Dynamically add/remove mDNSResponder chroot socket

mdnschroot.sh

#!/bin/bash
#
# This is an example script to dynamically update mDNSResponder on OSX to
# add/remove a chroot socket to enable DNS resolution inside it.
#
# If the chroot is not added, add it, otherwise delete it.
#
# You will probably want a more robust version for production.
#

chroot="${1%%/}"; shift

PB="/usr/libexec/PlistBuddy"
PLIST="/System/Library/LaunchDaemons/com.apple.mDNSResponder.plist"
ENTRY="Sockets:Listeners"

#
# Check that Sockets:Listeners is an array (it is not by default), if not
# then make it so.
#
${PB} -c "Print ${ENTRY}:0" ${PLIST} >/dev/null 2>&1
if [ $? -ne 0 ]; then
	echo "Converting ${ENTRY} to an array"

	tmplist=/tmp/import.$$.plist
	${PB} -x -c "Print ${ENTRY}" ${PLIST} >${tmplist}
	${PB} -c "Delete ${ENTRY}" ${PLIST}
	${PB} -c "Add ${ENTRY} array" ${PLIST}
	${PB} -c "Add ${ENTRY}:0 dict" ${PLIST}
	${PB} -c "Merge ${tmplist} ${ENTRY}:0" ${PLIST}

	rm -f ${tmplist}
fi

#
# Get size of Listeners array
#
entries=$(${PB} -c "Print ${ENTRY}" ${PLIST} | grep -c "Dict {")

#
# Find an entry which corresponds to our chroot.
#
e=0
while [ ${e} -lt ${entries} ]; do
	sockpath="$(${PB} -c 'Print '${ENTRY}':'$e':SockPathName' ${PLIST})" 
	if [ "${sockpath}" = "${chroot}/var/run/mDNSResponder" ]; then
		break
	fi
	e=$((e + 1))
done

#
# If we didn't find an entry, add it, otherwise remove it.
#
if [ ${e} -eq ${entries} ]; then
	addlist="/tmp/add.$$.plist"
	cat >${addlist} <<-EOF
		<array>
			<dict>
				<key>SockFamily</key>
				<string>Unix</string>
				<key>SockPathName</key>
				<string>${chroot}/var/run/mDNSResponder</string>
				<key>SockPathMode</key>
				<integer>438</integer>
			</dict>
		</array>
	EOF
	echo "Adding ${chroot}/var/run/mDNSResponder"
	${PB} -c "Merge ${addlist} ${ENTRY}" ${PLIST}
	rm -f ${addlist}
else
	echo "Deleting ${chroot}/var/run/mDNSResponder"
	${PB} -c "Delete ${ENTRY}:${e}" ${PLIST}
fi

#
# Reload mDNSResponder
#
launchctl unload ${PLIST}
launchctl load -w ${PLIST}

On Mojave, need sudo to modify /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist