test/t-21-dkim: Add driusan/dkim's dkimverify check

This patch adds a cross-tool integration check that uses
driusan/dkim's dkimverify to confirm it can verify our own DKIM signatures.

It is optional, since the tool may not be present.

test/t-21-dkim/from_B_to_A.expected

@@ -2,12 +2,18 @@ From user-a@srv-a
 Authentication-Results: srv-a
 	;spf=none (no DNS record found)
 	;dkim=pass  header.b=*
-DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed;
+DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
 	d=srv-b; s=sel77; *
 	h=from:subject:to:from:subject:date:to:cc:message-id;
 	bh=*
 	b=*
 	  *
+	  *
+	  *
+	  *
+	  *
+	  *
+	  *
 From: user-b@srv-b
 To: user-a@srv-a
 Subject: Feliz primavera!

test/t-21-dkim/run.sh

@@ -16,7 +16,7 @@ export GOTAGS="dnsoverride"
 # Use a fixed selector so we can be more thorough in from_B_to_A.expected.
 rm -f B/domains/srv-b/*.pem
 mkdir -p B/domains/srv-b/
-CONFDIR=B chasquid-util dkim-keygen srv-b sel77 --algo=ed25519 > /dev/null
+CONFDIR=B chasquid-util dkim-keygen srv-b sel77 > /dev/null
 
 cp zones .zones
 CONFDIR=B chasquid-util dkim-dns srv-b | sed 's/"//g' >> .zones
@@ -65,5 +65,14 @@ smtpc --addr=localhost:2465 \
 wait_for_file .mail/user-a@srv-a
 mail_diff from_B_to_A.expected .mail/user-a@srv-a
 
+# If driusan/dkim's dkimverify is available, use it to check the generated
+# signature.
+if dkimverify --help 2>&1 < /dev/null | grep -q -- "-txt string"; then
+	# Verify B's signature only, because dkimverify only supports RSA.
+	cat .zones | grep _domainkey.srv-b | sed 's/.*TXT//g' > .srv-b.dns.txt
+	dkimverify -txt .srv-b.dns.txt < .mail/user-a@srv-a
+else
+	skipped "driusan's dkimverify cross-check (binary not available)"
+fi
 
 success

test/util/minidns/minidns.go

@@ -290,8 +290,17 @@ func (m *miniDNS) loadZones(f *os.File) {
 			}
 		case "txt":
 			qType = dnsmessage.TypeTXT
+
+			// Cut value in chunks of 255 bytes.
+			chunks := []string{}
+			v := value
+			for len(v) > 254 {
+				chunks = append(chunks, v[:254])
+				v = v[254:]
+			}
+			chunks = append(chunks, v)
 			body = &dnsmessage.TXTResource{
-				TXT: []string{value},
+				TXT: chunks,
 			}
 		default:
 			log.Fatalf("line %d: unknown type %q", lineno, t)