extensions/libxt_bpf.man: clarify BPF code generation with tcpdump

The xt_bpf module applies BPF bytecode to the packet. Depending on
where the module is invoked, the kernel may pass a packet with or
without link layer header. Iptables has no such header.

A common `tcpdump -ddd <string>` compilation command may revert to
a physical device that generates code for packets starting from the
mac layer up (e.g., E10MB data link type: Ethernet).

Clarify in the man page that when using this tool for code generation,
a suitable target device must be chosen.

Netfilter Bugzilla Bug #1048

Reported-by: Lorenzo Pistone <blaffablaffa@gmail.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

diff --git a/extensions/libxt_bpf.man b/extensions/libxt_bpf.man
index 5b1d0424f80122138d70321d4fe789f2dc09c7a0..302e0103459ad1f20e4cba1d4fa5b36eea0f4e50 100644 (file)
--- a/extensions/libxt_bpf.man
+++ b/extensions/libxt_bpf.man
@@ -31,4 +31,17 @@ 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
+Or use tcpdump -ddd. In that case, generate BPF targeting a device with the
+same data link type as the xtables match. Iptables passes packets from the
+network layer up, without mac layer. Select a device with data link type RAW,
+such as a tun device:
+.IP
+ip tuntap add tun0 mode tun
+.br
+ip link set tun0 up
+.br
+tcpdump -ddd -i tun0 ip proto 6
+.PP
+See tcpdump -L -i $dev for a list of known data link types for a given device.
+.PP
 You may want to learn more about BPF from FreeBSD's bpf(4) manpage.