1 $Id: README.linux,v 1.6 2000/02/19 21:44:13 guy Exp $
3 In order to capture packets (with Ethereal/Tethereal, tcpdump, or any
4 other packet capture program) on a Linux system, the "packet" protocol
5 must be supported by your kernel. If it is not, you may get error
8 modprobe: can't locate module net-pf-17
10 in "/var/adm/messages". The following note is from the Linux
11 "Configure.help" file:
15 The Packet protocol is used by applications which communicate
16 directly with network devices without an intermediate network
17 protocol implemented in the kernel, e.g. tcpdump. If you want them
20 This driver is also available as a module called af_packet.o ( =
21 code which can be inserted in and removed from the running kernel
22 whenever you want). If you want to compile it as a module, say M
23 here and read Documentation/modules.txt; if you use modprobe or
24 kmod, you may also want to add "alias net-pf-17 af_packet" to
27 In addition, the standard libpcap compiled for Linux has a timeout
28 problem; it doesn't support the timeout argument to "pcap_open_live()".
30 The current version of Ethereal attempts to work around this, so its GUI
31 shouldn't freeze when capturing on a not-so-busy network. If its GUI
32 does freeze when that happens, please send a note about this, indicating
33 which version of which distribution of Linux you're using, and which
34 version of libpcap you're using, to ethereal-dev@zing.org.
36 The current version of Ethereal should work with versions of libpcap
37 that have been patched to fix the timeout problem, as well as working
38 with unpatched versions.
40 An additional problem, on Linux, with current versions of libpcap, is
41 that capture filters do not work when snooping loopback devices; if
42 you're capturing on a Linux loopback device, do not use a capture
43 filter, as it will probably reject most if not all packets, including
44 the packets it's intended to accept - instead, capture all packets and
45 use a display filter to select the packets you want to see.
47 In addition, current versions of libpcap on at least some Linux
48 distributions will not turn promiscuous mode off on a network device
49 until the program using promiscuous mode exits, so if you start a
50 capture with Ethereal on some Linux distributions, the network interface
51 will be put in promiscuous mode and will remain in promiscuous mode
52 until Ethereal exits. There might be additional libpcap bugs that cause
53 it not to be turned off even when Ethereal exits; if your network is
54 busy, this could cause the Linux networking stack to do a lot more work
55 discarding packets not intended for the machine, so you may want to
56 check, after running Ethereal, whether any network interfaces are in
57 promiscuous mode (the output of "ifconfig -a" will say something such as
59 eth0 Link encap:Ethernet HWaddr 00:00:66:66:66:66
60 inet addr:66.66.66.66 Bcast:66.66.66.255 Mask:255.255.255.0
61 UP BROADCAST RUNNING PROMISC MULTICAST MTU:1500 Metric:1
62 RX packets:6493 errors:0 dropped:0 overruns:0 frame:0
63 TX packets:3380 errors:0 dropped:0 overruns:0 carrier:0
64 collisions:0 txqueuelen:100
65 Interrupt:18 Base address:0xfc80
67 with "PROMISC" indicating that the interface is in promiscuous mode),
68 and, if any interfaces are in promiscuous mode and no capture is being
69 done on that interface, turn promiscuous mode off by hand with
71 ifconfig <ifname> -promisc
73 where "<ifname>" is the name of the interface.