Make receiver hostname configurable via --receiver-hostname flag

Remove the package-level global hostname from parser.go.

Adds a log warning when the last Received hop doesn't match the
expected receiver hostname.

Bug: https://github.com/happyDomain/happydeliver/issues/11

internal/config/cli.go

@@ -34,6 +34,7 @@ func declareFlags(o *Config) {
 	flag.StringVar(&o.Email.Domain, "domain", o.Email.Domain, "Domain used to receive emails")
 	flag.StringVar(&o.Email.TestAddressPrefix, "address-prefix", o.Email.TestAddressPrefix, "Expected email adress prefix (deny address that doesn't start with this prefix)")
 	flag.StringVar(&o.Email.LMTPAddr, "lmtp-addr", o.Email.LMTPAddr, "LMTP server listen address")
+	flag.StringVar(&o.Email.ReceiverHostname, "receiver-hostname", o.Email.ReceiverHostname, "Hostname used to filter Authentication-Results headers (defaults to os.Hostname())")
 	flag.DurationVar(&o.Analysis.DNSTimeout, "dns-timeout", o.Analysis.DNSTimeout, "Timeout when performing DNS query")
 	flag.DurationVar(&o.Analysis.HTTPTimeout, "http-timeout", o.Analysis.HTTPTimeout, "Timeout when performing HTTP query")
 	flag.Var(&StringArray{&o.Analysis.RBLs}, "rbl", "Append a RBL (use this option multiple time to append multiple RBLs)")

internal/config/config.go

@@ -34,6 +34,11 @@ import (
 	openapi_types "github.com/oapi-codegen/runtime/types"
 )
 
+func getHostname() string {
+	h, _ := os.Hostname()
+	return h
+}
+
 // Config represents the application configuration
 type Config struct {
 	DevProxy        string
@@ -58,6 +63,7 @@ type EmailConfig struct {
 	Domain            string
 	TestAddressPrefix string
 	LMTPAddr          string
+	ReceiverHostname  string
 }
 
 // AnalysisConfig contains timeout and behavior settings for email analysis
@@ -84,6 +90,7 @@ func DefaultConfig() *Config {
 			Domain:            "happydeliver.local",
 			TestAddressPrefix: "test-",
 			LMTPAddr:          "127.0.0.1:2525",
+			ReceiverHostname:  getHostname(),
 		},
 		Analysis: AnalysisConfig{
 			DNSTimeout:  5 * time.Second,