/share/doc/smm/18.net/b.t

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 32.\"	@(#)b.t	8.1 (Berkeley) 6/8/93
 33.\"
 34.nr H2 1
 35.\".ds RH "Raw sockets
 36.br
 37.ne 2i
 38.NH
 39\s+2Raw sockets\s0
 40.PP
 41A raw socket is an object which allows users direct access
 42to a lower-level protocol.  Raw sockets are intended for knowledgeable
 43processes which wish to take advantage of some protocol
 44feature not directly accessible through the normal interface, or 
 45for the development of new protocols built atop existing lower level
 46protocols.  For example, a new version of TCP might be developed at the
 47user level by utilizing a raw IP socket for delivery of packets.
 48The raw IP socket interface attempts to provide an identical interface
 49to the one a protocol would have if it were resident in the kernel.
 50.PP
 51The raw socket support is built around a generic raw socket interface,
 52(possibly) augmented by protocol-specific processing routines.
 53This section will describe the core of the raw socket interface.
 54.NH 2
 55Control blocks
 56.PP
 57Every raw socket has a protocol control block of the following form:
 58.DS
 59.ta \w'struct  'u +\w'caddr_t  'u +\w'sockproto rcb_proto;    'u 
 60struct rawcb {
 61	struct	rawcb *rcb_next;	/* doubly linked list */
 62	struct	rawcb *rcb_prev;
 63	struct	socket *rcb_socket;	/* back pointer to socket */
 64	struct	sockaddr rcb_faddr;	/* destination address */
 65	struct	sockaddr rcb_laddr;	/* socket's address */
 66	struct	sockproto rcb_proto;	/* protocol family, protocol */
 67	caddr_t	rcb_pcb;		/* protocol specific stuff */
 68	struct	mbuf *rcb_options;	/* protocol specific options */
 69	struct	route rcb_route;	/* routing information */
 70	short	rcb_flags;
 71};
 72.DE
 73All the control blocks are kept on a doubly linked list for
 74performing lookups during packet dispatch.  Associations may
 75be recorded in the control block and used by the output routine
 76in preparing packets for transmission.
 77The \fIrcb_proto\fP structure contains the protocol family and protocol
 78number with which the raw socket is associated.
 79The protocol, family and addresses are
 80used to filter packets on input; this will be described in more
 81detail shortly.  If any protocol-specific information is required,
 82it may be attached to the control block using the \fIrcb_pcb\fP
 83field.
 84Protocol-specific options for transmission in outgoing packets
 85may be stored in \fIrcb_options\fP.
 86.PP
 87A raw socket interface is datagram oriented.  That is, each send
 88or receive on the socket requires a destination address.  This
 89address may be supplied by the user or stored in the control block
 90and automatically installed in the outgoing packet by the output
 91routine.  Since it is not possible to determine whether an address
 92is present or not in the control block, two flags, RAW_LADDR and
 93RAW_FADDR, indicate if a local and foreign address are present.
 94Routing is expected to be performed by the underlying protocol
 95if necessary.
 96.NH 2
 97Input processing
 98.PP
 99Input packets are ``assigned'' to raw sockets based on a simple
100pattern matching scheme.  Each network interface or protocol
101gives unassigned packets
102to the raw input routine with the call:
103.DS
104raw_input(m, proto, src, dst)
105struct mbuf *m; struct sockproto *proto, struct sockaddr *src, *dst;
106.DE
107The data packet then has a generic header prepended to it of the
108form
109.DS
110._f
111struct raw_header {
112	struct	sockproto raw_proto;
113	struct	sockaddr raw_dst;
114	struct	sockaddr raw_src;
115};
116.DE
117and it is placed in a packet queue for the ``raw input protocol'' module.
118Packets taken from this queue are copied into any raw sockets that
119match the header according to the following rules,
120.IP 1)
121The protocol family of the socket and header agree.
122.IP 2)
123If the protocol number in the socket is non-zero, then it agrees
124with that found in the packet header.
125.IP 3)
126If a local address is defined for the socket, the address format
127of the local address is the same as the destination address's and
128the two addresses agree bit for bit.
129.IP 4)
130The rules of 3) are applied to the socket's foreign address and the packet's
131source address.
132.LP
133A basic assumption is that addresses present in the
134control block and packet header (as constructed by the network
135interface and any raw input protocol module) are in a canonical
136form which may be ``block compared''.
137.NH 2
138Output processing
139.PP
140On output the raw \fIpr_usrreq\fP routine 
141passes the packet and a pointer to the raw control block to the
142raw protocol output routine for any processing required before
143it is delivered to the appropriate network interface.  The
144output routine is normally the only code required to implement
145a raw socket interface.