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Willem
Asked: 2014-07-01 03:47:45 +0800 CST

tcpdump: "ethertype Unknown"?

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While debugging a DNS problem (apparently UDP packets get lost on the network), I'm stumbling upon many (but not all) "ethertype Unknown" messages such as follows. Tcpdump is run with "-n -i any port 53"

17:34:43.826196 IP 127.0.0.1.46508 > 127.0.0.1.53: 57861+ A? collector-123.newrelic.com. (44) 17:34:43.826221 IP 127.0.0.1.46508 > 127.0.0.1.53: 20016+ AAAA? collector-123.newrelic.com. (44) 17:34:43.826314 IP 127.0.0.1.53 > 127.0.0.1.46508: 57861 1/0/0 A 50.31.164.223 (60) 17:34:43.826505 IP 82.94.181.250.38280 > 208.78.70.21.53: 4554 AAAA? collector-123.newrelic.com. (44) 17:34:43.826510 Out 00:25:90:4b:94:dc ethertype Unknown (0x0067), length 92: 0x0000: 0000 0800 4500 0048 ef3d 4000 4011 2cab ....E..H.=@.@.,. 0x0010: 525e b5fa d04e 4615 9588 0035 0034 1f02 R^...NF....5.4.. 0x0020: 11ca 0000 0001 0000 0000 0000 0d63 6f6c .............col 0x0030: 6c65 6374 6f72 2d31 3233 086e 6577 7265 lector-123.newre 0x0040: 6c69 6303 636f 6d00 001c 0001 lic.com..... 17:34:43.826512 Out 00:25:90:4b:94:dc ethertype Unknown (0x0067), length 92: 0x0000: 0000 0800 4500 0048 ef3d 4000 4011 2cab ....E..H.=@.@.,. 0x0010: 525e b5fa d04e 4615 9588 0035 0034 1f02 R^...NF....5.4.. 0x0020: 11ca 0000 0001 0000 0000 0000 0d63 6f6c .............col 0x0030: 6c65 6374 6f72 2d31 3233 086e 6577 7265 lector-123.newre 0x0040: 6c69 6303 636f 6d00 001c 0001 lic.com..... 17:34:43.828629 In 00:1e:13:c1:52:00 ethertype Unknown (0x0067), length 159: 0x0000: 0000 0800 4500 008b b1dd 0000 3611 b3c8 ....E.......6... 0x0010: d04e 4615 525e b5fa 0035 9588 0077 8a5a .NF.R^...5...w.Z 0x0020: 11ca 8400 0001 0000 0001 0000 0d63 6f6c .............col 0x0030: 6c65 6374 6f72 2d31 3233 086e 6577 7265 lector-123.newre 0x0040: 6c69 6303 636f 6d00 001c 0001 c01a 0006 lic.com......... 0x0050: 0001 0000 0e10 0037 036e 7331 0370 3231 .......7.ns1.p21 0x0060: 0664 796e 6563 7403 6e65 7400 0c64 6f6d .dynect.net..dom 0x0070: 6169 6e2d 6164 6d69 6ec0 1a00 0002 3400 ain-admin.....4. 0x0080: 000e 1000 0002 5800 093a 8000 000e 10 ......X..:.....

What would cause these? Could it be related to hardware offloading to the nic? Is it harmful?

linux

1 Answers

  1. Best Answer

    It looks like there is an offset of 4 bytes in the dump. The ethertype is "0800" but tcpdump believes the ethertype is "0045" which is really the beginning of the IP header.

    Maybe you are using a VLAN (which adds 4 bytes to the frame). Yes, it can be related to offloading, or to the interface (eg. use eth0.X instead of eth0 where X is the VLAN). And no, it does not sound harmful.

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