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+.\" Copyright (c) 1985, 1991, 1993
+.\"	The Regents of the University of California.  All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. All advertising materials mentioning features or use of this software
+.\"    must display the following acknowledgement:
+.\"	This product includes software developed by the University of
+.\"	California, Berkeley and its contributors.
+.\" 4. Neither the name of the University nor the names of its contributors
+.\"    may be used to endorse or promote products derived from this software
+.\"    without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\"     @(#)nsip.4	8.2 (Berkeley) 11/30/93
+.\"
+.Dd November 30, 1993
+.Dt NSIP 4
+.Os BSD 4.3
+.Sh NAME
+.Nm nsip
+.Nd software network interface encapsulating NS packets in IP packets
+.Sh SYNOPSIS
+.Cd options NSIP
+.Fd #include <netns/ns_if.h>
+.Sh DESCRIPTION
+The
+.Nm nsip
+interface is a software mechanism which may be
+used to transmit Xerox
+.Tn NS Ns (tm)
+packets through otherwise uncooperative
+networks.
+It functions by prepending an
+.Tn IP
+header, and resubmitting the packet
+through the
+.Tn UNIX
+.Tn IP
+machinery.
+.Pp
+The super-user can advise the operating system of a willing partner
+by naming an
+.Tn IP
+address to be associated with an
+.Tn NS
+address.
+Presently, only specific hosts pairs are allowed, and for each host
+pair, an artificial point-to-point interface is constructed.
+At some future date,
+.Tn IP
+broadcast addresses or hosts may be paired
+with
+.Tn NS
+networks or hosts.
+.Pp
+Specifically, a socket option of
+.Dv SO_NSIP_ROUTE
+is set on a socket
+of family
+.Dv AF_NS ,
+type
+.Dv SOCK_DGRAM ,
+passing the following structure:
+.Bd -literal
+struct nsip_req {
+	struct sockaddr rq_ns;	/* must be ns format destination */
+	struct sockaddr rq_ip;	/* must be ip format gateway */
+	short rq_flags;
+};
+.Ed
+.Sh DIAGNOSTICS
+.Bl -diag
+.It nsip%d: can't handle af%d.
+The interface was handed
+a message with addresses formatted in an unsuitable address
+family; the packet was dropped.
+.El
+.Sh SEE ALSO
+.Xr intro 4 ,
+.Xr ns 4
+.Sh HISTORY
+The
+.Nm
+interface appeared in
+.Bx 4.3 .
+.Sh BUGS
+It is absurd to have a separate pseudo-device for each pt-to-pt
+link.
+There is no way to change the
+.Tn IP
+address for an
+.Tn NS
+host once the
+encapsulation interface is set up.
+The request should honor flags of
+.Dv RTF_GATEWAY
+to indicate
+remote networks, and the absence of
+.Dv RTF_UP
+should be a clue
+to remove that partner.
+This was intended to postpone the necessity of rewriting reverse
+.Tn ARP
+for the 
+.Xr en 4
+device, and to allow passing
+.Tn XNS
+packets through an
+Arpanet-Milnet gateway, to facilitate testing between some co-operating
+universities.