libalias(3)

NAME

libalias - packet aliasing library for masquerading and net
work address
translation

SYNOPSIS

#include <sys/types.h>
#include <netinet/in.h>
#include <alias.h>
Function  prototypes are given in the main body of the text.

DESCRIPTION

The libalias library is a collection of functions for alias
ing and dealiasing of IP packets, intended for masquerading and net
work address
translation (NAT).

INTRODUCTION

This library is a moderately portable set of functions de
signed to assist
in the process of IP masquerading and network address trans
lation. Outgoing packets from a local network with unregistered IP ad
dresses can be
aliased to appear as if they came from an accessible IP ad
dress. Incoming packets are then de-aliased so that they are sent to the
correct
machine on the local network.
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 be
tween local and
public addresses can also be implemented, which is known as
static NAT.
In between these extremes, different groups of private ad
dresses can be
linked to different public addresses, comprising several
distinct manyto-one mappings. Also, a given public address and port can
be statically
redirected to a private address/port.
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.

INITIALIZATION AND CONTROL

One special function, LibAliasInit(), must always be called
before any
packet handling may be performed and the returned instance
pointer passed
to all the other functions. Normally, the

LibAliasSetAddress

is called afterwards, to set the default aliasing address.
In addition,
the operating mode of the packet aliasing engine can be cus
tomized by
calling LibAliasSetMode().
struct libalias * LibAliasInit(struct libalias *)

This function is used to initialize internal data
structures. When
called the first time, a NULL pointer should be passed
as an argument. The following mode bits are always set after
calling
LibAliasInit(). See the description of

LibAliasSetMode

the meaning of these mode bits.
PKT_ALIAS_SAME_PORTS
PKT_ALIAS_USE_SOCKETS
PKT_ALIAS_RESET_ON_ADDR_CHANGE
This function will always return the packet aliasing
engine to the
same initial state. The LibAliasSetAddress() function
is normally
called afterwards, and any desired changes from the
default mode
bits listed above require a call to LibAliasSetMode().
It is mandatory that this function be called at the
beginning of a
program prior to any packet handling.
void LibAliasUninit(struct libalias *)

This function has no return value and is used to clear
any
resources attached to internal data structures.
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 atexit(3) chain by LibAliasInit().
void LibAliasSetAddress(struct libalias *, struct in_addr
addr)

This function sets the source address to which outgo
ing packets
from the local area network are aliased. All outgoing
packets are
re-mapped to this address unless overridden by a stat
ic address
mapping established by LibAliasRedirectAddr(). If
this function is
not called, and no static rules match, an outgoing
packet retains
its source address.
If the 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 ppp(8), where the IP address may or
may not
change on successive dial-up attempts.
If the 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 over
head call).
It is mandatory that this function be called prior to
any packet
handling.
unsigned int LibAliasSetMode(struct libalias *, unsigned int
flags,
unsigned int mask)

This function sets or clears mode bits according to
the value of
flags. Only bits marked in mask are affected. The
following mode
bits are defined in
PKT_ALIAS_LOG
Enables logging into /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 tail(1).
PKT_ALIAS_DENY_INCOMING
If this mode bit is set, all incoming packets
associated
with new TCP connections or new UDP transac
tions will be
marked for being ignored (LibAliasIn() returns PKT_ALIAS_IGNORED code) by the calling pro
gram. Response
packets to connections or transactions initi
ated from the
packet aliasing host or local network will be
unaffected.
This mode bit is useful for implementing a
one-way firewall.
PKT_ALIAS_SAME_PORTS
If this mode bit is set, the packet aliasing
engine will
attempt to leave the alias port numbers un
changed from the
actual local port numbers. This can be done
as long as the
quintuple (proto, alias addr, alias port, re
mote addr,
remote port) is unique. If a conflict exists,
a new aliasing port number is chosen even if this mode
bit is set.
PKT_ALIAS_USE_SOCKETS
This bit should be set when the packet alias
ing host originates network traffic as well as forwards it.
When the
packet aliasing host is waiting for a connec
tion 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.
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 unregis
tered addresses
are:

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)
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.
PKT_ALIAS_RESET_ON_ADDR_CHANGE
When this mode bit is set and

LibAliasSetAddress

called to change the aliasing address, the in
ternal link
table of the packet aliasing engine will be
cleared. This
operating mode is useful for 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.
PKT_ALIAS_PUNCH_FW
This option makes libalias `punch holes' in an ipfirewall(4) based firewall for FTP/IRC DCC
connections.
The holes punched are bound by from/to IP ad
dress and port;
it will not be possible to use a hole for an
other connection. A hole is removed when the connection
that uses it
dies. To cater to unexpected death of a pro
gram using
libalias (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
LibAliasSetFWBase(). This call must happen
prior to setting this flag.
PKT_ALIAS_REVERSE
This option makes libalias reverse the way it
handles
incoming and outgoing packets, allowing it to
be fed with
data that passes through the internal inter
face rather than
the external one.
PKT_ALIAS_PROXY_ONLY
This option tells libalias to obey transparent
proxy rules
only. Normal packet aliasing is not per
formed. See
LibAliasProxyRule() below for details.
void LibAliasSetFWBase(struct libalias *, unsigned int base,
unsigned int
num)

Set firewall range allocated for punching firewall
holes (with the
PKT_ALIAS_PUNCH_FW flag). The range will be cleared
for all rules
on initialization.
void LibAliasSkinnyPort(struct libalias *, unsigned int
port)

Set the TCP port used by the Skinny Station protocol.
Skinny is
used by Cisco IP phones to communicate with Cisco Call
Managers to
set up voice over IP calls. If this is not set, Skin
ny aliasing
will not be done. The typical port used by Skinny is
2000.

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.
Along with LibAliasInit() and LibAliasSetAddress(), the two
packet handling functions, LibAliasIn() and LibAliasOut(), comprise
minimal set of
functions needed for a basic IP masquerading implementation.
int LibAliasIn(struct libalias *, char *buffer, int
maxpacketsize)

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 buffer, and maxpacketsize indicates the size of the
data structure containing the packet and should be at least as
large as the
actual packet size.
Return codes:
PKT_ALIAS_OK
The packet aliasing process was successful.
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 PKT_ALIAS_DENY_IN
COMING mode
bit was set by LibAliasSetMode()).
PKT_ALIAS_UNRESOLVED_FRAGMENT
This is returned when a fragment cannot be re
solved because
the header fragment has not been sent yet. In
this situation, fragments must be saved with

LibAliasSaveFragment

until a header fragment is found.
PKT_ALIAS_FOUND_HEADER_FRAGMENT
The packet aliasing process was successful,
and a header
fragment was found. This is a signal to re
trieve any unresolved fragments with LibAliasGetFragment()
and de-alias
them with LibAliasFragmentIn().
PKT_ALIAS_ERROR
An internal error within the packet aliasing
engine
occurred.
int LibAliasOut(struct libalias *, char *buffer, int
maxpacketsize)

An outgoing packet coming from the local network to a
remote
machine is aliased by this function. The IP packet is
pointed to
by buffer, and maxpacketsize indicates the maximum
packet size permissible should the packet length be changed. IP en
coding protocols place address and port information in the encap
sulated 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.
Return codes:
PKT_ALIAS_OK
The packet aliasing process was successful.
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.
PKT_ALIAS_ERROR
An internal error within the packet aliasing
engine
occurred.

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 connec
tions from the
external network. Individual ports can be re-mapped or
static network
address translations can be designated.
struct alias_link * LibAliasRedirectPort(struct libalias *, struct in_addr local_addr, u_short local_port, struct in_addr remote_addr, u_short remote_port, struct in_addr alias_addr, u_short alias_port, u_char proto)

This function specifies that traffic from a given re
mote
address/port to an alias address/port be redirected to
a specified
local address/port. The parameter proto can be either
IPPROTO_TCP
or IPPROTO_UDP, as defined in
If local_addr or alias_addr is zero, this indicates
that the packet
aliasing address as established by

LibAliasSetAddress

used. Even if LibAliasSetAddress() is called to
change the address
after LibAliasRedirectPort() is called, a zero refer
ence will track
this change.
If the link is further set up to operate for a load
sharing, then
local_addr and local_port are ignored, and are select
ed dynamically
from the server pool, as described in

LibAliasAddServer

If remote_addr is zero, this indicates to redirect
packets from any
remote address. Likewise, if 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 nonzero remote addresses can sometimes be useful for
firewalling. If
two calls to LibAliasRedirectPort() overlap in their
address/port
specifications, then the most recent call will have
precedence.
This function returns a pointer which can subsequently
be used by
LibAliasRedirectDelete(). If NULL is returned, then
the function
call did not complete successfully.
All port numbers should be in network address byte or
der, so it is
necessary to use htons(3) to convert these parameters
from internally readable numbers to network byte order. Ad
dresses are also
in network byte order, which is implicit in the use of
the struct
in_addr data type.
struct alias_link * LibAliasRedirectAddr(struct libalias *, struct in_addr local_addr, struct in_addr alias_addr)

This function designates that all incoming traffic to
alias_addr be
redirected to local_addr. Similarly, all outgoing
traffic from
local_addr is aliased to alias_addr.
If local_addr or alias_addr is zero, this indicates
that the packet
aliasing address as established by

LibAliasSetAddress

used. Even if LibAliasSetAddress() is called to
change the address
after LibAliasRedirectAddr() is called, a zero refer
ence will track
this change.
If the link is further set up to operate for a load
sharing, then
local_addr is ignored, and is selected dynamically
from the server
pool, as described in LibAliasAddServer() below.
If subsequent calls to LibAliasRedirectAddr() use the
same aliasing
address, all new incoming traffic to this aliasing ad
dress will be
redirected to the local address made in the last func
tion 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:

LibAliasRedirectAddr(la, in
et_aton("192.168.0.2"),
in
et_aton("141.221.254.101"));
LibAliasRedirectAddr(la, in
et_aton("192.168.0.3"),
in
et_aton("141.221.254.101"));
LibAliasRedirectAddr(la, in
et_aton("192.168.0.4"),
in
et_aton("141.221.254.101"));
Any outgoing connections such as telnet(1) or 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.
Any calls to LibAliasRedirectPort() will have prece
dence over
address mappings designated by LibAliasRedirectAddr().
This function returns a pointer which can subsequently
be used by
LibAliasRedirectDelete(). If NULL is returned, then
the function
call did not complete successfully.
int LibAliasAddServer(struct libalias *, struct alias_link
*link,
struct in_addr addr, u_short port)

This function sets the 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 re
al-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 ser
vices, the configuration on LSNAT could be defined to restrict load
share for just
the services desired.
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.
First, the link is created by either

LibAliasRedirectPort

LibAliasRedirectAddr(). Then, LibAliasAddServer() is
called multiple times to add entries to the link's server pool.
For links created with LibAliasRedirectAddr(), the
port argument is
ignored and could have any value, e.g. htons(~0).
This function returns 0 on success, -1 otherwise.
int LibAliasRedirectDynamic(struct libalias *, struct
alias_link *link)

This function marks the specified static redirect rule
entered by
LibAliasRedirectPort() as dynamic. This can be used
to e.g. dynamically redirect a single TCP connection, after which
the rule is
removed. Only fully specified links can be made dy
namic. (See the
STATIC AND DYNAMIC LINKS and PARTIALLY SPECIFIED
ALIASING LINKS
sections below for a definition of static vs. dynamic,
and partially vs. fully specified links.)
This function returns 0 on success, -1 otherwise.
void LibAliasRedirectDelete(struct libalias *, struct
alias_link *link)

This function will delete a specific static redirect
rule entered
by LibAliasRedirectPort() or LibAliasRedirectAddr().
The parameter
link is the pointer returned by either of the redirec
tion functions. If an invalid pointer is passed to
LibAliasRedirectDelete(), then a program crash or un
predictable
operation could result, so it is necessary to be care
ful using this
function.
int LibAliasProxyRule(struct libalias *, const char *cmd)

The passed 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:
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
encode_ip_hdr is specified, the original destination address and
port are passed
as an extra IP option. If encode_tcp_stream
is specified,
the original destination address and port are
passed as the
first piece of data in the TCP stream in the
format ``DEST
IP port''.
port portnum
Only packets with the destination port portnum
are proxied.
server host[:portnum]
This specifies the host and portnum that the
data is to be
redirected to. host must be an IP address
rather than a
DNS host name. If portnum is not specified,
the destination port number is not changed.
The server specification is mandatory unless
the delete
command is being used.
rule index
Normally, each call to LibAliasProxyRule() in
serts the next
rule at the start of a linear list of rules.
If an index
is specified, the new rule will be checked af
ter all rules
with lower indices. Calls to

LibAliasProxyRule

not specify a rule are assigned rule 0.
delete index
This token and its argument MUST NOT be used
with any other
tokens. When used, all existing rules with
the given index
are deleted.
proto tcp | udp
If specified, only packets of the given proto
col type are
matched.
src IP[/bits]
If specified, only packets with a source ad
dress matching
the given IP are matched. If bits is also
specified, then
the first bits bits of IP are taken as a net
work specification, and all IP addresses from that network
will be
matched.
dst IP[/bits]
If specified, only packets with a destination
address
matching the given IP are matched. If bits is
also specified, then the first bits bits of IP are taken
as a network
specification, and all IP addresses from that
network will
be matched.
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 ma
chines.
struct alias_link * LibAliasRedirectProto(struct libalias *, struct in_addr local_addr, struct in_addr remote_addr, struct in_addr alias_addr, u_char proto)

This function specifies that any IP packet with proto
col number of
proto from a given remote address to an alias address
be redirected
to a specified local address.
If local_addr or alias_addr is zero, this indicates
that the packet
aliasing address as established by

LibAliasSetAddress

used. Even if LibAliasSetAddress() is called to
change the address
after LibAliasRedirectProto() is called, a zero refer
ence will
track this change.
If remote_addr is zero, this indicates to redirect
packets from any
remote address. Non-zero remote addresses can some
times be useful
for firewalling.
If two calls to LibAliasRedirectProto() overlap in
their address
specifications, then the most recent call will have
precedence.
This function returns a pointer which can subsequently
be used by
LibAliasRedirectDelete(). If NULL is returned, then
the function
call did not complete successfully.

FRAGMENT HANDLING

The functions in this section are used to deal with incoming
fragments.
Outgoing fragments are handled within LibAliasOut() by
changing the
address according to any applicable mapping set by
LibAliasRedirectAddr(), or the default aliasing address set
by
LibAliasSetAddress().
Incoming fragments are handled in one of two ways. If the
header of a
fragmented IP packet has already been seen, then all subse
quent 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.
int LibAliasSaveFragment(struct libalias *, char *ptr)

When LibAliasIn() returns PKT_ALIAS_UNRESOLVED_FRAG
MENT, this function can be used to save the pointer to the unresolved
fragment.
It is implicitly assumed that ptr points to a block of
memory allocated by malloc(3). If the fragment is never re
solved, the packet
aliasing engine will automatically free the memory af
ter a timeout
period. [Eventually this function should be modified
so that a
callback function for freeing memory is passed as an
argument.]
This function returns PKT_ALIAS_OK if it was success
ful and
PKT_ALIAS_ERROR if there was an error.
char * LibAliasGetFragment(struct libalias *, char *buffer)

This function can be used to retrieve fragment point
ers saved by
LibAliasSaveFragment(). The IP header fragment point
ed to by
buffer is the header fragment indicated when

LibAliasIn

PKT_ALIAS_FOUND_HEADER_FRAGMENT. Once a fragment
pointer is
retrieved, it becomes the calling program's responsi
bility to free
the dynamically allocated memory for the fragment.
The LibAliasGetFragment() function can be called se
quentially until
there are no more fragments available, at which time
it returns
NULL.
void LibAliasFragmentIn(struct libalias *, char *header,
char *fragment)

When a fragment is retrieved with

LibAliasGetFragment

then be de-aliased with a call to

LibAliasFragmentIn

argument is the pointer to a header fragment used as a
template,
and fragment is the pointer to the packet to be de
aliased.

MISCELLANEOUS FUNCTIONS

void LibAliasSetTarget(struct libalias *, struct in_addr
addr)

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 LibAliasSetTarget().
If this function is called with an INADDR_NONE address
argument,
then all new incoming packets go to the address set by
LibAliasSetAddress().
If this function is not called, or is called with an
INADDR_ANY
address argument, then all new incoming packets go to
the address
specified in the packet. This allows external ma
chines to talk
directly to internal machines if they can route pack
ets to the
machine in question.
int LibAliasCheckNewLink(void)

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 LibAliasSetTarget() is called to
change the default
target address.
u_short LibAliasInternetChecksum(struct libalias *, u_short
*buffer, int
nbytes)

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-spe
cific headers
(TCP, UDP, ICMP).
The buffer argument points to the data block to be
checksummed, and
nbytes is the number of bytes. The 16-bit checksum
field should be
zeroed before computing the checksum.
Checksums can also be verified by operating on a block
of data
including its checksum. If the checksum is valid,
LibAliasInternetChecksum() will return zero.
int LibAliasUnaliasOut(struct libalias *, char *buffer, int maxpacketsize)

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 buffer, and 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 (e.g. logging).

AUTHORS

Charles Mott <cm@linktel.net>, versions 1.0 - 1.8, 2.0
2.4.
Eivind Eklund <eivind@FreeBSD.org>, versions 1.8b, 1.9 and
2.5. Added
IRC DCC support as well as contributing a number of archi
tectural
improvements; added the firewall bypass for FTP/IRC DCC.
Erik Salander <erik@whistle.com> added support for PPTP and
RTSP.
Junichi Satoh <junichi@junichi.org> added support for RT
SP/PNA.
Ruslan Ermilov <ru@FreeBSD.org> added support for PPTP and
LSNAT as well
as general hacking.

ACKNOWLEDGMENTS

Listed below, in approximate chronological order, are indi
viduals who
have provided valuable comments and/or debugging assistance.

Gary Roberts
Tom Torrance
Reto Burkhalter
Martin Renters
Brian Somers
Paul Traina
Ari Suutari
Dave Remien
J. Fortes
Andrzej Bialecki
Gordon Burditt

CONCEPTUAL BACKGROUND

This section is intended for those who are planning to modi
fy the source
code or want to create somewhat esoteric applications using
the packet
aliasing functions.
The conceptual framework under which the packet aliasing en
gine operates
is described here. Central to the discussion is the idea of
an aliasing
link which describes the relationship for a given packet
transaction
between the local machine, aliased identity and remote ma
chine. It is
discussed how such links come into existence and are de
stroyed.
ALIASING LINKS
There is a notion of an aliasing link, which is a 7-tuple
describing a
specific translation:

(local addr, local port, alias addr, alias port,
remote addr, remote port, protocol)
Outgoing packets have the local address and port number re
placed 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.
Protocols can be TCP, UDP or even ICMP in certain circum
stances. (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.)
Each aliasing link must have a unique combination of the
following five
quantities: alias address/port, remote address/port and pro
tocol. 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.
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 connec
tion 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 ac
tivity 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.
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.
For instance, a partially specified link might be

(192.168.0.4, 23, 204.228.203.215, 8066, 0, 0, tcp)
The zeros denote unspecified components for the remote ad
dress and port.
If this link were static it would have the effect of redi
recting 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.
DYNAMIC LINK CREATION
In addition to aliasing links, there are also address map
pings that can
be stored within the internal data table of the packet
aliasing mechanism.

(local addr, alias addr)
Address mappings are searched when creating new dynamic
links.
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.
The aliasing port number is determined such that the new dy
namic link
does not conflict with any existing links. In the default
operating
mode, the packet aliasing engine attempts to set the alias
ing 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.

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.
BSD January 17, 2004

BSD

Copyright © 2010-2024 Platon Technologies, s.r.o.           Home | Man pages | tLDP | Documents | Utilities | About
Design by styleshout