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tcpmssclamp: only check the first ipv4 fragment for tcp header

--- a/sys/src/9/ip/tcp.c

+++ b/sys/src/9/ip/tcp.c

@@ -3322,6 +3322,8 @@

 	if((h4->vihl&0xF0)==IP_VER4) {

 		if(h4->proto != IP_TCPPROTO)

 			return;

+		if((h4->frag[0] & (IP_FO>>8)) | (h4->frag[1] & IP_FO))

+			return;

 		if(!(h4->tcpflag[1] & SYN))

 			return;

 		hdrlen = (h4->tcpflag[0] >> 2) & ~3;