What About Port 2048 For GRE(47) Packaged In Ip Protocol

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    Hi, all: 

        I’m playing with Gre Tunnel programming in CentOS 6.
        
        I wrote my program in these steps: 
        a, open /dev/net/tun  device and ioctl with TUNSETIFF, get fd to tun device, say “tun0”;
        b, create one socket and use it to set tun0’s  ip address, namyly SIOCSIFADDR and SIOCSIFDSTADDR; and then turn it up;
        c, create one raw socket to send things which got from the tun0 device to other machine, and to receive packages which should be feeded into the tun0 device;

        Finally, I ran the program in two machines, say MA and MB, synchronously, and set route info in MA and MB too. The last environment is like this:

    10.0.3.1/24     192.168.0.31/24    192.168.0.124/24    10.0.4.1/24
         ———- MA ——————————————-  MB ———-
             tun0          eth0                                    eth0          tun0

         When I ping MB in MA, and tcpdump eth0 in MA, I get: 

15:06:39.712390 IP 192.168.0.31 > 192.168.0.124: GREv0, length 88: IP 10.0.3.1 > 10.0.4.1: ICMP echo request, id 28797, seq 1, length 64
15:06:39.713232 IP 192.168.0.124 > 192.168.0.31: ICMP 192.168.0.124 protocol 47 port 2048 unreachable, length 116
15:06:39.713667 IP 192.168.0.124 > 192.168.0.31: GREv0, length 88: IP 10.0.4.1 > 10.0.3.1: ICMP echo reply, id 28797, seq 1, length 64
15:06:39.713754 IP 192.168.0.31 > 192.168.0.124: ICMP 192.168.0.31 protocol 47 port 2048 unreachable, length 116

         The 1st and 3rd lines is easy to understand, but what about  the 2nd and 4th lines??? Is the raw package been sent up into gre protocol while caught by my program??

         I have seen ICMP_PORT_UNREACH  stuff been placed in ip_gre.c kernel code, but say it is “Impossible event”. So, what’s wrong,  have I missed something in my program? Any suggestions are appreciated, thanks.

 

3 thoughts on - What About Port 2048 For GRE(47) Packaged In Ip Protocol

  • I remember I saw this behavior before. Mostly important thing is to find out if the enc/dec goes well. I was having this problem that triggers kernel to send out the icmp.

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  • At 2013-04-10 18:35:12,”Banyan He”  wrote:
    enc/dec ??  You mean the content of ip header,  which has been added in my program while using raw socket??

    How to fix or hack it, is there some way to do research?? I feel the package is sent to the upper GRE protocol while been intercepted with raw socket. 
    Is there one way to prevent this?? Thanks.
     

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  • encryption/decryption I mean. The kernel can’t decrypt the packet eventually, kernel sends out the icmp. Just remember this part last time I track it down to the kernel side. You may have to take a look at your application why the gre doesn’t setup correctly.

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