Invalid SYN

I got quite a lot of this kind of log in /var/log/messages

Anyone know what it is ?

Oct 3 12:15:27 main kernel: ip_conntrack_tcp: INVALID: invalid SYN (ignored) SRC=203.170.229.241 DST=66.95.177.181 LEN=52 TOS=0x08 PREC=0xA0 TTL=45 ID=5176 DF PROTO=TCP SPT=38523 DPT=80 SEQ=4088497165 ACK=0 WINDOW=16384 RES=0x00 SYN URGP=0 OPT (020405B40103030001010402)

Thanks

 

 

 

 

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