libalias -- packet aliasing library for masquerading and network address
translation
#include <sys/types.h>
#include <netinet/in.h>
#include <alias.h>
Function prototypes are given in the main body of the text.
The libalias library is a collection of functions for aliasing and dealiasing
of IP packets, intended for masquerading and network address
translation (NAT).
This library is a moderately portable set of functions designed to assist
in the process of IP masquerading and network address translation. Outgoing
packets from a local network with unregistered IP addresses can be
aliased to appear as if they came from an accessible IP address. Incoming
packets are then de-aliased so that they are sent to the correct
machine on the local network.
A certain amount of flexibility is built into the packet aliasing engine.
In the simplest mode of operation, a many-to-one address mapping takes
place between local network and the packet aliasing host. This is known
as IP masquerading. In addition, one-to-one mappings between local and
public addresses can also be implemented, which is known as static NAT.
In between these extremes, different groups of private addresses can be
linked to different public addresses, comprising several distinct 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 [Toc] [Back] One special function, PacketAliasInit(), must always be called before any
packet handling may be performed. Normally, the PacketAliasSetAddress()
function is called afterwards, to set the default aliasing address. In
addition, the operating mode of the packet aliasing engine can be customized
by calling PacketAliasSetMode().
void PacketAliasInit(void)
This function has no arguments or return value and is used to initialize
internal data structures. The following mode bits are
always set after calling PacketAliasInit(). See the description of
PacketAliasSetMode() below for 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 PacketAliasSetAddress() function is normally
called afterwards, and any desired changes from the default
mode bits listed above require a call to PacketAliasSetMode().
It is mandatory that this function be called at the beginning of a
program prior to any packet handling.
void PacketAliasUninit(void)
This function has no arguments or 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 PacketAliasInit(). Calling
it multiple times is harmless.
void PacketAliasSetAddress(struct in_addr addr)
This function sets the source address to which outgoing packets
from the local area network are aliased. All outgoing packets are
re-mapped to this address unless overridden by a static address
mapping established by PacketAliasRedirectAddr(). 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 overhead call).
It is mandatory that this function be called prior to any packet
handling.
unsigned int PacketAliasSetMode(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 <alias.h>:
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 transactions will be
marked for being ignored (PacketAliasIn() returns
PKT_ALIAS_IGNORED code) by the calling program. Response
packets to connections or transactions initiated from the
packet aliasing host or local network will be unaffected.
This mode bit is useful for implementing a one-way firewall.
PKT_ALIAS_SAME_PORTS
If this mode bit is set, the packet aliasing engine will
attempt to leave the alias port numbers unchanged from the
actual local port numbers. This can be done as long as the
quintuple (proto, alias addr, alias port, remote addr,
remote port) is unique. If a conflict exists, a new aliasing
port number is chosen even if this mode bit is set.
PKT_ALIAS_USE_SOCKETS
This bit should be set when the packet aliasing host originates
network traffic as well as forwards it. When the
packet aliasing host is waiting for a connection from an
unknown host address or unknown port number (e.g. an FTP
data connection), this mode bit specifies that a socket be
allocated as a place holder to prevent port conflicts.
Once a connection is established, usually within a minute
or so, the socket is closed.
PKT_ALIAS_UNREGISTERED_ONLY
If this mode bit is set, traffic on the local network which
does not originate from unregistered address spaces will be
ignored. Standard Class A, B and C unregistered addresses
are:
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 PacketAliasSetAddress() is
called to change the aliasing address, the internal link
table of the packet aliasing engine will be cleared. This
operating mode is useful for 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 address and port;
it will not be possible to use a hole for another connection.
A hole is removed when the connection that uses it
dies. To cater to unexpected death of a program using
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
PacketAliasSetFWBase(). 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 interface rather than
the external one.
PKT_ALIAS_PROXY_ONLY
This option tells libalias to obey transparent proxy rules
only. Normal packet aliasing is not performed. See
PacketAliasProxyRule() below for details.
void PacketAliasSetFWBase(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 PacketAliasSkinnyPort(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, Skinny aliasing
will not be done. The typical port used by Skinny is 2000.
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 PacketAliasInit() and PacketAliasSetAddress(), the two packet
handling functions, PacketAliasIn() and PacketAliasOut(), comprise minimal
set of functions needed for a basic IP masquerading implementation.
int PacketAliasIn(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_INCOMING mode
bit was set by PacketAliasSetMode()).
PKT_ALIAS_UNRESOLVED_FRAGMENT
This is returned when a fragment cannot be resolved because
the header fragment has not been sent yet. In this situation,
fragments must be saved with
PacketAliasSaveFragment() 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 retrieve any unresolved
fragments with PacketAliasGetFragment() and de-alias
them with PacketAliasFragmentIn().
PKT_ALIAS_ERROR
An internal error within the packet aliasing engine
occurred.
int PacketAliasOut(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 encoding protocols
place address and port information in the encapsulated data
stream which has to be modified and can account for changes in
packet length. Well known examples of such protocols are FTP and
IRC DCC.
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 [Toc] [Back] The functions described in this section allow machines on the local network
to be accessible in some degree to new incoming connections from the
external network. Individual ports can be re-mapped or static network
address translations can be designated.
struct alias_link * PacketAliasRedirectPort(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 remote
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 <netinet/in.h>.
If local_addr or alias_addr is zero, this indicates that the packet
aliasing address as established by PacketAliasSetAddress() is to be
used. Even if PacketAliasSetAddress() is called to change the
address after PacketAliasRedirectPort() is called, a zero reference
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 selected dynamically
from the server pool, as described in PacketAliasAddServer() below.
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 PacketAliasRedirectPort() 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
PacketAliasRedirectDelete(). If NULL is returned, then the function
call did not complete successfully.
All port numbers should be in network address byte order, so it is
necessary to use htons(3) to convert these parameters from internally
readable numbers to network byte order. Addresses are also
in network byte order, which is implicit in the use of the struct
in_addr data type.
struct alias_link * PacketAliasRedirectAddr(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 PacketAliasSetAddress() is to be
used. Even if PacketAliasSetAddress() is called to change the
address after PacketAliasRedirectAddr() is called, a zero reference
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 PacketAliasAddServer() below.
If subsequent calls to PacketAliasRedirectAddr() use the same
aliasing address, all new incoming traffic to this aliasing address
will be redirected to the local address made in the last function
call. New traffic generated by any of the local machines, designated
in the several function calls, will be aliased to the same
address. Consider the following example:
PacketAliasRedirectAddr(inet_aton("192.168.0.2"),
inet_aton("141.221.254.101"));
PacketAliasRedirectAddr(inet_aton("192.168.0.3"),
inet_aton("141.221.254.101"));
PacketAliasRedirectAddr(inet_aton("192.168.0.4"),
inet_aton("141.221.254.101"));
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 PacketAliasRedirectPort() will have precedence over
address mappings designated by PacketAliasRedirectAddr().
This function returns a pointer which can subsequently be used by
PacketAliasRedirectDelete(). If NULL is returned, then the function
call did not complete successfully.
int PacketAliasAddServer(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 real-time load
sharing algorithm. Multiple sessions may be initiated from the
same client, and each session could be directed to a different host
based on load balance across server pool hosts at the time. If
load share is desired for just a few specific services, the configuration
on LSNAT could be defined to restrict load share for just
the services desired.
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 PacketAliasRedirectPort() or
PacketAliasRedirectAddr(). Then, PacketAliasAddServer() is called
multiple times to add entries to the link's server pool.
For links created with PacketAliasRedirectAddr(), the port argument
is ignored and could have any value, e.g. htons(~0).
This function returns 0 on success, -1 otherwise.
int PacketAliasRedirectDynamic(struct alias_link *link)
This function marks the specified static redirect rule entered by
PacketAliasRedirectPort() 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 dynamic. (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 PacketAliasRedirectDelete(struct alias_link *link)
This function will delete a specific static redirect rule entered
by PacketAliasRedirectPort() or PacketAliasRedirectAddr(). The
parameter link is the pointer returned by either of the redirection
functions. If an invalid pointer is passed to
PacketAliasRedirectDelete(), then a program crash or unpredictable
operation could result, so it is necessary to be careful using this
function.
int PacketAliasProxyRule(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 PacketAliasProxyRule() inserts the
next rule at the start of a linear list of rules. If an
index is specified, the new rule will be checked after all
rules with lower indices. Calls to PacketAliasProxyRule()
that do 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 protocol type are
matched.
src IP[/bits]
If specified, only packets with a source 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.
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 machines.
struct alias_link * PacketAliasRedirectProto(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 protocol 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 PacketAliasSetAddress() is to be
used. Even if PacketAliasSetAddress() is called to change the
address after PacketAliasRedirectProto() is called, a zero reference
will track this change.
If remote_addr is zero, this indicates to redirect packets from any
remote address. Non-zero remote addresses can sometimes be useful
for firewalling.
If two calls to PacketAliasRedirectProto() overlap in their address
specifications, then the most recent call will have precedence.
This function returns a pointer which can subsequently be used by
PacketAliasRedirectDelete(). If NULL is returned, then the function
call did not complete successfully.
The functions in this section are used to deal with incoming fragments.
Outgoing fragments are handled within PacketAliasOut() by changing the
address according to any applicable mapping set by
PacketAliasRedirectAddr(), or the default aliasing address set by
PacketAliasSetAddress().
Incoming fragments are handled in one of two ways. If the header of a
fragmented IP packet has already been seen, then all subsequent fragments
will be re-mapped in the same manner the header fragment was. Fragments
which arrive before the header are saved and then retrieved once the
header fragment has been resolved.
int PacketAliasSaveFragment(char *ptr)
When PacketAliasIn() returns PKT_ALIAS_UNRESOLVED_FRAGMENT, 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 resolved, the packet
aliasing engine will automatically free the memory after a timeout
period. [Eventually this function should be modified so that a
callback function for freeing memory is passed as an argument.]
This function returns PKT_ALIAS_OK if it was successful and
PKT_ALIAS_ERROR if there was an error.
char * PacketAliasGetFragment(char *buffer)
This function can be used to retrieve fragment pointers saved by
PacketAliasSaveFragment(). The IP header fragment pointed to by
buffer is the header fragment indicated when PacketAliasIn()
returns PKT_ALIAS_FOUND_HEADER_FRAGMENT. Once a fragment pointer
is retrieved, it becomes the calling program's responsibility to
free the dynamically allocated memory for the fragment.
The PacketAliasGetFragment() function can be called sequentially
until there are no more fragments available, at which time it
returns NULL.
void PacketAliasFragmentIn(char *header, char *fragment)
When a fragment is retrieved with PacketAliasGetFragment(), it can
then be de-aliased with a call to PacketAliasFragmentIn(). The
header 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 [Toc] [Back] void PacketAliasSetTarget(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 PacketAliasSetTarget().
If this function is called with an INADDR_NONE address argument,
then all new incoming packets go to the address set by
PacketAliasSetAddress().
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 machines to talk
directly to internal machines if they can route packets to the
machine in question.
int PacketAliasCheckNewLink(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 PacketAliasSetTarget() is called to change the
default target address.
u_short PacketAliasInternetChecksum(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-specific 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,
PacketAliasInternetChecksum() will return zero.
int PacketUnaliasOut(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 (eg. logging).
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.
Charles Mott <[email protected]>, versions 1.0 - 1.8, 2.0 - 2.4.
Eivind Eklund <[email protected]>, versions 1.8b, 1.9 and 2.5. Added
IRC DCC support as well as contributing a number of architectural
improvements; added the firewall bypass for FTP/IRC DCC.
Erik Salander <[email protected]> added support for PPTP and RTSP.
Junichi Satoh <[email protected]> added support for RTSP/PNA.
Ruslan Ermilov <[email protected]> added support for PPTP and LSNAT as well
as general hacking.
Listed below, in approximate chronological order, are individuals 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 [Toc] [Back] This section is intended for those who are planning to modify the source
code or want to create somewhat esoteric applications using the packet
aliasing functions.
The conceptual framework under which the packet aliasing engine 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 machine. It is
discussed how such links come into existence and are destroyed.
ALIASING LINKS [Toc] [Back]
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 replaced with the
alias address and port number. Incoming packets undergo the reverse
process. The packet aliasing engine attempts to match packets against an
internal table of aliasing links to determine how to modify a given IP
packet. Both the IP header and protocol dependent headers are modified
as necessary. Aliasing links are created and deleted as necessary
according to network traffic.
Protocols can be TCP, UDP or even ICMP in certain circumstances. (Some
types of ICMP packets can be aliased according to sequence or ID number
which acts as an equivalent port number for identifying how individual
packets should be handled.)
Each aliasing link must have a unique combination of the following five
quantities: alias address/port, remote address/port and protocol. This
ensures that several machines on a local network can share the same
aliasing IP address. In cases where conflicts might arise, the aliasing
port is chosen so that uniqueness is maintained.
STATIC AND DYNAMIC LINKS [Toc] [Back]
Aliasing links can either be static or dynamic. Static links persist
indefinitely and represent fixed rules for translating IP packets.
Dynamic links come into existence for a specific TCP connection or UDP
transaction or ICMP ECHO sequence. For the case of TCP, the connection
can be monitored to see when the associated aliasing link should be
deleted. Aliasing links for UDP transactions (and ICMP ECHO and TIMESTAMP
requests) work on a simple timeout rule. When no activity is
observed on a dynamic link for a certain amount of time it is automatically
deleted. Timeout rules also apply to TCP connections which do not
open or close properly.
PARTIALLY SPECIFIED ALIASING LINKS [Toc] [Back]
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 address and port.
If this link were static it would have the effect of redirecting all
incoming traffic from port 8066 of 204.228.203.215 to port 23 (telnet) of
machine 192.168.0.4 on the local network. Each individual telnet connection
would initiate the creation of a distinct dynamic link.
DYNAMIC LINK CREATION [Toc] [Back]
In addition to aliasing links, there are also address mappings 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 dynamic link
does not conflict with any existing links. In the default operating
mode, the packet aliasing engine attempts to set the aliasing port equal
to the local port number. If this results in a conflict, then port numbers
are randomly chosen until a unique aliasing link can be established.
In an alternate operating mode, the first choice of an aliasing port is
also random and unrelated to the local port number.
FreeBSD 5.2.1 April 13, 2000 FreeBSD 5.2.1 [ Back ] |
