I'd redo the last two field validations like so:

```go
        if conf.SourceIpField == "" {
                return errors.New("`source_ip_field` must be specified")
        }
        if conf.TargetField == "" {
                return errors.New("`target_field` must be specified")
        }
        ld.SourceIpField = conf.SourceIpField
        ld.TargetField = conf.TargetField
```