[SNMP][IPv6]: Fix to use link local IPv6 address as snmp agentAddress (…

…sonic-net#16013)


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#### Why I did it
fixes: sonic-net#16001
Caused by: sonic-net#15487

The above PR introduced change to use Management and Loopback Ipv4 and ipv6 addresses as snmpagent address in snmpd.conf file.
With this change, if Link local IP address is configured as management or Loopback IPv6 address, then snmpd tries to open socket on that ipv6 address and fails with the below error:
```
Error opening specified endpoint "udp6:[fe80::5054:ff:fe6f:16f0]:161"
Server Exiting with code 1
```
From RFC4007, if we need to specify non-global ipv6 address without ambiguity, we need to use zone id along with the ipv6 address: <address>%<zone_id>
Reference: https://datatracker.ietf.org/doc/html/rfc4007

##### Work item tracking
- Microsoft ADO **(number only)**:

#### How I did it
Modify snmpd.conf file to use the %zone_id representation for ipv6 address.
#### How to verify it
In VS testbed, modify config_db to use link local ipv6 address as management address:
    "MGMT_INTERFACE": {
        "eth0|10.250.0.101/24": {
            "forced_mgmt_routes": [
                "172.17.0.1/24"
            ],
            "gwaddr": "10.250.0.1"
        },
        "eth0|fe80::5054:ff:fe6f:16f0/64": {
            "gwaddr": "fe80::1"
        }
    },

Execute config_reload after the above change.
snmpd comes up and check if snmpd is listening on ipv4 and ipv6 addresses:
```
admin@vlab-01:~$ sudo netstat -tulnp | grep 161
tcp        0      0 127.0.0.1:3161          0.0.0.0:*               LISTEN      274060/snmpd        
udp        0      0 10.1.0.32:161           0.0.0.0:*                           274060/snmpd        
udp        0      0 10.250.0.101:161        0.0.0.0:*                           274060/snmpd        
udp6       0      0 fc00:1::32:161          :::*                                274060/snmpd        
udp6       0      0 fe80::5054:ff:fe6f::161 :::*                                274060/snmpd      -- Link local 
 
admin@vlab-01:~$ sudo ifconfig eth0
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 10.250.0.101  netmask 255.255.255.0  broadcast 10.250.0.255
        inet6 fe80::5054:ff:fe6f:16f0  prefixlen 64  scopeid 0x20<link>
        ether 52:54:00:6f:16:f0  txqueuelen 1000  (Ethernet)
        RX packets 36384  bytes 22878123 (21.8 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 261265  bytes 46585948 (44.4 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

admin@vlab-01:~$ docker exec -it snmp snmpget -v2c -c public fe80::5054:ff:fe6f:16f0 1.3.6.1.2.1.1.1.0
iso.3.6.1.2.1.1.1.0 = STRING: "SONiC Software Version: SONiC.master.327516-04a6031b2 - HwSku: Force10-S6000 - Distribution: Debian 11.7 - Kernel: 5.10.0-18-2-amd64"
```
Logs from snmpd:
```
Turning on AgentX master support.
NET-SNMP version 5.9
Connection from UDP/IPv6: [fe80::5054:ff:fe6f:16f0%eth0]:44308
```
Ran test_snmp_loopback test to check if loopback ipv4 and ipv6 works:
```
./run_tests.sh -n vms-kvm-t0 -d vlab-01 -c snmp/test_snmp_loopback.py  -f vtestbed.yaml -i ../ansible/veos_vtb -e "--skip_sanity --disable_loganalyzer" -u
=== Running tests in groups ===
Running: pytest snmp/test_snmp_loopback.py --inventory ../ansible/veos_vtb --host-pattern vlab-01 --testbed vms-kvm-t0 --testbed_file vtestbed.yaml --log-cli-level warning --log-file-level debug --kube_master unset --showlocals --assert plain --show-capture no -rav --allow_recover --ignore=ptftests --ignore=acstests --ignore=saitests --ignore=scripts --ignore=k8s --ignore=sai_qualify --junit-xml=logs/tr.xml --log-file=logs/test.log --skip_sanity --disable_loganalyzer
..                                                                        

snmp/test_snmp_loopback.py::test_snmp_loopback[vlab-01] PASSED 
```
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dockers/docker-snmp/snmpd.conf.j2

@@ -30,16 +30,27 @@ agentAddress {{ protocol(agentip) }}:[{{ agentip }}]{% if port %}:{{ port }}{% e
 {% endfor %}
 {% elif NAMESPACE_COUNT is not defined or NAMESPACE_COUNT|int <= 1 %}
 {% if MGMT_INTERFACE is defined %}
-{% for if, ip in MGMT_INTERFACE %}
-{% set agentip = ip.split('/')[0] %}
-agentAddress {{ protocol(agentip) }}:[{{ agentip }}]:161
+{% for intf, ip in MGMT_INTERFACE %}
+{% set agentip = ip.split('/')[0]|lower %}
+{% set zoneid = '' %}
+# Use interface as zoneid for link local ipv6
+{% if agentip.startswith('fe80') %}
+{% set zoneid = '%' + intf %}
+{% endif %}
+agentAddress {{ protocol(agentip) }}:[{{ agentip }}{{ zoneid }}]:161
 {% endfor %}
 {% endif %}
 {% if LOOPBACK_INTERFACE is defined %}
 {% for lo in LOOPBACK_INTERFACE %}
 {% if lo | length == 2 %}
-{% set agentip = lo[1].split('/')[0] %}
-agentAddress {{ protocol(agentip) }}:[{{ agentip }}]:161
+{% set intf = lo[0] %}
+{% set agentip = lo[1].split('/')[0]|lower %}
+{% set zoneid = '' %}
+# Use interface as zoneid for link local ipv6
+{% if agentip.startswith('fe80') %}
+{% set zoneid = '%' + intf %}
+{% endif %}
+agentAddress {{ protocol(agentip) }}:[{{ agentip }}{{ zoneid }}]:161
 {% endif %}
 {% endfor %}
 {% endif %}