Date: Fri, 7 Jan 2011 04:48:29 -0500 (EST) From: Daniel Eischen <deischen@freebsd.org> To: freebsd-current@freebsd.org Subject: UDP checksum broken, -head and releng_8 Message-ID: <Pine.GSO.4.64.1101070412140.19838@sea.ntplx.net>
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When sending multicast packets to a socket that is _not_ bound to the multicast address, this generates bad UDP checksums. This use to work and was broke sometime between the middle of October and late December as far as I can tell. A test program is here: http://people.freebsd.org/~deischen/test_net.c It will use multicast with the -b option, so run it with something like this: test_net -b 225.168.2.50 -p 5000 -m 64 And on another box, use tcpdump to see the bad checksums, as in: tcpdump -x -s0 -vvv host 225.168.2.50 and port 5000 This is a real problem for us because our applications cannot read the multicast data (both the Solaris and VxWork stacks refuse to deliver the packets with bad checksums). As a data point, if the sending socket is bound to the multicast address, then the checksums are calculated correctly. It is only if the socket is bound to a different address (the host IP and ephemeral port), that the checksum is invalid. To see this, you can change line 827 in test_net.c from this: if ((ss = init_mcast_socket(p, mcast_addr, if_addr, 1, 0)) < 0) to if ((ss = init_mcast_socket(p, mcast_addr, if_addr, 1, 1)) < 0) Here's a snippet from a tcpdump showing the problem: 04:46:13.640846 IP (tos 0x0, ttl 1, id 42777, offset 0, flags [none], proto UDP (17), length 92) 192.168.3.85.60133 > 225.168.2.50.5000: [bad udp cksum 3c02!] UDP, length 64 0x0000: 4500 005c a719 0000 0111 6aa0 c0a8 0355 0x0010: e1a8 0232 eae5 1388 0048 1cc6 0000 0038 0x0020: 0000 0018 0000 0000 0000 0000 0000 0000 0x0030: 0000 0000 0000 0000 0000 0000 0000 0000 0x0040: 0000 0000 0000 0000 0000 0000 0000 0000 0x0050: 0000 0000 0000 0000 0000 0000 04:46:13.741800 IP (tos 0x0, ttl 1, id 42778, offset 0, flags [none], proto UDP (17), length 92) 192.168.3.85.60133 > 225.168.2.50.5000: [bad udp cksum 3c02!] UDP, length 64 0x0000: 4500 005c a71a 0000 0111 6a9f c0a8 0355 0x0010: e1a8 0232 eae5 1388 0048 c76f 0000 0038 0x0020: 0000 0019 5555 5555 5555 5555 5555 5555 0x0030: 5555 5555 5555 5555 5555 5555 5555 5555 0x0040: 5555 5555 5555 5555 5555 5555 5555 5555 0x0050: 5555 5555 5555 5555 5555 5555 -- DE
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