Re-add 127.0.0.1/8 when bringing down the interfaces (#15080) (#15462)

* Re-add 127.0.0.1/8 when bringing down the interfaces

With #5353, 127.0.0.1/16 was added to the lo interface, and then
127.0.0.1/8 was removed. However, when bringing down the lo interface,
like during a config reload, 127.0.0.1/16 gets removed, but 127.0.0.1/8
isn't added back to the interface. This means that there's a period of
time where 127.0.0.1 is not available at all, and services that need to
connect to 127.0.01 (such as for redis DB) will fail.

To fix this, when going down, add 127.0.0.1/8. Add this address before
the existing configuration gets removed, so that 127.0.0.1 is available
at all times.

Note that running `ifdown lo` doesn't actually bring down the loopback
interface; the interface always stays "physically" up.

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

files/image_config/interfaces/interfaces.j2

@@ -26,6 +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
 {% endblock loopback %}
 {% block mgmt_interface %}
 

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

@@ -10,6 +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
 
 # The management network interface
 auto eth0

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

@@ -19,6 +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
 
 # The management network interface
 auto eth0

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

@@ -10,6 +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
 
 # The management network interface
 auto eth0

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

@@ -19,6 +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
 
 # The management network interface
 auto eth0