Expand allowedUsers email field to support comma-separated and domains (#9)

* Expand allowedUsers email field to support comma-separated and domains

Closes #8

* Refactor AuthFetch() to return AuthUser struct

Also, this breaks out a parseLine() function which can be easily tested.

* Ignore empty addrs after splitting commas

This ignores a trailing comma

* Add tests for auth parseLine()

* Update documentation in smtprelay.ini

* Fix bug where addrAllowed() was incorrectly case-sensitive

* Update allowedUsers allowed domain format to require leading @

This disambiguates a local user ('john.smith') from a domain ('example.com')

main.go

@@ -36,15 +36,58 @@ func connectionChecker(peer smtpd.Peer) error {
 	return smtpd.Error{Code: 421, Message: "Denied"}
 }
 
+func addrAllowed(addr string, allowedAddrs []string) bool {
+	if allowedAddrs == nil {
+		// If absent, all addresses are allowed
+		return true
+	}
+
+	addr = strings.ToLower(addr)
+
+	// Extract optional domain part
+	domain := ""
+	if idx := strings.LastIndex(addr, "@"); idx != -1 {
+		domain = strings.ToLower(addr[idx+1:])
+	}
+
+	// Test each address from allowedUsers file
+	for _, allowedAddr := range allowedAddrs {
+		allowedAddr = strings.ToLower(allowedAddr)
+
+		// Three cases for allowedAddr format:
+		if idx := strings.Index(allowedAddr, "@"); idx == -1 {
+			// 1. local address (no @) -- must match exactly
+			if allowedAddr == addr {
+				return true
+			}
+		} else {
+			if idx != 0 {
+				// 2. email address (user@domain.com) -- must match exactly
+				if allowedAddr == addr {
+					return true
+				}
+			} else {
+				// 3. domain (@domain.com) -- must match addr domain
+				allowedDomain := allowedAddr[idx+1:]
+				if allowedDomain == domain {
+					return true
+				}
+			}
+		}
+	}
+
+	return false
+}
+
 func senderChecker(peer smtpd.Peer, addr string) error {
 	// check sender address from auth file if user is authenticated
 	if *allowedUsers != "" && peer.Username != "" {
-		_, email, err := AuthFetch(peer.Username)
+		user, err := AuthFetch(peer.Username)
 		if err != nil {
 			return smtpd.Error{Code: 451, Message: "Bad sender address"}
 		}
 
-		if email != "" && strings.ToLower(addr) != strings.ToLower(email) {
+		if !addrAllowed(addr, user.allowedAddresses) {
 			return smtpd.Error{Code: 451, Message: "Bad sender address"}
 		}
 	}