LIBALIAS(3) | Library Functions Manual | LIBALIAS(3) |
NAME¶
libalias — packet aliasing library for masquerading and network address translationSYNOPSIS¶
#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 the 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.INITIALIZATION AND CONTROL¶
One special function, LibAliasInit(), must always be called before any packet handling may be performed, and the returned instance pointer must be 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
void
LibAliasUninit(struct libalias *)
NULL
pointer should be passed as an argument. 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
This function has no return value
and is used to clear any resources attached to internal data structures.
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
unsigned int
LibAliasSetMode(struct libalias *,
unsigned int flags, unsigned int
mask)
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.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>:
void
LibAliasSetFWBase(struct libalias *,
unsigned int base, unsigned int
num)
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. 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)
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. 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.
Set the firewall range allocated
for punching firewall holes (with the
void
LibAliasSkinnyPort(struct libalias *,
unsigned int port)
PKT_ALIAS_PUNCH_FW
flag). The range is cleared for all
rules on initialization.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 handling functions, LibAliasIn() and LibAliasOut(), comprise the 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 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:
int LibAliasOut(struct
libalias *, char *buffer, int
maxpacketsize)
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, as for an ICMP message type that is not
handled, or if incoming packets for new connections are being ignored (if
the
PKT_ALIAS_DENY_INCOMING
mode bit was set using 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 situation, 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 unresolved fragments with LibAliasGetFragment() and de-alias them with LibAliasFragmentIn().
PKT_ALIAS_ERROR
- An internal error within the packet aliasing engine occurred.
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 * 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
struct alias_link *
LibAliasRedirectAddr(struct libalias
*, struct in_addr local_addr,
struct in_addr alias_addr);
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 with 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. The remote port specification will almost always 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 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.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:
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
int
LibAliasAddServer(struct libalias *,
struct alias_link *link, struct in_addr
addr, u_short port);
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"));
NULL
is
returned, then the function call did not complete successfully.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's server pool.
For links created with LibAliasRedirectAddr(), the
port argument is ignored and could have any value, e.g.
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. dynamically redirect a single TCP
connection, after which the rule is removed. Only fully specified links can be
made dynamic. (See the STATIC
AND DYNAMIC LINKS and
PARTIALLY SPECIFIED
ALIASING LINKS sections below for a definition of static vs. dynamic, and
partially vs. fully specified links.)
This function returns 0 on success, -1 otherwise.
void
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:
struct alias_link *
LibAliasRedirectProto(struct libalias
*, struct in_addr local_addr,
struct in_addr remote_addr, struct
in_addr alias_addr, u_char proto);
- 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 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.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
char *
LibAliasGetFragment(struct libalias *,
char *buffer)
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.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
void
LibAliasFragmentIn(struct libalias *,
char *header, char *fragment)
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
.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¶
struct alias_link * AddLink(struct libalias *, struct in_addr src_addr, struct in_addr dst_addr, struct in_addr alias_addr, u_short src_port, u_short dst_port, int alias_param, int link_type)This function adds new state to the
instance hash table. The dst_address and/or dst_port may be given as zero,
which introduces some dynamic character into the link, since
LibAliasSetAddress can change the address that is used. However, in the
current implementation, such links can only be used for inbound (ext ->
int) traffic.
void
LibAliasSetTarget(struct libalias *,
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
LibAliasSetTarget().
If this function is called with an
int
LibAliasCheckNewLink(struct libalias
*)
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.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 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 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,
LibAliasInternetChecksum() will return zero.
int
LibAliasUnaliasOut(struct libalias *,
char *buffer, int maxpacketsize)
An outgoing packet, which has
already been aliased, has its private address/port information restored by
this function. The IP packet is pointed to by buffer,
and maxpacketsize is provided for error checking
purposes. This function can be used if an already-aliased packet needs to have
its original IP header restored for further processing (e.g. logging).
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.
Paolo Pisati ⟨piso@FreeBSD.org⟩ made the library modular, moving support for all protocols (except for IP, TCP and UDP) to external modules.
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)
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)
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)
MODULAR ARCHITECTURE (AND ipfw(4) Sh SUPPORT)¶
One of the latest improvements to libalias was to make its support for new protocols independent from the rest of the library, giving it the ability to load/unload support for new protocols at run-time. To achieve this feature, all the code for protocol handling was moved to a series of modules outside of the main library. These modules are compiled from the same sources but work in different ways, depending on whether they are compiled to work inside a kernel or as part of the userland library.LIBALIAS MODULES IN KERNEL LAND¶
When compiled for the kernel, libalias modules are plain KLDs recognizable with the alias_ prefix. To add support for a new protocol, load the corresponding module. For example:kldload alias_ftp
kldunload alias_ftp
LIBALIAS MODULES IN USERLAND¶
Due to the differences between kernel and userland (no KLD mechanism, many different address spaces, etc.), we had to change a bit how to handle module loading/tracking/unloading in userland. While compiled for a userland libalias, all the modules are plain libraries, residing in /usr/lib, and recognizable with the libalias_ prefix. There is a configuration file, /etc/libalias.conf, with the following contents (by default):/usr/lib/libalias_cuseeme.so /usr/lib/libalias_ftp.so /usr/lib/libalias_irc.so /usr/lib/libalias_nbt.so /usr/lib/libalias_pptp.so /usr/lib/libalias_skinny.so /usr/lib/libalias_smedia.so
SIGHUP
signal handler, add a
call to LibAliasRefreshModules() inside the handler, and
everytime you want to refresh the loaded modules, send it the
SIGHUP
signal:
kill -HUP <process_pid>
MODULAR ARCHITECURE: HOW IT WORKS¶
The modular architecture of libalias works similar whether it is running inside the kernel or in userland. From alias_mod.c:/* Protocol and userland module handlers chains. */ LIST_HEAD(handler_chain, proto_handler) handler_chain ... ... SLIST_HEAD(dll_chain, dll) dll_chain ...
struct proto_handler { u_int pri; int16_t dir; uint8_t proto; int (*fingerprint)(struct libalias *la, struct ip *pip, struct alias_data *ah); int (*protohandler)(struct libalias *la, struct ip *pip, struct alias_data *ah); LIST_ENTRY(proto_handler) entries; };
- pri
- is the priority assigned to a protocol handler; lower priority is better.
- dir
- is the direction of packets: ingoing or outgoing.
- proto
- indicates to which protocol this packet belongs: IP, TCP or UDP.
- fingerprint
- points to the fingerprint function while protohandler points to the protocol handler function.
struct alias_data ad = { lnk, &original_address, &alias_address, &alias_port, &ud->uh_sport, /* original source port */ &ud->uh_dport, /* original dest port */ 256 /* maxpacketsize */ }; ... /* walk out chain */ err = find_handler(IN, UDP, la, pip, &ad);
- IN
- direction
- UDP
- working protocol
- la
- pointer to this instance of libalias
- pip
- pointer to a struct ip
- ad
- pointer to struct alias_data (see above)
struct dll { /* name of module */ char name[DLL_LEN]; /* * ptr to shared obj obtained through * dlopen() - use this ptr to get access * to any symbols from a loaded module * via dlsym() */ void *handle; struct dll *next; };
- name
- is the name of the module
- handle
- is a pointer to the module obtained through dlopen(3)
HOW TO WRITE A MODULE FOR LIBALIAS¶
There is a module (called alias_dummy.[ch]) in libalias that can be used as a skeleton for future work. Here we analyse some parts of that module. From alias_dummy.c:struct proto_handler handlers [] = {{666, IN|OUT, UDP|TCP, &fingerprint, &protohandler}};
static int mod_handler(module_t mod, int type, void *data) { int error; switch (type) { case MOD_LOAD: error = 0; attach_handlers(handlers); break; case MOD_UNLOAD: error = 0; detach_handlers(handlers; break; default: error = EINVAL; } return (error); }
#ifdef _KERNEL static #endif int fingerprint(struct libalias *la, struct ip *pip, struct alias_data *ah) { ... } #ifdef _KERNEL static #endif int protohandler(struct libalias *la, struct ip *pip, struct alias_data *ah) { ... }
PATCHING AN APPLICATION FOR USERLAND LIBALIAS MODULES¶
To add module support into an application that uses libalias, the following simple steps can be followed.- Find the main file of an application (let us call it main.c).
- Add this to the header section of
main.c, if not already present:
and this just after the header section:
#include <signal.h>
static void signal_handler(int);
- Add the following line to the init function of an
application or, if it does not have any init function, put it in
main():
and place the signal_handler() function somewhere in main.c:
signal(SIGHUP, signal_handler);
static void signal_handler(int sig) { LibAliasRefreshModules(); }
SIGHUP
signal, just add a call to LibAliasRefreshModules() in the signal handler function.
LibAliasRefreshModules()
LOGGING SUPPORT IN KERNEL LAND¶
When working as KLD, libalias now has log support that happens on a buffer allocated inside struct libalias (from alias_local.h):struct libalias { ... /* log descriptor */ #ifdef KERNEL_LOG char *logDesc; /* * ptr to an auto-malloced * memory buffer when libalias * works as kld */ #else FILE *logDesc; /* * ptr to /var/log/alias.log * when libalias runs as a * userland lib */ #endif ... }
LOG_SECURITY
facility and the LOG_INFO
level.July 4, 2011 | Debian |