There are a number of instances throughout the code where the constant 28 is

used with no explanation. Some of this was introduced as part of RT#1929. The
value 28 is the length of the IP header (20 bytes) plus the UDP header (8
bytes). However use of this constant is incorrect because there may be
instances where a different value is needed, e.g. an IPv4 header is 20 bytes
but an IPv6 header is 40. Similarly you may not be using UDP (e.g. SCTP).
This commit introduces a new BIO_CTRL that provides the value to be used for
this mtu "overhead". It will be used by subsequent commits.
Reviewed-by: NTim Hudson <tjh@openssl.org>

crypto/bio/bio.h

@@ -176,6 +176,8 @@ extern "C" {
                                               * adjust socket timeouts */
 #define BIO_CTRL_DGRAM_SET_DONT_FRAG      48
 
+#define BIO_CTRL_DGRAM_GET_MTU_OVERHEAD   49
+
 #ifndef OPENSSL_NO_SCTP
 /* SCTP stuff */
 #define BIO_CTRL_DGRAM_SCTP_SET_IN_HANDSHAKE	50
@@ -608,6 +610,8 @@ int BIO_ctrl_reset_read_request(BIO *b);
          (int)BIO_ctrl(b, BIO_CTRL_DGRAM_GET_PEER, 0, (char *)peer)
 #define BIO_dgram_set_peer(b,peer) \
          (int)BIO_ctrl(b, BIO_CTRL_DGRAM_SET_PEER, 0, (char *)peer)
+#define BIO_dgram_get_mtu_overhead(b) \
+         (unsigned int)BIO_ctrl((b), BIO_CTRL_DGRAM_GET_MTU_OVERHEAD, 0, NULL)
 
 /* These two aren't currently implemented */
 /* int BIO_get_ex_num(BIO *bio); */

crypto/bio/bss_dgram.c

@@ -458,6 +458,36 @@ static int dgram_write(BIO *b, const char *in, int inl)
 	return(ret);
 	}
 
+static long dgram_get_mtu_overhead(bio_dgram_data *data)
+	{
+	long ret;
+
+	switch (data->peer.sa.sa_family)
+		{
+		case AF_INET:
+			/* Assume this is UDP - 20 bytes for IP, 8 bytes for UDP */
+			ret = 28;
+			break;
+#if OPENSSL_USE_IPV6
+		case AF_INET6:
+#ifdef IN6_IS_ADDR_V4MAPPED
+			if (IN6_IS_ADDR_V4MAPPED(&data->peer.sa_in6.sin6_addr))
+				/* Assume this is UDP - 20 bytes for IP, 8 bytes for UDP */
+				ret = 28;
+			else
+#endif
+				/* Assume this is UDP - 40 bytes for IP, 8 bytes for UDP */
+				ret = 48;
+			break;
+#endif
+		default:
+			/* We don't know. Go with the historical default */
+			ret = 28;
+			break;
+		}
+	return ret;
+	}
+
 static long dgram_ctrl(BIO *b, int cmd, long num, void *ptr)
 	{
 	long ret=1;
@@ -634,23 +664,24 @@ static long dgram_ctrl(BIO *b, int cmd, long num, void *ptr)
 #endif
 		break;
 	case BIO_CTRL_DGRAM_GET_FALLBACK_MTU:
+		ret = -dgram_get_mtu_overhead(data);
 		switch (data->peer.sa.sa_family)
 			{
 			case AF_INET:
-				ret = 576 - 20 - 8;
+				ret += 576;
 				break;
 #if OPENSSL_USE_IPV6
 			case AF_INET6:
 #ifdef IN6_IS_ADDR_V4MAPPED
 				if (IN6_IS_ADDR_V4MAPPED(&data->peer.sa_in6.sin6_addr))
-					ret = 576 - 20 - 8;
+					ret += 576;
 				else
 #endif
-					ret = 1280 - 40 - 8;
+					ret += 1280;
 				break;
 #endif
 			default:
-				ret = 576 - 20 - 8;
+				ret += 576;
 				break;
 			}
 		break;
@@ -895,6 +926,9 @@ static long dgram_ctrl(BIO *b, int cmd, long num, void *ptr)
 				break;
 			}
 		break;
+	case BIO_CTRL_DGRAM_GET_MTU_OVERHEAD:
+		ret = dgram_get_mtu_overhead(data);
+		break;
 	default:
 		ret=0;
 		break;
@@ -1415,6 +1449,10 @@ static long dgram_sctp_ctrl(BIO *b, int cmd, long num, void *ptr)
 		 * Returns always 1.
 		 */
 		break;
+	case BIO_CTRL_DGRAM_GET_MTU_OVERHEAD:
+		/* We allow transport protocol fragmentation so this is irrelevant */
+		ret = 0;
+		break;
 	case BIO_CTRL_DGRAM_SCTP_SET_IN_HANDSHAKE:
 		if (num > 0)
 			data->in_handshake = 1;