Clear DNS configuration received from DHCP during networking reconfig…

…uration in Linux. (sonic-net#13516) (sonic-net#13695)

- Why I did it
fixes sonic-net#12907

When the management interface IP address configuration changes from dynamic to static the DNS configuration (retrieved from the DHCP server) in /etc/resolv.conf remains uncleared. This leads to a DNS configuration pointing to the wrong nameserver. To make the behavior clear DNS configuration received from DHCP should be cleared.

- How I did it
Use resolvconf package for managing DNS configuration. It is capable of tracking the source of DNS configuration and puts the configuration retrieved from the DHCP servers into a separate file. This allows the implementation of DNS configuration cleanup retrieved from DHCP during networking reconfiguration.

- How to verify it
Ensure that the management interface has no static configuration.
Check that /etc/resolv.conf has DNS configuration.
Configure a static IP address on the management interface.
Verify that /etc/resolv.conf has no DNS configuration.
Remove the static IP address from the management interface.
Verify that /etc/resolv.conf has DNS configuration retrieved form DHCP server.

build_debian.sh

@@ -385,7 +385,8 @@ sudo LANG=C DEBIAN_FRONTEND=noninteractive chroot $FILESYSTEM_ROOT apt-get -y in
     gpg                     \
     jq                      \
     auditd                  \
-    linux-perf
+    linux-perf              \
+    resolvconf
 
 # Have systemd create the auditd log directory
 sudo mkdir -p ${FILESYSTEM_ROOT}/etc/systemd/system/auditd.service.d
@@ -661,7 +662,11 @@ sudo rm -f $ONIE_INSTALLER_PAYLOAD $FILESYSTEM_SQUASHFS
 ## Note: -x to skip directories on different file systems, such as /proc
 sudo du -hsx $FILESYSTEM_ROOT
 sudo mkdir -p $FILESYSTEM_ROOT/var/lib/docker
-sudo cp files/image_config/resolv-config/resolv.conf $FILESYSTEM_ROOT/etc/resolv.conf
+
+## Clear DNS configuration inherited from the build server
+sudo rm -f $FILESYSTEM_ROOT/etc/resolvconf/resolv.conf.d/original
+sudo cp files/image_config/resolv-config/resolv.conf.head $FILESYSTEM_ROOT/etc/resolvconf/resolv.conf.d/head
+
 sudo mksquashfs $FILESYSTEM_ROOT $FILESYSTEM_SQUASHFS -e boot -e var/lib/docker -e $PLATFORM_DIR
 
 # Ensure admin gid is 1000

files/image_config/interfaces/interfaces-config.sh

@@ -60,6 +60,9 @@ for intf_pid in $(ls -1 /var/run/dhclient*.Ethernet*.pid 2> /dev/null); do
     [[ -f ${intf_pid} ]] && kill `cat ${intf_pid}` && rm -f ${intf_pid}
 done
 
+[[ -f /var/run/resolvconf/interface/eth0.dhclient ]] && rm -f /var/run/resolvconf/interface/eth0.dhclient
+[[ -f /var/run/resolvconf/interface/eth0.ip6.dhclient ]] && rm -f /var/run/resolvconf/interface/eth0.ip6.dhclient
+
 # Read sysctl conf files again
 sysctl -p /etc/sysctl.d/90-dhcp6-systcl.conf
 

files/image_config/resolv-config/resolv.conf.head

@@ -0,0 +1,2 @@
+# Dynamic resolv.conf(5) file generated by resolvconf(8)
+# The content of this file may be overwritten during a config reload.