| Match using Linux Socket Filter. Expects a BPF program in decimal format. This |
| is the format generated by the \fBnfbpf_compile\fP utility. |
| .TP |
| \fB\-\-bytecode\fP \fIcode\fP |
| Pass the BPF byte code format (described in the example below). |
| .PP |
| The code format is similar to the output of the tcpdump -ddd command: one line |
| that stores the number of instructions, followed by one line for each |
| instruction. Instruction lines follow the pattern 'u16 u8 u8 u32' in decimal |
| notation. Fields encode the operation, jump offset if true, jump offset if |
| false and generic multiuse field 'K'. Comments are not supported. |
| .PP |
| For example, to read only packets matching 'ip proto 6', insert the following, |
| without the comments or trailing whitespace: |
| .IP |
| 4 # number of instructions |
| .br |
| 48 0 0 9 # load byte ip->proto |
| .br |
| 21 0 1 6 # jump equal IPPROTO_TCP |
| .br |
| 6 0 0 1 # return pass (non-zero) |
| .br |
| 6 0 0 0 # return fail (zero) |
| .PP |
| You can pass this filter to the bpf match with the following command: |
| .IP |
| iptables \-A OUTPUT \-m bpf \-\-bytecode '4,48 0 0 9,21 0 1 6,6 0 0 1,6 0 0 0' \-j ACCEPT |
| .PP |
| Or instead, you can invoke the nfbpf_compile utility. |
| .IP |
| iptables \-A OUTPUT \-m bpf \-\-bytecode "`nfbpf_compile RAW 'ip proto 6'`" \-j ACCEPT |
| .PP |
| You may want to learn more about BPF from FreeBSD's bpf(4) manpage. |
