1.\"- 2.\" Copyright (c) 2001 Charles Mott <cm@linktel.net> 3.\" All rights reserved. 4.\" 5.\" Redistribution and use in source and binary forms, with or without 6.\" modification, are permitted provided that the following conditions 7.\" are met: 8.\" 1. Redistributions of source code must retain the above copyright 9.\" notice, this list of conditions and the following disclaimer. 10.\" 2. Redistributions in binary form must reproduce the above copyright 11.\" notice, this list of conditions and the following disclaimer in the 12.\" documentation and/or other materials provided with the distribution. 13.\" 14.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 15.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 16.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 17.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 18.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 19.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 20.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 21.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 22.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 23.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 24.\" SUCH DAMAGE. 25.\" 26.\" $FreeBSD: src/lib/libalias/libalias.3,v 1.23.2.11 2001/12/17 10:08:22 ru Exp $ 27.\" $DragonFly: src/lib/libalias/libalias.3,v 1.4 2006/05/26 19:39:36 swildner Exp $ 28.\" 29.Dd April 13, 2000 30.Dt LIBALIAS 3 31.Os 32.Sh NAME 33.Nm libalias 34.Nd packet aliasing library for masquerading and network address translation 35.Sh SYNOPSIS 36.In sys/types.h 37.In netinet/in.h 38.In alias.h 39.Pp 40Function prototypes are given in the main body of the text. 41.Sh DESCRIPTION 42The 43.Nm 44library is a collection of functions for aliasing and de-aliasing of IP 45packets, intended for masquerading and network address translation (NAT). 46.Sh INTRODUCTION 47This library is a moderately portable set of functions designed to assist 48in the process of IP masquerading and network address translation. 49Outgoing packets from a local network with unregistered IP addresses can 50be aliased to appear as if they came from an accessible IP address. 51Incoming packets are then de-aliased so that they are sent to the correct 52machine on the local network. 53.Pp 54A certain amount of flexibility is built into the packet aliasing engine. 55In the simplest mode of operation, a many-to-one address mapping takes 56place between local network and the packet aliasing host. 57This is known as IP masquerading. 58In addition, one-to-one mappings between local and public addresses can 59also be implemented, which is known as static NAT. 60In between these extremes, different groups of private addresses can be 61linked to different public addresses, comprising several distinct 62many-to-one mappings. 63Also, a given public address and port can be statically redirected to a 64private address/port. 65.Pp 66The packet aliasing engine was designed to operate in user space outside 67of the kernel, without any access to private kernel data structure, but 68the source code can also be ported to a kernel environment. 69.Sh INITIALIZATION AND CONTROL 70Two special functions, 71.Fn PacketAliasInit 72and 73.Fn PacketAliasSetAddress , 74must always be called before any packet handling may be performed. 75In addition, the operating mode of the packet aliasing engine can be 76customized by calling 77.Fn PacketAliasSetMode . 78.Pp 79.Ft void 80.Fn PacketAliasInit void 81.Bd -ragged -offset indent 82This function has no arguments or return value and is used to initialize 83internal data structures. 84The following mode bits are always set after calling 85.Fn PacketAliasInit . 86See the description of 87.Fn PacketAliasSetMode 88below for the meaning of these mode bits. 89.Pp 90.Bl -item -offset indent -compact 91.It 92.Dv PKT_ALIAS_SAME_PORTS 93.It 94.Dv PKT_ALIAS_USE_SOCKETS 95.It 96.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE 97.El 98.Pp 99This function will always return the packet aliasing engine to the same 100initial state. 101.Fn PacketAliasSetAddress 102must be called afterwards, and any desired changes from the default mode 103bits listed above require a call to 104.Fn PacketAliasSetMode . 105.Pp 106It is mandatory that this function be called at the beginning of a program 107prior to any packet handling. 108.Ed 109.Pp 110.Ft void 111.Fn PacketAliasUninit void 112.Bd -ragged -offset indent 113This function has no arguments or return value and is used to clear any 114resources attached to internal data structures. 115.Pp 116This functions should be called when a program stops using the aliasing 117engine; it does, amongst other things, clear out any firewall holes. 118To provide backwards compatibility and extra security, it is added to 119the 120.Xr atexit 3 121chain by 122.Fn PacketAliasInit . 123Calling it multiple times is harmless. 124.Ed 125.Pp 126.Ft void 127.Fn PacketAliasSetAddress "struct in_addr addr" 128.Bd -ragged -offset indent 129This function sets the source address to which outgoing packets from the 130local area network are aliased. 131All outgoing packets are re-mapped to this address unless overridden by a 132static address mapping established by 133.Fn PacketAliasRedirectAddr . 134.Pp 135If the 136.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE 137mode bit is set (the default mode of operation), then the internal aliasing 138link tables will be reset any time the aliasing address changes. 139This is useful for interfaces such as 140.Xr ppp 8 , 141where the IP 142address may or may not change on successive dial-up attempts. 143.Pp 144If the 145.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE 146mode bit is set to zero, this function can also be used to dynamically change 147the aliasing address on a packet to packet basis (it is a low overhead call). 148.Pp 149It is mandatory that this function be called prior to any packet handling. 150.Ed 151.Pp 152.Ft unsigned int 153.Fn PacketAliasSetMode "unsigned int flags" "unsigned int mask" 154.Bd -ragged -offset indent 155This function sets or clears mode bits 156according to the value of 157.Fa flags . 158Only bits marked in 159.Fa mask 160are affected. 161The following mode bits are defined in 162.In alias.h : 163.Bl -tag -width indent 164.It Dv PKT_ALIAS_LOG 165Enables logging into 166.Pa /var/log/alias.log . 167Each time an aliasing link is created or deleted, the log file is appended 168with the current number of ICMP, TCP and UDP links. 169Mainly useful for debugging when the log file is viewed continuously with 170.Xr tail 1 . 171.It Dv PKT_ALIAS_DENY_INCOMING 172If this mode bit is set, all incoming packets associated with new TCP 173connections or new UDP transactions will be marked for being ignored 174.Fn ( PacketAliasIn 175returns 176.Dv PKT_ALIAS_IGNORED 177code) 178by the calling program. 179Response packets to connections or transactions initiated from the packet 180aliasing host or local network will be unaffected. 181This mode bit is useful for implementing a one-way firewall. 182.It Dv PKT_ALIAS_SAME_PORTS 183If this mode bit is set, the packet aliasing engine will attempt to leave 184the alias port numbers unchanged from the actual local port numbers. 185This can be done as long as the quintuple (proto, alias addr, alias port, 186remote addr, remote port) is unique. 187If a conflict exists, a new aliasing port number is chosen even if this 188mode bit is set. 189.It Dv PKT_ALIAS_USE_SOCKETS 190This bit should be set when the packet aliasing host originates network 191traffic as well as forwards it. 192When the packet aliasing host is waiting for a connection from an unknown 193host address or unknown port number (e.g. an FTP data connection), this 194mode bit specifies that a socket be allocated as a place holder to prevent 195port conflicts. 196Once a connection is established, usually within a minute or so, the socket 197is closed. 198.It Dv PKT_ALIAS_UNREGISTERED_ONLY 199If this mode bit is set, traffic on the local network which does not 200originate from unregistered address spaces will be ignored. 201Standard Class A, B and C unregistered addresses are: 202.Bd -literal -offset indent 20310.0.0.0 -> 10.255.255.255 (Class A subnet) 204172.16.0.0 -> 172.31.255.255 (Class B subnets) 205192.168.0.0 -> 192.168.255.255 (Class C subnets) 206.Ed 207.Pp 208This option is useful in the case that packet aliasing host has both 209registered and unregistered subnets on different interfaces. 210The registered subnet is fully accessible to the outside world, so traffic 211from it does not need to be passed through the packet aliasing engine. 212.It Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE 213When this mode bit is set and 214.Fn PacketAliasSetAddress 215is called to change the aliasing address, the internal link table of the 216packet aliasing engine will be cleared. 217This operating mode is useful for 218.Xr ppp 8 219links where the interface address can sometimes change or remain the same 220between dial-up attempts. 221If this mode bit is not set, the link table will never be reset in the event 222of an address change. 223.It Dv PKT_ALIAS_PUNCH_FW 224This option makes 225.Nm 226`punch holes' in an 227.Xr ipfirewall 4 228based firewall for FTP/IRC DCC connections. 229The holes punched are bound by from/to IP address and port; it will not be 230possible to use a hole for another connection. 231A hole is removed when the connection that uses it dies. 232To cater to unexpected death of a program using 233.Nm 234(e.g. kill -9), 235changing the state of the flag will clear the entire firewall range 236allocated for holes. 237This will also happen on the initial call to 238.Fn PacketAliasSetFWBase . 239This call must happen prior to setting this flag. 240.It Dv PKT_ALIAS_REVERSE 241This option makes 242.Nm 243reverse the way it handles incoming and outgoing packets, allowing it 244to be fed with data that passes through the internal interface rather 245than the external one. 246.It Dv PKT_ALIAS_PROXY_ONLY 247This option tells 248.Nm 249to obey transparent proxy rules only. 250Normal packet aliasing is not performed. 251See 252.Fn PacketAliasProxyRule 253below for details. 254.El 255.Ed 256.Pp 257.Ft void 258.Fn PacketAliasSetFWBase "unsigned int base" "unsigned int num" 259.Bd -ragged -offset indent 260Set firewall range allocated for punching firewall holes (with the 261.Dv PKT_ALIAS_PUNCH_FW 262flag). 263The range will be cleared for all rules on initialization. 264.Ed 265.Sh PACKET HANDLING 266The packet handling functions are used to modify incoming (remote to local) 267and outgoing (local to remote) packets. 268The calling program is responsible for receiving and sending packets via 269network interfaces. 270.Pp 271Along with 272.Fn PacketAliasInit 273and 274.Fn PacketAliasSetAddress , 275the two packet handling functions, 276.Fn PacketAliasIn 277and 278.Fn PacketAliasOut , 279comprise minimal set of functions needed for a basic IP masquerading 280implementation. 281.Pp 282.Ft int 283.Fn PacketAliasIn "char *buffer" "int maxpacketsize" 284.Bd -ragged -offset indent 285An incoming packet coming from a remote machine to the local network is 286de-aliased by this function. 287The IP packet is pointed to by 288.Fa buffer , 289and 290.Fa maxpacketsize 291indicates the size of the data structure containing the packet and should 292be at least as large as the actual packet size. 293.Pp 294Return codes: 295.Bl -tag -width indent 296.It Dv PKT_ALIAS_OK 297The packet aliasing process was successful. 298.It Dv PKT_ALIAS_IGNORED 299The packet was ignored and not de-aliased. 300This can happen if the protocol is unrecognized, possibly an ICMP message 301type is not handled or if incoming packets for new connections are being 302ignored (if 303.Dv PKT_ALIAS_DENY_INCOMING 304mode bit was set by 305.Fn PacketAliasSetMode ) . 306.It Dv PKT_ALIAS_UNRESOLVED_FRAGMENT 307This is returned when a fragment cannot be resolved because the header 308fragment has not been sent yet. 309In this situation, fragments must be saved with 310.Fn PacketAliasSaveFragment 311until a header fragment is found. 312.It Dv PKT_ALIAS_FOUND_HEADER_FRAGMENT 313The packet aliasing process was successful, and a header fragment was found. 314This is a signal to retrieve any unresolved fragments with 315.Fn PacketAliasGetFragment 316and de-alias them with 317.Fn PacketAliasFragmentIn . 318.It Dv PKT_ALIAS_ERROR 319An internal error within the packet aliasing engine occurred. 320.El 321.Ed 322.Pp 323.Ft int 324.Fn PacketAliasOut "char *buffer" "int maxpacketsize" 325.Bd -ragged -offset indent 326An outgoing packet coming from the local network to a remote machine is 327aliased by this function. 328The IP packet is pointed to by 329.Fa buffer , 330and 331.Fa maxpacketsize 332indicates the maximum packet size permissible should the packet length be 333changed. 334IP encoding protocols place address and port information in the encapsulated 335data stream which has to be modified and can account for changes in packet 336length. 337Well known examples of such protocols are FTP and IRC DCC. 338.Pp 339Return codes: 340.Bl -tag -width indent 341.It Dv PKT_ALIAS_OK 342The packet aliasing process was successful. 343.It Dv PKT_ALIAS_IGNORED 344The packet was ignored and not aliased. 345This can happen if the protocol is unrecognized, or possibly an ICMP message 346type is not handled. 347.It Dv PKT_ALIAS_ERROR 348An internal error within the packet aliasing engine occurred. 349.El 350.Ed 351.Sh PORT AND ADDRESS REDIRECTION 352The functions described in this section allow machines on the local network 353to be accessible in some degree to new incoming connections from the external 354network. 355Individual ports can be re-mapped or static network address translations can 356be designated. 357.Pp 358.Ft struct alias_link * 359.Fo PacketAliasRedirectPort 360.Fa "struct in_addr local_addr" 361.Fa "u_short local_port" 362.Fa "struct in_addr remote_addr" 363.Fa "u_short remote_port" 364.Fa "struct in_addr alias_addr" 365.Fa "u_short alias_port" 366.Fa "u_char proto" 367.Fc 368.Bd -ragged -offset indent 369This function specifies that traffic from a given remote address/port to 370an alias address/port be redirected to a specified local address/port. 371The parameter 372.Fa proto 373can be either 374.Dv IPPROTO_TCP 375or 376.Dv IPPROTO_UDP , 377as defined in 378.In netinet/in.h . 379.Pp 380If 381.Fa local_addr 382or 383.Fa alias_addr 384is zero, this indicates that the packet aliasing address as established 385by 386.Fn PacketAliasSetAddress 387is to be used. 388Even if 389.Fn PacketAliasSetAddress 390is called to change the address after 391.Fn PacketAliasRedirectPort 392is called, a zero reference will track this change. 393.Pp 394If the link is further set up to operate for a load sharing, then 395.Fa local_addr 396and 397.Fa local_port 398are ignored, and are selected dynamically from the server pool, as described in 399.Fn PacketAliasAddServer 400below. 401.Pp 402If 403.Fa remote_addr 404is zero, this indicates to redirect packets from any remote address. 405Likewise, if 406.Fa remote_port 407is zero, this indicates to redirect packets originating from any remote 408port number. 409Almost always, the remote port specification will be zero, but non-zero 410remote addresses can sometimes be useful for firewalling. 411If two calls to 412.Fn PacketAliasRedirectPort 413overlap in their address/port specifications, then the most recent call 414will have precedence. 415.Pp 416This function returns a pointer which can subsequently be used by 417.Fn PacketAliasRedirectDelete . 418If 419.Dv NULL 420is returned, then the function call did not complete successfully. 421.Pp 422All port numbers should be in network address byte order, so it is necessary 423to use 424.Xr htons 3 425to convert these parameters from internally readable numbers to network byte 426order. 427Addresses are also in network byte order, which is implicit in the use of the 428.Fa struct in_addr 429data type. 430.Ed 431.Pp 432.Ft struct alias_link * 433.Fo PacketAliasRedirectAddr 434.Fa "struct in_addr local_addr" 435.Fa "struct in_addr alias_addr" 436.Fc 437.Bd -ragged -offset indent 438This function designates that all incoming traffic to 439.Fa alias_addr 440be redirected to 441.Fa local_addr . 442Similarly, all outgoing traffic from 443.Fa local_addr 444is aliased to 445.Fa alias_addr . 446.Pp 447If 448.Fa local_addr 449or 450.Fa alias_addr 451is zero, this indicates that the packet aliasing address as established by 452.Fn PacketAliasSetAddress 453is to be used. 454Even if 455.Fn PacketAliasSetAddress 456is called to change the address after 457.Fn PacketAliasRedirectAddr 458is called, a zero reference will track this change. 459.Pp 460If the link is further set up to operate for a load sharing, then 461.Fa local_addr 462is ignored, and is selected dynamically from the server pool, as described in 463.Fn PacketAliasAddServer 464below. 465.Pp 466If subsequent calls to 467.Fn PacketAliasRedirectAddr 468use the same aliasing address, all new incoming traffic to this aliasing 469address will be redirected to the local address made in the last function 470call. 471New traffic generated by any of the local machines, designated in the 472several function calls, will be aliased to the same address. 473Consider the following example: 474.Bd -literal -offset indent 475PacketAliasRedirectAddr(inet_aton("192.168.0.2"), 476 inet_aton("141.221.254.101")); 477PacketAliasRedirectAddr(inet_aton("192.168.0.3"), 478 inet_aton("141.221.254.101")); 479PacketAliasRedirectAddr(inet_aton("192.168.0.4"), 480 inet_aton("141.221.254.101")); 481.Ed 482.Pp 483Any outgoing connections such as 484.Xr telnet 1 485or 486.Xr ftp 1 487from 192.168.0.2, 192.168.0.3 and 192.168.0.4 will appear to come from 488141.221.254.101. 489Any incoming connections to 141.221.254.101 will be directed to 192.168.0.4. 490.Pp 491Any calls to 492.Fn PacketAliasRedirectPort 493will have precedence over address mappings designated by 494.Fn PacketAliasRedirectAddr . 495.Pp 496This function returns a pointer which can subsequently be used by 497.Fn PacketAliasRedirectDelete . 498If 499.Dv NULL 500is returned, then the function call did not complete successfully. 501.Ed 502.Pp 503.Ft int 504.Fo PacketAliasAddServer 505.Fa "struct alias_link *link" 506.Fa "struct in_addr addr" 507.Fa "u_short port" 508.Fc 509.Bd -ragged -offset indent 510This function sets the 511.Fa link 512up for Load Sharing using IP Network Address Translation (RFC 2391, LSNAT). 513LSNAT operates as follows. 514A client attempts to access a server by using the server virtual address. 515The LSNAT router transparently redirects the request to one of the hosts 516in server pool, selected using a real-time load sharing algorithm. 517Multiple sessions may be initiated from the same client, and each session 518could be directed to a different host based on load balance across server 519pool hosts at the time. 520If load share is desired for just a few specific services, the configuration 521on LSNAT could be defined to restrict load share for just the services 522desired. 523.Pp 524Currently, only the simplest selection algorithm is implemented, where a 525host is selected on a round-robin basis only, without regard to load on 526the host. 527.Pp 528First, the 529.Fa link 530is created by either 531.Fn PacketAliasRedirectPort 532or 533.Fn PacketAliasRedirectAddr . 534Then, 535.Fn PacketAliasAddServer 536is called multiple times to add entries to the 537.Fa link Ns 's 538server pool. 539.Pp 540For links created with 541.Fn PacketAliasRedirectAddr , 542the 543.Fa port 544argument is ignored and could have any value, e.g. htons(~0). 545.Pp 546This function returns 0 on success, -1 otherwise. 547.Ed 548.Pp 549.Ft void 550.Fn PacketAliasRedirectDelete "struct alias_link *link" 551.Bd -ragged -offset indent 552This function will delete a specific static redirect rule entered by 553.Fn PacketAliasRedirectPort 554or 555.Fn PacketAliasRedirectAddr . 556The parameter 557.Fa link 558is the pointer returned by either of the redirection functions. 559If an invalid pointer is passed to 560.Fn PacketAliasRedirectDelete , 561then a program crash or unpredictable operation could result, so it is 562necessary to be careful using this function. 563.Ed 564.Pp 565.Ft int 566.Fn PacketAliasProxyRule "const char *cmd" 567.Bd -ragged -offset indent 568The passed 569.Fa cmd 570string consists of one or more pairs of words. 571The first word in each pair is a token and the second is the value that 572should be applied for that token. 573Tokens and their argument types are as follows: 574.Bl -tag -width indent 575.It Cm type encode_ip_hdr | encode_tcp_stream | no_encode 576In order to support transparent proxying, it is necessary to somehow 577pass the original address and port information into the new destination 578server. 579If 580.Cm encode_ip_hdr 581is specified, the original address and port is passed as an extra IP 582option. 583If 584.Cm encode_tcp_stream 585is specified, the original address and port is passed as the first 586piece of data in the TCP stream in the format 587.Dq DEST Ar IP port . 588.It Cm port Ar portnum 589Only packets with the destination port 590.Ar portnum 591are proxied. 592.It Cm server Ar host Ns Xo 593.Op : Ns Ar portnum 594.Xc 595This specifies the 596.Ar host 597and 598.Ar portnum 599that the data is to be redirected to. 600.Ar host 601must be an IP address rather than a DNS host name. 602If 603.Ar portnum 604is not specified, the destination port number is not changed. 605.Pp 606The 607.Ar server 608specification is mandatory unless the 609.Cm delete 610command is being used. 611.It Cm rule Ar index 612Normally, each call to 613.Fn PacketAliasProxyRule 614inserts the next rule at the start of a linear list of rules. 615If an 616.Ar index 617is specified, the new rule will be checked after all rules with lower 618indices. 619Calls to 620.Fn PacketAliasProxyRule 621that do not specify a rule are assigned rule 0. 622.It Cm delete Ar index 623This token and its argument MUST NOT be used with any other tokens. 624When used, all existing rules with the given 625.Ar index 626are deleted. 627.It Cm proto tcp | udp 628If specified, only packets of the given protocol type are matched. 629.It Cm src Ar IP Ns Xo 630.Op / Ns Ar bits 631.Xc 632If specified, only packets with a source address matching the given 633.Ar IP 634are matched. 635If 636.Ar bits 637is also specified, then the first 638.Ar bits 639bits of 640.Ar IP 641are taken as a network specification, and all IP addresses from that 642network will be matched. 643.It Cm dst Ar IP Ns Xo 644.Op / Ns Ar bits 645.Xc 646If specified, only packets with a destination address matching the given 647.Ar IP 648are matched. 649If 650.Ar bits 651is also specified, then the first 652.Ar bits 653bits of 654.Ar IP 655are taken as a network specification, and all IP addresses from that 656network will be matched. 657.El 658.Pp 659This function is usually used to redirect outgoing connections for 660internal machines that are not permitted certain types of internet 661access, or to restrict access to certain external machines. 662.Ed 663.Pp 664.Ft struct alias_link * 665.Fo PacketAliasRedirectProto 666.Fa "struct in_addr local_addr" 667.Fa "struct in_addr remote_addr" 668.Fa "struct in_addr alias_addr" 669.Fa "u_char proto" 670.Fc 671.Bd -ragged -offset indent 672This function specifies that any IP packet with protocol number of 673.Fa proto 674from a given remote address to an alias address be 675redirected to a specified local address. 676.Pp 677If 678.Fa local_addr 679or 680.Fa alias_addr 681is zero, this indicates that the packet aliasing address as established 682by 683.Fn PacketAliasSetAddress 684is to be used. 685Even if 686.Fn PacketAliasSetAddress 687is called to change the address after 688.Fn PacketAliasRedirectProto 689is called, a zero reference will track this change. 690.Pp 691If 692.Fa remote_addr 693is zero, this indicates to redirect packets from any remote address. 694Non-zero remote addresses can sometimes be useful for firewalling. 695.Pp 696If two calls to 697.Fn PacketAliasRedirectProto 698overlap in their address specifications, then the most recent call 699will have precedence. 700.Pp 701This function returns a pointer which can subsequently be used by 702.Fn PacketAliasRedirectDelete . 703If 704.Dv NULL 705is returned, then the function call did not complete successfully. 706.Ed 707.Sh FRAGMENT HANDLING 708The functions in this section are used to deal with incoming fragments. 709.Pp 710Outgoing fragments are handled within 711.Fn PacketAliasOut 712by changing the address according to any applicable mapping set by 713.Fn PacketAliasRedirectAddr , 714or the default aliasing address set by 715.Fn PacketAliasSetAddress . 716.Pp 717Incoming fragments are handled in one of two ways. 718If the header of a fragmented IP packet has already been seen, then all 719subsequent fragments will be re-mapped in the same manner the header 720fragment was. 721Fragments which arrive before the header are saved and then retrieved 722once the header fragment has been resolved. 723.Pp 724.Ft int 725.Fn PacketAliasSaveFragment "char *ptr" 726.Bd -ragged -offset indent 727When 728.Fn PacketAliasIn 729returns 730.Dv PKT_ALIAS_UNRESOLVED_FRAGMENT , 731this function can be used to save the pointer to the unresolved fragment. 732.Pp 733It is implicitly assumed that 734.Fa ptr 735points to a block of memory allocated by 736.Xr malloc 3 . 737If the fragment is never resolved, the packet aliasing engine will 738automatically free the memory after a timeout period. 739[Eventually this function should be modified so that a callback function 740for freeing memory is passed as an argument.] 741.Pp 742This function returns 743.Dv PKT_ALIAS_OK 744if it was successful and 745.Dv PKT_ALIAS_ERROR 746if there was an error. 747.Ed 748.Pp 749.Ft char * 750.Fn PacketAliasGetFragment "char *buffer" 751.Bd -ragged -offset indent 752This function can be used to retrieve fragment pointers saved by 753.Fn PacketAliasSaveFragment . 754The IP header fragment pointed to by 755.Fa buffer 756is the header fragment indicated when 757.Fn PacketAliasIn 758returns 759.Dv PKT_ALIAS_FOUND_HEADER_FRAGMENT . 760Once a fragment pointer is retrieved, it becomes the calling program's 761responsibility to free the dynamically allocated memory for the fragment. 762.Pp 763.Fn PacketAliasGetFragment 764can be called sequentially until there are no more fragments available, 765at which time it returns 766.Dv NULL . 767.Ed 768.Pp 769.Ft void 770.Fn PacketAliasFragmentIn "char *header" "char *fragment" 771.Bd -ragged -offset indent 772When a fragment is retrieved with 773.Fn PacketAliasGetFragment , 774it can then be de-aliased with a call to 775.Fn PacketAliasFragmentIn . 776The 777.Fa header 778argument is the pointer to a header fragment used as a template, and 779.Fa fragment 780is the pointer to the packet to be de-aliased. 781.Ed 782.Sh MISCELLANEOUS FUNCTIONS 783.Ft void 784.Fn PacketAliasSetTarget "struct in_addr addr" 785.Bd -ragged -offset indent 786When an incoming packet not associated with any pre-existing aliasing link 787arrives at the host machine, it will be sent to the address indicated by a 788call to 789.Fn PacketAliasSetTarget . 790.Pp 791If this function is called with an 792.Dv INADDR_NONE 793address argument, then all new incoming packets go to the address set by 794.Fn PacketAliasSetAddress . 795.Pp 796If this function is not called, or is called with an 797.Dv INADDR_ANY 798address argument, then all new incoming packets go to the address specified 799in the packet. 800This allows external machines to talk directly to internal machines if they 801can route packets to the machine in question. 802.Ed 803.Pp 804.Ft int 805.Fn PacketAliasCheckNewLink void 806.Bd -ragged -offset indent 807This function returns a non-zero value when a new aliasing link is created. 808In circumstances where incoming traffic is being sequentially sent to 809different local servers, this function can be used to trigger when 810.Fn PacketAliasSetTarget 811is called to change the default target address. 812.Ed 813.Pp 814.Ft u_short 815.Fn PacketAliasInternetChecksum "u_short *buffer" "int nbytes" 816.Bd -ragged -offset indent 817This is a utility function that does not seem to be available elsewhere and 818is included as a convenience. 819It computes the internet checksum, which is used in both IP and 820protocol-specific headers (TCP, UDP, ICMP). 821.Pp 822The 823.Fa buffer 824argument points to the data block to be checksummed, and 825.Fa nbytes 826is the number of bytes. 827The 16-bit checksum field should be zeroed before computing the checksum. 828.Pp 829Checksums can also be verified by operating on a block of data including 830its checksum. 831If the checksum is valid, 832.Fn PacketAliasInternetChecksum 833will return zero. 834.Ed 835.Pp 836.Ft int 837.Fn PacketUnaliasOut "char *buffer" "int maxpacketsize" 838.Bd -ragged -offset indent 839An outgoing packet, which has already been aliased, 840has its private address/port information restored by this function. 841The IP packet is pointed to by 842.Fa buffer , 843and 844.Fa maxpacketsize 845is provided for error checking purposes. 846This function can be used if an already-aliased packet needs to have its 847original IP header restored for further processing (eg. logging). 848.Ed 849.Sh AUTHORS 850.An Charles Mott Aq cm@linktel.net , 851versions 1.0 - 1.8, 2.0 - 2.4. 852.An Eivind Eklund Aq eivind@FreeBSD.org , 853versions 1.8b, 1.9 and 2.5. 854Added IRC DCC support as well as contributing a number of architectural 855improvements; added the firewall bypass for FTP/IRC DCC. 856.An Erik Salander Aq erik@whistle.com 857added support for PPTP and RTSP. 858.An Junichi Satoh Aq junichi@junichi.org 859added support for RTSP/PNA. 860.Sh ACKNOWLEDGMENTS 861Listed below, in approximate chronological order, are individuals who 862have provided valuable comments and/or debugging assistance. 863.Pp 864.Bd -ragged -offset indent 865.An -split 866.An Gary Roberts 867.An Tom Torrance 868.An Reto Burkhalter 869.An Martin Renters 870.An Brian Somers 871.An Paul Traina 872.An Ari Suutari 873.An Dave Remien 874.An J. Fortes 875.An Andrzej Bialecki 876.An Gordon Burditt 877.Ed 878.Sh CONCEPTUAL BACKGROUND 879This section is intended for those who are planning to modify the source 880code or want to create somewhat esoteric applications using the packet 881aliasing functions. 882.Pp 883The conceptual framework under which the packet aliasing engine operates 884is described here. 885Central to the discussion is the idea of an 886.Em aliasing link 887which describes the relationship for a given packet transaction between 888the local machine, aliased identity and remote machine. 889It is discussed how such links come into existence and are destroyed. 890.Ss ALIASING LINKS 891There is a notion of an 892.Em aliasing link , 893which is a 7-tuple describing a specific translation: 894.Bd -literal -offset indent 895(local addr, local port, alias addr, alias port, 896 remote addr, remote port, protocol) 897.Ed 898.Pp 899Outgoing packets have the local address and port number replaced with the 900alias address and port number. 901Incoming packets undergo the reverse process. 902The packet aliasing engine attempts to match packets against an internal 903table of aliasing links to determine how to modify a given IP packet. 904Both the IP header and protocol dependent headers are modified as necessary. 905Aliasing links are created and deleted as necessary according to network 906traffic. 907.Pp 908Protocols can be TCP, UDP or even ICMP in certain circumstances. 909(Some types of ICMP packets can be aliased according to sequence or ID 910number which acts as an equivalent port number for identifying how 911individual packets should be handled.) 912.Pp 913Each aliasing link must have a unique combination of the following five 914quantities: alias address/port, remote address/port and protocol. 915This ensures that several machines on a local network can share the 916same aliasing IP address. 917In cases where conflicts might arise, the aliasing port is chosen so that 918uniqueness is maintained. 919.Ss STATIC AND DYNAMIC LINKS 920Aliasing links can either be static or dynamic. 921Static links persist indefinitely and represent fixed rules for translating 922IP packets. 923Dynamic links come into existence for a specific TCP connection or UDP 924transaction or ICMP ECHO sequence. 925For the case of TCP, the connection can be monitored to see when the 926associated aliasing link should be deleted. 927Aliasing links for UDP transactions (and ICMP ECHO and TIMESTAMP requests) 928work on a simple timeout rule. 929When no activity is observed on a dynamic link for a certain amount of time 930it is automatically deleted. 931Timeout rules also apply to TCP connections which do not open or close 932properly. 933.Ss PARTIALLY SPECIFIED ALIASING LINKS 934Aliasing links can be partially specified, meaning that the remote address 935and/or remote port are unknown. 936In this case, when a packet matching the incomplete specification is found, 937a fully specified dynamic link is created. 938If the original partially specified link is dynamic, it will be deleted 939after the fully specified link is created, otherwise it will persist. 940.Pp 941For instance, a partially specified link might be 942.Bd -literal -offset indent 943(192.168.0.4, 23, 204.228.203.215, 8066, 0, 0, tcp) 944.Ed 945.Pp 946The zeros denote unspecified components for the remote address and port. 947If this link were static it would have the effect of redirecting all 948incoming traffic from port 8066 of 204.228.203.215 to port 23 (telnet) 949of machine 192.168.0.4 on the local network. 950Each individual telnet connection would initiate the creation of a distinct 951dynamic link. 952.Ss DYNAMIC LINK CREATION 953In addition to aliasing links, there are also address mappings that can be 954stored within the internal data table of the packet aliasing mechanism. 955.Bd -literal -offset indent 956(local addr, alias addr) 957.Ed 958.Pp 959Address mappings are searched when creating new dynamic links. 960.Pp 961All outgoing packets from the local network automatically create a dynamic 962link if they do not match an already existing fully specified link. 963If an address mapping exists for the outgoing packet, this determines 964the alias address to be used. 965If no mapping exists, then a default address, usually the address of the 966packet aliasing host, is used. 967If necessary, this default address can be changed as often as each individual 968packet arrives. 969.Pp 970The aliasing port number is determined such that the new dynamic link does 971not conflict with any existing links. 972In the default operating mode, the packet aliasing engine attempts to set 973the aliasing port equal to the local port number. 974If this results in a conflict, then port numbers are randomly chosen until 975a unique aliasing link can be established. 976In an alternate operating mode, the first choice of an aliasing port is also 977random and unrelated to the local port number. 978.Sh BUGS 979PPTP aliasing does not work when more than one internal client 980connects to the same external server at the same time, because 981PPTP requires a single TCP control connection to be established 982between any two IP addresses. 983