diff --git a/slirp/tcp.h b/slirp/tcp.h
index 82996031e0..2e2b4033a6 100644
--- a/slirp/tcp.h
+++ b/slirp/tcp.h
@@ -79,20 +79,23 @@ struct tcphdr {
 #define	TCPOPT_EOL		0
 #define	TCPOPT_NOP		1
 #define	TCPOPT_MAXSEG		2
-#define    TCPOLEN_MAXSEG		4
 #define TCPOPT_WINDOW		3
-#define    TCPOLEN_WINDOW		3
 #define TCPOPT_SACK_PERMITTED	4		/* Experimental */
-#define    TCPOLEN_SACK_PERMITTED	2
 #define TCPOPT_SACK		5		/* Experimental */
 #define TCPOPT_TIMESTAMP	8
-#define    TCPOLEN_TIMESTAMP		10
-#define    TCPOLEN_TSTAMP_APPA		(TCPOLEN_TIMESTAMP+2) /* appendix A */
 
 #define TCPOPT_TSTAMP_HDR	\
     (TCPOPT_NOP<<24|TCPOPT_NOP<<16|TCPOPT_TIMESTAMP<<8|TCPOLEN_TIMESTAMP)
 #endif
 
+#ifndef TCPOLEN_MAXSEG
+#define    TCPOLEN_MAXSEG		4
+#define    TCPOLEN_WINDOW		3
+#define    TCPOLEN_SACK_PERMITTED	2
+#define    TCPOLEN_TIMESTAMP		10
+#define    TCPOLEN_TSTAMP_APPA		(TCPOLEN_TIMESTAMP+2) /* appendix A */
+#endif
+
 /*
  * Default maximum segment size for TCP.
  * With an IP MSS of 576, this is 536,