linux – Why does it go through netfilter postrouting when visiting local ports?

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Environment：

[root@VM-32-4-centos ~]# uname -r
3.10.0-514.26.2.el7.x86_64

I have set log printing in the following chain：

[root@VM-32-4-centos ~]# iptables -A INPUT -p tcp --dport 8000 -j LOG --log-prefix "INPUT DPORT 8000 LOGS: "
[root@VM-32-4-centos ~]# iptables -A INPUT -p tcp --sport 8000 -j LOG --log-prefix "INPUT SPORT 8000 LOGS: "
[root@VM-32-4-centos ~]# iptables -A OUTPUT -p tcp --dport 8000 -j LOG --log-prefix "OUTPUT DPORT 8000 LOGS: "
[root@VM-32-4-centos ~]# iptables -A OUTPUT -p tcp --sport 8000 -j LOG --log-prefix "OUTPUT SPORT 8000 LOGS: "
[root@VM-32-4-centos ~]# iptables -t nat -A POSTROUTING -p tcp --sport 8000 -j LOG --log-prefix "POSTROUTING SPORT 8000 LOGS: "
[root@VM-32-4-centos ~]# iptables -t nat -A POSTROUTING -p tcp --dport 8000 -j LOG --log-prefix "POSTROUTING DPORT 8000 LOGS: "

Then I execute the telnet command：

telnet 127.0.0.1 8000

tail -f /var/log/message, show the following：

May  2 23:54:47 VM-32-4-centos kernel: OUTPUT DPORT 8000 LOGS: IN= OUT=lo SRC=127.0.0.1 DST=127.0.0.1 LEN=60 TOS=0x10 PREC=0x00 TTL=64 ID=6163 DF PROTO=TCP SPT=51096 DPT=8000 WINDOW=43690 RES=0x00 SYN URGP=0 
May  2 23:54:47 VM-32-4-centos kernel: POSTROUTING DPORT 8000 LOGS: IN= OUT=lo SRC=127.0.0.1 DST=127.0.0.1 LEN=60 TOS=0x10 PREC=0x00 TTL=64 ID=6163 DF PROTO=TCP SPT=51096 DPT=8000 WINDOW=43690 RES=0x00 SYN URGP=0 
May  2 23:54:47 VM-32-4-centos kernel: INPUT DPORT 8000 LOGS: IN=lo OUT= MAC=00:00:00:00:00:00:00:00:00:00:00:00:08:00 SRC=127.0.0.1 DST=127.0.0.1 LEN=60 TOS=0x10 PREC=0x00 TTL=64 ID=6163 DF PROTO=TCP SPT=51096 DPT=8000 WINDOW=43690 RES=0x00 SYN URGP=0 
May  2 23:54:47 VM-32-4-centos kernel: OUTPUT SPORT 8000 LOGS: IN= OUT=lo SRC=127.0.0.1 DST=127.0.0.1 LEN=60 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=TCP SPT=8000 DPT=51096 WINDOW=43690 RES=0x00 ACK SYN URGP=0 
May  2 23:54:47 VM-32-4-centos kernel: INPUT SPORT 8000 LOGS: IN=lo OUT= MAC=00:00:00:00:00:00:00:00:00:00:00:00:08:00 SRC=127.0.0.1 DST=127.0.0.1 LEN=60 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=TCP SPT=8000 DPT=51096 WINDOW=43690 RES=0x00 ACK SYN URGP=0 
May  2 23:54:47 VM-32-4-centos kernel: OUTPUT DPORT 8000 LOGS: IN= OUT=lo SRC=127.0.0.1 DST=127.0.0.1 LEN=52 TOS=0x10 PREC=0x00 TTL=64 ID=6164 DF PROTO=TCP SPT=51096 DPT=8000 WINDOW=342 RES=0x00 ACK URGP=0 
May  2 23:54:47 VM-32-4-centos kernel: INPUT DPORT 8000 LOGS: IN=lo OUT= MAC=00:00:00:00:00:00:00:00:00:00:00:00:08:00 SRC=127.0.0.1 DST=127.0.0.1 LEN=52 TOS=0x10 PREC=0x00 TTL=64 ID=6164 DF PROTO=TCP SPT=51096 DPT=8000 WINDOW=342 RES=0x00 ACK URGP=0

Excuse me, when accessing locally, why does SYN packet go through POSTROUTING?

Subsequent ACK+SYN packets did not go through POSTROUTING, why?

May I ask why this is so, is there any information about it?

Thanks.

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