[dns] Add support for static DNS configuration. (sonic-net#14549)

- Why I did it
Add support for static DNS configuration. According to sonic-net/SONiC#1262 HLD.

- How I did it
Add a new resolv-config.service that is responsible for transferring configuration from Config DB into /etc/resolv.conf file that is consumed by various subsystems in Linux to resolve domain names into IP addresses.

- How to verify it
Run the image compilation. Each component related to the static DNS feature is covered with the unit tests.
Run sonic-mgmt tests. Static DNS feature will be covered with the system tests.
Install the image and run manual tests.

Author: oleksandrivantsiv

build_debian.sh

@@ -400,6 +400,7 @@ sudo LANG=C DEBIAN_FRONTEND=noninteractive chroot $FILESYSTEM_ROOT apt-get -y in
     jq                      \
     auditd                  \
     linux-perf              \
+    resolvconf              \
 	lsof                    \
 	sysstat
 
@@ -762,7 +763,11 @@ 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

files/build_templates/sonic_debian_extension.j2

@@ -430,6 +430,15 @@ j2 files/dhcp/dhclient.conf.j2 | sudo tee $FILESYSTEM_ROOT/etc/dhcp/dhclient.con
 sudo cp files/dhcp/ifupdown2_policy.json $FILESYSTEM_ROOT/etc/network/ifupdown2/policy.d
 sudo cp files/dhcp/90-dhcp6-systcl.conf.j2 $FILESYSTEM_ROOT_USR_SHARE_SONIC_TEMPLATES/
 
+# Copy DNS configuration files and templates
+sudo cp $IMAGE_CONFIGS/resolv-config/resolv-config.service $FILESYSTEM_ROOT_USR_LIB_SYSTEMD_SYSTEM
+sudo cp $IMAGE_CONFIGS/resolv-config/resolv-config.sh $FILESYSTEM_ROOT/usr/bin/
+sudo cp $IMAGE_CONFIGS/resolv-config/resolv.conf.j2 $FILESYSTEM_ROOT_USR_SHARE_SONIC_TEMPLATES/
+echo "resolv-config.service" | sudo tee -a $GENERATED_SERVICE_FILE
+sudo LANG=C chroot $FILESYSTEM_ROOT systemctl disable resolvconf.service
+sudo mkdir -p $FILESYSTEM_ROOT/etc/resolvconf/update-libc.d/
+sudo cp $IMAGE_CONFIGS/resolv-config/update-containers $FILESYSTEM_ROOT/etc/resolvconf/update-libc.d/
+
 # Copy initial interfaces configuration file, will be overwritten on first boot
 sudo cp $IMAGE_CONFIGS/interfaces/init_interfaces $FILESYSTEM_ROOT/etc/network/interfaces
 sudo mkdir -p $FILESYSTEM_ROOT/etc/network/interfaces.d

files/image_config/interfaces/interfaces-config.sh

@@ -60,6 +60,8 @@ 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
 
+/usr/bin/resolv-config.sh cleanup
+
 # Read sysctl conf files again
 sysctl -p /etc/sysctl.d/90-dhcp6-systcl.conf
 

files/image_config/resolv-config/resolv-config.service

@@ -0,0 +1,15 @@
+[Unit]
+Description=Update DNS configuration
+Requires=updategraph.service
+After=updategraph.service
+BindsTo=sonic.target
+After=sonic.target
+StartLimitIntervalSec=0
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart=/usr/bin/resolv-config.sh start
+
+[Install]
+WantedBy=sonic.target

files/image_config/resolv-config/resolv-config.sh

@@ -0,0 +1,61 @@
+#!/bin/bash
+
+WD=/var/run/resolvconf/
+CONFIG_DIR=${WD}/interface/
+STATIC_CONFIG_FILE=mgmt.static
+DYNAMIC_CONFIG_FILE_TEMPLATE=*.dhclient
+
+update_symlink()
+{
+    ln -sf /run/resolvconf/resolv.conf /etc/resolv.conf
+}
+
+start()
+{
+    update_symlink
+
+    redis-dump -d 4 -k "DNS_NAMESERVER*" -y > /tmp/dns.json
+    if [[ $? -eq 0 && "$(cat /tmp/dns.json)" != "{}" ]]; then
+        # Apply static DNS configuration and disable updates
+        /sbin/resolvconf --disable-updates
+        pushd ${CONFIG_DIR}
+        # Backup dynamic configuration to restore it when the static configuration is removed
+        mv ${DYNAMIC_CONFIG_FILE_TEMPLATE} ${WD} || true
+
+        sonic-cfggen -d -t /usr/share/sonic/templates/resolv.conf.j2,${STATIC_CONFIG_FILE}
+
+        /sbin/resolvconf --enable-updates
+        /sbin/resolvconf -u
+        /sbin/resolvconf --disable-updates
+        popd
+    else
+        # Dynamic DNS configuration. Enable updates. It is expected to receive configuraution for DHCP server
+        /sbin/resolvconf --disable-updates
+        pushd ${CONFIG_DIR}
+        rm -f ${STATIC_CONFIG_FILE}
+        # Restore dynamic configuration if it exists
+        mv ${WD}/${DYNAMIC_CONFIG_FILE_TEMPLATE} ${CONFIG_DIR} || true
+
+        /sbin/resolvconf --enable-updates
+        /sbin/resolvconf -u
+    fi
+}
+
+clean-dynamic-conf()
+{
+    rm -f ${WD}/${DYNAMIC_CONFIG_FILE_TEMPLATE}
+    rm -f ${WD}/postponed-update
+}
+
+case $1 in
+    start)
+        start
+        ;;
+    cleanup)
+        clean-dynamic-conf
+        ;;
+    *)
+        echo "Usage: $0 {start|clean-dynamic-conf}"
+        exit 2
+        ;;
+esac

files/image_config/resolv-config/resolv.conf

@@ -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.

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

@@ -0,0 +1,3 @@
+{% for ip in DNS_NAMESERVER|sort %}
+nameserver {{ ip }}
+{% endfor -%}

files/image_config/resolv-config/resolv.conf.j2

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+for container in $(docker ps -a --format=" {{ .ID }}"); do
+    docker cp -L /etc/resolv.conf ${container}:/_resolv.conf
+    docker exec -t ${container} bash -c "cat /_resolv.conf > /etc/resolv.conf"
+    docker exec -t ${container} bash -c "rm /_resolv.conf"
+done

files/image_config/resolv-config/update-containers

@@ -993,6 +993,7 @@ def parse_meta(meta, hname):
     dhcp_servers = []
     dhcpv6_servers = []
     ntp_servers = []
+    dns_nameservers = []
     tacacs_servers = []
     mgmt_routes = []
     erspan_dst = []
@@ -1023,6 +1024,8 @@ def parse_meta(meta, hname):
                     dhcp_servers = value_group
                 elif name == "NtpResources":
                     ntp_servers = value_group
+                elif name == "DnsNameserverResources":
+                    dns_nameservers = value_group
                 elif name == "SyslogResources":
                     syslog_servers = value_group
                 elif name == "TacacsServer":
@@ -1061,7 +1064,7 @@ def parse_meta(meta, hname):
                     qos_profile = value
                 elif name == "RackMgmtMap":
                     rack_mgmt_map = value
-    return syslog_servers, dhcp_servers, dhcpv6_servers, ntp_servers, tacacs_servers, mgmt_routes, erspan_dst, deployment_id, region, cloudtype, resource_type, downstream_subrole, switch_id, switch_type, max_cores, kube_data, macsec_profile, downstream_redundancy_types, redundancy_type, qos_profile, rack_mgmt_map
+    return syslog_servers, dhcp_servers, dhcpv6_servers, ntp_servers, dns_nameservers, tacacs_servers, mgmt_routes, erspan_dst, deployment_id, region, cloudtype, resource_type, downstream_subrole, switch_id, switch_type, max_cores, kube_data, macsec_profile, downstream_redundancy_types, redundancy_type, qos_profile, rack_mgmt_map
 
 
 def parse_linkmeta(meta, hname):
@@ -1488,6 +1491,7 @@ def parse_xml(filename, platform=None, port_config_file=None, asic_name=None, hw
     dhcp_servers = []
     dhcpv6_servers = []
     ntp_servers = []
+    dns_nameservers = []
     tacacs_servers = []
     mgmt_routes = []
     erspan_dst = []
@@ -1543,7 +1547,7 @@ def parse_xml(filename, platform=None, port_config_file=None, asic_name=None, hw
             elif child.tag == str(QName(ns, "UngDec")):
                 (u_neighbors, u_devices, _, _, _, _, _, _) = parse_png(child, hostname, None)
             elif child.tag == str(QName(ns, "MetadataDeclaration")):
-                (syslog_servers, dhcp_servers, dhcpv6_servers, ntp_servers, tacacs_servers, mgmt_routes, erspan_dst, deployment_id, region, cloudtype, resource_type, downstream_subrole, switch_id, switch_type, max_cores, kube_data, macsec_profile, downstream_redundancy_types, redundancy_type, qos_profile, rack_mgmt_map) = parse_meta(child, hostname)
+                (syslog_servers, dhcp_servers, dhcpv6_servers, ntp_servers, dns_nameservers, tacacs_servers, mgmt_routes, erspan_dst, deployment_id, region, cloudtype, resource_type, downstream_subrole, switch_id, switch_type, max_cores, kube_data, macsec_profile, downstream_redundancy_types, redundancy_type, qos_profile, rack_mgmt_map) = parse_meta(child, hostname)
             elif child.tag == str(QName(ns, "LinkMetadataDeclaration")):
                 linkmetas = parse_linkmeta(child, hostname)
             elif child.tag == str(QName(ns, "DeviceInfos")):
@@ -1995,6 +1999,7 @@ def parse_xml(filename, platform=None, port_config_file=None, asic_name=None, hw
     results['DHCP_SERVER'] = dict((item, {}) for item in dhcp_servers)
     results['DHCP_RELAY'] = dhcp_relay_table
     results['NTP_SERVER'] = dict((item, {}) for item in ntp_servers)
+    results['DNS_NAMESERVER'] = dict((item, {}) for item in dns_nameservers)
     results['TACPLUS_SERVER'] = dict((item, {'priority': '1', 'tcp_port': '49'}) for item in tacacs_servers)
     if len(acl_table_types) > 0:
         results['ACL_TABLE_TYPE'] = acl_table_types