Fix potentially not having any loopback address on lo interface (#16490) (#16628)

In #15080, there was a command added to re-add 127.0.0.1/8 to the lo
interface when the networking configuration is being brought down.
However, the trigger for that command is `down`, which, looking at
ifupdown2 configuration files, runs immediately after 127.0.0.1/16 is
removed. This means there may be a period of time where there are no
loopback addresses assigned to the lo interface, and redis commands will
fail.

Fix this by changing this to pre-down, which should run well before
127.0.0.1/16 is removed, and should always leave lo with a loopback
address.

Signed-off-by: Saikrishna Arcot <sarcot@microsoft.com>
Co-authored-by: Saikrishna Arcot <sarcot@microsoft.com>

files/image_config/interfaces/interfaces.j2

@@ -26,7 +26,7 @@ iface lo inet loopback
    netmask 255.255.0.0
    scope host
    post-up ip addr del 127.0.0.1/8 dev lo
-   down ip addr add 127.0.0.1/8 dev lo
+   pre-down ip addr add 127.0.0.1/8 dev lo
 {% endblock loopback %}
 {% block mgmt_interface %}
 

src/sonic-config-engine/tests/sample_output/py2/interfaces

@@ -10,7 +10,7 @@ iface lo inet loopback
    netmask 255.255.0.0
    scope host
    post-up ip addr del 127.0.0.1/8 dev lo
-   down ip addr add 127.0.0.1/8 dev lo
+   pre-down ip addr add 127.0.0.1/8 dev lo
 
 # The management network interface
 auto eth0

src/sonic-config-engine/tests/sample_output/py2/interfaces_nomgmt

@@ -10,7 +10,7 @@ iface lo inet loopback
    netmask 255.255.0.0
    scope host
    post-up ip addr del 127.0.0.1/8 dev lo
-   down ip addr add 127.0.0.1/8 dev lo
+   pre-down ip addr add 127.0.0.1/8 dev lo
 
 # The management network interface
 auto eth0

src/sonic-config-engine/tests/sample_output/py2/interfaces_nomgmt_ztp

@@ -10,7 +10,7 @@ iface lo inet loopback
    netmask 255.255.0.0
    scope host
    post-up ip addr del 127.0.0.1/8 dev lo
-   down ip addr add 127.0.0.1/8 dev lo
+   pre-down ip addr add 127.0.0.1/8 dev lo
 
 # The management network interface
 auto eth0

src/sonic-config-engine/tests/sample_output/py2/interfaces_nomgmt_ztp_inband

@@ -10,7 +10,7 @@ iface lo inet loopback
    netmask 255.255.0.0
    scope host
    post-up ip addr del 127.0.0.1/8 dev lo
-   down ip addr add 127.0.0.1/8 dev lo
+   pre-down ip addr add 127.0.0.1/8 dev lo
 
 # The management network interface
 auto eth0

src/sonic-config-engine/tests/sample_output/py2/interfaces_nomgmt_ztp_inband_ip

@@ -10,7 +10,7 @@ iface lo inet loopback
    netmask 255.255.0.0
    scope host
    post-up ip addr del 127.0.0.1/8 dev lo
-   down ip addr add 127.0.0.1/8 dev lo
+   pre-down ip addr add 127.0.0.1/8 dev lo
 
 # The management network interface
 auto eth0

src/sonic-config-engine/tests/sample_output/py2/interfaces_nomgmt_ztp_ip

@@ -10,7 +10,7 @@ iface lo inet loopback
    netmask 255.255.0.0
    scope host
    post-up ip addr del 127.0.0.1/8 dev lo
-   down ip addr add 127.0.0.1/8 dev lo
+   pre-down ip addr add 127.0.0.1/8 dev lo
 
 # The management network interface
 auto eth0

src/sonic-config-engine/tests/sample_output/py2/mvrf_interfaces

@@ -19,7 +19,7 @@ iface lo inet loopback
    netmask 255.255.0.0
    scope host
    post-up ip addr del 127.0.0.1/8 dev lo
-   down ip addr add 127.0.0.1/8 dev lo
+   pre-down ip addr add 127.0.0.1/8 dev lo
 
 # The management network interface
 auto eth0

src/sonic-config-engine/tests/sample_output/py2/mvrf_interfaces_nomgmt

@@ -19,7 +19,7 @@ iface lo inet loopback
    netmask 255.255.0.0
    scope host
    post-up ip addr del 127.0.0.1/8 dev lo
-   down ip addr add 127.0.0.1/8 dev lo
+   pre-down ip addr add 127.0.0.1/8 dev lo
 
 # The management network interface
 auto eth0

src/sonic-config-engine/tests/sample_output/py2/two_mgmt_interfaces

@@ -10,7 +10,7 @@ iface lo inet loopback
    netmask 255.255.0.0
    scope host
    post-up ip addr del 127.0.0.1/8 dev lo
-   down ip addr add 127.0.0.1/8 dev lo
+   pre-down ip addr add 127.0.0.1/8 dev lo
 
 # The management network interface
 auto eth1

src/sonic-config-engine/tests/sample_output/py3/interfaces

@@ -10,7 +10,7 @@ iface lo inet loopback
    netmask 255.255.0.0
    scope host
    post-up ip addr del 127.0.0.1/8 dev lo
-   down ip addr add 127.0.0.1/8 dev lo
+   pre-down ip addr add 127.0.0.1/8 dev lo
 
 # The management network interface
 auto eth0

src/sonic-config-engine/tests/sample_output/py3/interfaces_nomgmt

@@ -10,7 +10,7 @@ iface lo inet loopback
    netmask 255.255.0.0
    scope host
    post-up ip addr del 127.0.0.1/8 dev lo
-   down ip addr add 127.0.0.1/8 dev lo
+   pre-down ip addr add 127.0.0.1/8 dev lo
 
 # The management network interface
 auto eth0

src/sonic-config-engine/tests/sample_output/py3/interfaces_nomgmt_ztp

@@ -10,7 +10,7 @@ iface lo inet loopback
    netmask 255.255.0.0
    scope host
    post-up ip addr del 127.0.0.1/8 dev lo
-   down ip addr add 127.0.0.1/8 dev lo
+   pre-down ip addr add 127.0.0.1/8 dev lo
 
 # The management network interface
 auto eth0

src/sonic-config-engine/tests/sample_output/py3/interfaces_nomgmt_ztp_inband

@@ -10,7 +10,7 @@ iface lo inet loopback
    netmask 255.255.0.0
    scope host
    post-up ip addr del 127.0.0.1/8 dev lo
-   down ip addr add 127.0.0.1/8 dev lo
+   pre-down ip addr add 127.0.0.1/8 dev lo
 
 # The management network interface
 auto eth0

src/sonic-config-engine/tests/sample_output/py3/interfaces_nomgmt_ztp_inband_ip

@@ -10,7 +10,7 @@ iface lo inet loopback
    netmask 255.255.0.0
    scope host
    post-up ip addr del 127.0.0.1/8 dev lo
-   down ip addr add 127.0.0.1/8 dev lo
+   pre-down ip addr add 127.0.0.1/8 dev lo
 
 # The management network interface
 auto eth0

src/sonic-config-engine/tests/sample_output/py3/interfaces_nomgmt_ztp_ip

@@ -10,7 +10,7 @@ iface lo inet loopback
    netmask 255.255.0.0
    scope host
    post-up ip addr del 127.0.0.1/8 dev lo
-   down ip addr add 127.0.0.1/8 dev lo
+   pre-down ip addr add 127.0.0.1/8 dev lo
 
 # The management network interface
 auto eth0

src/sonic-config-engine/tests/sample_output/py3/mvrf_interfaces

@@ -19,7 +19,7 @@ iface lo inet loopback
    netmask 255.255.0.0
    scope host
    post-up ip addr del 127.0.0.1/8 dev lo
-   down ip addr add 127.0.0.1/8 dev lo
+   pre-down ip addr add 127.0.0.1/8 dev lo
 
 # The management network interface
 auto eth0

src/sonic-config-engine/tests/sample_output/py3/mvrf_interfaces_nomgmt

@@ -19,7 +19,7 @@ iface lo inet loopback
    netmask 255.255.0.0
    scope host
    post-up ip addr del 127.0.0.1/8 dev lo
-   down ip addr add 127.0.0.1/8 dev lo
+   pre-down ip addr add 127.0.0.1/8 dev lo
 
 # The management network interface
 auto eth0

src/sonic-config-engine/tests/sample_output/py3/two_mgmt_interfaces

@@ -10,7 +10,7 @@ iface lo inet loopback
    netmask 255.255.0.0
    scope host
    post-up ip addr del 127.0.0.1/8 dev lo
-   down ip addr add 127.0.0.1/8 dev lo
+   pre-down ip addr add 127.0.0.1/8 dev lo
 
 # The management network interface
 auto eth0