.TH IP 3 
.SH NAME
ip \- network protocols over IP
.SH SYNOPSIS
.nf
.B bind -a #I\fIspec\fP /net

.B /net/ipifc
.B /net/ipifc/clone
.B /net/ipifc/stats
.BI /net/ipifc/ n 
.BI /net/ipifc/ n /status
.BI /net/ipifc/ n /ctl
\&...

.B /net/arp
.B /net/log
.B /net/ndb
.B /net/iproute
.B /net/ipselftab

.B /net/esp
.B /net/gre
.B /net/icmp
.B /net/il
.B /net/ipmux
.B /net/rudp
.B /net/tcp
.B /net/udp

.B /net/tcp/clone
.B /net/tcp/stats
.BI /net/tcp/ n 
.BI /net/tcp/ n /data
.BI /net/tcp/ n /ctl
.BI /net/tcp/ n /local
.BI /net/tcp/ n /remote
.BI /net/tcp/ n /status
.BI /net/tcp/ n /listen
\&...
.fi
.SH DESCRIPTION
The IP device provides the interface to Internet protocol stacks.
.I Spec
is an integer from 0 to 15 identifying a stack.
Each stack is physically independent of all others:
the only information transfer between them is via programs that
mount multiple stacks.
Normally a system uses only one stack.
However multiple stacks can be used for debugging
new IP networks or implementing firewalls or proxy
services.
.PP
All addresses used are 16-byte IPv6 addresses.
We implement IPv4 and IPv6.
IPv4 addresses are a subset of the IPv6 addresses and both standard
.SM ASCII
formats
are accepted.  In binary, all v4 addresses start with the
12 bytes:
.EX
	00 00 00 00 00 00 00 00 00 00 ff ff
.EE
.SS "Configuring interfaces
.PP
Each stack may have multiple interfaces and each interface
may have multiple addresses.
The
.B /net/ipifc
directory contains a
.B clone
file, a
.B stats
file, and numbered subdirectories for each physical interface.
.PP
Opening the
.B clone
file reserves an interface.
The file descriptor returned from the
.IR open (2)
will point to the control file,
.BR ctl ,
of the newly allocated interface.
Reading
.B ctl
returns a text string representing the number of the interface.
Writing
.B ctl
alters aspects of the interface.
The possible
.I ctl
messages are:
.TP
.BI "bind ether " path
Treat the device mounted at
.I path
as an Ethernet medium carrying IP and ARP packets
and associate it with this interface.
The kernel will
.IR dial (2)
.IR path !0x800
and
.IR path !0x806
and use the two connections for IP and
ARP respectively.
.TP
.B "bind pkt
Treat this interface as a packet interface.  Assume
a user program will read and write the
.I data
file to receive and transmit IP packets to the kernel.
This is used by programs such as
.IR ppp (8)
to mediate IP packet transfer between the kernel and
a PPP encoded device.
.TP
.BI "bind netdev " path
Treat this interface as a packet interface.
The kernel will open
.I path
and read and write the resulting file descriptor
to receive and transmit IP packets.
.TP
.BI "bind loopback "
Treat this interface as a local loopback.  Anything
written to it will be looped back.
.TP
.B "unbind
Disassociate the physical device from an IP interface.
.TP
.BI add\  "local mask remote mtu " proxy
Add a local IP address to the interface.  The
.IR mask ,
.IR remote ,
.IR mtu ,
and
.B proxy
arguments are all optional.  The default mask is
the class mask for the local address.  The default
remote address is
.I local
ANDed with
.IR mask .
The default mtu is 1514 for Ethernet and 4096 for packet
media.
.IR Proxy ,
if specified, means that this machine should answer
ARP requests for the remote address.
.IR Ppp (8)
does this to make remote machines appear
to be connected to the local Ethernet.
.TP
.BI remove\  "local mask"
Remove a local IP address from an interface.
.TP
.BI mtu\  n
Set the maximum transfer unit for this device to
.IR n .
The mtu is the maximum size of the packet including any
medium-specific headers.
.TP
.BI reassemble
Reassemble IP fragments before forwarding to this interface
.TP
.BI iprouting\  n
Allow
.RI ( n
is missing or non-zero) or disallow
.RI ( n
is 0) forwarding packets between this interface and
others.
.TP
.BI addmulti\  addr
Treat the multicast
.I addr
on this interface as a local address.
.TP
.BI remmulti\  addr
Remove the multicast address
.I addr
from this interface.
.PP
Reading the interface's
.I status
file returns information about the interface, one line for each
local address on that interface.  The first line
has 9 white-space-separated fields: device, mtu, local address,
mask, remote or network address, packets in, packets out, input errors,
output errors.  Each subsequent line contains all but the device and mtu.
See
.B readipifc
in
.IR ip (2).
.SS "Routing
.PP
The file
.I iproute
controls information about IP routing.
When read, it returns one line per routing entry.
Each line contains six white-space-separated fields:
target address, target mask, address of next hop, flags,
tag, and interface number.
The entry used for routing an IP packet is the one with
the longest mask for which destination address ANDed with
target mask equals the target address.
The one character flags are:
.TP
.B 4
IPv4 route
.TP
.B 6
IPv6 route
.TP
.B i
local interface
.TP
.B b
broadcast address
.TP
.B u
local unicast address
.TP
.B m
multicast route
.TP
.B p
point-to-point route
.PP
The tag is an arbitrary, up to 4 character, string.  It is normally used to
indicate what routing protocol originated the route.
.PP
Writing to
.B /net/iproute
changes the route table.  The messages are:
.TP
.B flush
Remove all routes.
.TP
.BI tag\  string
Associate the tag,
.IR string ,
with all subsequent routes added via this file descriptor.
.TP
.BI add\  "target mask nexthop"
Add the route to the table.  If one already exists with the
same target and mask, replace it.
.TP
.BI remove\  "target mask"
Remove a route with a matching target and mask.
.SS "Address resolution
.PP
The file
.B /net/arp
controls information about address resolution.
The kernel automatically updates the ARP information for Ethernet
interfaces.
When read, the file returns one line per address containing the
type of medium, the status of the entry (OK, WAIT), the IP
address, and the medium address.
Writing to
.B /net/arp
administers the ARP information.  The control messages are:
.TP
.B flush
Remove all entries.
.TP
.BI add\  "type IP-addr Media-addr"
Add an entry or replace an existing one for the
same IP address.
.TP
.BI del\  "IP-addr"
Delete an individual entry.
.PP
ARP entries do not time out.  The ARP table is a
cache with an LRU replacement policy.  The IP stack
listens for all ARP requests and, if the requester is in
the table, the entry is updated.
Also, whenever a new address is configured onto an
Ethernet, an ARP request is sent to help
update the table on other systems.
.PP
Currently, the only medium type is
.BR ether .
.SS "Debugging and stack information
.PP
If any process is holding
.B /net/log
open, the IP stack queues debugging information to it.
This is intended primarily for debugging the IP stack.
The information provided is implementation-defined;
see the source for details.  Generally, what is returned is error messages
about bad packets.
.PP
Writing to
.B /net/log
controls debugging.  The control messages
are:
.TP
.BI set\  arglist
.I Arglist
is a space-separated list of items for which to enable debugging.
The possible items are:
.BR ppp ,
.BR ip ,
.BR fs ,
.BR tcp ,
.BR il ,
.BR icmp ,
.BR udb ,
.BR compress ,
.BR ilmsg ,
.BR gre ,
.BR tcpmsg ,
.BR udpmsg ,
.BR ipmsg ,
and
.BR esp .
.TP
.BI clear\  arglist
.I Arglist
is a space-separated list of items for which to disable debugging.
.TP
.BI only\  addr
If
.I addr
is non-zero, restrict debugging to only those
packets whose source or destination is that
address.
.PP
The file
.B /net/ndb
can be read or written by
programs.  It is normally used by
.IR ipconfig (8)
to leave configuration information for other programs
such as
.B dns
and
.B cs
(see
.IR ndb (8)).
.B /net/ndb
may contain up to 1024 bytes.
.PP
The file
.B /net/ipselftab
is a read-only file containing all the IP addresses
considered local.  Each line in the file contains
three white-space-separated fields: IP address, usage count,
and flags.  The usage count is the number of interfaces to which
the address applies.  The flags are the same as for routing
entries.
.SS "Protocol directories
.PP
The
.I ip
device
supports IP as well as several protocols that run over it:
TCP, IL, UDP, GRE, ESP, ICMP, and RUDP.
TCP and UDP provide the standard Internet
protocols for reliable stream and unreliable datagram
communication.
IL provides a reliable datagram service for communication
between Plan 9 machines.
GRE is a general encapsulation protocol.
ESP is the encapsulation protocol for IPSEC.
ICMP is IP's catch-all control protocol used to send
low level error messages and to implement
.IR ping (8).
RUDP is a locally developed reliable datagram protocol based on
UDP.
.PP
Each protocol is a subdirectory of the IP stack.
The top level directory of each protocol contains a
.B clone
file, a
.B stats
file, and subdirectories numbered from zero to the number of connections
opened for this protocol.
.PP
Opening the
.B clone
file reserves a connection.  The file descriptor returned from the
.IR open (2)
will point to the control file,
.BR ctl ,
of the newly allocated connection.
Reading
.B ctl
returns a text
string representing the number of the
connection.
Connections may be used either to listen for incoming calls
or to initiate calls to other machines.
.PP
A connection is controlled by writing text strings to the associated
.B ctl
file.
After a connection has been established data may be read from
and written to
.BR data .
A connection can be actively established using the
.B connect
message (see also
.IR dial (2)).
A connection can be established passively by first
using an
.B announce
message (see
.IR dial (2))
to bind to a local port and then
opening the
.B listen
file (see
.IR dial (2))
to receive incoming calls.
.PP
The following control messages are supported:
.TP
.BI connect\  ipaddress ! port "!r " local
Establish a connection to the remote address
.I ipaddress
and remote port
.IR port .
If
.I local
is specified, it is used as the local port number.
If
.I local
is not specified but
.B !r
is, the system will allocate
a restricted port number (less than 1024) for the connection to allow communication
with Unix  
.B login
and
.B exec
services.
Otherwise a free port number starting at 5000 is chosen.
The connect fails if the combination of local and remote address/port pairs
are already assigned to another port.
.TP
.BI announce\  X
.I X
is a decimal port number or
.LR * .
Set the local port
number to
.I X
and accept calls to
.IR X .
If
.I X
is
.LR * ,
accept
calls for any port that no process has explicitly announced.
The local IP address cannot be set.
.B Announce
fails if the connection is already announced or connected.
.TP
.BI bind\  X
.I X
is a decimal port number or
.LR * .
Set the local port number to
.IR X .
This exists to support emulation
of BSD sockets by the APE libraries (see
.IR pcc (1))
and is not otherwise used.
.TP
.BI backlog\  n
Set the maximum number of unanswered (queued) incoming
connections to an announced port to
.IR n .
By default
.I n
is set to five.  If more than
.I n
connections are pending,
further requests for a service will be rejected.
.TP
.BI ttl\  n
Set the time to live IP field in outgoing packets to
.IR n .
.TP
.BI tos\  n
Set the service type IP field in outgoing packets to
.IR n .
.PP
Port numbers must be in the range 1 to 32767.
.PP
Several files report the status of a
connection.
The
.B remote
and
.B local
files contain the IP address and port number for the remote and local side of the
connection.  The
.B status
file contains protocol-dependent information to help debug network connections.
On receiving and error or EOF reading or writing the
.B data
file, the
.B err
file contains the reason for error.
.PP
A process may accept incoming connections by
.IR open (2)ing
the
.B listen
file.
The
.B open
will block until a new connection request arrives.
Then
.B open
will return an open file descriptor which points to the control file of the
newly accepted connection.
This procedure will accept all calls for the
given protocol.
See
.IR dial (2).
.SS TCP
.PP
TCP connections are reliable point-to-point byte streams; there are no
message delimiters.
A connection is determined by the address and port numbers of the two
ends.
TCP 
.B ctl
files support the following additional messages:
.TP
.B hangup
close down a TCP connection
.TP
.BI keepalive \ n
turn on keep alive messages.
.IR N ,
if given, is the milliseconds between keepalives
(default 30000).
.SS UDP
.PP
UDP connections carry unreliable and unordered datagrams.  A read from
.B data
will return the next datagram, discarding anything
that doesn't fit in the read buffer.
A write is sent as a single datagram.
.PP
By default, a UDP connection is a point-to-point link.
Either a
.B connect
establishes a local and remote address/port pair or
after an
.BR announce ,
each datagram coming from a different remote address/port pair
establishes a new incoming connection.
However, many-to-one semantics is also possible.
.PP
If, after an
.BR announce ,
one of the following messages is written to
.BR ctl ,
then all messages sent to the announced port
are received on the announced connection prefixed with the given structure.
.B headers
.EX
typedef struct Udphdr Udphdr;
struct Udphdr
{
	uchar	raddr[16];	/* v6 remote address and port */
	uchar	laddr[16];	/* v6 local address and port */
	uchar	ifcaddr[16];	/* interface address (receive only) */
	uchar	rport[2];
	uchar	lport[2];
};
.EE
.PP
The only difference in the two is the type of address, IPv4 or IPv6.
Before a write, a user must prefix a similar structure to each message.
The system overrides the user specified local port with the announced
one.  If the user specifies an address that isn't a unicast address in
.BR /net/ipselftab ,
that too is overridden.
Since the prefixed structure is the same in read and write, it is relatively
easy to write a server that responds to client requests by just copying new
data into the message body and then writing back the same buffer that was
written.
.SS RUDP
.PP
RUDP is a reliable datagram protocol based on UDP.
Packets are delivered in order.
RUDP does not support
.BR listen .
One must use either
.B connect
or
.B announce
followed immediately by
.B headers .
.PP
Unlike IL or TCP, the reboot of one end of a connection does
not force a closing of the connection.  Communications will
resume when the rebooted machine resumes talking.  Any unacknowledged
packets queued before the reboot will be lost.  A reboot can
be detected by reading the
.B err
file.  It will have the message
.IP
.BI hangup\  address ! port
.PP
where
.I address
and
.I port
are of the far side of the connection.
Retransmitting a datagram more than 10 times
is treated like a reboot:
all queued messages are dropped, an error is queued to the
.B err
file, and the conversation resumes.
.SS IL
.PP
IL is a reliable point-to-point datagram protocol.  Like TCP, IL delivers datagrams
reliably and in order. Also like TCP, a connection is
determined by the address and port numbers of the two ends.
Like UDP, each read and write transfers a single datagram.
.PP
IL is efficient for LANs but doesn't have the
congestion control features needed for use through
the Internet.
.SS GRE
.PP
GRE is the encapsulation protocol used by PPTP.
The kernel implements just enough of the protocol
to multiplex it.
.B Announce
is not allowed in GRE, only
.BR connect .
Since GRE has no port numbers, the port number in the connect
is actually the 16 bit
.B eproto
field in the GRE header.
.PP
Reads and writes transfer a
GRE datagram starting at the GRE header.
On write, the kernel fills in the
.B eproto
field with the port number specified
in the connect message.
.SS ESP
.PP
ESP is the Encapsulating Security Payload (RFC 1827).
It is used to set up an encrypted tunnel between machines.
Like GRE, ESP has no port numbers.  Instead, the
port number in the
.B connect
message is the SPI (Security Association Identifier (sic)).
IP packets are written to and read from
.BR data .
The kernel encrypts any packets written to
.BR data ,
appends a MAC, and prefixes an ESP header before
sending to the other end of the tunnel.
Received packets are checked against their MAC's,
decrypted, and queued for reading from
.BR data .
The control messages are:
.TP
.BI esp\  "alg secret
Encrypt with the algorithm,
.IR alg ,
using
.I secret
as the key.
Possible algorithms are:
.BR null ,
.BR des_56_cbc ,
and
.BR rc4_128 .
.TP
.BI ah\  "alg secret
Use the hash algorithm,
.IR alg ,
with
.I secret
as the key for generating the MAC.
Possible algorithms are:
.BR null ,
.BR hmac_sha1_96 ,
and
.BR hmac_md5_96 .
.TP
.B header
Turn on header mode.  Every buffer read from
.B data
starts with 4 unused bytes, and the first 4 bytes
of every buffer written to
.B data
are ignored.
.TP
.B noheader
Turn off header mode.
.SS "IP packet filter
.PP
The directory
.B /net/ipmux
looks like another protocol directory.
It is a packet filter built on top of IP.  Each numbered
subdirectory represents a different filter.
The connect messages written to the
.I ctl
file describe the filter. Packets matching the filter can be read on the
.B data
file.  Packets written to the
.B data
file are routed to an interface and transmitted.
.PP
A filter is a semicolon-separated list of
relations.  Each relation describes a portion
of a packet to match.  The possible relations are:
.TP
.BI proto= n
the IP protocol number must be
.IR n .
.TP
.BI dat[ n : m ]= expr
bytes
.I n
through
.I m
following the IP packet must match
.IR expr .
.TP
.BI ifc= expr
the packet must have been received on an interface whose address
matches
.IR expr .
.TP
.BI src= expr
The source address in the packet must match
.IR expr .
.TP
.BI dst= expr
The destination address in the packet must match
.IR expr .
.PP
.I Expr
is of the form:
.TP
.I \	value
.TP
.IB \	value | value | ...
.TP
.IB \	value & mask
.TP
.IB \	value | value & mask
.PP
If a mask is given, the relevant field is first ANDed with
the mask.  The result is compared against the value or list
of values for a match.  In the case of
.BR ifc ,
.BR dst ,
and
.B src
the value is a dot-formatted IP address and the mask is a dot-formatted
IP mask.  In the case of
.BR dat ,
both value and mask are strings of 2 character hexadecimal digits representing
8 bit values.
.PP
A packet is delivered to only one filter.
The filters are merged into a single comparison tree.
If two filters match the same packet, the following
rules apply in order (here '>' means is preferred to):
.IP 1)
protocol > data > source > destination > interface
.IP 2)
lower data offsets > higher data offsets
.IP 3)
longer matches > shorter matches
.IP 4)
older > younger
.PP
So far this has just been used to implement a version of
OSPF in Inferno.
.SS Statistics
.PP
The
.B stats
files are read only and contain statistics useful to network
monitoring.
.PP
Reading
.B /net/ipifc/stats
returns a list of 19 tagged and new line separated fields representing:
.EX
.ft 1
	forwarding status (0 and 2 mean forwarding off, 1 means on)
	default TTL
	input packets
	input header errors
	input address errors
	packets forwarded
	input packets for unknown protocols
	input packets discarded
	input packets delivered to higher level protocols
	output packets
	output packets discarded
	output packets with no route
	timed out fragments in reassembly queue
	requested reassemblies
	successful reassemblies
	failed reassemblies
	successful fragmentations
	unsuccessful fragmentations
	fragments created
.ft
.EE
.PP
Reading
.B /net/icmp/stats
returns a list of 25 tagged and new line separated fields representing:
.EX
.ft 1
	messages received 
	bad received messages
	unreachables received
	time exceededs received
	input parameter problems received
	source quenches received
	redirects received
	echo requests received
	echo replies received
	timestamps received
	timestamp replies received
	address mask requests received
	address mask replies received
	messages sent
	transmission errors
	unreachables sent
	time exceededs sent
	input parameter problems sent
	source quenches sent
	redirects sent
	echo requests sent
	echo replies sent
	timestamps sent
	timestamp replies sent
	address mask requests sent
	address mask replies sent
.EE
.PP
Reading
.B /net/tcp/stats
returns a list of 11 tagged and new line separated fields representing:
.EX
.ft 1
	maximum number of connections
	total outgoing calls
	total incoming calls
	number of established connections to be reset
	number of currently established connections
	segments received
	segments sent
	segments retransmitted
	retransmit timeouts
	bad received segments
	transmission failures
.EE
.PP
Reading
.B /net/udp/stats
returns a list of 4 tagged and new line separated fields representing:
.EX
.ft 1
	datagrams received
	datagrams received for bad ports
	malformed datagrams received
	datagrams sent
.EE
.PP
Reading
.B /net/il/stats
returns a list of 7 tagged and new line separated fields representing:
.EX
.ft 1
	checksum errors
	header length errors
	out of order messages
	retransmitted messages
	duplicate messages
	duplicate bytes
.EE
.PP
Reading
.B /net/gre/stats
returns a list of 1 tagged number representing:
.EX
.ft 1
	header length errors
.EE
.SH "SEE ALSO"
.IR listen (8),
.IR dial (2),
.IR ndb (6)
.SH SOURCE
.B /sys/src/9/ip
.SH BUGS
.I Ipmux
has not been heavily used and should be considered experimental.
It may disappear in favor of a more traditional packet filter in the future.