Implement RFC Config #307 (#311)

RFCs/2022-06-23-307-config.md

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+# pktvisor Config
+
+## Status
+**Experimental**: implemented and available for beta testing but the interface may still change
+
+## Summary
+
+Pktvisord binary allows multiple [command line arguments](https://github.com/ns1labs/pktvisor#agent-usage) on its start up. Almost all the parameters can be passed to pktvisord 
+via yaml configuration file. Config file can also be combined with command line arguments.
+
+Moreover, the config file has a feature to allow setup of global configuration for specific handlers (`global_handler_config`). 
+The configuration set for a specific handler in this tag will be applied to every handler created in this agent which has the same specified type.
+
+
+`config.yaml`
+
+```yaml
+version: "1.0"
+
+visor:
+ # visor/config supports almost all pktvisord command line arguments 
+ config:
+ verbose: true
+ daemon: false
+ no_track: true
+ max_deep_sample: 50
+ periods: 3
+ syslog: false
+ log_file: "/path/to/logfile"
+ prometheus: false
+ prom_instance: "prom-ID"
+ geo_city: "/path/to/geoCityDB"
+ geo_asn: "/path/to/geoASNDB"
+ admin_api: true
+ tls: false
+ port: 10853
+ host: localhost
+ tls_cert: "/path/to/tlsCert"
+ tls_key: "/path/to/tlsKey"
+ module_list: false
+ module_dir: "/path/to/moduleDir"
+ cp_disable: true
+ cp_token: "crashpadToken"
+ cp_url: "crashpadURL"
+ cp_path: "/path/to/crashpad/executable"
+ #configuration inserted here will be applied to every specified handler in the agent provisioned with this config file 
+ global_handler_config:
+ #specifies a global configuration for DNS handlers 
+ dns:
+ #if a policy with a DNS handler specifies different value for 'only_rcode', this global config will be overwritten
+ only_rcode: 2
+
+```
+
+## REST API
+
+Config setup is only allowed at start up. Therefore, it is not supported on the REST API.
+
+## Standalone Command Line Example
+
+```shell
+$ pktvisord --config config.yaml
+```