B.2 DSL - PPPD and Active Filter

fli4l uses the expression:

        'outbound and not icmp[0] != 8 and not tcp[13] & 4 != 0'

and accomplishes that generally only packets sent from the local network to the internet keep the connection open, with a few exceptions:

TCP-RST: Answers to rejected connection from outside do not reset the timeout,
ICMP: ICMP messages sent do not reset the timeout unless an echo request is sent.

This expression is converted by the PPPD into a packet-filter usable by the kernel. In this example it looks like this:

#
# Expression: outbound and not icmp[0] != 8 and not tcp[13] & 4 != 0
#
(000) ldb      [0]
(001) jeq      #0x0             jt 17   jf 2
(002) ldh      [2]
(003) jeq      #0x21            jt 4    jf 18
(004) ldb      [13]
(005) jeq      #0x1             jt 6    jf 11
(006) ldh      [10]
(007) jset     #0x1fff          jt 18   jf 8
(008) ldxb     4*([4]&0xf)
(009) ldb      [x + 4]
(010) jeq      #0x8             jt 18   jf 17
(011) jeq      #0x6             jt 12   jf 18
(012) ldh      [10]
(013) jset     #0x1fff          jt 18   jf 14
(014) ldxb     4*([4]&0xf)
(015) ldb      [x + 17]
(016) jset     #0x4             jt 17   jf 18
(017) ret      #0
(018) ret      #4