freebsd-dev/share/man/man9/ifnet.9

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.Dd December 16, 1996
.Os FreeBSD 2.2
.Dt IFNET 9
.Sh NAME
.Nm ifnet ,
.Nm ifaddr ,
.Nm ifqueue ,
.Nm if_data
.Nd kernel interfaces for manipulating network interfaces
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/time.h>
.Fd #include <sys/socket.h>
.Fd #include <net/if.h>
.Fd #include <net/if_types.h>
.\"
.Ss "Interface manipulation functions"
.Ft void
.Fn if_attach "struct ifnet *ifp"
.Ft void
.Fn if_down "struct ifnet *ifp"
.Ft int
.Fn ifioctl "struct socket *so" "int cmd" "caddr_t data" "struct proc *p"
.Ft int
.Fn ifpromisc "struct ifnet *ifp" "int pswitch"
.Ft "struct ifnet *"
.Fn ifunit "char *name"
.Ft void
.Fn if_up "struct ifnet *ifp"
.\"
.Ss "Interface address functions"
.Ft "struct ifaddr *"
.Fn ifa_ifwithaddr "struct sockaddr *addr"
.Ft "struct ifaddr *"
.Fn ifa_ifwithdstaddr "struct sockaddr *addr"
.Ft "struct ifaddr *"
.Fn ifa_ifwithnet "struct sockaddr *addr"
.Ft "struct ifaddr *"
.Fn ifaof_ifpforaddr "struct sockaddr *addr" "struct ifnet *ifp"
.Ft void
.Fn ifafree "struct ifaddr *ifa"
.Ft void \"macro
.Fn IFAFREE "struct ifaddr *ifa"
.\"
.Ss "Output queue macros"
.Ft void \"macro
.Fn IF_ENQ_DROP "struct ifqueue *ifq" "struct mbuf *m"
.Ft void \"macro
.Fn IF_DEQUEUE "struct ifqueue *ifq" "struct mbuf *m"
.\"
.Ss "struct ifnet member functions"
.Ft int
.Fn (*if_output) "struct ifnet *ifp" "struct mbuf *m" "struct sockaddr *dst" "struct rtentry *rt"
.Ft void
.Fn (*if_start) "struct ifnet *ifp"
.Ft int
.Fn (*if_dont) "struct ifnet *ifp"
.Ft int
.Fn (*if_ioctl) "struct ifnet *ifp" "int cmd" "caddr_t data"
.Ft void
.Fn (*if_watchdog) "struct ifnet *ifp"
.Ft int
.Fn (*if_poll_recv) "struct ifnet *ifp" "int *quotap"
.Ft int
.Fn (*if_poll_xmit) "struct ifnet *ifp" "int *quotap"
.Ft void
.Fn (*if_poll_inttrn) "struct ifnet *ifp"
.Ft void
.Fn (*if_poll_slowinput) "struct ifnet *ifp" "struct mbuf *m"
.Ft void
.Fn (*if_init) "void *wtf_is_this"
.Ss "struct ifaddr member function"
.Ft void
.Fn (*ifa_rtrequest) "int cmd" "struct rtentry *rt" "struct sockaddr *dst"
.Ss "Global variables"
.Fd extern struct ifnethead ifnet;
.Fd extern struct ifaddr \&**ifnet_addrs;
.Fd extern int if_index;
.Fd extern int ifqmaxlen;
.Sh DATA STRUCTURES
The kernel mechanisms for handling network interfaces reside primarily
in the
.Li ifnet ,
.Li if_data ,
and
.Li ifaddr
structures in
.Aq Pa net/if.h ,
and the functions named above and defined in
.Pa /sys/net/if.c .
The system keeps a linked list of interfaces using the
.Li TAILQ
macros defined in
.Xr queue 3 ;
this list is headed by a
.Li "struct ifnethead"
called
.Li ifnet .
The elements of this list are of type
.Li "struct ifnet" ,
and most kernel routines which manipulate interface as such accept or
return pointers to these structures.  Each interface structure
contains an
.Li if_data
structure, which contains statistics and identifying information used
by management programs, and which is exported to user programs by way
of the
.Xr ifmib 4
branch of the
.Xr sysctl 3
MIB.
Each interface also has a
.Li TAILQ
of interface addresses, described by
.Li ifaddr
structures; the head of the queue is always an
.Dv AF_LINK
address
(see
.Xr link_addr 3 )
describing the link layer implemented by the interface (if any).
(Some trivial interfaces do not provide any link layer addresses;
this structure, while still present, serves only to identify the
interface name and index.)
.Pp
Interfaces are also associated with an output queue, defined as a
.Li "struct ifqueue" ;
this structure is used to hold packets while the interface is in the
process of sending another.  The current implementation implements a
drop-tail queuing discipline, but in the future a Random Early Drop
discipline is expected to be used.  For this reason, kernel code
should not depend on the internals of the queue structure; in
particular, only the
.Fn IF_ENQ_DROP
and
.Fn IF_DEQUEUE
macros will be supported in future implementations.
.\" The old structure will probably be retained for compatibility
.\" under a different name.
.Pp
.Ss The ifnet structure
The fields of
.Li "struct ifnet"
are as follows:
.Pp
.Bl -tag -width "if_poll_slowq" -offset indent
.It Li "if_softc"
.Pq Li "void *"
A pointer to the driver's private state block.  (Initialized by
driver.)
.It Li if_name
.Pq Li "char *"
The name of the interface, not including the unit number
(e.g.,
.Dq Li de
or
.Dq Li lo ) .
(Initialized by driver.)
.It Li if_link
.Pq Li "TAILQ_ENTRY(ifnet)"
.Xr queue 3
macro glue.
.It Li if_addrhead
.Pq Li "struct ifaddrhead"
The head of the
.Xr queue 3
.Li TAILQ
containing the list of addresses assigned to this interface.
.It Li if_pcount
.Pq Li "int"
A count of promiscuous listeners on this interface, used to
reference-count the
.Dv IFF_PROMISC
flag.
.It Li "if_bpf"
.Pq Li "struct bpf_if *"
Opaque per-interface data for the packet filter,
.Xr bpf 4 .
(Initialized by
.Fn bpf_attach . )
.It Li "if_index"
.Pq Li "u_short"
A unique number assigned to each interface in sequence as it is
attached.  This number can be used in a
.Li "struct sockaddr_dl"
to refer to a particular interface by index
(see
.Xr link_addr 3 ) .
.It Li "if_unit"
.Pq Li "short"
A unique number assigned to each interface managed by a particular
driver, usually related to the unit number of a physical device in the
kernel configuration file
(see
.Xr config 8 ) .
(Initialized by driver.)
.It Li "if_timer"
.Pq Li "short"
Number of seconds until the watchdog timer
.Fn if_watchdog
is called, or zero if the timer is disabled.  (Set by driver,
decremented by generic watchdog code.)
.It Li "if_flags"
.Pq Li "short"
Flags describing operational parameters of this interface (see below).
(Manipulated by both driver and generic code.)
.\" .It Li "if_ipending"
.\" Interrupt-pending bits for polled operation:
.\" .Dv IFI_XMIT
.\" (transmit complete interrupt)
.\" and
.\" .Dv IFI_RECV
.\" (received packet ready interrupt).  See the
.\" .Sx Polling
.\" section, below.  (Manipulated by driver.)
.It Li "if_linkmib"
.Pq Li "void *"
A pointer to an interface-specific MIB structure exported by
.Xr ifmib 4 .
(Initialized by driver.)
.It Li "if_linkmiblen"
.Pq Li "size_t"
The size of said structure.  (Initialized by driver.)
.It Li "if_data"
.Pq Li "struct if_data"
More statistics and information; see
.Dq Sx "The if_data structure" ,
below.  (Initialized by driver, manipulated by both driver and generic
code.)
.It Li "if_snd"
.Pq Li "struct ifqueue"
The output queue.  (Manipulated by driver.)
.\".It Li "if_poll_slowq"
.\".Pq Li "struct ifqueue *"
.\"A pointer to the input queue for devices which do not support polling
.\"well.  See the
.\".Sx Polling 
.\"section, below.  (Initialized by driver.)
.El
.Pp
There are in addition a number of function pointers which the driver
must initialize to complete its interface with the generic interface
layer:
.Bl -ohang -offset indent
.It Fn if_output
Output a packet on interface
.Ar ifp ,
or queue it on the output queue if the interface is already active.
.It Fn if_start
Start queued output on an interface.  This function is exposed in
order to provide for some interface classes to share a
.Fn if_output
among all drivers.
.Fn if_start
may only be called when the
.Dv IFF_OACTIVE
flag is not set.  (Thus,
.Dv IFF_OACTIVE
does not literally mean that output is active, but rather that the
device's internal output queue is full.)
.It Fn if_done
Not used.  We're not even sure what it was ever for.
.It Fn if_ioctl
Process interface-related
.Xr ioctl 2
requests
(defined in
.Aq Pa sys/sockio.h ) .
Preliminary processing is done by the generic routine
.Fn ifioctl
to check for appropriate privileges, locate the interface being
manipulated, and perform certain generic operations like twiddling
flags and flushing queues.  See the description of
.Fn ifioctl
below for more information.
.It Fn if_watchdog
Routine called by the generic code when the watchdog timer,
.Li if_timer ,
expires.  Usually this will reset the interface.
.\" .It Fn if_poll_recv
.\" .It Fn if_poll_xmit
.\" .It Fn if_poll_slowinput
.\" .It Fn if_poll_intren
.\" See the
.\" .Sx Polling
.\" section, below.
.It Fn if_init
XXX fill me in
.El
.Ss "Interface flags"
Interface flags are used for a number of different purposes.  Some
flags simply indicate information about the type of interface and its
capabilities; others are dynamically manipulated to reflect the
current state of the interface.  Flags of the former kind are marked
.Aq S
in this table; the latter are marked
.Aq D .
.Pp
.Bl -tag -width "IFF_POINTOPOINT" -compact -offset indent
.It Dv IFF_UP
.Aq D
The interface has been configured up by the user-level code.
.It Dv IFF_BROADCAST
.Aq S*
The interface supports broadcast.
.It Dv IFF_DEBUG
.Aq D
Used to enable/disable driver debugging code.
.It Dv IFF_LOOPBACK
.Aq S
The interface is a loopback device.
.It Dv IFF_POINTOPOINT
.Aq S*
The interface is point-to-point;
.Dq broadcast
addresses are actually the address of the other end.
.It Dv IFF_RUNNING
.Aq D*
The interface has been configured and dynamic resources were
successfully allocated.  Probably only useful internal to the
interface.
.It Dv IFF_NOARP
.Aq D
Disable network address resolution on this interface.
.It Dv IFF_PROMISC
.Aq D
This interface is in promiscuous mode.
.It Dv IFF_ALLMULTI
.Aq D*
This interface is in all-multicasts mode (used by multicast routers).
.It Dv IFF_OACTIVE
.Aq D*
The interface's hardware output queue (if any) is full; output packets
are to be queued.
.It Dv IFF_SIMPLEX
.Aq S*
The interface cannot hear its own transmissions.
.It Dv IFF_LINK0
.It Dv IFF_LINK1
.It Dv IFF_LINK2
.Aq D
Control flags for the link layer.  (Currently abused to select among
multiple physical layers on some devices.)
.It Dv IFF_MULTICAST
.Aq S*
This interface supports multicast.
.El
.Pp
The macro
.Dv IFF_CANTCHANGE
defines the bits which cannot be set by a user program using the
.Dv SIOCSIFFLAGS
command to
.Xr ioctl 2 ;
these are indicated by an asterisk in the listing above.
.Ss The if_data structure
In
.Bx 4.4 ,
a subset of the interface information believed to be of interest to
management stations was segregated from the
.Li ifnet
structure and moved into its own
.Li if_data
structure to facilitate its use by user programs.  The following
elements of the
.Li if_data
structure are initialized by the interface and are not expected to change
significantly over the course of normal operation:
.Bl -tag -width "ifi_lastchange" -offset indent
.It Li ifi_type
.Pq Li u_char
The type of the interface, as defined in
.Aq Pa net/if_types.h
and described below in the
.Dq Sx "Interface types"
section.
.It Li ifi_physical
.Pq Li u_char
Intended to represent a selection of physical layers on devices which
support more than one; never implemented.
.It Li ifi_addrlen
.Pq Li u_char
Length of a link-layer address on this device, or zero if there are
none.  Used to initialized the address length field in
.Li "sockaddr_dl"
structures referring to this interface.
.It Li ifi_hdrlen
.Pq Li u_char
Maximum length of any link-layer header which might be prepended by
the driver to a packet before transmission.  The generic code computes
the maximum over all interfaces and uses that value to influence the
placement of data in
.Li mbuf Ns s
to attempt to ensure that there is always
sufficient space to prepend a link-layer header without allocating an
additional 
.Li mbuf .
.\" (See
.\" .Xr mbuf 9 . )
.\" .It Li ifi_recvquota
.\" .Pq Li u_char
.\" Number of packets the interface is permitted to receive at one time
.\" when in polled mode.
.\" .It Li ifi_xmitquota
.\" .Pq Li u_char
.\" Number of packets the interface is permitted to queue for transmission
.\" at one time when in polled mode.  There is some controversy over
.\" whether such a restriction makes any sense at all.
.It Li ifi_mtu
.Pq Li u_long
The maximum transmission unit of the medium, exclusive of any
link-layer overhead.
.It Li ifi_metric
.Pq Li u_long
A dimensionless metric interpreted by a user-mode routing process.
.It Li ifi_baudrate
.Pq Li u_long
The line rate of the interface, in bits per second.
.El
.Pp
The structure additionally contains generic statistics applicable to a
variety of different interface types (except as noted, all members are
of type
.Li u_long ) :
.Bl -tag -width "ifi_lastchange" -offset indent
.It Li ifi_ipackets
Number of packets received.
.It Li ifi_ierrors
Number of receive errors detected (e.g., FCS errors, DMA overruns,
etc.).  More detailed breakdowns can often be had by way of a
link-specific MIB.
.It Li ifi_opackets
Number of packets transmitted.
.It Li ifi_oerrors
Number of output errors detected (e.g., late collisions, DMA overruns,
etc.).  More detailed breakdowns can often be had by way of a
link-specific MIB.
.It Li ifi_collisions
Total number of collisions detected on output for CSMA interfaces.
(This member is sometimes [ab]used by other types of interfaces for
other output error counts.)
.It Li ifi_ibytes
Total traffic received, in bytes.
.It Li ifi_obytes
Total traffic transmitted, in bytes.
.It Li ifi_imcasts
Number of packets received which were sent by link-layer multicast.
.It Li ifi_omcasts
Number of packets sent by link-layer multicast.
.It Li ifi_iqdrops
Number of packets dropped on input.  Rarely implemented.
.It Li ifi_noproto
Number of packets received for unknown network-layer protocol.
.\" .It Li ifi_recvtiming
.\" Amount of time, in microseconds, spent to receive an average packet on
.\" this interface.  See the
.\" .Sx Polling
.\" section, below.
.\" .It Li ifi_xmittiming
.\" Amount of time, in microseconds, spent to service a transmit-complete
.\" interrupt on this interface.  See the
.\" .Sx Polling
.\" section, below.
.It Li ifi_lastchange
.Pq Li "struct timeval"
The time of the last administrative change to the interface (as required
for
.Tn SNMP ) .
.El
.Ss Interface types
The header file
.Aq Pa net/if_types.h
defines symbolic constants for a number of different types of
interfaces.  The most common are:
.Pp
.Bl -tag -compact -offset indent -width IFT_PROPVIRTUAL
.It Dv IFT_OTHER
none of the following
.It Dv IFT_ETHER
Ethernet
.It Dv IFT_ISO88023
ISO 8802-3 CSMA/CD
.It Dv IFT_ISO88024
ISO 8802-4 Token Bus
.It Dv IFT_ISO88025
ISO 8802-5 Token Ring
.It Dv IFT_ISO88026
ISO 8802-6 DQDB MAN
.It Dv IFT_FDDI
FDDI
.It Dv IFT_PPP
Internet Point-to-Point Protocol
.Pq Xr ppp 8
.It Dv IFT_LOOP
The loopback
.Pq Xr lo 4
interface.
.It Dv IFT_SLIP
Serial Line IP
.It Dv IFT_PARA
Parallel-port IP
.Pq Dq Tn PLIP
.It Dv IFT_ATM
Asynchronous Transfer Mode
.El
.Ss The ifaddr structure
Every interface is associated with a list
(or, rather, a
.Dv TAILQ )
of addresses, rooted at the interface structure's
.Li if_addrlist
member.  The first element in this list is always an
.Dv AF_LINK
address representing the interface itself; multi-access network
drivers should complete this structure by filling in their link-layer
addresses after calling
.Fn if_attach .
Other members of the structure represent network-layer addresses which
have been configured by means of the
.Dv SIOCAIFADDR
command to
.Xr ioctl 2 ,
called on a socket of the appropriate protocol family.
The elements of this list consist of
.Li ifaddr
structures.  Most protocols will declare their own protocol-specific
interface address structures, but all begin with a
.Li "struct ifaddr"
which provides the most-commonly-needed functionality across all
protocols.  Interface addresses are reference-counted.
.Pp
The members of
.Li "struct ifaddr"
are as follows:
.Bl -tag -width ifa_rtrequest -offset indent
.It Li ifa_addr
.Pq Li "struct sockaddr *"
The local address of the interface.
.It Li ifa_dstaddr
.Pq Li "struct sockaddr *"
The remote address of point-to-point interfaces, and the broadcast
address of broadcast interfaces.
.Po
.Li ifa_broadaddr
is a macro for
.Li ifa_dstaddr .
.Pc
.It Li ifa_netmask
.Pq Li "struct sockaddr *"
The network mask for multi-access interfaces, and the confusion
generator for point-to-point interfaces.
.It Li ifa_ifp
.Pq Li "struct ifnet *"
A link back to the interface structure.
.It Li ifa_link
.Pq Li TAILQ_ENTRY(ifaddr)
.Xr queue 3
glue for list of addresses on each interface.
.It Li ifa_rtrequest
See below.
.It Li ifa_flags
.Pq Li u_short
Some of the flags which would be used for a route representing this
address in the route table.
.It Li ifa_refcnt
.Pq Li short
The reference count.
.It Li ifa_metric
.Pq Li int
A metric associated with this interface address, for the use of some
external routing protocol.
.El
.Pp
References to
.Li ifaddr
structures are gained manually, by incrementing the
.Li ifa_refcnt
member.  References are released by calling either the
.Fn ifafree
function or the
.Fn IFAFREE
macro.
.Pp
.Fn ifa_rtrequest
is a pointer to a function which receives callouts from the routing
code
.Pq Fn rtrequest
to perform link-layer-specific actions upon requests to add, resolve,
or delete routes.  The
.Ar cmd
argument indicates the request in question:
.Dv RTM_ADD ,
.Dv RTM_RESOLVE ,
or
.Dv RTM_DELETE .
The
.Ar rt
argument is the route in question; the
.Ar sa
argument is the specific destination being manipulated
for
.Dv RTM_RESOLVE ,
or a null pointer otherwise.
.Sh FUNCTIONS
The functions provided by the generic interface code can be divided
into two groups: those which manipulate interfaces, and those which
manipulate interface addresses.  In addition to these functions, there
may also be link-layer support routines which are used by a number of
drivers implementing a specific link layer over different hardware;
see the documentation for that link layer for more details.
.Ss Interface manipulation functions
.Bl -ohang -offset indent
.It Fn if_attach
Link the specified interface
.Ar ifp
into the list of network interfaces.  Also initialize the list of
addresses on that interface, and create a link-layer 
.Li ifaddr
structure to be the first element in that list.  (A pointer to
this address structure is saved in the global array
.Li ifnet_addrs . )
.It Fn if_down
Mark the interface
.Ar ifp
as down (i.e.,
.Dv IFF_UP
is not set),
flush its output queue, and call the interface's
.Fn if_ioctl
routine to notify the driver of the shutdown request.
.It Fn if_up
Mark the interface
.Ar ifp
as up, and call the interface's
.Fn if_ioctl
routine to (re-)initialize the driver.
.It Fn ifpromisc
Add or remove a promiscuous reference to
.Ar ifp .
If 
.Ar pswitch
is true, add a reference;
if it is false, remove a reference.  On reference count transitions
from zero to one and one to zero, set the
.Dv IFF_PROMISC
flag appropriately and call
.Fn if_ioctl
to set up the interface in the desired mode.
.It Fn ifunit
Return an 
.Li ifnet
pointer for the interface named
.Ar name .
.It Fn ifioctl
Process the ioctl request
.Ar cmd ,
issued on socket
.Ar so 
by process
.Ar p ,
with data parameter
.Ar data .
This is the main routine for handling all interface configuration
requests from user mode.
It is ordinarily only called from the socket-layer
.Xr ioctl 2
handler, and only for commands with class
.Sq Li i .
Any unrecognized commands will be passed down to socket
.Ar so Ns 's
protocol for
further interpretation.  The following commands are handled by
.Fn ifioctl :
.Pp
.Bl -tag -width OSIOCGIFNETMASK -compact -offset indent
.It Dv SIOCGIFCONF
.It Dv OSIOCGIFCONF
Get interface configuration.  (No call-down to driver.)
.It Dv SIOCGIFFLAGS
.It Dv SIOCGIFMETRIC
.It Dv SIOCGIFMTU
.It Dv SIOCGIFPHYS
Get interface flags, metric, MTU, medium selection.  (No call-down to
driver.)
.Pp
.It Dv SIOCSIFFLAGS
Change interface flags.  Caller must have appropriate privilege.  If
requested a change to the IFF_UP flag is requested,
.Fn if_up
or
.Fn if_down
is called as appropriate.  Flags listed in
.Dv IFF_CANTCHANGE
are masked off, and the driver
.Fn if_ioctl
routine is called to perform any setup
requested.
.It Dv SIOCSIFMETRIC
.It Dv SIOCSIFPHYS
Change interface metric or medium.  Caller must have appropriate privilege.
.Pp
.It Dv SIOCSIFMTU
Change interface MTU.  Caller must have appropriate privilege.  MTU
values less than 72 or greater than 65535 are considered invalid.  The
driver
.Fn if_ioctl
routine is called to implement the change; it is responsible for any
additional sanity checking and for actually modifying the MTU in the
interface structure.
.It Dv SIOCADDMULTI
.It Dv SIOCDELMULTI
Add or delete permanent multicast group memberships on the interface.
Caller must have appropriate privilege.  The driver
.Fn if_ioctl
routine is called to perform the requested action.
.It Dv SIOCSIFDSTADDR
.It Dv SIOCSIFADDR
.It Dv SIOCSIFBRDADDR
.It Dv SIOCSIFNETMASK
The socket's protocol control routine is called to implement the
requested action.
.It Dv OSIOGIFADDR
.It Dv OSIOCGIFDSTADDR
.It Dv OSIOCGIFBRDADDR
.It Dv OSIOCGIFNETMASK
The socket's protocol control routine is called to implement the
requested action.  On return,
.Li sockaddr
structures are converted into old-style (no
.Li sa_len
member).
.El
.Pp
.Fn if_down ,
.Fn ifioctl ,
.Fn ifpromisc ,
and
.Fn if_up
must be called at
.Fn splnet
or higher.
.Ss "Interface address functions"
Several functions exist to look up an interface address structure
given an address.
.Fn ifa_ifwithaddr
returns an interface address with either a local address or a
broadcast address precisely matching the parameter
.Ar addr .
.Fn ifa_ifwithdstaddr
returns an interface address for a point-to-point interface whose
remote (``destination'') address is
.Ar addr .
.Pp
.Fn ifa_ifwithnet
returns the most specific interface address which matches the
specified address,
.Ar addr ,
subject to its configured netmask, or a point-to-point interface
address whose remote address is
.Ar addr
if one is found.
.Pp
.Fn ifaof_ifpforaddr
returns the most specific address configured on interface
.Ar ifp
which matches address
.Ar addr ,
subject to its configured netmask.  If the interface is
point-to-point, only an interface address whose remote address is
precisely
.Ar addr
will be returned.
.Pp
All of these functions return a null pointer if no such address can be
found.
.\" .Sh POLLING
.\" XXX write me!
.Sh SEE ALSO
.Xr ioctl 2 ,
.Xr link_addr 3 ,
.Xr queue 3 ,
.Xr sysctl 3 ,
.Xr bpf 4 ,
.Xr ifmib 4 ,
.Xr lo 4 ,
.Xr netintro 4 ,
.Xr config 8 ,
.Xr ppp 8 ,
.Xr rtentry 9
.\" .Xr mbuf 9 ,
.Rs
.%A Gary R. Wright and W. Richard Stevens
.%B TCP/IP Illustrated
.%V vol. 2
.%O Addison-Wesley, ISBN 0-201-63354-X
.Re
.Sh AUTHOR
This manual page was written by Garrett A. Wollman