$Id: README.linux,v 1.7 2001/01/10 09:54:44 guy Exp $

In order to capture packets (with Ethereal/Tethereal, tcpdump, or any
other packet capture program) on a Linux system, the "packet" protocol
must be supported by your kernel.  If it is not, you may get error
messages such as

	modprobe: can't locate module net-pf-17

in "/var/adm/messages".  The following note is from the Linux
"Configure.help" file:

	Packet socket
	CONFIG_PACKET
	  The Packet protocol is used by applications which communicate
	  directly with network devices without an intermediate network
	  protocol implemented in the kernel, e.g. tcpdump. If you want them
	  to work, choose Y. 

	  This driver is also available as a module called af_packet.o ( =
	  code which can be inserted in and removed from the running kernel
	  whenever you want). If you want to compile it as a module, say M
	  here and read Documentation/modules.txt; if you use modprobe or
	  kmod, you may also want to add "alias net-pf-17 af_packet" to 
	  /etc/modules.conf.

In addition, the standard libpcap compiled for Linux has a timeout
problem; it doesn't support the timeout argument to "pcap_open_live()".

The current version of Ethereal attempts to work around this, so its GUI
shouldn't freeze when capturing on a not-so-busy network.  If its GUI
does freeze when that happens, please send a note about this, indicating
which version of which distribution of Linux you're using, and which
version of libpcap you're using, to ethereal-dev@ethereal.com.

The current version of Ethereal should work with versions of libpcap
that have been patched to fix the timeout problem, as well as working
with unpatched versions.

An additional problem, on Linux, with current versions of libpcap, is
that capture filters do not work when snooping loopback devices; if
you're capturing on a Linux loopback device, do not use a capture
filter, as it will probably reject most if not all packets, including
the packets it's intended to accept - instead, capture all packets and
use a display filter to select the packets you want to see.

In addition, current versions of libpcap on at least some Linux
distributions will not turn promiscuous mode off on a network device
until the program using promiscuous mode exits, so if you start a
capture with Ethereal on some Linux distributions, the network interface
will be put in promiscuous mode and will remain in promiscuous mode
until Ethereal exits.  There might be additional libpcap bugs that cause
it not to be turned off even when Ethereal exits; if your network is
busy, this could cause the Linux networking stack to do a lot more work
discarding packets not intended for the machine, so you may want to
check, after running Ethereal, whether any network interfaces are in
promiscuous mode (the output of "ifconfig -a" will say something such as

eth0      Link encap:Ethernet  HWaddr 00:00:66:66:66:66
          inet addr:66.66.66.66  Bcast:66.66.66.255  Mask:255.255.255.0
          UP BROADCAST RUNNING PROMISC MULTICAST  MTU:1500  Metric:1
          RX packets:6493 errors:0 dropped:0 overruns:0 frame:0
          TX packets:3380 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100 
          Interrupt:18 Base address:0xfc80 

with "PROMISC" indicating that the interface is in promiscuous mode),
and, if any interfaces are in promiscuous mode and no capture is being
done on that interface, turn promiscuous mode off by hand with

	ifconfig <ifname> -promisc

where "<ifname>" is the name of the interface.
