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Help with TCP Options in C


 
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Old 07-24-2009
Help with TCP Options in C

Hello everybody,
It's me again, i need your help!
I was almost finishing a program when it crashed, because of TCP segments with the TCP Options enabled.

I am able to control every field of every packet i receive, except of those using TCP Options. Is there a way to determine if a packet is using TCP Options, and the Options size to reduce it to the TCP payload?

there are no more fields after Urgent Pointer (urg_ptr) in the /usr/include/linux/tcp.h file, Please, Help!!

Thank you.

---------- Post updated at 03:46 PM ---------- Previous update was at 03:27 PM ----------

I've found the solution, the TCP Offset determines the size of the header, and with no options it's normal value is twenty. If tcp->doff is greater than 20, it means that the TCP Segment has the Options field enabled, and the Options field size is the value of the offset minus 20.

Thanks anyway.
Zykl0n-B

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FAITH(4)						   BSD Kernel Interfaces Manual 						  FAITH(4)

NAME
faith -- IPv6-to-IPv4 TCP relay capturing interface SYNOPSIS
device faith DESCRIPTION
The faith interface captures IPv6 TCP traffic, for implementing userland IPv6-to-IPv4 TCP relay like faithd(8). Each faith interface is created at runtime using interface cloning. This is most easily done with the ifconfig(8) create command or using the cloned_interfaces variable in rc.conf(5). Special action will be taken when IPv6 TCP traffic is seen on a router, and the routing table suggests to route it to the faith interface. In this case, the packet will be accepted by the router, regardless of the list of IPv6 interface addresses assigned to the router. The packet will be captured by an IPv6 TCP socket, if it has the IN6P_FAITH flag turned on and matching address/port pairs. As a result, faith will let you capture IPv6 TCP traffic to some specific destination addresses. Userland programs, such as faithd(8) can use this behavior to relay IPv6 TCP traffic to IPv4 TCP traffic. The program can accept some specific IPv6 TCP traffic, perform getsockname(2) to get the IPv6 destination address specified by the client, and perform application-specific address mapping to relay IPv6 TCP to IPv4 TCP. The IN6P_FAITH flag on a IPv6 TCP socket can be set by using setsockopt(2), with level IPPROTO_IPV6 and optname IPv6_FAITH. To handle error reports by ICMPv6, some ICMPv6 packets routed to an faith interface will be delivered to IPv6 TCP, as well. To understand how faith can be used, take a look at the source code of faithd(8). As the faith interface implements potentially dangerous operations, great care must be taken when configuring it. To avoid possible misuse, the sysctl(8) variable net.inet6.ip6.keepfaith must be set to 1 prior to using the interface. When net.inet6.ip6.keepfaith is 0, no packets will be captured by the faith interface. The faith interface is intended to be used on routers, not on hosts. SEE ALSO
inet(4), inet6(4), faithd(8) Jun-ichiro itojun Hagino and Kazu Yamamoto, An IPv6-to-IPv4 transport relay translator, RFC3142. HISTORY
The FAITH IPv6-to-IPv4 TCP relay translator first appeared in the WIDE hydrangea IPv6 stack. BSD
April 10, 1999 BSD

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