Date: Sat, 24 Mar 2001 17:10:04 +0530 (IST) From: Madhavi Suram <madhavis@sasken.com> To: freebsd-hackers@FreeBSD.ORG Subject: udp checksum problem.. Message-ID: <Pine.LNX.4.21.0103241655060.15455-100000@pcs111.sasi.com> In-Reply-To: <20010324012954.A31961@xor.obsecurity.org>
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Hi I am modifying FreeBSD 4.1 kernel. I am hacking all UDP packets in ip_input, changing some headers and finding the udp checksum using in_pseudo() and setting the packet header csum_flags to CSUM_UDP (I know this is a dirty way of doing it.. but, had to do it for efficiency reasons). When I try to see the checksum after the checksum calculation is complete in ip_output, I am getting the checksum 65535(0xffff). Is this a special checksum(error or something like that..)? I have seen INVERT doing something like this in the file /sys/alpha/alpha/in_cksum.c. sum == 0xffff ? 0xffff : ~sum & 0xffff. After modifying the packet in ip_input(), I call ip_forward() and the packet should go and reach the intended destination as usual. But, this destination address is the one that is changed in ip_input(). But, the destination host is not getting the packet. I think this is a checksum problem. Could anyone please tell me what checksum 0xffff means, or give me some pointers to where I can find this information? Also, could anyone tell me if there are any csum_flags other than CSUM_UDP and CSUM_DELAY_DATA in the mbuf packet header that can be set before a udp packet reaches ip_output()? thanks in advance. regards Madhavi. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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