问题
I was looking into raw sockets. so I created one raw socket client and other one is server. on two different computers. I am using ISP provided router. so I sent spoofed packet (with iphdr struct's saddr of different computer on same network) from client but when the packet received at the server the source ip in packet's ip header was correct (the real sender address=client address) plus the source port and destination port of tcphdr were something different too (which really didnt make sense). So I assumed that my ISP provided router is doing something funny -- OR I am completely wrong correct if this is a normal ip protocol. if I am correct then what steps should i take to configure my router so it does not mess up with tcp and ip headers of incoming packets plus the sync=1 at sender's end became sync=0 of tcphdr. I am using local IPs plus I am trying to implement tcp using raw socket server. I have configured iptables on server as well with sync allowed on my bounded port
iptables -I INPUT -p tcp --dport xxxx --syn -j ACCEPT
WHat settings do I need to do on server in iptables so my socket incoming packets wont be dropped nor any header be touched so I can get all packets required for tcp to keep coming in. plus what setting do I need to do on router so,if its cause problem then, it wont
Update 1:
after spending some time to the problem I found that one cause could be my kernel tcp stack dropping the incoming sync packets. so I looked around and found that in the tcp stack there is work done to make this happening. So I changed the sysctl.conf in /etc to allow sync packets to come in through by adding or uncommenting this line
net.ipv4.tcp_syncookies=1
Then I rebooted the system but still no difference. is it because somehow I have to reload my sysctl.conf file. if this is the case or any related case then please fill in.
Update 2
I have reloaded sysctl.conf after allowing syn packets and now I am getting syn packets from client application. but the destination port different. also along with syn=1, ack=1 is also coming in same packet. is this also something kernel is doing. please explain. as long as I know this is not how tcp works (tcp handshake) and I am connecting from client using stream socket
来源：https://stackoverflow.com/questions/64972073/does-router-or-linux-kernel-change-the-tcp-headers-and-ip-headers-of-packets