09e9a903df
is not called, and no static rules match an outgoing packet, the latter retains its source IP address. This is in support of the "static NAT only" mode.
985 lines
32 KiB
Groff
985 lines
32 KiB
Groff
.\"-
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.\" Copyright (c) 2001 Charles Mott <cm@linktel.net>
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.\" All rights reserved.
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.\"
|
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.\" Redistribution and use in source and binary forms, with or without
|
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.\" modification, are permitted provided that the following conditions
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.\" are met:
|
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.\" 1. Redistributions of source code must retain the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer.
|
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.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
|
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
|
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
|
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
|
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
|
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
|
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
|
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
|
|
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
|
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
|
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
|
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.Dd April 13, 2000
|
|
.Dt LIBALIAS 3
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|
.Os
|
|
.Sh NAME
|
|
.Nm libalias
|
|
.Nd packet aliasing library for masquerading and network address translation
|
|
.Sh SYNOPSIS
|
|
.In sys/types.h
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|
.In netinet/in.h
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|
.In alias.h
|
|
.Pp
|
|
Function prototypes are given in the main body of the text.
|
|
.Sh DESCRIPTION
|
|
The
|
|
.Nm
|
|
library is a collection of functions for aliasing and de-aliasing of IP
|
|
packets, intended for masquerading and network address translation (NAT).
|
|
.Sh INTRODUCTION
|
|
This library is a moderately portable set of functions designed to assist
|
|
in the process of IP masquerading and network address translation.
|
|
Outgoing packets from a local network with unregistered IP addresses can
|
|
be aliased to appear as if they came from an accessible IP address.
|
|
Incoming packets are then de-aliased so that they are sent to the correct
|
|
machine on the local network.
|
|
.Pp
|
|
A certain amount of flexibility is built into the packet aliasing engine.
|
|
In the simplest mode of operation, a many-to-one address mapping takes
|
|
place between local network and the packet aliasing host.
|
|
This is known as IP masquerading.
|
|
In addition, one-to-one mappings between local and public addresses can
|
|
also be implemented, which is known as static NAT.
|
|
In between these extremes, different groups of private addresses can be
|
|
linked to different public addresses, comprising several distinct
|
|
many-to-one mappings.
|
|
Also, a given public address and port can be statically redirected to a
|
|
private address/port.
|
|
.Pp
|
|
The packet aliasing engine was designed to operate in user space outside
|
|
of the kernel, without any access to private kernel data structure, but
|
|
the source code can also be ported to a kernel environment.
|
|
.Sh INITIALIZATION AND CONTROL
|
|
One special function,
|
|
.Fn PacketAliasInit ,
|
|
must always be called before any packet handling may be performed.
|
|
Normally, the
|
|
.Fn PacketAliasSetAddress
|
|
function is called afterwards, to set the default aliasing address.
|
|
In addition, the operating mode of the packet aliasing engine can be
|
|
customized by calling
|
|
.Fn PacketAliasSetMode .
|
|
.Pp
|
|
.Ft void
|
|
.Fn PacketAliasInit void
|
|
.Bd -ragged -offset indent
|
|
This function has no arguments or return value and is used to initialize
|
|
internal data structures.
|
|
The following mode bits are always set after calling
|
|
.Fn PacketAliasInit .
|
|
See the description of
|
|
.Fn PacketAliasSetMode
|
|
below for the meaning of these mode bits.
|
|
.Pp
|
|
.Bl -item -offset indent -compact
|
|
.It
|
|
.Dv PKT_ALIAS_SAME_PORTS
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|
.It
|
|
.Dv PKT_ALIAS_USE_SOCKETS
|
|
.It
|
|
.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
|
|
.El
|
|
.Pp
|
|
This function will always return the packet aliasing engine to the same
|
|
initial state.
|
|
.Fn PacketAliasSetAddress
|
|
is normally called afterwards, and any desired changes from the default mode
|
|
bits listed above require a call to
|
|
.Fn PacketAliasSetMode .
|
|
.Pp
|
|
It is mandatory that this function be called at the beginning of a program
|
|
prior to any packet handling.
|
|
.Ed
|
|
.Pp
|
|
.Ft void
|
|
.Fn PacketAliasUninit void
|
|
.Bd -ragged -offset indent
|
|
This function has no arguments or return value and is used to clear any
|
|
resources attached to internal data structures.
|
|
.Pp
|
|
This functions should be called when a program stops using the aliasing
|
|
engine; it does, amongst other things, clear out any firewall holes.
|
|
To provide backwards compatibility and extra security, it is added to
|
|
the
|
|
.Xr atexit 3
|
|
chain by
|
|
.Fn PacketAliasInit .
|
|
Calling it multiple times is harmless.
|
|
.Ed
|
|
.Pp
|
|
.Ft void
|
|
.Fn PacketAliasSetAddress "struct in_addr addr"
|
|
.Bd -ragged -offset indent
|
|
This function sets the source address to which outgoing packets from the
|
|
local area network are aliased.
|
|
All outgoing packets are re-mapped to this address unless overridden by a
|
|
static address mapping established by
|
|
.Fn PacketAliasRedirectAddr .
|
|
If this function is not called, and no static rules match, an outgoing
|
|
packet retains its source address.
|
|
.Pp
|
|
If the
|
|
.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
|
|
mode bit is set (the default mode of operation), then the internal aliasing
|
|
link tables will be reset any time the aliasing address changes.
|
|
This is useful for interfaces such as
|
|
.Xr ppp 8 ,
|
|
where the IP
|
|
address may or may not change on successive dial-up attempts.
|
|
.Pp
|
|
If the
|
|
.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
|
|
mode bit is set to zero, this function can also be used to dynamically change
|
|
the aliasing address on a packet to packet basis (it is a low overhead call).
|
|
.Pp
|
|
It is mandatory that this function be called prior to any packet handling.
|
|
.Ed
|
|
.Pp
|
|
.Ft unsigned int
|
|
.Fn PacketAliasSetMode "unsigned int flags" "unsigned int mask"
|
|
.Bd -ragged -offset indent
|
|
This function sets or clears mode bits
|
|
according to the value of
|
|
.Fa flags .
|
|
Only bits marked in
|
|
.Fa mask
|
|
are affected.
|
|
The following mode bits are defined in
|
|
.Aq Pa alias.h :
|
|
.Bl -tag -width indent
|
|
.It Dv PKT_ALIAS_LOG
|
|
Enables logging into
|
|
.Pa /var/log/alias.log .
|
|
Each time an aliasing link is created or deleted, the log file is appended
|
|
with the current number of ICMP, TCP and UDP links.
|
|
Mainly useful for debugging when the log file is viewed continuously with
|
|
.Xr tail 1 .
|
|
.It Dv PKT_ALIAS_DENY_INCOMING
|
|
If this mode bit is set, all incoming packets associated with new TCP
|
|
connections or new UDP transactions will be marked for being ignored
|
|
.Fn ( PacketAliasIn
|
|
returns
|
|
.Dv PKT_ALIAS_IGNORED
|
|
code)
|
|
by the calling program.
|
|
Response packets to connections or transactions initiated from the packet
|
|
aliasing host or local network will be unaffected.
|
|
This mode bit is useful for implementing a one-way firewall.
|
|
.It Dv PKT_ALIAS_SAME_PORTS
|
|
If this mode bit is set, the packet aliasing engine will attempt to leave
|
|
the alias port numbers unchanged from the actual local port numbers.
|
|
This can be done as long as the quintuple (proto, alias addr, alias port,
|
|
remote addr, remote port) is unique.
|
|
If a conflict exists, a new aliasing port number is chosen even if this
|
|
mode bit is set.
|
|
.It Dv PKT_ALIAS_USE_SOCKETS
|
|
This bit should be set when the packet aliasing host originates network
|
|
traffic as well as forwards it.
|
|
When the packet aliasing host is waiting for a connection from an unknown
|
|
host address or unknown port number (e.g. an FTP data connection), this
|
|
mode bit specifies that a socket be allocated as a place holder to prevent
|
|
port conflicts.
|
|
Once a connection is established, usually within a minute or so, the socket
|
|
is closed.
|
|
.It Dv PKT_ALIAS_UNREGISTERED_ONLY
|
|
If this mode bit is set, traffic on the local network which does not
|
|
originate from unregistered address spaces will be ignored.
|
|
Standard Class A, B and C unregistered addresses are:
|
|
.Bd -literal -offset indent
|
|
10.0.0.0 -> 10.255.255.255 (Class A subnet)
|
|
172.16.0.0 -> 172.31.255.255 (Class B subnets)
|
|
192.168.0.0 -> 192.168.255.255 (Class C subnets)
|
|
.Ed
|
|
.Pp
|
|
This option is useful in the case that packet aliasing host has both
|
|
registered and unregistered subnets on different interfaces.
|
|
The registered subnet is fully accessible to the outside world, so traffic
|
|
from it does not need to be passed through the packet aliasing engine.
|
|
.It Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
|
|
When this mode bit is set and
|
|
.Fn PacketAliasSetAddress
|
|
is called to change the aliasing address, the internal link table of the
|
|
packet aliasing engine will be cleared.
|
|
This operating mode is useful for
|
|
.Xr ppp 8
|
|
links where the interface address can sometimes change or remain the same
|
|
between dial-up attempts.
|
|
If this mode bit is not set, the link table will never be reset in the event
|
|
of an address change.
|
|
.It Dv PKT_ALIAS_PUNCH_FW
|
|
This option makes
|
|
.Nm
|
|
`punch holes' in an
|
|
.Xr ipfirewall 4
|
|
based firewall for FTP/IRC DCC connections.
|
|
The holes punched are bound by from/to IP address and port; it will not be
|
|
possible to use a hole for another connection.
|
|
A hole is removed when the connection that uses it dies.
|
|
To cater to unexpected death of a program using
|
|
.Nm
|
|
(e.g. kill -9),
|
|
changing the state of the flag will clear the entire firewall range
|
|
allocated for holes.
|
|
This will also happen on the initial call to
|
|
.Fn PacketAliasSetFWBase .
|
|
This call must happen prior to setting this flag.
|
|
.It Dv PKT_ALIAS_REVERSE
|
|
This option makes
|
|
.Nm
|
|
reverse the way it handles incoming and outgoing packets, allowing it
|
|
to be fed with data that passes through the internal interface rather
|
|
than the external one.
|
|
.It Dv PKT_ALIAS_PROXY_ONLY
|
|
This option tells
|
|
.Nm
|
|
to obey transparent proxy rules only.
|
|
Normal packet aliasing is not performed.
|
|
See
|
|
.Fn PacketAliasProxyRule
|
|
below for details.
|
|
.El
|
|
.Ed
|
|
.Pp
|
|
.Ft void
|
|
.Fn PacketAliasSetFWBase "unsigned int base" "unsigned int num"
|
|
.Bd -ragged -offset indent
|
|
Set firewall range allocated for punching firewall holes (with the
|
|
.Dv PKT_ALIAS_PUNCH_FW
|
|
flag).
|
|
The range will be cleared for all rules on initialization.
|
|
.Ed
|
|
.Sh PACKET HANDLING
|
|
The packet handling functions are used to modify incoming (remote to local)
|
|
and outgoing (local to remote) packets.
|
|
The calling program is responsible for receiving and sending packets via
|
|
network interfaces.
|
|
.Pp
|
|
Along with
|
|
.Fn PacketAliasInit
|
|
and
|
|
.Fn PacketAliasSetAddress ,
|
|
the two packet handling functions,
|
|
.Fn PacketAliasIn
|
|
and
|
|
.Fn PacketAliasOut ,
|
|
comprise minimal set of functions needed for a basic IP masquerading
|
|
implementation.
|
|
.Pp
|
|
.Ft int
|
|
.Fn PacketAliasIn "char *buffer" "int maxpacketsize"
|
|
.Bd -ragged -offset indent
|
|
An incoming packet coming from a remote machine to the local network is
|
|
de-aliased by this function.
|
|
The IP packet is pointed to by
|
|
.Fa buffer ,
|
|
and
|
|
.Fa maxpacketsize
|
|
indicates the size of the data structure containing the packet and should
|
|
be at least as large as the actual packet size.
|
|
.Pp
|
|
Return codes:
|
|
.Bl -tag -width indent
|
|
.It Dv PKT_ALIAS_OK
|
|
The packet aliasing process was successful.
|
|
.It Dv PKT_ALIAS_IGNORED
|
|
The packet was ignored and not de-aliased.
|
|
This can happen if the protocol is unrecognized, possibly an ICMP message
|
|
type is not handled or if incoming packets for new connections are being
|
|
ignored (if
|
|
.Dv PKT_ALIAS_DENY_INCOMING
|
|
mode bit was set by
|
|
.Fn PacketAliasSetMode ) .
|
|
.It Dv PKT_ALIAS_UNRESOLVED_FRAGMENT
|
|
This is returned when a fragment cannot be resolved because the header
|
|
fragment has not been sent yet.
|
|
In this situation, fragments must be saved with
|
|
.Fn PacketAliasSaveFragment
|
|
until a header fragment is found.
|
|
.It Dv PKT_ALIAS_FOUND_HEADER_FRAGMENT
|
|
The packet aliasing process was successful, and a header fragment was found.
|
|
This is a signal to retrieve any unresolved fragments with
|
|
.Fn PacketAliasGetFragment
|
|
and de-alias them with
|
|
.Fn PacketAliasFragmentIn .
|
|
.It Dv PKT_ALIAS_ERROR
|
|
An internal error within the packet aliasing engine occurred.
|
|
.El
|
|
.Ed
|
|
.Pp
|
|
.Ft int
|
|
.Fn PacketAliasOut "char *buffer" "int maxpacketsize"
|
|
.Bd -ragged -offset indent
|
|
An outgoing packet coming from the local network to a remote machine is
|
|
aliased by this function.
|
|
The IP packet is pointed to by
|
|
.Fa buffer ,
|
|
and
|
|
.Fa maxpacketsize
|
|
indicates the maximum packet size permissible should the packet length be
|
|
changed.
|
|
IP encoding protocols place address and port information in the encapsulated
|
|
data stream which has to be modified and can account for changes in packet
|
|
length.
|
|
Well known examples of such protocols are FTP and IRC DCC.
|
|
.Pp
|
|
Return codes:
|
|
.Bl -tag -width indent
|
|
.It Dv PKT_ALIAS_OK
|
|
The packet aliasing process was successful.
|
|
.It Dv PKT_ALIAS_IGNORED
|
|
The packet was ignored and not aliased.
|
|
This can happen if the protocol is unrecognized, or possibly an ICMP message
|
|
type is not handled.
|
|
.It Dv PKT_ALIAS_ERROR
|
|
An internal error within the packet aliasing engine occurred.
|
|
.El
|
|
.Ed
|
|
.Sh PORT AND ADDRESS REDIRECTION
|
|
The functions described in this section allow machines on the local network
|
|
to be accessible in some degree to new incoming connections from the external
|
|
network.
|
|
Individual ports can be re-mapped or static network address translations can
|
|
be designated.
|
|
.Pp
|
|
.Ft struct alias_link *
|
|
.Fo PacketAliasRedirectPort
|
|
.Fa "struct in_addr local_addr"
|
|
.Fa "u_short local_port"
|
|
.Fa "struct in_addr remote_addr"
|
|
.Fa "u_short remote_port"
|
|
.Fa "struct in_addr alias_addr"
|
|
.Fa "u_short alias_port"
|
|
.Fa "u_char proto"
|
|
.Fc
|
|
.Bd -ragged -offset indent
|
|
This function specifies that traffic from a given remote address/port to
|
|
an alias address/port be redirected to a specified local address/port.
|
|
The parameter
|
|
.Fa proto
|
|
can be either
|
|
.Dv IPPROTO_TCP
|
|
or
|
|
.Dv IPPROTO_UDP ,
|
|
as defined in
|
|
.Aq Pa netinet/in.h .
|
|
.Pp
|
|
If
|
|
.Fa local_addr
|
|
or
|
|
.Fa alias_addr
|
|
is zero, this indicates that the packet aliasing address as established
|
|
by
|
|
.Fn PacketAliasSetAddress
|
|
is to be used.
|
|
Even if
|
|
.Fn PacketAliasSetAddress
|
|
is called to change the address after
|
|
.Fn PacketAliasRedirectPort
|
|
is called, a zero reference will track this change.
|
|
.Pp
|
|
If the link is further set up to operate for a load sharing, then
|
|
.Fa local_addr
|
|
and
|
|
.Fa local_port
|
|
are ignored, and are selected dynamically from the server pool, as described in
|
|
.Fn PacketAliasAddServer
|
|
below.
|
|
.Pp
|
|
If
|
|
.Fa remote_addr
|
|
is zero, this indicates to redirect packets from any remote address.
|
|
Likewise, if
|
|
.Fa remote_port
|
|
is zero, this indicates to redirect packets originating from any remote
|
|
port number.
|
|
Almost always, the remote port specification will be zero, but non-zero
|
|
remote addresses can sometimes be useful for firewalling.
|
|
If two calls to
|
|
.Fn PacketAliasRedirectPort
|
|
overlap in their address/port specifications, then the most recent call
|
|
will have precedence.
|
|
.Pp
|
|
This function returns a pointer which can subsequently be used by
|
|
.Fn PacketAliasRedirectDelete .
|
|
If
|
|
.Dv NULL
|
|
is returned, then the function call did not complete successfully.
|
|
.Pp
|
|
All port numbers should be in network address byte order, so it is necessary
|
|
to use
|
|
.Xr htons 3
|
|
to convert these parameters from internally readable numbers to network byte
|
|
order.
|
|
Addresses are also in network byte order, which is implicit in the use of the
|
|
.Fa struct in_addr
|
|
data type.
|
|
.Ed
|
|
.Pp
|
|
.Ft struct alias_link *
|
|
.Fo PacketAliasRedirectAddr
|
|
.Fa "struct in_addr local_addr"
|
|
.Fa "struct in_addr alias_addr"
|
|
.Fc
|
|
.Bd -ragged -offset indent
|
|
This function designates that all incoming traffic to
|
|
.Fa alias_addr
|
|
be redirected to
|
|
.Fa local_addr .
|
|
Similarly, all outgoing traffic from
|
|
.Fa local_addr
|
|
is aliased to
|
|
.Fa alias_addr .
|
|
.Pp
|
|
If
|
|
.Fa local_addr
|
|
or
|
|
.Fa alias_addr
|
|
is zero, this indicates that the packet aliasing address as established by
|
|
.Fn PacketAliasSetAddress
|
|
is to be used.
|
|
Even if
|
|
.Fn PacketAliasSetAddress
|
|
is called to change the address after
|
|
.Fn PacketAliasRedirectAddr
|
|
is called, a zero reference will track this change.
|
|
.Pp
|
|
If the link is further set up to operate for a load sharing, then
|
|
.Fa local_addr
|
|
is ignored, and is selected dynamically from the server pool, as described in
|
|
.Fn PacketAliasAddServer
|
|
below.
|
|
.Pp
|
|
If subsequent calls to
|
|
.Fn PacketAliasRedirectAddr
|
|
use the same aliasing address, all new incoming traffic to this aliasing
|
|
address will be redirected to the local address made in the last function
|
|
call.
|
|
New traffic generated by any of the local machines, designated in the
|
|
several function calls, will be aliased to the same address.
|
|
Consider the following example:
|
|
.Bd -literal -offset indent
|
|
PacketAliasRedirectAddr(inet_aton("192.168.0.2"),
|
|
inet_aton("141.221.254.101"));
|
|
PacketAliasRedirectAddr(inet_aton("192.168.0.3"),
|
|
inet_aton("141.221.254.101"));
|
|
PacketAliasRedirectAddr(inet_aton("192.168.0.4"),
|
|
inet_aton("141.221.254.101"));
|
|
.Ed
|
|
.Pp
|
|
Any outgoing connections such as
|
|
.Xr telnet 1
|
|
or
|
|
.Xr ftp 1
|
|
from 192.168.0.2, 192.168.0.3 and 192.168.0.4 will appear to come from
|
|
141.221.254.101.
|
|
Any incoming connections to 141.221.254.101 will be directed to 192.168.0.4.
|
|
.Pp
|
|
Any calls to
|
|
.Fn PacketAliasRedirectPort
|
|
will have precedence over address mappings designated by
|
|
.Fn PacketAliasRedirectAddr .
|
|
.Pp
|
|
This function returns a pointer which can subsequently be used by
|
|
.Fn PacketAliasRedirectDelete .
|
|
If
|
|
.Dv NULL
|
|
is returned, then the function call did not complete successfully.
|
|
.Ed
|
|
.Pp
|
|
.Ft int
|
|
.Fo PacketAliasAddServer
|
|
.Fa "struct alias_link *link"
|
|
.Fa "struct in_addr addr"
|
|
.Fa "u_short port"
|
|
.Fc
|
|
.Bd -ragged -offset indent
|
|
This function sets the
|
|
.Fa link
|
|
up for Load Sharing using IP Network Address Translation (RFC 2391, LSNAT).
|
|
LSNAT operates as follows.
|
|
A client attempts to access a server by using the server virtual address.
|
|
The LSNAT router transparently redirects the request to one of the hosts
|
|
in server pool, selected using a real-time load sharing algorithm.
|
|
Multiple sessions may be initiated from the same client, and each session
|
|
could be directed to a different host based on load balance across server
|
|
pool hosts at the time.
|
|
If load share is desired for just a few specific services, the configuration
|
|
on LSNAT could be defined to restrict load share for just the services
|
|
desired.
|
|
.Pp
|
|
Currently, only the simplest selection algorithm is implemented, where a
|
|
host is selected on a round-robin basis only, without regard to load on
|
|
the host.
|
|
.Pp
|
|
First, the
|
|
.Fa link
|
|
is created by either
|
|
.Fn PacketAliasRedirectPort
|
|
or
|
|
.Fn PacketAliasRedirectAddr .
|
|
Then,
|
|
.Fn PacketAliasAddServer
|
|
is called multiple times to add entries to the
|
|
.Fa link Ns 's
|
|
server pool.
|
|
.Pp
|
|
For links created with
|
|
.Fn PacketAliasRedirectAddr ,
|
|
the
|
|
.Fa port
|
|
argument is ignored and could have any value, e.g. htons(~0).
|
|
.Pp
|
|
This function returns 0 on success, -1 otherwise.
|
|
.Ed
|
|
.Pp
|
|
.Ft void
|
|
.Fn PacketAliasRedirectDelete "struct alias_link *link"
|
|
.Bd -ragged -offset indent
|
|
This function will delete a specific static redirect rule entered by
|
|
.Fn PacketAliasRedirectPort
|
|
or
|
|
.Fn PacketAliasRedirectAddr .
|
|
The parameter
|
|
.Fa link
|
|
is the pointer returned by either of the redirection functions.
|
|
If an invalid pointer is passed to
|
|
.Fn PacketAliasRedirectDelete ,
|
|
then a program crash or unpredictable operation could result, so it is
|
|
necessary to be careful using this function.
|
|
.Ed
|
|
.Pp
|
|
.Ft int
|
|
.Fn PacketAliasProxyRule "const char *cmd"
|
|
.Bd -ragged -offset indent
|
|
The passed
|
|
.Fa cmd
|
|
string consists of one or more pairs of words.
|
|
The first word in each pair is a token and the second is the value that
|
|
should be applied for that token.
|
|
Tokens and their argument types are as follows:
|
|
.Bl -tag -width indent
|
|
.It Cm type encode_ip_hdr | encode_tcp_stream | no_encode
|
|
In order to support transparent proxying, it is necessary to somehow
|
|
pass the original address and port information into the new destination
|
|
server.
|
|
If
|
|
.Cm encode_ip_hdr
|
|
is specified, the original address and port is passed as an extra IP
|
|
option.
|
|
If
|
|
.Cm encode_tcp_stream
|
|
is specified, the original address and port is passed as the first
|
|
piece of data in the TCP stream in the format
|
|
.Dq DEST Ar IP port .
|
|
.It Cm port Ar portnum
|
|
Only packets with the destination port
|
|
.Ar portnum
|
|
are proxied.
|
|
.It Cm server Ar host Ns Xo
|
|
.Op : Ns Ar portnum
|
|
.Xc
|
|
This specifies the
|
|
.Ar host
|
|
and
|
|
.Ar portnum
|
|
that the data is to be redirected to.
|
|
.Ar host
|
|
must be an IP address rather than a DNS host name.
|
|
If
|
|
.Ar portnum
|
|
is not specified, the destination port number is not changed.
|
|
.Pp
|
|
The
|
|
.Ar server
|
|
specification is mandatory unless the
|
|
.Cm delete
|
|
command is being used.
|
|
.It Cm rule Ar index
|
|
Normally, each call to
|
|
.Fn PacketAliasProxyRule
|
|
inserts the next rule at the start of a linear list of rules.
|
|
If an
|
|
.Ar index
|
|
is specified, the new rule will be checked after all rules with lower
|
|
indices.
|
|
Calls to
|
|
.Fn PacketAliasProxyRule
|
|
that do not specify a rule are assigned rule 0.
|
|
.It Cm delete Ar index
|
|
This token and its argument MUST NOT be used with any other tokens.
|
|
When used, all existing rules with the given
|
|
.Ar index
|
|
are deleted.
|
|
.It Cm proto tcp | udp
|
|
If specified, only packets of the given protocol type are matched.
|
|
.It Cm src Ar IP Ns Xo
|
|
.Op / Ns Ar bits
|
|
.Xc
|
|
If specified, only packets with a source address matching the given
|
|
.Ar IP
|
|
are matched.
|
|
If
|
|
.Ar bits
|
|
is also specified, then the first
|
|
.Ar bits
|
|
bits of
|
|
.Ar IP
|
|
are taken as a network specification, and all IP addresses from that
|
|
network will be matched.
|
|
.It Cm dst Ar IP Ns Xo
|
|
.Op / Ns Ar bits
|
|
.Xc
|
|
If specified, only packets with a destination address matching the given
|
|
.Ar IP
|
|
are matched.
|
|
If
|
|
.Ar bits
|
|
is also specified, then the first
|
|
.Ar bits
|
|
bits of
|
|
.Ar IP
|
|
are taken as a network specification, and all IP addresses from that
|
|
network will be matched.
|
|
.El
|
|
.Pp
|
|
This function is usually used to redirect outgoing connections for
|
|
internal machines that are not permitted certain types of internet
|
|
access, or to restrict access to certain external machines.
|
|
.Ed
|
|
.Pp
|
|
.Ft struct alias_link *
|
|
.Fo PacketAliasRedirectProto
|
|
.Fa "struct in_addr local_addr"
|
|
.Fa "struct in_addr remote_addr"
|
|
.Fa "struct in_addr alias_addr"
|
|
.Fa "u_char proto"
|
|
.Fc
|
|
.Bd -ragged -offset indent
|
|
This function specifies that any IP packet with protocol number of
|
|
.Fa proto
|
|
from a given remote address to an alias address be
|
|
redirected to a specified local address.
|
|
.Pp
|
|
If
|
|
.Fa local_addr
|
|
or
|
|
.Fa alias_addr
|
|
is zero, this indicates that the packet aliasing address as established
|
|
by
|
|
.Fn PacketAliasSetAddress
|
|
is to be used.
|
|
Even if
|
|
.Fn PacketAliasSetAddress
|
|
is called to change the address after
|
|
.Fn PacketAliasRedirectProto
|
|
is called, a zero reference will track this change.
|
|
.Pp
|
|
If
|
|
.Fa remote_addr
|
|
is zero, this indicates to redirect packets from any remote address.
|
|
Non-zero remote addresses can sometimes be useful for firewalling.
|
|
.Pp
|
|
If two calls to
|
|
.Fn PacketAliasRedirectProto
|
|
overlap in their address specifications, then the most recent call
|
|
will have precedence.
|
|
.Pp
|
|
This function returns a pointer which can subsequently be used by
|
|
.Fn PacketAliasRedirectDelete .
|
|
If
|
|
.Dv NULL
|
|
is returned, then the function call did not complete successfully.
|
|
.Ed
|
|
.Sh FRAGMENT HANDLING
|
|
The functions in this section are used to deal with incoming fragments.
|
|
.Pp
|
|
Outgoing fragments are handled within
|
|
.Fn PacketAliasOut
|
|
by changing the address according to any applicable mapping set by
|
|
.Fn PacketAliasRedirectAddr ,
|
|
or the default aliasing address set by
|
|
.Fn PacketAliasSetAddress .
|
|
.Pp
|
|
Incoming fragments are handled in one of two ways.
|
|
If the header of a fragmented IP packet has already been seen, then all
|
|
subsequent fragments will be re-mapped in the same manner the header
|
|
fragment was.
|
|
Fragments which arrive before the header are saved and then retrieved
|
|
once the header fragment has been resolved.
|
|
.Pp
|
|
.Ft int
|
|
.Fn PacketAliasSaveFragment "char *ptr"
|
|
.Bd -ragged -offset indent
|
|
When
|
|
.Fn PacketAliasIn
|
|
returns
|
|
.Dv PKT_ALIAS_UNRESOLVED_FRAGMENT ,
|
|
this function can be used to save the pointer to the unresolved fragment.
|
|
.Pp
|
|
It is implicitly assumed that
|
|
.Fa ptr
|
|
points to a block of memory allocated by
|
|
.Xr malloc 3 .
|
|
If the fragment is never resolved, the packet aliasing engine will
|
|
automatically free the memory after a timeout period.
|
|
[Eventually this function should be modified so that a callback function
|
|
for freeing memory is passed as an argument.]
|
|
.Pp
|
|
This function returns
|
|
.Dv PKT_ALIAS_OK
|
|
if it was successful and
|
|
.Dv PKT_ALIAS_ERROR
|
|
if there was an error.
|
|
.Ed
|
|
.Pp
|
|
.Ft char *
|
|
.Fn PacketAliasGetFragment "char *buffer"
|
|
.Bd -ragged -offset indent
|
|
This function can be used to retrieve fragment pointers saved by
|
|
.Fn PacketAliasSaveFragment .
|
|
The IP header fragment pointed to by
|
|
.Fa buffer
|
|
is the header fragment indicated when
|
|
.Fn PacketAliasIn
|
|
returns
|
|
.Dv PKT_ALIAS_FOUND_HEADER_FRAGMENT .
|
|
Once a fragment pointer is retrieved, it becomes the calling program's
|
|
responsibility to free the dynamically allocated memory for the fragment.
|
|
.Pp
|
|
.Fn PacketAliasGetFragment
|
|
can be called sequentially until there are no more fragments available,
|
|
at which time it returns
|
|
.Dv NULL .
|
|
.Ed
|
|
.Pp
|
|
.Ft void
|
|
.Fn PacketAliasFragmentIn "char *header" "char *fragment"
|
|
.Bd -ragged -offset indent
|
|
When a fragment is retrieved with
|
|
.Fn PacketAliasGetFragment ,
|
|
it can then be de-aliased with a call to
|
|
.Fn PacketAliasFragmentIn .
|
|
The
|
|
.Fa header
|
|
argument is the pointer to a header fragment used as a template, and
|
|
.Fa fragment
|
|
is the pointer to the packet to be de-aliased.
|
|
.Ed
|
|
.Sh MISCELLANEOUS FUNCTIONS
|
|
.Ft void
|
|
.Fn PacketAliasSetTarget "struct in_addr addr"
|
|
.Bd -ragged -offset indent
|
|
When an incoming packet not associated with any pre-existing aliasing link
|
|
arrives at the host machine, it will be sent to the address indicated by a
|
|
call to
|
|
.Fn PacketAliasSetTarget .
|
|
.Pp
|
|
If this function is called with an
|
|
.Dv INADDR_NONE
|
|
address argument, then all new incoming packets go to the address set by
|
|
.Fn PacketAliasSetAddress .
|
|
.Pp
|
|
If this function is not called, or is called with an
|
|
.Dv INADDR_ANY
|
|
address argument, then all new incoming packets go to the address specified
|
|
in the packet.
|
|
This allows external machines to talk directly to internal machines if they
|
|
can route packets to the machine in question.
|
|
.Ed
|
|
.Pp
|
|
.Ft int
|
|
.Fn PacketAliasCheckNewLink void
|
|
.Bd -ragged -offset indent
|
|
This function returns a non-zero value when a new aliasing link is created.
|
|
In circumstances where incoming traffic is being sequentially sent to
|
|
different local servers, this function can be used to trigger when
|
|
.Fn PacketAliasSetTarget
|
|
is called to change the default target address.
|
|
.Ed
|
|
.Pp
|
|
.Ft u_short
|
|
.Fn PacketAliasInternetChecksum "u_short *buffer" "int nbytes"
|
|
.Bd -ragged -offset indent
|
|
This is a utility function that does not seem to be available elsewhere and
|
|
is included as a convenience.
|
|
It computes the internet checksum, which is used in both IP and
|
|
protocol-specific headers (TCP, UDP, ICMP).
|
|
.Pp
|
|
The
|
|
.Fa buffer
|
|
argument points to the data block to be checksummed, and
|
|
.Fa nbytes
|
|
is the number of bytes.
|
|
The 16-bit checksum field should be zeroed before computing the checksum.
|
|
.Pp
|
|
Checksums can also be verified by operating on a block of data including
|
|
its checksum.
|
|
If the checksum is valid,
|
|
.Fn PacketAliasInternetChecksum
|
|
will return zero.
|
|
.Ed
|
|
.Pp
|
|
.Ft int
|
|
.Fn PacketUnaliasOut "char *buffer" "int maxpacketsize"
|
|
.Bd -ragged -offset indent
|
|
An outgoing packet, which has already been aliased,
|
|
has its private address/port information restored by this function.
|
|
The IP packet is pointed to by
|
|
.Fa buffer ,
|
|
and
|
|
.Fa maxpacketsize
|
|
is provided for error checking purposes.
|
|
This function can be used if an already-aliased packet needs to have its
|
|
original IP header restored for further processing (eg. logging).
|
|
.Ed
|
|
.Sh BUGS
|
|
PPTP aliasing does not work when more than one internal client
|
|
connects to the same external server at the same time, because
|
|
PPTP requires a single TCP control connection to be established
|
|
between any two IP addresses.
|
|
.Sh AUTHORS
|
|
.An Charles Mott Aq cm@linktel.net ,
|
|
versions 1.0 - 1.8, 2.0 - 2.4.
|
|
.An Eivind Eklund Aq eivind@FreeBSD.org ,
|
|
versions 1.8b, 1.9 and 2.5.
|
|
Added IRC DCC support as well as contributing a number of architectural
|
|
improvements; added the firewall bypass for FTP/IRC DCC.
|
|
.An Erik Salander Aq erik@whistle.com
|
|
added support for PPTP and RTSP.
|
|
.An Junichi Satoh Aq junichi@junichi.org
|
|
added support for RTSP/PNA.
|
|
.Sh ACKNOWLEDGMENTS
|
|
Listed below, in approximate chronological order, are individuals who
|
|
have provided valuable comments and/or debugging assistance.
|
|
.Pp
|
|
.Bd -ragged -offset indent
|
|
.An -split
|
|
.An Gary Roberts
|
|
.An Tom Torrance
|
|
.An Reto Burkhalter
|
|
.An Martin Renters
|
|
.An Brian Somers
|
|
.An Paul Traina
|
|
.An Ari Suutari
|
|
.An Dave Remien
|
|
.An J. Fortes
|
|
.An Andrzej Bialecki
|
|
.An Gordon Burditt
|
|
.Ed
|
|
.Sh CONCEPTUAL BACKGROUND
|
|
This section is intended for those who are planning to modify the source
|
|
code or want to create somewhat esoteric applications using the packet
|
|
aliasing functions.
|
|
.Pp
|
|
The conceptual framework under which the packet aliasing engine operates
|
|
is described here.
|
|
Central to the discussion is the idea of an
|
|
.Em aliasing link
|
|
which describes the relationship for a given packet transaction between
|
|
the local machine, aliased identity and remote machine.
|
|
It is discussed how such links come into existence and are destroyed.
|
|
.Ss ALIASING LINKS
|
|
There is a notion of an
|
|
.Em aliasing link ,
|
|
which is a 7-tuple describing a specific translation:
|
|
.Bd -literal -offset indent
|
|
(local addr, local port, alias addr, alias port,
|
|
remote addr, remote port, protocol)
|
|
.Ed
|
|
.Pp
|
|
Outgoing packets have the local address and port number replaced with the
|
|
alias address and port number.
|
|
Incoming packets undergo the reverse process.
|
|
The packet aliasing engine attempts to match packets against an internal
|
|
table of aliasing links to determine how to modify a given IP packet.
|
|
Both the IP header and protocol dependent headers are modified as necessary.
|
|
Aliasing links are created and deleted as necessary according to network
|
|
traffic.
|
|
.Pp
|
|
Protocols can be TCP, UDP or even ICMP in certain circumstances.
|
|
(Some types of ICMP packets can be aliased according to sequence or ID
|
|
number which acts as an equivalent port number for identifying how
|
|
individual packets should be handled.)
|
|
.Pp
|
|
Each aliasing link must have a unique combination of the following five
|
|
quantities: alias address/port, remote address/port and protocol.
|
|
This ensures that several machines on a local network can share the
|
|
same aliasing IP address.
|
|
In cases where conflicts might arise, the aliasing port is chosen so that
|
|
uniqueness is maintained.
|
|
.Ss STATIC AND DYNAMIC LINKS
|
|
Aliasing links can either be static or dynamic.
|
|
Static links persist indefinitely and represent fixed rules for translating
|
|
IP packets.
|
|
Dynamic links come into existence for a specific TCP connection or UDP
|
|
transaction or ICMP ECHO sequence.
|
|
For the case of TCP, the connection can be monitored to see when the
|
|
associated aliasing link should be deleted.
|
|
Aliasing links for UDP transactions (and ICMP ECHO and TIMESTAMP requests)
|
|
work on a simple timeout rule.
|
|
When no activity is observed on a dynamic link for a certain amount of time
|
|
it is automatically deleted.
|
|
Timeout rules also apply to TCP connections which do not open or close
|
|
properly.
|
|
.Ss PARTIALLY SPECIFIED ALIASING LINKS
|
|
Aliasing links can be partially specified, meaning that the remote address
|
|
and/or remote port are unknown.
|
|
In this case, when a packet matching the incomplete specification is found,
|
|
a fully specified dynamic link is created.
|
|
If the original partially specified link is dynamic, it will be deleted
|
|
after the fully specified link is created, otherwise it will persist.
|
|
.Pp
|
|
For instance, a partially specified link might be
|
|
.Bd -literal -offset indent
|
|
(192.168.0.4, 23, 204.228.203.215, 8066, 0, 0, tcp)
|
|
.Ed
|
|
.Pp
|
|
The zeros denote unspecified components for the remote address and port.
|
|
If this link were static it would have the effect of redirecting all
|
|
incoming traffic from port 8066 of 204.228.203.215 to port 23 (telnet)
|
|
of machine 192.168.0.4 on the local network.
|
|
Each individual telnet connection would initiate the creation of a distinct
|
|
dynamic link.
|
|
.Ss DYNAMIC LINK CREATION
|
|
In addition to aliasing links, there are also address mappings that can be
|
|
stored within the internal data table of the packet aliasing mechanism.
|
|
.Bd -literal -offset indent
|
|
(local addr, alias addr)
|
|
.Ed
|
|
.Pp
|
|
Address mappings are searched when creating new dynamic links.
|
|
.Pp
|
|
All outgoing packets from the local network automatically create a dynamic
|
|
link if they do not match an already existing fully specified link.
|
|
If an address mapping exists for the outgoing packet, this determines
|
|
the alias address to be used.
|
|
If no mapping exists, then a default address, usually the address of the
|
|
packet aliasing host, is used.
|
|
If necessary, this default address can be changed as often as each individual
|
|
packet arrives.
|
|
.Pp
|
|
The aliasing port number is determined such that the new dynamic link does
|
|
not conflict with any existing links.
|
|
In the default operating mode, the packet aliasing engine attempts to set
|
|
the aliasing port equal to the local port number.
|
|
If this results in a conflict, then port numbers are randomly chosen until
|
|
a unique aliasing link can be established.
|
|
In an alternate operating mode, the first choice of an aliasing port is also
|
|
random and unrelated to the local port number.
|