LIBALIAS(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. 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 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
Two special functions, PacketAliasInit() and PacketAliasSetAddress(), must always be called before any packet handling may be performed. 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. PacketAliasSetAddress() must be 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 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.

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 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
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 non-zero 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.

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 address and port is passed as an extra IP option. If encode_tcp_stream is specified, the original address and port is passed as the first piece of data in the TCP stream in the format “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.

int PacketAliasPptp(struct in_addr addr)

This function causes any PPTP packets to be aliased using addr rather than the address set via PacketAliasSetAddress(). This allows the uses of the PPTP on a single machine on the internal network.

If the passed address is INADDR_NONE, then PPTP aliasing is disabled.

This function is made obsolete by PacketAliasRedirectProto(), and is provided only for backward compatibility.

FRAGMENT HANDLING
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.

PacketAliasGetFragment() 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
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.

AUTHORS
Charles Mott <cmott@scientech.com>, 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.

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 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
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
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
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.