Add documentation about listening on port < 1024 (influxdata#6785)

plugins/inputs/snmp_trap/README.md

@@ -17,7 +17,11 @@ the SNMP [README.md](../snmp/README.md) for details.
   ## Transport, local address, and port to listen on.  Transport must
   ## be "udp://".  Omit local address to listen on all interfaces.
   ##   example: "udp://127.0.0.1:1234"
-  # service_address = udp://:162
+  ##
+  ## Special permissions may be required to listen on a port less than
+  ## 1024.  See README.md for details
+  ##
+  # service_address = "udp://:162"
   ## Timeout running snmptranslate command
   # timeout = "5s"
 ```
@@ -41,3 +45,28 @@ the SNMP [README.md](../snmp/README.md) for details.
 snmp_trap,mib=SNMPv2-MIB,name=coldStart,oid=.1.3.6.1.6.3.1.1.5.1,source=192.168.122.102,version=2c snmpTrapEnterprise.0="linux",sysUpTimeInstance=1i 1574109187723429814
 snmp_trap,mib=NET-SNMP-AGENT-MIB,name=nsNotifyShutdown,oid=.1.3.6.1.4.1.8072.4.0.2,source=192.168.122.102,version=2c sysUpTimeInstance=5803i,snmpTrapEnterprise.0="netSnmpNotificationPrefix" 1574109186555115459
 ```
+
+### Using a Privileged Port
+
+On many operating systems, listening on a privileged port (a port
+number less than 1024) requires extra permission.  Since the default
+SNMP trap port 162 is in this category, using telegraf to receive SNMP
+traps may need extra permission.
+
+Instructions for listening on a privileged port vary by operating
+system. It is not recommended to run telegraf as superuser in order to
+use a privileged port. Instead follow the principle of least privilege
+and use a more specific operating system mechanism to allow telegraf to
+use the port.  You may also be able to have telegraf use an
+unprivileged port and then configure a firewall port forward rule from
+the privileged port.
+
+To use a privileged port on Linux, you can use setcap to enable the
+CAP_NET_BIND_SERVICE capability on the telegraf binary:
+
+```
+setcap cap_net_bind_service=+ep /usr/bin/telegraf
+```
+
+On Mac OS, listening on privileged ports is unrestricted on versions
+10.14 and later.

plugins/inputs/snmp_trap/snmp_trap.go

@@ -50,7 +50,11 @@ var sampleConfig = `
   ## Transport, local address, and port to listen on.  Transport must
   ## be "udp://".  Omit local address to listen on all interfaces.
   ##   example: "udp://127.0.0.1:1234"
-  # service_address = udp://:162
+  ##
+  ## Special permissions may be required to listen on a port less than
+  ## 1024.  See README.md for details
+  ##
+  # service_address = "udp://:162"
   ## Timeout running snmptranslate command
   # timeout = "5s"
 `