.TH IP 3
.SH NAME
ip, esp, gre, icmp, icmpv6, ipmux, rudp, tcp, udp \- network protocols over IP
.SH SYNOPSIS
.nf
.2C
.B bind -a #I\fIspec\fP /net
.sp 0.3v
.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
\&...
.sp 0.3v
.B /net/arp
.B /net/bootp
.B /net/iproute
.B /net/ipselftab
.B /net/log
.B /net/ndb
.sp 0.3v
.B /net/esp
.B /net/gre
.B /net/icmp
.B /net/icmpv6
.B /net/ipmux
.B /net/rudp
.B /net/tcp
.B /net/udp
.sp 0.3v
.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
\&...
.1C
.fi
.SH DESCRIPTION
The
.I ip
device provides the interface to Internet Protocol stacks.
.I Spec
is an integer from 0 to 15 identifying a stack.
Each stack implements IPv4 and IPv6.
Each stack is 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.
IPv4 addresses are a subset of the IPv6 addresses and both standard
.SM ASCII
formats are accepted.
In binary representation, all v4 addresses start with the 12 bytes, in hex:
.IP
.EX
00 00 00 00 00 00 00 00 00 00 ff ff
.EE
.
.SS "Configuring interfaces
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 those described under
.B "Protocol directories"
below and these:
.TF "\fLbind loopback\fR"
.PD
.
.\" from devip.c
.
.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,
.IR path !0x806
and
.IR path !0x86dd
and use the connections for IPv4, ARP and IPv6 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.
.
.\" from ipifc.c
.
.TP
.B "unbind
Disassociate the physical device from an IP interface.
.TP
.BI add\  "local [ mask remote mtu " proxy " ]
.PD 0
.TP
.BI try\  "local [ mask remote mtu " proxy " ]
.PD
Add a local IP address to the interface.
.I Try
adds the
.I local
address as a tentative address
if it's an IPv6 address.
The
.IR mask ,
.IR remote ,
.IR mtu ,
and
.B proxy
arguments are all optional.
The default
.I mask
is the class mask for the local address.
The default
.I remote
address is
.I local
ANDed with
.IR mask .
The default
.I mtu
(maximum transmission unit)
is 1514 for Ethernet and 4096 for packet media.
The
.I mtu
is the size in bytes of the largest packet that this interface can send.
.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.
.
.\" remainder from netif.c (thus called from devether.c),
.\" except add6 and ra6 from ipifc.c
.
.TP
.B bridge
Enable bridging (see
.IR bridge (3)).
.TP
.B promiscuous
Set the interface into promiscuous mode,
which makes it accept all incoming packets,
whether addressed to it or not.
.TP
.BI "connect " type
marks the Ethernet packet
.I type
as being in use, if not already in use
on this interface.
A
.I type
of -1 means `all' but appears to be a no-op.
.TP
.BI addmulti\  Media-addr
Treat the multicast
.I Media-addr
on this interface as a local address.
.TP
.BI remmulti\  Media-addr
Remove the multicast address
.I Media-addr
from this interface.
.TP
.B scanbs
Make the wireless interface scan for base stations.
.TP
.B headersonly
Set the interface to pass only packet headers, not data too.
.
.\" remainder from ipifc.c; tedious, so put them last
.
.TP
.BI "add6 " "v6addr pfx-len [onlink auto validlt preflt]"
Add the local IPv6 address
.I v6addr
with prefix length
.I pfx-len
to this interface.
See RFC 2461 §6.2.1 for more detail.
The remaining arguments are optional:
.RS
.TF "\fIonlink\fR"
.TP
.I onlink
flag: address is `on-link'
.TP
.I auto
flag: autonomous
.TP
.I validlt
valid life-time in seconds
.TP
.I preflt
preferred life-time in seconds
.RE
.PD
.TP
.BI "ra6 " "keyword value ..."
Set IPv6 router advertisement (RA) parameter
.IR keyword 's
.IR value .
Known
.IR keyword s
and the meanings of their values follow.
See RFC 2461 §6.2.1 for more detail.
Flags are true iff non-zero.
.RS
.TF "\fLreachtime\fR"
.TP
.B recvra
flag: receive and process RAs.
.TP
.B sendra
flag: generate and send RAs.
.TP
.B mflag
flag: ``Managed address configuration'',
goes into RAs.
.TP
.B oflag
flag: ``Other stateful configuration'',
goes into RAs.
.TP
.B maxraint
``maximum time allowed between sending unsolicited multicast''
RAs from the interface, in ms.
.TP
.B minraint
``minimum time allowed between sending unsolicited multicast''
RAs from the interface, in ms.
.TP
.B linkmtu
``value to be placed in MTU options sent by the router.''
Zero indicates none.
.TP
.B reachtime
sets the Reachable Time field in RAs sent by the router.
``Zero means unspecified (by this router).''
.TP
.B rxmitra
sets the Retrans Timer field in RAs sent by the router.
``Zero means unspecified (by this router).''
.TP
.B ttl
default value of the Cur Hop Limit field in RAs sent by the router.
Should be set to the ``current diameter of the Internet.''
``Zero means unspecified (by this router).''
.TP
.B routerlt
sets the Router Lifetime field of RAs sent from the interface, in ms.
Zero means the router is not to be used as a default router.
.PD
.RE
.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
.I readipifc
in
.IR ip (2).
.
.SS "Routing
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:
.TF m
.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
.PD
.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:
.TF "\fLroute \fItarget\fR"
.PD
.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.
.
.TP
.BI route\  target
Print on the console the route to address
.IR target ,
if any.
Primarily a debugging aid.
.
.SS "Address resolution
The file
.B /net/arp
controls information about address resolution.
The kernel automatically updates the v4 ARP and v6 Neighbour Discovery
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:
.TF "\fLdel \fIIP-addr\fR"
.PD
.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 .
.br
.ne 3
.
.SS "Debugging and stack information
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:
.TF "\fLclear \fIarglist\fR"
.PD
.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 icmp ,
.BR udp ,
.BR compress ,
.BR gre ,
.BR tcpwin ,
.BR tcprxmt ,
.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.
Note that the `IPv4 route' flag will never be set.
.br
.ne 3
.
.SS "Protocol directories
The
.I ip
device
supports IP as well as several protocols that run over it:
TCP, UDP, RUDP, ICMP, GRE, and ESP.
TCP and UDP provide the standard Internet
protocols for reliable stream and unreliable datagram
communication.
RUDP is a locally-developed reliable datagram protocol based on UDP.
ICMP is IP's catch-all control protocol used to send
low level error messages and to implement
.IR ping (8).
GRE is a general encapsulation protocol.
ESP is the encapsulation protocol for IPsec.
IL provided a reliable datagram service for communication
between Plan 9 machines over IPv4, but is no longer part of the system.
.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:
.TF "\fLremmulti \fIip\fR"
.PD
.TP
.BI connect\  ip-address ! port "!r " local
Establish a connection to the remote
.I ip-address
and
.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.
.\" this is gone
.\" .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 .
.TP
.B ignoreadvice
Don't break (UDP) connections because of ICMP errors.
.TP
.BI addmulti\  "ifc-ip [ mcast-ip ]"
Treat
.I ifc-ip
on this multicast interface as a local address.
If
.I mcast-ip
is present,
use it as the interface's multicast address.
.TP
.BI remmulti\  ip
Remove the address
.I ip
from this multicast interface.
.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
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:
.TF "\fLkeepalive\fI n\fR"
.PD
.TP
.BI hangup \ [direction]
close down this TCP connection. If specified, the
.I direction
can be used to perform a
.I half-duplex
close. Specifying
.B read
will close the incoming side of the connection; a
.I write
will close the outgoing side.
.TP
.BI keepalive \ n
turn on keep alive messages.
.IR N ,
if given, is the milliseconds between keepalives
(default 30000).
.TP
.BI checksum \ n
emit TCP checksums of zero if
.I n
is zero; otherwise, and by default,
TCP checksums are computed and sent normally.
.TP
.BI tcpporthogdefense \ onoff
.I onoff
of
.L on
enables the TCP port-hog defense for all TCP connections;
.I onoff
of
.L off
disables it.
The defense is a solution to hijacked systems staking out ports
as a form of denial-of-service attack.
To avoid stateless TCP conversation hogs,
.I ip
picks a TCP sequence number at random for keepalives.
If that number gets acked by the other end,
.I ip
shuts down the connection.
Some firewalls,
notably ones that perform stateful inspection,
discard such out-of-specification keepalives,
so connections through such firewalls
will be killed after five minutes
by the lack of keepalives.
.
.SS UDP
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 ,
the message
.L headers
is written to
.BR ctl ,
then all messages sent to the announced port
are received on the announced connection prefixed
with the corresponding structure,
declared in
.BR <ip.h> :
.IP
.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];	/* V6 interface address (receive only) */
	uchar	rport[2];	/* remote port */
	uchar	lport[2];	/* local port */
};
.EE
.PP
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
read.
.PP
In this case (writing
.L headers
to the
.I ctl
file),
no
.I listen
nor
.I accept
is needed;
otherwise,
the usual sequence of
.IR announce ,
.IR listen ,
.I accept
must be executed before performing I/O on the corresponding
.I data
file.
.
.SS RUDP
RUDP is a reliable datagram protocol based on UDP,
currently only for IPv4.
Packets are delivered in order.
RUDP does not support
.BR listen .
One must write either
.L connect
or
.L announce
followed immediately by
.L headers
to
.BR ctl .
.PP
Unlike 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 contain 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.
.PP
RUDP
.I ctl
files accept the following messages:
.TF "\fLranddrop \fI[ percent ]\fR"
.TP
.B headers
Corresponds to the
.L headers
format of UDP.
.TP
.BI "hangup " "IP port"
Drop the connection to address
.I IP
and
.IR port .
.TP
.BI "randdrop " "[ percent ]"
Randomly drop
.I percent
of outgoing packets.
Default is 10%.
.
.SS ICMP
ICMP is a datagram protocol for IPv4 used to exchange control requests and
their responses with other machines' IP implementations.
ICMP is primarily a kernel-to-kernel protocol, but it is possible
to generate `echo request' and read `echo reply' packets from user programs.
.
.SS ICMPV6
ICMPv6 is the IPv6 equivalent of ICMP.
If, after an
.BR announce ,
the message
.L headers
is written to
.BR ctl ,
then before a write,
a user must prefix each message with a corresponding structure,
declared in
.BR <ip.h> :
.IP
.EX
/*
 *  user level icmpv6 with control message "headers"
 */
typedef struct Icmp6hdr Icmp6hdr;
struct Icmp6hdr {
	uchar	unused[8];
	uchar	laddr[IPaddrlen];	/* local address */
	uchar	raddr[IPaddrlen];	/* remote address */
};
.EE
.PP
In this case (writing
.L headers
to the
.I ctl
file),
no
.I listen
nor
.I accept
is needed;
otherwise,
the usual sequence of
.IR announce ,
.IR listen ,
.I accept
must be executed before performing I/O on the corresponding
.I data
file.
.
.SS GRE
GRE is the encapsulation protocol used by PPTP.
The kernel implements just enough of the protocol
to multiplex it.
Our implementation encapsulates in IPv4, per RFC 1702.
.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.
.br
.ne 3
.
.SS ESP
ESP is the Encapsulating Security Payload (RFC 1827, obsoleted by RFC 4303)
for IPsec (RFC 4301).
We currently implement only tunnel mode, not transport mode.
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 .
In the following,
.I secret
is the hexadecimal encoding of a key,
without a leading
.LR 0x .
The control messages are:
.TF "\fLesp \fIalg secret\fR"
.PD
.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 ,
.BR des3_cbc ,
and eventually
.BR aes_128_cbc ,
and
.BR aes_ctr .
.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 ,
.BR hmac_md5_96 ,
and eventually
.BR aes_xcbc_mac_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
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:
.TF "\fLdata[\fIn\fL:\fIm\fL]=\fIexpr\fR "
.PD
.TP
.BI proto= n
the IP protocol number must be
.IR n .
.TP
.BI data[ n : m ]= expr
bytes
.I n
through
.I m
following the IP packet must match
.IR expr .
.TP
.BI iph[ n : m ]= expr
bytes
.I n
through
.I m
of the IP packet header 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 data ,
.B iph
and
.BR proto ,
both value and mask are strings of 2 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
and 6to4 tunnelling.
.br
.ne 5
.
.SS Statistics
The
.B stats
files are read only and contain statistics useful to network monitoring.
.br
.ne 12
.PP
Reading
.B /net/ipifc/stats
returns a list of 19 tagged and newline-separated fields representing:
.EX
.ft 1
.2C
.in +0.25i
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
.in -0.25i
.1C
.ft
.EE
.br
.ne 16
.PP
Reading
.B /net/icmp/stats
returns a list of 26 tagged and newline-separated fields representing:
.EX
.ft 1
.2C
.in +0.25i
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
.in -0.25i
.1C
.EE
.PP
Reading
.B /net/tcp/stats
returns a list of 11 tagged and newline-separated fields representing:
.EX
.ft 1
.2C
.in +0.25i
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
.in -0.25i
.1C
.EE
.PP
Reading
.B /net/udp/stats
returns a list of 4 tagged and newline-separated fields representing:
.EX
.ft 1
.2C
.in +0.25i
datagrams received
datagrams received for bad ports
malformed datagrams received
datagrams sent
.in -0.25i
.1C
.EE
.PP
Reading
.B /net/gre/stats
returns a list of 1 tagged number representing:
.EX
.ft 1
.in +0.25i
header length errors
.in -0.25i
.EE
.SH "SEE ALSO"
.IR dial (2),
.IR ip (2),
.IR bridge (3),
.\" .IR ike (4),
.IR ndb (6),
.IR listen (8)
.br
.PD 0
.TF "\fL/lib/rfc/rfc2822"
.TP
.B /lib/rfc/rfc2460
IPv6
.TP
.B /lib/rfc/rfc4291
IPv6 address architecture
.TP
.B /lib/rfc/rfc4443
ICMPv6
.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.