tst_checksum_enable - Set the IPV6_CHECKSUM socket option to have the kernel compute and store a checksum
To check that if IPV6_CHECKSUM option is set, the kernel compute and store a checksum for output, and verify the received checksum on input, discarding the packet if the checksum is in error.
./tst_checksum_enable [-tooloption ...] -tooloption : v6api tool option
1. Create an IPv6 RAW socket 2. Set the value of IPV6_CHECKSUM option to odd value 3. Check A: Fail to set value of IPV6_CHECKSUM option 4. Bind socket to address 5. Send ICMP message to itself 6. Receive ICMP message on socket 7. Check B: Message is received and the checksum is not changed by kernel 8. Set the value of IPV6_CHECKSUM option to 2 9. Send ICMP message to itself 10.Receive ICMP message on socket 11.Check C: Message is received and the checksum is changed by kernel 12.Create another IPv6 RAW socket 13.Bind the socket to address 14.Send ICMP message from the first socket 15.Receive ICMP message on the socket 16.Check D: Message is received and the checksum is changed by kernel 17.Set the value of IPV6_CHECKSUM option to 4 18.Send ICMP message from the first socket 19.Receive ICMP message on socket 20.Check E: Message can not be received on socket
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RFC 3542
3.1. Checksums
The kernel will calculate and insert the ICMPv6 checksum for ICMPv6 raw sockets, since this checksum is mandatory.
For other raw IPv6 sockets (that is, for raw IPv6 sockets created with a third argument other than IPPROTO_ICMPV6), the application must set the new IPV6_CHECKSUM socket option to have the kernel (1) compute and store a checksum for output, and (2) verify the received checksum on input, discarding the packet if the checksum is in error. This option prevents applications from having to perform source address selection on the packets they send. The checksum will incorporate the IPv6 pseudo-header, defined in Section 8.1 of [RFC- 2460]. This new socket option also specifies an integer offset into the user data of where the checksum is located.
int offset = 2; setsockopt(fd, IPPROTO_IPV6, IPV6_CHECKSUM, &offset, sizeof(offset));
By default, this socket option is disabled. Setting the offset to -1 also disables the option. By disabled we mean (1) the kernel will not calculate and store a checksum for outgoing packets, and (2) the kernel will not verify a checksum for received packets.
This option assumes the use of the 16-bit one's complement of the one's complement sum as the checksum algorithm and that the checksum field is aligned on a 16-bit boundary. Thus, specifying a positive odd value as offset is invalid, and setsockopt() will fail for such offset values.
An attempt to set IPV6_CHECKSUM for an ICMPv6 socket will fail. Also, an attempt to set or get IPV6_CHECKSUM for a non-raw IPv6 socket will fail.
(Note: Since the checksum is always calculated by the kernel for an ICMPv6 socket, applications are not able to generate ICMPv6 packets with incorrect checksums (presumably for testing purposes) using this API.)