libalias(3)

NAME
libalias Packet Aliasing Library. A collection of functions for aliasing and de-aliasing of IP packets, intended for masquerading and network address translation (NAT).

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

CONTENTS

    1. Introduction
    2. Initialization and Control
        2.1 PacketAliasInit()
        2.2 PacketAliasSetAddress()
        2.3 PacketAliasSetMode()
    3. Packet Handling
        3.1 PacketAliasIn()
        3.2 PacketAliasOut()
    4. Port and Address Redirection
        4.1 PacketAliasRedirectPort()
        4.2 PacketAliasRedirectAddr()
        4.3 PacketAliasRedirectDelete()
    5. Fragment Handling
        5.1 PacketAliasSaveFragment()
        5.2 PacketAliasGetFragment()
        5.3 PacketAliasFragmentIn()
    6. Miscellaneous Functions
        6.1 PacketAliasSetTarget()
        6.2 PacketAliasCheckNewLink()
        6.3 PacketAliasInternetChecksum()
    7. Authors
    8. Acknowledgments
    Appendix A: Conceptual Background
        A.1 Aliasing Links
        A.2 Static and Dynamic Links
        A.3 Partially Specified Links
        A.4 Dynamic Link Creation

1. 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.

2. Initialization and Control
Two specific 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().

  2.1 PacketAliasInit()
void PacketAliasInit(void) This function has no argument or return value and is used to initialize internal data structures. The following mode bits are always set after calling PacketAliasInit(). See section 2.3 for the meaning of these mode bits.

        PKT_ALIAS_USE_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.

  2.2 PacketAliasSetAddress()
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 remapped 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, as if PacketAliasReset() were called. This is useful for interfaces such as ppp 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.

  2.3 PacketAliasSetMode()
void PacketAliasSetMode(int mode, int mask) This function sets or clears mode bits according to the value of mode. Only bits marked in mask are affected. The following mode bits are defined in alias.h:

PKT_ALIAS_LOG.
    Enables logging /var/log/alias.log. The log file shows total numbers of links (icmp, tcp, udp) each time an aliasing link is created or deleted. Mainly useful for debugging when the log file is viewed continuously with "tail -f".

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() return code PKT_ALIAS_IGNORED) 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 number. This can be done as long as the quintuple (proto, alias addr, alias port, remote addr, remote port) is unique. If a conflict exists, an new aliasing port number is chosen even if this mode bit is set.

PKT_ALIAS_USE_SOCKETS.
    This bit should be set when the 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 doesn't 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 links where the interface address can sometimes change or remain the same between dial-ups. If this mode bit is not set, it the link table will never be reset in the event of an address change.

3. Packet Handling
The packet handling functions are used to modify incoming (remote->local) and outgoing (local->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.

  3.1 PacketAliasIn()
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_ERROR.
    An internal error within the packet aliasing engine occurred.

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 protocal is unrecognized, possibly an ICMP message type is not handled or if incoming packets for new connections are being ignored (see PKT_ALIAS_DENY_INCOMING in section 2.2).

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().

  3.2 PacketAliasOut()
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 r, 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 have to be modified and can account for changes in packet length. Well known examples of such protocols are FTP and IRC. Return codes:

PKT_ALIAS_ERROR.
    An internal error within the packet aliasing engine occurred.

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 protocal is unrecognized, or possibly an ICMP message type is not handled.

4. 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.

  4.1 PacketAliasRedirectPort()
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 PacketAliasAddress() is called to change the address after PacketAliasRedirectPort() is called, a zero reference will track this change. 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 be 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 are in network address byte order, so it is necessary to use htons() 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.

  4.2 PacketAliasRedirectAddr()
struct alias_link * PacketAliasRedirectAddress(struct in_addr local_addr, struct in_addr alias_addr) This function desgnates 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 PacketAliasAddress() is called to change the address after PacketAliasRedirectAddr() is called, a zero reference will track this change. 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, but new traffic all 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 or ftp from 192.168.0.2, 102.168.0.3, 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.

  4.3 PacketAliasRedirectDelete()
void PacketAliasRedirectDelete(struct alias_link *ptr) This function will delete a specific static redirect rule entered by PacketAliasRedirectPort() or PacketAliasRedirectAddr(). The parameter ptr 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.

5. 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 PacketAliasRedirectAddress(), 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.

  5.1 PacketAliasSaveFragment()
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(). 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.

  5.2 PacketAliasGetNextFragment()
char * PacketAliasGetFragment(char *buffer) This function can be used to retrieve fragment pointers saved by PacketAliasSaveFragment(). The IP header fragment pointed to by Em buffer is the header fragment indicated when PacketAliasIn() returns PKT_ALIAS_FOUND_HEADER_FRAGMENT. Once a 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.

  5.3 PacketAliasFragmentIn()
void PacketAliasFragmentIn(char *header, char *fragment) When a fragment is retrieved with PacketAliasGetFragment(), it can then be de-aliased with a call to PacketAliasFragmentIn(). header is the pointer to a header fragment used as a template, and fragment is the pointer to the packet to be de-aliased.

6. Miscellaneous Functions

  6.1 PacketAliasSetTarget()
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 not called, or is called with a zero address argument, then all new incoming packets go to the address set by PacketAliasSetAddress.

  6.2 PacketAliasCheckNewLink()
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.

  6.3 PacketAliasInternetChecksum()
u_short PacketAliasInternetChecksum(char *buffer, int nbytes) This is a utility function that does not seem to be available elswhere and is included as a convenience. It computes the internet checksum, which is used in both IP and protocol-specific headers (TCP, UDP, ICMP). buffer 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.

7. Authors
Charles Mott (cmott@srv.net), versions 1.0 - 1.8, 2.0 - 2.4. Eivind Eiklund (eivind@freebsd.org), versions 1.8b and 1.9. Added IRC support as well as contributing a number of architectural improvements.

8. 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 Bialeki
Gordon Burditt

Appendix: Conceptual Background
This appendix 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.

  A.1 Aliasing Links
There is a notion of an "aliasing link", which is 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 aliased IP address. In cases where conflicts might arise, the aliasing port is chosen so that uniqueness is maintained.

  A.2 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.

  A.3 Partially Specified Aliasing Links
Aliasing links can be partially specified, meaning that the remote address and/or remote ports 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.

  A.4 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 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.