[CentOS] What about port 2048 for GRE(47) packaged in ip protocol

[Banyan He]

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.

------------
Banyan He
Blog: http://www.rootong.com
Email: banyan at rootong.com

On 4/10/2013 5:27 PM, yihect wrote:
>    
>      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.
>
>   
>
>
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