diff options
author | Willem de Bruijn <willemb@google.com> | 2016-08-10 15:23:07 -0400 |
---|---|---|
committer | Pablo Neira Ayuso <pablo@netfilter.org> | 2016-08-12 12:46:26 +0200 |
commit | bd1f88bf85100554d35e74273d907db1fdcd250f (patch) | |
tree | 6bbf358fd8dba3157872d43a1d0f23a58372a1f4 /extensions | |
parent | a44bee8c3582cb72868a3b7f703494dd2b24bf7d (diff) |
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>
Diffstat (limited to 'extensions')
-rw-r--r-- | extensions/libxt_bpf.man | 13 |
1 files changed, 13 insertions, 0 deletions
diff --git a/extensions/libxt_bpf.man b/extensions/libxt_bpf.man index 5b1d0424..302e0103 100644 --- 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. |