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On Ubuntu Xenial I want to use tcpdump to filter DNS responses with errors RCODes (ServFail, NXDomain, etc.). So every RCODEs except '0'.

(See IANA DNS RCodes)

So I have to use a filter based on the 11th byte in the UDP packet. The 4 low bits contains the RCODE. I want to test if the value is different than 0. So I left-shift by 4 bits to get only these 4 low bits.

This command doesn't work:

tcpdump port 53 and 'udp[11] << 4 > 0'

listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
  14:07:04.148693 IP ns-cache0.oleane.net.domain > 192.168.151.53.50372: 17799 2/0/0 CNAME video-stats.l.google.com., A 216.58.204.142 (82)
  14:07:04.150768 IP ns-cache0.oleane.net.domain > 192.168.151.53.50372: 39389 2/0/0 CNAME video-stats.l.google.com., AAAA 2a00:1450:4007:812::200e (94)
  14:07:04.150798 IP ns-cache0.oleane.net.domain > 192.168.151.53.60464: 60219 NXDomain* 0/1/0 (104)
  14:07:04.152703 IP ns-cache0.oleane.net.domain > 192.168.151.53.40235: 3472 1/0/0 PTR ns-cache0.oleane.net. (75)

I also get NoError DNS responses...

But using this command works:

tcpdump port 53 and 'udp[11]&0x0F > 0'

listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
  14:07:42.014626 IP ns-cache0.oleane.net.domain > 192.168.151.53.54270: 44447 NXDomain 0/1/1 (109)
  14:07:42.016611 IP ns-cache0.oleane.net.domain > 192.168.151.53.41840: 57750 NXDomain* 0/1/0 (104)

Why is the left-shifting not working?

0

It's nowhere specified that the left shift is executed in an 8-bit register, and that the high bits are dropped. (In fact, the shift probably uses a register of whatever native size the machine has, e.g. 32 bit or 64 bit).

So don't use left shifts to try to mask out values...

  • Indeed you're right... The filtering seems to be done by the kernel : kernel.org/doc/Documentation/networking/filter.txt github.com/torvalds/linux/blob/… So yes, I will avoid left-shift. Thanks! – Jacques Aug 30 '17 at 7:27
  • Yet, it is used in the tcpdump manpage... :/ tcpdump.org/tcpdump_man.html#lbAF – Jacques Aug 30 '17 at 7:33
  • This example (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0) does something different (hint: left shift is not used in a ... > 0 expression, but in a ... - ... expression). So don't use left shifts and comparison with zero to mask out values. Use left shift in other circumstances as appropriate. – dirkt Aug 30 '17 at 9:29

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