linux - Heavily confused by candump (SocketCAN) ID filtering feature -

in nutshell, want candump show me frames ids 0x00200200 or 0x255.

so this:

candump can0,00200200:0,255:0 

but gives frames, , each frame shown twice. i.e. output of:

cansend can0 256#112233 

would this:

can0      256  [3] 11 22 33 can0      256  [3] 11 22 33 

aside filter not behaving expected , passing through 0x256, fact shows twice suggests frames matched both filters, makes less sense me. can explain why happening, , perhaps show me correct way it?

from of candump:

<can_id>:<can_mask> (matches when <received_can_id> & mask == can_id & mask) 

now, when mask 0, every can id match it. can_id has no real effect, why messages pass, each required bit in can id should set 1 in mask.

regarding duplication problem, happens because use 2 filters, although not sure this.

what want is:

candump can0,00200200:1fffffff,255:7ff 

example (provided op):

enyquist:~$ candump vcan0,00200200:1fffffff,255:7ff & [1] 7339  enyquist:~$ cansend vcan0 002001fe#1122  enyquist:~$ cansend vcan0 002001ff#1122  enyquist:~$ cansend vcan0 00200200#1122 vcan0 00200200 [2] 11 22  enyquist:~$ cansend vcan0 00200201#1122  enyquist:~$ cansend vcan0 00200202#1122  enyquist:~$  enyquist:~$ cansend vcan0 253#1122  enyquist:~$ cansend vcan0 254#1122  enyquist:~$ cansend vcan0 255#1122 vcan0 255 [2] 11 22  enyquist:~$ cansend vcan0 256#1122  enyquist:~$ cansend vcan0 257#1122  enyquist:~$