The top level directory has a single directory named ethern, where n is the interface number, starting from 0, assigned in some platform-dependent way. That directory contains a conversation directory for each of 8 Ethernet packet types, and a clone file.

Opening the clone file returns a file descriptor open on the ctl file of an unused conversation directory. Reading the ctl file returns a text string representing the number of the connection. The connection is controlled by writing textual commands to the associated ctl file:

connect type

Set the type of Ethernet packets received on the connection; the type is expressed as an integer constant (in hexadecimal if it has a leading 0x, octal if it has a leading 0, and otherwise decimal). For instance, Ethernet packets carrying IP version 4 use type 0x800. The value -1 stands for all types. The value -2 causes at most the first 64 bytes of all types of packets to be copied to the conversation. If several conversations are assigned the same packet type, a copy of the packet is given to each.

promiscuous

Set the interface to capture all packets regardless of destination address. An interface normally receives only packets whose destination address is that of the interface or is the broadcast address, ffffffffffff. The interface remains promiscuous until the control file is closed. The extra packets are received only by conversations of the same type as the incoming packet (or of type -1).

addmulti address

Add the given MAC multicast address to the set of multicast addresses accepted by the interface. Address is a twelve digit MAC address in hexadecimal.

remmulti address

Remove address from the set of multicast addresses accepted by the interface.

Incoming Ethernet packets are demultiplexed by destination address and packet type and queued for reading by the corresponding open connection(s). Each read of the data file returns each packet in turn, including the Ethernet header. A read will terminate at packet boundaries. Each write to the data file causes a packet to be sent. The Ethernet address of the interface is inserted into the packet header as the source address.

Reading the type file returns the decimal value of the assigned Ethernet packet type.

Reading the stats file returns status information, the value of counters, the `promiscuous' state, and the Ethernet MAC address of the interface.

Reading the ifstats file returns statistics and status information specific to a given hardware interface.