these can span several lines.  Then we add a new
# forwarders statement at the beginning of the options statement.
#
echo "// named.conf fragment automatically generated by $0" > "$TMP_FILE"
echo "// DO NOT EDIT THIS FILE.  Instead edit /etc/bind/named.conf.options ." >> "$TMP_FILE"
cat /etc/bind/named.conf.options \
	| sed -e 's%\*/%\*/\
%g' \
	| sed -e '\%/\*%{ :x ; s%\*/%\*/% ; t y ; N ; b x ; :y ; s%/\*.*\*/%% ; }' \
	| sed -e 's%//.*%%' -e 's%#.*%%' \
	| sed -e '/forwarders/{ :x ; s/}/}/ ; t y ; N ; b x ; :y ; s/}[[:space:]]*;/};/ ; t z ; N ; b y ; :z s/forwarders[[:space:]]*{[^}]*};// ; }' \
	| sed -e 's/options[[:space:]]*{/options {\
	forwarders { '"${NMSRVRS}"'};/' | sed -e '/^[[:space:]]*$/{ d ; }' \
	>> "$TMP_FILE"

# Reload named unless we know its options haven't changed
if [ -x /usr/bin/diff ] && [ -f "$DYNAMIC_OPTS_FILE" ] && /usr/bin/diff -q "$DYNAMIC_OPTS_FILE" "$TMP_FILE" > /dev/null ; then
	# No change
	rm -f "$TMP_FILE"
else
	mv -f "$TMP_FILE" "$DYNAMIC_OPTS_FILE"
	[ -x /etc/init.d/bind9 ] && /etc/init.d/bind9 reload > /dev/null 2>&1 || :
fi