From bf609a4cf8b0946331be0de9922dcd75ea22530c Mon Sep 17 00:00:00 2001
From: David Fifield <david@bamsoftware.com>
Date: Fri, 20 Mar 2020 23:25:07 -0600
Subject: [PATCH] Set MTU based on domain length.

---
 dnstt-client/dns_test.go | 27 +++++++++++++++++++++++++++
 dnstt-client/main.go     | 26 +++++++++++++++++++++++++-
 2 files changed, 52 insertions(+), 1 deletion(-)
 create mode 100644 dnstt-client/dns_test.go

diff --git a/dnstt-client/dns_test.go b/dnstt-client/dns_test.go
new file mode 100644
index 0000000..071fc50
--- /dev/null
+++ b/dnstt-client/dns_test.go
@@ -0,0 +1,27 @@
+package main
+
+import (
+	"bytes"
+	"testing"
+
+	"www.bamsoftware.com/git/dnstt.git/dns"
+)
+
+func TestDNSNameCapacity(t *testing.T) {
+	for domainLen := 0; domainLen < 255; domainLen++ {
+		domain, err := dns.NewName(chunks(bytes.Repeat([]byte{'x'}, domainLen), 63))
+		if err != nil {
+			continue
+		}
+		capacity := dnsNameCapacity(domain)
+		if capacity <= 0 {
+			continue
+		}
+		prefix := []byte(base32Encoding.EncodeToString(bytes.Repeat([]byte{'y'}, capacity)))
+		labels := append(chunks(prefix, 63), domain...)
+		_, err = dns.NewName(labels)
+		if err != nil {
+			t.Errorf("length %v  capacity %v  %v", domainLen, capacity, err)
+		}
+	}
+}
diff --git a/dnstt-client/main.go b/dnstt-client/main.go
index a098b7f..45abe11 100644
--- a/dnstt-client/main.go
+++ b/dnstt-client/main.go
@@ -239,6 +239,25 @@ func handle(local *net.TCPConn, sess *smux.Session) error {
 	return err
 }
 
+// dnsNameCapacity returns the number of bytes remaining for encoded data after
+// including domain in a DNS name.
+func dnsNameCapacity(domain dns.Name) int {
+	// https://tools.ietf.org/html/rfc1035#section-2.3.4
+	// Names must be 255 octets or shorter in total length.
+	capacity := 255
+	// Subtract the length of the null terminator.
+	capacity -= 1
+	for _, label := range domain {
+		// Subtract the length of the label and the length octet.
+		capacity -= len(label) + 1
+	}
+	// Each label may be up to 63 bytes long and requires 64
+	capacity = capacity * 63 / 64
+	// Base32 expands every 5 bytes to 8.
+	capacity = capacity * 5 / 8
+	return capacity
+}
+
 func run(domain dns.Name, localAddr, udpAddr string) error {
 	var sess *smux.Session
 
@@ -272,7 +291,12 @@ func run(domain dns.Name, localAddr, udpAddr string) error {
 			0, // default resend
 			1, // nc=1 => congestion window off
 		)
-		conn.SetMtu(100) // TODO: MTU appropriate for length of domain
+		mtu := dnsNameCapacity(domain) - 8 // clientid
+		if mtu < 80 {
+			return fmt.Errorf("domain %s leaves only %d bytes for payload", domain, mtu)
+		}
+		fmt.Printf("MTU %d\n", mtu)
+		conn.SetMtu(mtu)
 
 		// Start a smux session on the KCP conn.
 		smuxConfig := smux.DefaultConfig()