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BPF boolean logic

asked 2020-05-22 19:18:21 +0000

balderman gravatar image

updated 2020-05-22 19:18:55 +0000

Are the 2 filters below identical?

  1. tcp && ((port 56 && host 1.2.3.4) or (port 57 && host 1.2.3.5))
  2. (tcp && port 56 && host 1.2.3.4) or (tcp && port 57 && host 1.2.3.5))
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answered 2020-05-22 20:22:51 +0000

grahamb gravatar image

updated 2020-05-22 20:24:52 +0000

If you look at the compiled BPF (using the Compile BPFs button in the Capture Options dialog) for each filter you can compare the result. The examples shown are for my WiFi interface:

tcp && ((port 56 && host 1.2.3.4) or (port 57 && host 1.2.3.5))

(000) ldh      [12]
(001) jeq      #0x86dd          jt 25   jf 2
(002) jeq      #0x800           jt 3    jf 25
(003) ldb      [23]
(004) jeq      #0x6             jt 5    jf 25
(005) ldh      [20]
(006) jset     #0x1fff          jt 25   jf 7
(007) ldxb     4*([14]&0xf)
(008) ldh      [x + 14]
(009) jeq      #0x38            jt 12   jf 10
(010) ldh      [x + 16]
(011) jeq      #0x38            jt 12   jf 16
(012) ld       [26]
(013) jeq      #0x1020304       jt 24   jf 14
(014) ld       [30]
(015) jeq      #0x1020304       jt 24   jf 16
(016) ldh      [x + 14]
(017) jeq      #0x39            jt 20   jf 18
(018) ldh      [x + 16]
(019) jeq      #0x39            jt 20   jf 25
(020) ld       [26]
(021) jeq      #0x1020305       jt 24   jf 22
(022) ld       [30]
(023) jeq      #0x1020305       jt 24   jf 25
(024) ret      #262144
(025) ret      #0

and the second, with the errant trailing paren removed:

(tcp && port 56 && host 1.2.3.4) or (tcp && port 57 && host 1.2.3.5)

(000) ldh      [12]
(001) jeq      #0x86dd          jt 25   jf 2
(002) jeq      #0x800           jt 3    jf 25
(003) ldb      [23]
(004) jeq      #0x6             jt 5    jf 25
(005) ldh      [20]
(006) jset     #0x1fff          jt 25   jf 7
(007) ldxb     4*([14]&0xf)
(008) ldh      [x + 14]
(009) jeq      #0x38            jt 12   jf 10
(010) ldh      [x + 16]
(011) jeq      #0x38            jt 12   jf 16
(012) ld       [26]
(013) jeq      #0x1020304       jt 24   jf 14
(014) ld       [30]
(015) jeq      #0x1020304       jt 24   jf 16
(016) ldh      [x + 14]
(017) jeq      #0x39            jt 20   jf 18
(018) ldh      [x + 16]
(019) jeq      #0x39            jt 20   jf 25
(020) ld       [26]
(021) jeq      #0x1020305       jt 24   jf 22
(022) ld       [30]
(023) jeq      #0x1020305       jt 24   jf 25
(024) ret      #262144
(025) ret      #0
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Comments

So it looks identical... thanks.

balderman gravatar imagebalderman ( 2020-05-22 20:30:00 +0000 )edit

Note: You can also check the compiled BPF using dumpcap or tcpdump.

Examples:

dumpcap.exe -d -f "(tcp && port 56 && host 1.2.3.4) or (tcp && port 57 && host 1.2.3.5)"  
tcpdump -d "tcp && ((port 56 && host 1.2.3.4) or (port 57 && host 1.2.3.5))"
cmaynard gravatar imagecmaynard ( 2020-05-22 22:31:27 +0000 )edit

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Asked: 2020-05-22 19:18:21 +0000

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Last updated: May 22