FreeBSD Man Pageslibalias(3)
NAME
libalias -- packet aliasing library for masquerading and network 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 aliasing and de-
aliasing of IP packets, intended for masquerading and network address
translation (NAT).
INTRODUCTION
This library is a moderately portable set of functions designed to assist
in the process of IP masquerading and network address translation. Out-
going packets from a local network with unregistered IP addresses can be
aliased to appear as if they came from an accessible IP address. Incom-
ing 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 many-
to-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() 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 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 argu-
ment. The following mode bits are always set after calling
LibAliasInit(). See the description of LibAliasSetMode() below for
the meaning of these mode bits.
PKT_ALIAS_SAME_PORTS
PKT_ALIAS_USE_SOCKETS
PKT_ALIAS_RESET_ON_ADDR_CHANGE
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 fire-
wall 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 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 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 overhead 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 <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 (LibAliasIn() 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 fire-
wall.
PKT_ALIAS_SAME_PORTS
This bit should be set when the packet aliasing host origi-
nates 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 LibAliasSetAddress() 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 inter-
face 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 connec-
tion. 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
LibAliasSetFWBase(). This call must happen prior to set-
ting 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
LibAliasProxyRule() below for details.
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, Skinny 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 han-
dling 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 net-
work is de-aliased by this function. The IP packet is pointed to
by buffer, and maxpacketsize indicates the size of the data struc-
ture 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 connec-
tions are being ignored (if PKT_ALIAS_DENY_INCOMING mode
bit was set by LibAliasSetMode()).
PKT_ALIAS_UNRESOLVED_FRAGMENT
This is returned when a fragment cannot be resolved because
the header fragment has not been sent yet. In this situa-
tion, 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 retrieve any unre-
solved 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 per-
missible should the packet length be changed. IP encoding proto-
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 net-
work 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 * 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 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 LibAliasSetAddress() is to be
used. Even if LibAliasSetAddress() is called to change the address
after LibAliasRedirectPort() 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 LibAliasAddServer() 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 non-
zero 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 order, so it is
necessary to use htons(3) to convert these parameters from inter-
nally 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 * LibAliasRedirectAddr(struct libalias *,
struct in_addr local_addr, struct in_addr alias_addr)
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 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:
LibAliasRedirectAddr(la, inet_aton("192.168.0.2"),
inet_aton("141.221.254.101"));
LibAliasRedirectAddr(la, inet_aton("192.168.0.3"),
inet_aton("141.221.254.101"));
LibAliasRedirectAddr(la, 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 LibAliasRedirectPort() will have precedence 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 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 config-
uration 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() or
LibAliasRedirectAddr(). Then, LibAliasAddServer() is called multi-
ple 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).
STATIC AND DYNAMIC LINKS and PARTIALLY SPECIFIED ALIASING LINKS
sections below for a definition of static vs. dynamic, and par-
tially 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 redirection func-
tions. If an invalid pointer is passed to
LibAliasRedirectDelete(), then a program crash or unpredictable
operation could result, so it is necessary to be careful 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 spec-
ified, 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 destina-
tion port number is not changed.
The server specification is mandatory unless the delete
command is being used.
rule index
Normally, each call to LibAliasProxyRule() 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 LibAliasProxyRule() 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
matched.
dst IP[/bits]
If specified, only packets with a destination address
matching the given IP are matched. If bits is also speci-
fied, 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 * 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 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 LibAliasSetAddress() is to be
used. Even if LibAliasSetAddress() is called to change the address
after LibAliasRedirectProto() 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 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 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 LibAliasSaveFragment(struct libalias *, char *ptr)
When LibAliasIn() returns PKT_ALIAS_UNRESOLVED_FRAGMENT, this func-
tion can be used to save the pointer to the unresolved fragment.
It is implicitly assumed that ptr points to a block of memory allo-
char * LibAliasGetFragment(struct libalias *, char *buffer)
This function can be used to retrieve fragment pointers saved by
LibAliasSaveFragment(). The IP header fragment pointed to by
buffer is the header fragment indicated when LibAliasIn() 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 LibAliasGetFragment() function can be called sequentially 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(), it can
then be de-aliased with a call to LibAliasFragmentIn(). 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
void LibAliasSetTarget(struct libalias *, struct in_addr addr)
When an incoming packet not associated with any pre-existing alias-
ing 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 machines to talk
directly to internal machines if they can route packets 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 sequen-
tially 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 else-
where 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
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).
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.
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 architectural
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 RTSP/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 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
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
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
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
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 TIME-
STAMP requests) work on a simple timeout rule. When no activity is
observed on a dynamic link for a certain amount of time it is automati-
cally 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 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 connec-
tion would initiate the creation of a distinct dynamic link.
DYNAMIC LINK CREATION
In addition to aliasing links, there are also address mappings that can
be stored within the internal data table of the packet aliasing mecha-
nism.
(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 deter-
mines 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 individ-
ual 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
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