From 5ef488f808f0b92ff5aadf4f7c106e06163e276c Mon Sep 17 00:00:00 2001
From: Oleksandr Ivantsiv <oivantsiv@nvidia.com>
Date: Mon, 30 Jan 2023 21:13:10 +0100
Subject: [PATCH] Clear DNS configuration received from DHCP during networking
 reconfiguration in Linux. (#13516)

- Why I did it
fixes #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                                    | 9 +++++++--
 files/image_config/interfaces/interfaces-config.sh | 3 +++
 files/image_config/resolv-config/resolv.conf       | 0
 files/image_config/resolv-config/resolv.conf.head  | 2 ++
 4 files changed, 12 insertions(+), 2 deletions(-)
 delete mode 100644 files/image_config/resolv-config/resolv.conf
 create mode 100644 files/image_config/resolv-config/resolv.conf.head

diff --git a/build_debian.sh b/build_debian.sh
index 1461b176c7..180a427cac 100755
--- a/build_debian.sh
+++ b/build_debian.sh
@@ -386,7 +386,8 @@ sudo LANG=C DEBIAN_FRONTEND=noninteractive chroot $FILESYSTEM_ROOT apt-get -y in
     gpg                     \
     jq                      \
     auditd                  \
-    linux-perf
+    linux-perf              \
+    resolvconf
 
 # default rsyslog version is 8.2110.0 which has a bug on log rate limit,
 # use backport version
@@ -674,7 +675,11 @@ 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 -comp zstd -b 1M -e boot -e var/lib/docker -e $PLATFORM_DIR
 
 # Ensure admin gid is 1000
diff --git a/files/image_config/interfaces/interfaces-config.sh b/files/image_config/interfaces/interfaces-config.sh
index f6aa4147a4..9613493846 100755
--- a/files/image_config/interfaces/interfaces-config.sh
+++ b/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
 
diff --git a/files/image_config/resolv-config/resolv.conf b/files/image_config/resolv-config/resolv.conf
deleted file mode 100644
index e69de29bb2..0000000000
diff --git a/files/image_config/resolv-config/resolv.conf.head b/files/image_config/resolv-config/resolv.conf.head
new file mode 100644
index 0000000000..db81bded75
--- /dev/null
+++ b/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.