888 lines
28 KiB
Groff
888 lines
28 KiB
Groff
.\" $FreeBSD$
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.\"
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.Dd July, 1997
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.Dt LIBALIAS 3
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.Os
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.Sh NAME
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.Nm libalias
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.Nd packet aliasing library for masquerading and address translation (NAT)
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.Sh SYNOPSIS
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.Fd #include <sys/types.h>
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.Fd #include <netinet/in.h>
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.Fd #include <alias.h>
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Function prototypes are given in the main body
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of the text.
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.Sh DESCRIPTION
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The
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.Nm
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library is a collection of
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functions for aliasing and de-aliasing
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of IP packets, intended for masquerading and
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network address translation (NAT).
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.Sh CONTENTS
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.Bd -literal -offset left
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1. Introduction
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2. Initialization and Control
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2.1 PacketAliasInit()
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2.2 PacketAliasUninit()
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2.3 PacketAliasSetAddress()
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2.4 PacketAliasSetMode()
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2.5 PacketAliasSetFWBase()
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3. Packet Handling
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3.1 PacketAliasIn()
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3.2 PacketAliasOut()
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4. Port and Address Redirection
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4.1 PacketAliasRedirectPort()
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4.2 PacketAliasRedirectAddr()
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4.3 PacketAliasRedirectDelete()
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4.4 PacketAliasProxyRule()
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4.5 PacketAliasPptp()
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5. Fragment Handling
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5.1 PacketAliasSaveFragment()
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5.2 PacketAliasGetFragment()
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5.3 PacketAliasFragmentIn()
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6. Miscellaneous Functions
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6.1 PacketAliasSetTarget()
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6.2 PacketAliasCheckNewLink()
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6.3 PacketAliasInternetChecksum()
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7. Authors
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8. Acknowledgments
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Appendix A: Conceptual Background
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A.1 Aliasing Links
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A.2 Static and Dynamic Links
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A.3 Partially Specified Links
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A.4 Dynamic Link Creation
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.Ed
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.Sh 1. Introduction
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This library is a moderately portable
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set of functions designed to assist
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in the process of IP masquerading and
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network address translation. Outgoing
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packets from a local network with
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unregistered IP addresses can be aliased
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to appear as if they came from an
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accessible IP address. Incoming packets
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are then de-aliased so that they are sent
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to the correct machine on the local network.
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A certain amount of flexibility is built
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into the packet aliasing engine. In
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the simplest mode of operation, a
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many-to-one address mapping takes place
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between local network and the packet
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aliasing host. This is known as IP
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masquerading. In addition, one-to-one
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mappings between local and public addresses
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can also be implemented, which is known as
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static NAT. In between these extremes,
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different groups of private addresses
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can be linked to different public addresses,
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comprising several distinct many-to-one
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mappings. Also, a given public address
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and port can be statically redirected to
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a private address/port.
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The packet aliasing engine was designed
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to operate in user space outside of the
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kernel, without any access to private
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kernel data structure, but the source code
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can also be ported to a kernel environment.
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.Sh 2. Initialization and Control
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Two specific functions, PacketAliasInit()
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and PacketAliasSetAddress(), must always be
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called before any packet handling may be
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performed. In addition, the operating mode
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of the packet aliasing engine can be customized
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by calling PacketAliasSetMode().
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.Ss 2.1 PacketAliasInit()
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.Ft void
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.Fn PacketAliasInit "void"
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This function has no argument or return
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value and is used to initialize internal
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data structures. The following mode bits
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are always set after calling
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PacketAliasInit(). See section 2.3 for
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the meaning of these mode bits.
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.Bd -literal -offset indent
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PKT_ALIAS_USE_SAME_PORTS
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PKT_ALIAS_USE_SOCKETS
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PKT_ALIAS_RESET_ON_ADDR_CHANGE
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.Ed
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This function will always return the packet
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aliasing engine to the same initial state.
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PacketAliasSetAddress() must be called afterwards,
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and any desired changes from the default mode
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bits listed above require a call to
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PacketAliasSetMode().
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It is mandatory that this function be called
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at the beginning of a program prior to any
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packet handling.
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.Ss 2.2 PacketAliasUninit()
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.Ft void
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.Fn PacketAliasUninit "void"
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This function has no argument or return
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value and is used to clear any resources
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attached to internal data structures.
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This functions should be called when a
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program stop using the aliasing engine;
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it do, among other things, clear out any
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firewall holes. To provide backwards
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compatibility and extra security, it is
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added to the atexit() chain by
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PacketAliasInit(). Calling it multiple
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times is harmless.
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.Ss 2.3 PacketAliasSetAddress()
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.Ft void
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.Fn PacketAliasSetAddress "struct in_addr addr"
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This function sets the source address to which
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outgoing packets from the local area network
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are aliased. All outgoing packets are remapped
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to this address unless overridden by a static
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address mapping established by
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PacketAliasRedirectAddr().
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If the PKT_ALIAS_RESET_ON_ADDR_CHANGE mode bit
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is set (the default mode of operation), then
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the internal aliasing link tables will be reset
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any time the aliasing address changes, as if
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PacketAliasReset() were called. This is useful
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for interfaces such as ppp where the IP
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address may or may not change on successive
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dial-up attempts.
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If the PKT_ALIAS_RESET_ON_ADDR_CHANGE mode bit
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is set to zero, this function can also be used to
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dynamically change the aliasing address on a
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packet to packet basis (it is a low overhead
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call).
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It is mandatory that this function be called
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prior to any packet handling.
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.Ss 2.4 PacketAliasSetMode()
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.Ft unsigned int
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.Fn PacketAliasSetMode "unsigned int mode" "unsigned int mask"
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This function sets or clears mode bits
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according to the value of
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.Em mode .
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Only bits marked in
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.Em mask
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are affected. The following mode bits are
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defined in alias.h:
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.Bl -hang -offset left
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.It PKT_ALIAS_LOG.
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Enables logging /var/log/alias.log. The log file
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shows total numbers of links (icmp, tcp, udp) each
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time an aliasing link is created or deleted. Mainly
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useful for debugging when the log file is viewed
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continuously with "tail -f".
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.It PKT_ALIAS_DENY_INCOMING.
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If this mode bit is set, all incoming packets
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associated with new TCP connections or new
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UDP transactions will be marked for being
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ignored (PacketAliasIn() return code
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PKT_ALIAS_IGNORED) by the calling program.
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Response packets to connections or transactions
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initiated from the packet aliasing host or
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local network will be unaffected. This mode
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bit is useful for implementing a one-way firewall.
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.It PKT_ALIAS_SAME_PORTS.
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If this mode bit is set, the packet aliasing
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engine will attempt to leave the alias port
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numbers unchanged from the actual local port
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number. This can be done as long as the
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quintuple (proto, alias addr, alias port,
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remote addr, remote port) is unique. If a
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conflict exists, an new aliasing port number is
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chosen even if this mode bit is set.
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.It PKT_ALIAS_USE_SOCKETS.
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This bit should be set when the the packet
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aliasing host originates network traffic as
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well as forwards it. When the packet aliasing
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host is waiting for a connection from an
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unknown host address or unknown port number
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(e.g. an FTP data connection), this mode bit
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specifies that a socket be allocated as a place
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holder to prevent port conflicts. Once a
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connection is established, usually within a
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minute or so, the socket is closed.
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.It PKT_ALIAS_UNREGISTERED_ONLY.
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If this mode bit is set, traffic on the
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local network which does not originate from
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unregistered address spaces will be ignored.
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Standard Class A, B and C unregistered addresses
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are:
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.Bd -literal -offset indent
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10.0.0.0 -> 10.255.255.255 (Class A subnet)
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172.16.0.0 -> 172.31.255.255 (Class B subnets)
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192.168.0.0 -> 192.168.255.255 (Class C subnets)
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.Ed
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This option is useful in the case that
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packet aliasing host has both registered and
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unregistered subnets on different interfaces.
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The registered subnet is fully accessible to
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the outside world, so traffic from it doesn't
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need to be passed through the packet aliasing
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engine.
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.It PKT_ALIAS_RESET_ON_ADDR_CHANGE.
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When this mode bit is set and
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PacketAliasSetAddress() is called to change
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the aliasing address, the internal link table
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of the packet aliasing engine will be cleared.
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This operating mode is useful for ppp links
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where the interface address can sometimes
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change or remain the same between dial-ups.
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If this mode bit is not set, it the link table
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will never be reset in the event of an
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address change.
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.It PKT_ALIAS_PUNCH_FW.
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This option makes libalias `punch holes' in an
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ipfw based firewall for FTP/IRC DCC connections.
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The holes punched are bound by from/to IP address
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and port; it will not be possible to use a hole
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for another connection. A hole is removed when
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the connection that use it die. To cater for
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unexpected death of a program using libalias (e.g
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kill -9), changing the state of the flag will
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clear the entire ipfw range allocated for holes.
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This will also happen on the initial call to
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PacketAliasSetFWBase(). This call must happen
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prior to setting this flag.
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.It PKT_ALIAS_REVERSE.
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This option makes libalias reverse the way it
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handles incoming and outgoing packets, allowing
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it to be fed data that passes through the internal
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interface rather than the external one.
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.El
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.Ss 2.5 PacketAliasSetFWBase()
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.Ft void
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.Fn PacketAliasSetFWBase "unsigned int base" "unsigned int num"
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Set IPFW range allocated for punching firewall holes (with the
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PKT_ALIAS_PUNCH_FW flag). The range will be cleared for all rules on
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initialization.
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.Sh 3. Packet Handling
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The packet handling functions are used to
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modify incoming (remote->local) and outgoing
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(local->remote) packets. The calling program
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is responsible for receiving and sending
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packets via network interfaces.
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Along with PacketAliasInit() and PacketAliasSetAddress(),
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the two packet handling functions, PacketAliasIn()
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and PacketAliasOut(), comprise minimal set of functions
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needed for a basic IP masquerading implementation.
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.Ss 3.1 PacketAliasIn()
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.Ft int
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.Fn PacketAliasIn "char *buffer" "int maxpacketsize"
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An incoming packet coming from a remote machine to
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the local network is de-aliased by this function.
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The IP packet is pointed to by
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.Em buffer ,
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and
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.Em maxpacketsize
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indicates the size of the data structure containing
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the packet and should be at least as large as the
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actual packet size.
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Return codes:
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.Bl -hang -offset left
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.It PKT_ALIAS_ERROR.
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An internal error within the packet aliasing
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engine occurred.
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.It PKT_ALIAS_OK.
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The packet aliasing process was successful.
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.It PKT_ALIAS_IGNORED.
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The packet was ignored and not de-aliased.
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This can happen if the protocal is unrecognized,
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possibly an ICMP message type is not handled or
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if incoming packets for new connections are being
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ignored (see PKT_ALIAS_DENY_INCOMING in section
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2.2).
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.It PKT_ALIAS_UNRESOLVED_FRAGMENT.
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This is returned when a fragment cannot be
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resolved because the header fragment has not
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been sent yet. In this situation, fragments
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must be saved with PacketAliasSaveFragment()
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until a header fragment is found.
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.It PKT_ALIAS_FOUND_HEADER_FRAGMENT.
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The packet aliasing process was successful,
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and a header fragment was found. This is a
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signal to retrieve any unresolved fragments
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with PacketAliasGetFragment() and de-alias
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them with PacketAliasFragmentIn().
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.El
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.Ss 3.2 PacketAliasOut()
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.Ft int
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.Fn PacketAliasOut "char *buffer" "int maxpacketsize"
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An outgoing packet coming from the local network
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to a remote machine is aliased by this function.
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The IP packet is pointed to by
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.Em buffer r,
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and
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.Em maxpacketsize
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indicates the maximum packet size permissible
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should the packet length be changed. IP encoding
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protocols place address and port information in
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the encapsulated data stream which have to be
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modified and can account for changes in packet
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length. Well known examples of such protocols
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are FTP and IRC DCC.
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Return codes:
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.Bl -hang -offset left
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.It PKT_ALIAS_ERROR.
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An internal error within the packet aliasing
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engine occurred.
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.It PKT_ALIAS_OK.
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The packet aliasing process was successful.
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.It PKT_ALIAS_IGNORED.
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The packet was ignored and not de-aliased.
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This can happen if the protocal is unrecognized,
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or possibly an ICMP message type is not handled.
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.El
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.Sh 4. Port and Address Redirection
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The functions described in this section allow machines
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on the local network to be accessible in some degree
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to new incoming connections from the external network.
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Individual ports can be re-mapped or static network
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address translations can be designated.
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.Ss 4.1 PacketAliasRedirectPort()
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.Ft struct alias_link *
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.Fo PacketAliasRedirectPort
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.Fa "struct in_addr local_addr"
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.Fa "u_short local_port"
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.Fa "struct in_addr remote_addr"
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.Fa "u_short remote_port"
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.Fa "struct in_addr alias_addr"
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.Fa "u_short alias_port"
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.Fa "u_char proto"
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.Fc
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This function specifies that traffic from a
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given remote address/port to an alias address/port
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be redirected to a specified local address/port.
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The parameter
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.Em proto
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can be either IPPROTO_TCP or IPPROTO_UDP, as
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defined in <netinet/in.h>.
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If
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.Em local_addr
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or
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.Em alias_addr
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is zero, this indicates that the packet aliasing
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address as established by PacketAliasSetAddress()
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is to be used. Even if PacketAliasAddress() is
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called to change the address after PacketAliasRedirectPort()
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is called, a zero reference will track this change.
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If
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.Em remote_addr
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is zero, this indicates to redirect packets from
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any remote address. Likewise, if
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.Em remote_port
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is zero, this indicates to redirect packets originating
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from any remote port number. Almost always, the remote
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port specification will be zero, but non-zero remote
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addresses can be sometimes be useful for firewalling.
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If two calls to PacketAliasRedirectPort() overlap in
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their address/port specifications, then the most recent
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call will have precedence.
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This function returns a pointer which can subsequently
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be used by PacketAliasRedirectDelete(). If NULL is
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returned, then the function call did not complete
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successfully.
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All port numbers are in network address byte order,
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so it is necessary to use htons() to convert these
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parameters from internally readable numbers to
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network byte order. Addresses are also in network
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byte order, which is implicit in the use of the
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.Em struct in_addr
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data type.
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.Ss 4.2 PacketAliasRedirectAddr()
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.Ft struct alias_link *
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.Fo PacketAliasRedirectAddr
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.Fa "struct in_addr local_addr"
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.Fa "struct in_addr alias_addr"
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.Fc
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This function desgnates that all incoming
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traffic to
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.Em alias_addr
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be redirected to
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.Em local_addr.
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Similarly, all outgoing traffic from
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.Em local_addr
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is aliased to
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.Em alias_addr .
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If
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.Em local_addr
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or
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.Em alias_addr
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is zero, this indicates that the packet aliasing
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address as established by PacketAliasSetAddress()
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is to be used. Even if PacketAliasAddress() is
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called to change the address after PacketAliasRedirectAddr()
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is called, a zero reference will track this change.
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If subsequent calls to PacketAliasRedirectAddr()
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use the same aliasing address, all new incoming
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traffic to this aliasing address will be redirected
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to the local address made in the last function call,
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but new traffic all of the local machines designated
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in the several function calls will be aliased to
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the same address. Consider the following example:
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.Bd -literal -offset left
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PacketAliasRedirectAddr(inet_aton("192.168.0.2"),
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inet_aton("141.221.254.101"));
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PacketAliasRedirectAddr(inet_aton("192.168.0.3"),
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inet_aton("141.221.254.101"));
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PacketAliasRedirectAddr(inet_aton("192.168.0.4"),
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inet_aton("141.221.254.101"));
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.Ed
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Any outgoing connections such as telnet or ftp
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from 192.168.0.2, 192.168.0.3, 192.168.0.4 will
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appear to come from 141.221.254.101. Any incoming
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connections to 141.221.254.101 will be directed
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to 192.168.0.4.
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Any calls to PacketAliasRedirectPort() will
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have precedence over address mappings designated
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by PacketAliasRedirectAddr().
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This function returns a pointer which can subsequently
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be used by PacketAliasRedirectDelete(). If NULL is
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returned, then the function call did not complete
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successfully.
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.Ss 4.3 PacketAliasRedirectDelete()
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.Ft void
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.Fn PacketAliasRedirectDelete "struct alias_link *ptr"
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This function will delete a specific static redirect
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rule entered by PacketAliasRedirectPort() or
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PacketAliasRedirectAddr(). The parameter
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.Em ptr
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is the pointer returned by either of the redirection
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functions. If an invalid pointer is passed to
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PacketAliasRedirectDelete(), then a program crash
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or unpredictable operation could result, so it is
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necessary to be careful using this function.
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.Sh 5. Fragment Handling
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The functions in this section are used to deal with
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incoming fragments.
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Outgoing fragments are handled within PacketAliasOut()
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by changing the address according to any
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applicable mapping set by PacketAliasRedirectAddress(),
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or the default aliasing address set by
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PacketAliasSetAddress().
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Incoming fragments are handled in one of two ways.
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If the header of a fragmented IP packet has already
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been seen, then all subsequent fragments will be
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re-mapped in the same manner the header fragment
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was. Fragments which arrive before the header
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are saved and then retrieved once the header fragment
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has been resolved.
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.Ss 4.4 PacketAliasProxyRule()
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.Ft int
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.Fn PacketAliasProxyRule "const char *cmd"
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The passed
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.Ar cmd
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string consists of one or more pairs of words. The first word in each
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pair is a token and the second is the value that should be applied for
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that token. Tokens and their argument types are as follows:
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.Bl -tag -offset XXX -width XXX
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.It type encode_ip_hdr|encode_tcp_stream|no_encode
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In order to support transparent proxying, it is necessary to somehow
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pass the original address and port information into the new destination
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server. If
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.Dq encode_ip_hdr
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is specified, the original address and port is passed as an extra IP
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option. If
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.Dq encode_tcp_stream
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is specified, the original address and port is passed as the first
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piece of data in the tcp stream in the format
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.Dq DEST Ar IP port .
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.It port Ar portnum
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Only packets with the destination port
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.Ar portnum
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are proxied.
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.It server Ar host[:portnum]
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This specifies the
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.Ar host
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and
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.Ar portnum
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that the data is to be redirected to.
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.Ar host
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must be an IP address rather than a DNS host name. If
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.Ar portnum
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is not specified, the destination port number is not changed.
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.Pp
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The
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.Ar server
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specification is mandatory unless the
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.Dq delete
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command is being used.
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.It rule Ar index
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Normally, each call to
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.Fn PacketAliasProxyRule
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inserts the next rule at the start of a linear list of rules. If an
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.Ar index
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is specified, the new rule will be checked after all rules with lower
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indices. Calls to
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.Fn PacketAliasProxyRule
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that do not specify a rule are assigned rule 0.
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.It delete Ar index
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This token and its argument must not be used with any other tokens. When
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used, all existing rules with the given
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.Ar index
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are deleted.
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.It proto tcp|udp
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If specified, only packets of the given protocol type are matched.
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.It src Ar IP[/bits]
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If specified, only packets with a source address matching the given
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.Ar IP
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are matched. If
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.Ar bits
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is also specified, then the first
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.Ar bits
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bits of
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.Ar IP
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are taken as a network specification, and all IP addresses from that
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network will be matched.
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.It dst Ar IP[/bits]
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If specified, only packets with a destination address matching the given
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.Ar IP
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are matched. If
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.Ar bits
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is also specified, then the first
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.Ar bits
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bits of
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.Ar IP
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are taken as a network specification, and all IP addresses from that
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network will be matched.
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.El
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This function is usually used to redirect outgoing connections for
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internal machines that are not permitted certain types of internet
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access, or to restrict access to certain external machines.
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.Ss 4.5 PacketAliasPptp()
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.Ft extern int
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.Fn PacketAliasPptp "struct in_addr addr"
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This function causes any
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.Em G Ns No eneral
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.Em R Ns No outing
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.Em E Ns No encapsulated
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.Pq Dv IPPROTO_GRE
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packets to be aliased using
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.Ar addr
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rather than the address set via
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.Fn PacketAliasSetAddress .
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This allows the uses of the
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.Em P Ns No oint
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to
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.Em P Ns No oint
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.Em T Ns No unneling
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.Em P Ns No rotocol
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on a machine on the internal network.
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.Pp
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If the passed address is
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.Dv INADDR_NONE
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.Pq 255.255.255.255 ,
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.Dv PPTP
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aliasing is disabled.
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.Ss 5.1 PacketAliasSaveFragment()
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.Ft int
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.Fn PacketAliasSaveFragment "char *ptr"
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When PacketAliasIn() returns
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PKT_ALIAS_UNRESOLVED_FRAGMENT, this
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function can be used to save the pointer to
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the unresolved fragment.
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It is implicitly assumed that
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.Em ptr
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points to a block of memory allocated by
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malloc(). If the fragment is never
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resolved, the packet aliasing engine will
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automatically free the memory after a
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timeout period. [Eventually this function
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should be modified so that a callback
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function for freeing memory is passed as
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an argument.]
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This function returns PKT_ALIAS_OK if it
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was successful and PKT_ALIAS_ERROR if there
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was an error.
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.Ss 5.2 PacketAliasGetFragment()
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.Ft char *
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.Fn PacketAliasGetFragment "char *buffer"
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This function can be used to retrieve fragment
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pointers saved by PacketAliasSaveFragment().
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The IP header fragment pointed to by
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.Em buffer
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is the header fragment indicated when
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PacketAliasIn() returns PKT_ALIAS_FOUND_HEADER_FRAGMENT.
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Once a a fragment pointer is retrieved, it
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becomes the calling program's responsibility
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to free the dynamically allocated memory for
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the fragment.
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PacketAliasGetFragment() can be called
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sequentially until there are no more fragments
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available, at which time it returns NULL.
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.Ss 5.3 PacketAliasFragmentIn()
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.Ft void
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.Fn PacketAliasFragmentIn "char *header" "char *fragment"
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When a fragment is retrieved with
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PacketAliasGetFragment(), it can then be
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de-aliased with a call to PacketAliasFragmentIn().
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.Em header
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is the pointer to a header fragment used as a
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template, and
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.Em fragment
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is the pointer to the packet to be de-aliased.
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.Sh 6. Miscellaneous Functions
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.Ss 6.1 PacketAliasSetTarget()
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.Ft void
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.Fn PacketAliasSetTarget "struct in_addr addr"
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When an incoming packet not associated with
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any pre-existing aliasing link arrives at the
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host machine, it will be sent to the address
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indicated by a call to PacketAliasSetTarget().
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If this function is not called, or is called
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with a zero address argument, then all new
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incoming packets go to the address set by
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PacketAliasSetAddress.
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.Ss 6.2 PacketAliasCheckNewLink()
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.Ft int
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.Fn PacketAliasCheckNewLink "void"
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This function returns a non-zero value when
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a new aliasing link is created. In circumstances
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where incoming traffic is being sequentially
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sent to different local servers, this function
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can be used to trigger when PacketAliasSetTarget()
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is called to change the default target address.
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.Ss 6.3 PacketAliasInternetChecksum()
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.Ft u_short
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.Fn PacketAliasInternetChecksum "u_short *buffer" "int nbytes"
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This is a utility function that does not seem
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to be available elswhere and is included as a
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convenience. It computes the internet checksum,
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which is used in both IP and protocol-specific
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headers (TCP, UDP, ICMP).
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.Em buffer
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points to the data block to be checksummed, and
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.Em nbytes
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is the number of bytes. The 16-bit checksum
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field should be zeroed before computing the checksum.
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Checksums can also be verified by operating on a block
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of data including its checksum. If the checksum is
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valid, PacketAliasInternetChecksum() will return zero.
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.Sh 7. Authors
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Charles Mott (cmott@srv.net), versions 1.0 - 1.8, 2.0 - 2.4.
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Eivind Eklund (eivind@freebsd.org), versions 1.8b, 1.9 and
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2.5. Added IRC DCC support as well as contributing a number of
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architectural improvements; added the firewall bypass
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for FTP/IRC DCC.
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.Sh 8. Acknowledgments
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Listed below, in approximate chronological
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order, are individuals who have provided
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valuable comments and/or debugging assistance.
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.Bl -inset -compact -offset left
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.It Gary Roberts
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.It Tom Torrance
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.It Reto Burkhalter
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.It Martin Renters
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.It Brian Somers
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.It Paul Traina
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.It Ari Suutari
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.It Dave Remien
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.It J. Fortes
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.It Andrzej Bialeki
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.It Gordon Burditt
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.El
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.Sh Appendix: Conceptual Background
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This appendix is intended for those who
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are planning to modify the source code or want
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to create somewhat esoteric applications using
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the packet aliasing functions.
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The conceptual framework under which the
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packet aliasing engine operates is described here.
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Central to the discussion is the idea of an
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"aliasing link" which describes the relationship
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for a given packet transaction between the local
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machine, aliased identity and remote machine. It
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is discussed how such links come into existence
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and are destroyed.
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.Ss A.1 Aliasing Links
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There is a notion of an "aliasing link",
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which is 7-tuple describing a specific
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translation:
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.Bd -literal -offset indent
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(local addr, local port, alias addr, alias port,
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remote addr, remote port, protocol)
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.Ed
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Outgoing packets have the local address and
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port number replaced with the alias address
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and port number. Incoming packets undergo the
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reverse process. The packet aliasing engine
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attempts to match packets against an internal
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table of aliasing links to determine how to
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modify a given IP packet. Both the IP
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header and protocol dependent headers are
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modified as necessary. Aliasing links are
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created and deleted as necessary according
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to network traffic.
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Protocols can be TCP, UDP or even ICMP in
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certain circumstances. (Some types of ICMP
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packets can be aliased according to sequence
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or id number which acts as an equivalent port
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number for identifying how individual packets
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should be handled.)
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Each aliasing link must have a unique
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combination of the following five quantities:
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alias address/port, remote address/port
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and protocol. This ensures that several
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machines on a local network can share the
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same aliased IP address. In cases where
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conflicts might arise, the aliasing port
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is chosen so that uniqueness is maintained.
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.Ss A.2 Static and Dynamic Links
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Aliasing links can either be static or dynamic.
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Static links persist indefinitely and represent
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fixed rules for translating IP packets. Dynamic
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links come into existence for a specific TCP
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connection or UDP transaction or ICMP echo
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sequence. For the case of TCP, the connection
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can be monitored to see when the associated
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aliasing link should be deleted. Aliasing links
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for UDP transactions (and ICMP echo and timestamp
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requests) work on a simple timeout rule. When
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no activity is observed on a dynamic link for
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a certain amount of time it is automatically
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deleted. Timeout rules also apply to TCP
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connections which do not open or close
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properly.
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.Ss A.3 Partially Specified Aliasing Links
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Aliasing links can be partially specified,
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meaning that the remote address and/or remote
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ports are unknown. In this case, when a packet
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matching the incomplete specification is found,
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a fully specified dynamic link is created. If
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the original partially specified link is dynamic,
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it will be deleted after the fully specified link
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is created, otherwise it will persist.
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For instance, a partially specified link might
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be
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.Bd -literal -offset indent
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(192.168.0.4, 23, 204.228.203.215, 8066, 0, 0, tcp)
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.Ed
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The zeros denote unspecified components for
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the remote address and port. If this link were
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static it would have the effect of redirecting
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all incoming traffic from port 8066 of
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204.228.203.215 to port 23 (telnet) of machine
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192.168.0.4 on the local network. Each
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individual telnet connection would initiate
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the creation of a distinct dynamic link.
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.Ss A.4 Dynamic Link Creation
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In addition to aliasing links, there are
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also address mappings that can be stored
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within the internal data table of the packet
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aliasing mechanism.
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.Bd -literal -offset indent
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(local addr, alias addr)
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.Ed
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Address mappings are searched when creating
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new dynamic links.
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All outgoing packets from the local network
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automatically create a dynamic link if
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they do not match an already existing fully
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specified link. If an address mapping exists
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for the the outgoing packet, this determines
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the alias address to be used. If no mapping
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exists, then a default address, usually the
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address of the packet aliasing host, is used.
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If necessary, this default address can be
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changed as often as each individual packet
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arrives.
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The aliasing port number is determined
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such that the new dynamic link does not
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conflict with any existing links. In the
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default operating mode, the packet aliasing
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engine attempts to set the aliasing port
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equal to the local port number. If this
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results in a conflict, then port numbers
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are randomly chosen until a unique aliasing
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link can be established. In an alternate
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operating mode, the first choice of an
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aliasing port is also random and unrelated
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to the local port number.
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