Manual Pages  — LIBALIAS

This function should be called when a program stops using the aliasing engine; amongst other things, it clears out any firewall holes. To provide backwards compatibility and extra security, it is added to the atexit(3) chain by LibAliasInit().

void LibAliasSetAddress(struct libalias *, struct in_addr addr) This function sets the source address to which 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 has not been 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 to 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 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.amp; 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 the 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_UNREGISTERED_CGN
	Like PKT_ALIAS_UNREGISTERED_ONLY, but includes the RFC 6598 (Carrier Grade NAT) subnet as follows: 100.64.0.0 -> 100.127.255.255 (RFC 6598 subnet)
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 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.amp; kill -9), changing the state of the flag will clear the entire firewall range allocated for holes. This clearing will also happen on the initial call to LibAliasSetFWBase(), which 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 LibAliasProxyRule() below for details.
PKT_ALIAS_SKIP_GLOBAL
	This option is used by ipfw_nat only. Specifying it as a flag to LibAliasSetMode() has no effect. See section NETWORK ADDRESS TRANSLATION in ipfw(8) for more details.

void LibAliasSetFWBase(struct libalias *, unsigned int base, unsigned int num) Set the firewall range allocated for punching firewall holes (with the PKT_ALIAS_PUNCH_FW flag). The range is cleared for all rules on initialization.

void LibAliasSkinnyPort(struct libalias *, unsigned int port) Set the TCP port used by the Skinny Station protocol. Skinny is used by Cisco IP phones to communicate with Cisco Call Managers to set up voice over IP calls. If this is not set, Skinny aliasing will not be done. The typical port used by Skinny is 2000.

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 * LibAliasRedirectAddr(struct libalias *, struct in_addr local_addr, struct in_addr alias_addr) This function designates that all incoming traffic to alias_addr be redirected to local_addr. Similarly, all outgoing traffic from local_addr is aliased to alias_addr.

If local_addr or alias_addr is zero, this indicates that the packet aliasing address as established by LibAliasSetAddress() is to be used. Even if LibAliasSetAddress() is called to change the address after LibAliasRedirectAddr() is called, a zero reference will track this change.

If the link is further set up to operate with load sharing, then the local_addr argument 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 the server pool, 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 the load balance across server pool hosts when the sessions are initiated. If load sharing is desired for just a few specific services, the configuration on LSNAT could be defined to restrict load sharing to 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 multiple times to add entries to the link, Ns, 's server pool.

For links created with LibAliasRedirectAddr(), the port argument is ignored and could have any value, e.g.amp; htons(~0).

This function returns 0 on success, -1 otherwise.

int LibAliasRedirectDynamic(struct libalias *, struct alias_link *link) This function marks the specified static redirect rule entered by LibAliasRedirectPort() as dynamic. This can be used to e.g.amp; 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.amp; dynamic, and partially vs.amp; 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 functions. If an invalid pointer is passed to LibAliasRedirectDelete(), then a program crash or unpredictable operation could result, so care is needed when using this function.

int LibAliasProxyRule(struct libalias *, const char *cmd) The passed cmd string consists of one or more pairs of words. The first word in each pair is a token and the second is the value that should be applied for that token. Tokens and their argument types are as follows:

type encode_ip_hdr | encode_tcp_stream | no_encode
	In order to support transparent proxying, it is necessary to somehow pass the original address and port information into the new destination server. If encode_ip_hdr is specified, the original destination address and port are passed as an extra IP option. If encode_tcp_stream is specified, the original destination address and port are passed as the first piece of data in the TCP stream in the format "DEST IP port".
port portnum
	Only packets with the destination port portnum are proxied.
server host[:portnum]
	This specifies the host and portnum that the data is to be redirected to. host must be an IP address rather than a DNS host name. If portnum is not specified, the destination port number is not changed. The server specification is mandatory unless the delete command is being used.
rule index
	Normally, each call to LibAliasProxyRule() 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
	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 * 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 will 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.

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.

void * LibAliasGetFragment(struct libalias *, void *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 *, void *header, void *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.

    LibAliasRefreshModules()

recompile and you are done.