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.\" 28.Dd April 13, 2000 29.Dt LIBALIAS 3 30.Os 31.Sh NAME 32.Nm libalias 33.Nd packet aliasing library for masquerading and network address translation 34.Sh LIBRARY 35.Lb libalias 36.Sh SYNOPSIS 37.In sys/types.h 38.In netinet/in.h 39.In alias.h 40.Pp 41Function prototypes are given in the main body of the text. 42.Sh DESCRIPTION 43The 44.Nm 45library is a collection of functions for aliasing and de-aliasing of IP 46packets, intended for masquerading and network address translation (NAT). 47.Sh INTRODUCTION 48This library is a moderately portable set of functions designed to assist 49in the process of IP masquerading and network address translation. 50Outgoing packets from a local network with unregistered IP addresses can 51be aliased to appear as if they came from an accessible IP address. 52Incoming packets are then de-aliased so that they are sent to the correct 53machine on the local network. 54.Pp 55A certain amount of flexibility is built into the packet aliasing engine. 56In the simplest mode of operation, a many-to-one address mapping takes 57place between local network and the packet aliasing host. 58This is known as IP masquerading. 59In addition, one-to-one mappings between local and public addresses can 60also be implemented, which is known as static NAT. 61In between these extremes, different groups of private addresses can be 62linked to different public addresses, comprising several distinct 63many-to-one mappings. 64Also, a given public address and port can be statically redirected to a 65private address/port. 66.Pp 67The packet aliasing engine was designed to operate in user space outside 68of the kernel, without any access to private kernel data structure, but 69the source code can also be ported to a kernel environment. 70.Sh INITIALIZATION AND CONTROL 71Two special functions, 72.Fn PacketAliasInit 73and 74.Fn PacketAliasSetAddress , 75must always be called before any packet handling may be performed. 76In addition, the operating mode of the packet aliasing engine can be 77customized by calling 78.Fn PacketAliasSetMode . 79.Pp 80.Ft void 81.Fn PacketAliasInit void 82.Bd -ragged -offset indent 83This function has no arguments or return value and is used to initialize 84internal data structures. 85The following mode bits are always set after calling 86.Fn PacketAliasInit . 87See the description of 88.Fn PacketAliasSetMode 89below for the meaning of these mode bits. 90.Pp 91.Bl -item -offset indent -compact 92.It 93.Dv PKT_ALIAS_SAME_PORTS 94.It 95.Dv PKT_ALIAS_USE_SOCKETS 96.It 97.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE 98.El 99.Pp 100This function will always return the packet aliasing engine to the same 101initial state. 102.Fn PacketAliasSetAddress 103must be called afterwards, and any desired changes from the default mode 104bits listed above require a call to 105.Fn PacketAliasSetMode . 106.Pp 107It is mandatory that this function be called at the beginning of a program 108prior to any packet handling. 109.Ed 110.Pp 111.Ft void 112.Fn PacketAliasUninit void 113.Bd -ragged -offset indent 114This function has no arguments or return value and is used to clear any 115resources attached to internal data structures. 116.Pp 117This functions should be called when a program stops using the aliasing 118engine; it does, amongst other things, clear out any firewall holes. 119To provide backwards compatibility and extra security, it is added to 120the 121.Xr atexit 3 122chain by 123.Fn PacketAliasInit . 124Calling it multiple times is harmless. 125.Ed 126.Pp 127.Ft void 128.Fn PacketAliasSetAddress "struct in_addr addr" 129.Bd -ragged -offset indent 130This function sets the source address to which outgoing packets from the 131local area network are aliased. 132All outgoing packets are re-mapped to this address unless overridden by a 133static address mapping established by 134.Fn PacketAliasRedirectAddr . 135.Pp 136If the 137.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE 138mode bit is set (the default mode of operation), then the internal aliasing 139link tables will be reset any time the aliasing address changes. 140This is useful for interfaces such as 141.Xr ppp 8 , 142where the IP 143address may or may not change on successive dial-up attempts. 144.Pp 145If the 146.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE 147mode bit is set to zero, this function can also be used to dynamically change 148the aliasing address on a packet to packet basis (it is a low overhead call). 149.Pp 150It is mandatory that this function be called prior to any packet handling. 151.Ed 152.Pp 153.Ft unsigned int 154.Fn PacketAliasSetMode "unsigned int flags" "unsigned int mask" 155.Bd -ragged -offset indent 156This function sets or clears mode bits 157according to the value of 158.Fa flags . 159Only bits marked in 160.Fa mask 161are affected. 162The following mode bits are defined in 163.In alias.h : 164.Bl -tag -width indent 165.It Dv PKT_ALIAS_LOG 166Enables logging into 167.Pa /var/log/alias.log . 168Each time an aliasing link is created or deleted, the log file is appended 169with the current number of ICMP, TCP and UDP links. 170Mainly useful for debugging when the log file is viewed continuously with 171.Xr tail 1 . 172.It Dv PKT_ALIAS_DENY_INCOMING 173If this mode bit is set, all incoming packets associated with new TCP 174connections or new UDP transactions will be marked for being ignored 175.Fn ( PacketAliasIn 176returns 177.Dv PKT_ALIAS_IGNORED 178code) 179by the calling program. 180Response packets to connections or transactions initiated from the packet 181aliasing host or local network will be unaffected. 182This mode bit is useful for implementing a one-way firewall. 183.It Dv PKT_ALIAS_SAME_PORTS 184If this mode bit is set, the packet aliasing engine will attempt to leave 185the alias port numbers unchanged from the actual local port numbers. 186This can be done as long as the quintuple (proto, alias addr, alias port, 187remote addr, remote port) is unique. 188If a conflict exists, a new aliasing port number is chosen even if this 189mode bit is set. 190.It Dv PKT_ALIAS_USE_SOCKETS 191This bit should be set when the packet aliasing host originates network 192traffic as well as forwards it. 193When the packet aliasing host is waiting for a connection from an unknown 194host address or unknown port number (e.g. an FTP data connection), this 195mode bit specifies that a socket be allocated as a place holder to prevent 196port conflicts. 197Once a connection is established, usually within a minute or so, the socket 198is closed. 199.It Dv PKT_ALIAS_UNREGISTERED_ONLY 200If this mode bit is set, traffic on the local network which does not 201originate from unregistered address spaces will be ignored. 202Standard Class A, B and C unregistered addresses are: 203.Pp 20410.0.0.0 -> 10.255.255.255 (Class A subnet) 205172.16.0.0 -> 172.31.255.255 (Class B subnets) 206192.168.0.0 -> 192.168.255.255 (Class C subnets) 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.Pp 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.Pp 482Any outgoing connections such as 483.Xr telnet 1 484or 485.Xr ftp 1 486from 192.168.0.2, 192.168.0.3 and 192.168.0.4 will appear to come from 487141.221.254.101. 488Any incoming connections to 141.221.254.101 will be directed to 192.168.0.4. 489.Pp 490Any calls to 491.Fn PacketAliasRedirectPort 492will have precedence over address mappings designated by 493.Fn PacketAliasRedirectAddr . 494.Pp 495This function returns a pointer which can subsequently be used by 496.Fn PacketAliasRedirectDelete . 497If 498.Dv NULL 499is returned, then the function call did not complete successfully. 500.Ed 501.Pp 502.Ft int 503.Fo PacketAliasAddServer 504.Fa "struct alias_link *link" 505.Fa "struct in_addr addr" 506.Fa "u_short port" 507.Fc 508.Bd -ragged -offset indent 509This function sets the 510.Fa link 511up for Load Sharing using IP Network Address Translation (RFC 2391, LSNAT). 512LSNAT operates as follows. 513A client attempts to access a server by using the server virtual address. 514The LSNAT router transparently redirects the request to one of the hosts 515in server pool, selected using a real-time load sharing algorithm. 516Multiple sessions may be initiated from the same client, and each session 517could be directed to a different host based on load balance across server 518pool hosts at the time. 519If load share is desired for just a few specific services, the configuration 520on LSNAT could be defined to restrict load share for just the services 521desired. 522.Pp 523Currently, only the simplest selection algorithm is implemented, where a 524host is selected on a round-robin basis only, without regard to load on 525the host. 526.Pp 527First, the 528.Fa link 529is created by either 530.Fn PacketAliasRedirectPort 531or 532.Fn PacketAliasRedirectAddr . 533Then, 534.Fn PacketAliasAddServer 535is called multiple times to add entries to the 536.Fa link Ns 's 537server pool. 538.Pp 539For links created with 540.Fn PacketAliasRedirectAddr , 541the 542.Fa port 543argument is ignored and could have any value, e.g. htons(~0). 544.Pp 545This function returns 0 on success, -1 otherwise. 546.Ed 547.Pp 548.Ft void 549.Fn PacketAliasRedirectDelete "struct alias_link *link" 550.Bd -ragged -offset indent 551This function will delete a specific static redirect rule entered by 552.Fn PacketAliasRedirectPort 553or 554.Fn PacketAliasRedirectAddr . 555The parameter 556.Fa link 557is the pointer returned by either of the redirection functions. 558If an invalid pointer is passed to 559.Fn PacketAliasRedirectDelete , 560then a program crash or unpredictable operation could result, so it is 561necessary to be careful using this function. 562.Ed 563.Pp 564.Ft int 565.Fn PacketAliasProxyRule "const char *cmd" 566.Bd -ragged -offset indent 567The passed 568.Fa cmd 569string consists of one or more pairs of words. 570The first word in each pair is a token and the second is the value that 571should be applied for that token. 572Tokens and their argument types are as follows: 573.Bl -tag -width indent 574.It Cm type encode_ip_hdr | encode_tcp_stream | no_encode 575In order to support transparent proxying, it is necessary to somehow 576pass the original address and port information into the new destination 577server. 578If 579.Cm encode_ip_hdr 580is specified, the original address and port is passed as an extra IP 581option. 582If 583.Cm encode_tcp_stream 584is specified, the original address and port is passed as the first 585piece of data in the TCP stream in the format 586.Dq DEST Ar IP port . 587.It Cm port Ar portnum 588Only packets with the destination port 589.Ar portnum 590are proxied. 591.It Cm server Ar host Ns Xo 592.Op : Ns Ar portnum 593.Xc 594This specifies the 595.Ar host 596and 597.Ar portnum 598that the data is to be redirected to. 599.Ar host 600must be an IP address rather than a DNS host name. 601If 602.Ar portnum 603is not specified, the destination port number is not changed. 604.Pp 605The 606.Ar server 607specification is mandatory unless the 608.Cm delete 609command is being used. 610.It Cm rule Ar index 611Normally, each call to 612.Fn PacketAliasProxyRule 613inserts the next rule at the start of a linear list of rules. 614If an 615.Ar index 616is specified, the new rule will be checked after all rules with lower 617indices. 618Calls to 619.Fn PacketAliasProxyRule 620that do not specify a rule are assigned rule 0. 621.It Cm delete Ar index 622This token and its argument MUST NOT be used with any other tokens. 623When used, all existing rules with the given 624.Ar index 625are deleted. 626.It Cm proto tcp | udp 627If specified, only packets of the given protocol type are matched. 628.It Cm src Ar IP Ns Xo 629.Op / Ns Ar bits 630.Xc 631If specified, only packets with a source address matching the given 632.Ar IP 633are matched. 634If 635.Ar bits 636is also specified, then the first 637.Ar bits 638bits of 639.Ar IP 640are taken as a network specification, and all IP addresses from that 641network will be matched. 642.It Cm dst Ar IP Ns Xo 643.Op / Ns Ar bits 644.Xc 645If specified, only packets with a destination address matching the given 646.Ar IP 647are matched. 648If 649.Ar bits 650is also specified, then the first 651.Ar bits 652bits of 653.Ar IP 654are taken as a network specification, and all IP addresses from that 655network will be matched. 656.El 657.Pp 658This function is usually used to redirect outgoing connections for 659internal machines that are not permitted certain types of internet 660access, or to restrict access to certain external machines. 661.Ed 662.Pp 663.Ft struct alias_link * 664.Fo PacketAliasRedirectProto 665.Fa "struct in_addr local_addr" 666.Fa "struct in_addr remote_addr" 667.Fa "struct in_addr alias_addr" 668.Fa "u_char proto" 669.Fc 670.Bd -ragged -offset indent 671This function specifies that any IP packet with protocol number of 672.Fa proto 673from a given remote address to an alias address be 674redirected to a specified local address. 675.Pp 676If 677.Fa local_addr 678or 679.Fa alias_addr 680is zero, this indicates that the packet aliasing address as established 681by 682.Fn PacketAliasSetAddress 683is to be used. 684Even if 685.Fn PacketAliasSetAddress 686is called to change the address after 687.Fn PacketAliasRedirectProto 688is called, a zero reference will track this change. 689.Pp 690If 691.Fa remote_addr 692is zero, this indicates to redirect packets from any remote address. 693Non-zero remote addresses can sometimes be useful for firewalling. 694.Pp 695If two calls to 696.Fn PacketAliasRedirectProto 697overlap in their address specifications, then the most recent call 698will have precedence. 699.Pp 700This function returns a pointer which can subsequently be used by 701.Fn PacketAliasRedirectDelete . 702If 703.Dv NULL 704is returned, then the function call did not complete successfully. 705.Ed 706.Sh FRAGMENT HANDLING 707The functions in this section are used to deal with incoming fragments. 708.Pp 709Outgoing fragments are handled within 710.Fn PacketAliasOut 711by changing the address according to any applicable mapping set by 712.Fn PacketAliasRedirectAddr , 713or the default aliasing address set by 714.Fn PacketAliasSetAddress . 715.Pp 716Incoming fragments are handled in one of two ways. 717If the header of a fragmented IP packet has already been seen, then all 718subsequent fragments will be re-mapped in the same manner the header 719fragment was. 720Fragments which arrive before the header are saved and then retrieved 721once the header fragment has been resolved. 722.Pp 723.Ft int 724.Fn PacketAliasSaveFragment "char *ptr" 725.Bd -ragged -offset indent 726When 727.Fn PacketAliasIn 728returns 729.Dv PKT_ALIAS_UNRESOLVED_FRAGMENT , 730this function can be used to save the pointer to the unresolved fragment. 731.Pp 732It is implicitly assumed that 733.Fa ptr 734points to a block of memory allocated by 735.Xr malloc 3 . 736If the fragment is never resolved, the packet aliasing engine will 737automatically free the memory after a timeout period. 738[Eventually this function should be modified so that a callback function 739for freeing memory is passed as an argument.] 740.Pp 741This function returns 742.Dv PKT_ALIAS_OK 743if it was successful and 744.Dv PKT_ALIAS_ERROR 745if there was an error. 746.Ed 747.Pp 748.Ft char * 749.Fn PacketAliasGetFragment "char *buffer" 750.Bd -ragged -offset indent 751This function can be used to retrieve fragment pointers saved by 752.Fn PacketAliasSaveFragment . 753The IP header fragment pointed to by 754.Fa buffer 755is the header fragment indicated when 756.Fn PacketAliasIn 757returns 758.Dv PKT_ALIAS_FOUND_HEADER_FRAGMENT . 759Once a fragment pointer is retrieved, it becomes the calling program's 760responsibility to free the dynamically allocated memory for the fragment. 761.Pp 762.Fn PacketAliasGetFragment 763can be called sequentially until there are no more fragments available, 764at which time it returns 765.Dv NULL . 766.Ed 767.Pp 768.Ft void 769.Fn PacketAliasFragmentIn "char *header" "char *fragment" 770.Bd -ragged -offset indent 771When a fragment is retrieved with 772.Fn PacketAliasGetFragment , 773it can then be de-aliased with a call to 774.Fn PacketAliasFragmentIn . 775The 776.Fa header 777argument is the pointer to a header fragment used as a template, and 778.Fa fragment 779is the pointer to the packet to be de-aliased. 780.Ed 781.Sh MISCELLANEOUS FUNCTIONS 782.Ft void 783.Fn PacketAliasSetTarget "struct in_addr addr" 784.Bd -ragged -offset indent 785When an incoming packet not associated with any pre-existing aliasing link 786arrives at the host machine, it will be sent to the address indicated by a 787call to 788.Fn PacketAliasSetTarget . 789.Pp 790If this function is called with an 791.Dv INADDR_NONE 792address argument, then all new incoming packets go to the address set by 793.Fn PacketAliasSetAddress . 794.Pp 795If this function is not called, or is called with an 796.Dv INADDR_ANY 797address argument, then all new incoming packets go to the address specified 798in the packet. 799This allows external machines to talk directly to internal machines if they 800can route packets to the machine in question. 801.Ed 802.Pp 803.Ft int 804.Fn PacketAliasCheckNewLink void 805.Bd -ragged -offset indent 806This function returns a non-zero value when a new aliasing link is created. 807In circumstances where incoming traffic is being sequentially sent to 808different local servers, this function can be used to trigger when 809.Fn PacketAliasSetTarget 810is called to change the default target address. 811.Ed 812.Pp 813.Ft u_short 814.Fn PacketAliasInternetChecksum "u_short *buffer" "int nbytes" 815.Bd -ragged -offset indent 816This is a utility function that does not seem to be available elsewhere and 817is included as a convenience. 818It computes the internet checksum, which is used in both IP and 819protocol-specific headers (TCP, UDP, ICMP). 820.Pp 821The 822.Fa buffer 823argument points to the data block to be checksummed, and 824.Fa nbytes 825is the number of bytes. 826The 16-bit checksum field should be zeroed before computing the checksum. 827.Pp 828Checksums can also be verified by operating on a block of data including 829its checksum. 830If the checksum is valid, 831.Fn PacketAliasInternetChecksum 832will return zero. 833.Ed 834.Pp 835.Ft int 836.Fn PacketUnaliasOut "char *buffer" "int maxpacketsize" 837.Bd -ragged -offset indent 838An outgoing packet, which has already been aliased, 839has its private address/port information restored by this function. 840The IP packet is pointed to by 841.Fa buffer , 842and 843.Fa maxpacketsize 844is provided for error checking purposes. 845This function can be used if an already-aliased packet needs to have its 846original IP header restored for further processing (eg. logging). 847.Ed 848.Sh AUTHORS 849.An Charles Mott Aq Mt cm@linktel.net , 850versions 1.0 - 1.8, 2.0 - 2.4. 851.An Eivind Eklund Aq Mt eivind@FreeBSD.org , 852versions 1.8b, 1.9 and 2.5. 853Added IRC DCC support as well as contributing a number of architectural 854improvements; added the firewall bypass for FTP/IRC DCC. 855.An Erik Salander Aq Mt erik@whistle.com 856added support for PPTP and RTSP. 857.An Junichi Satoh Aq Mt junichi@junichi.org 858added support for RTSP/PNA. 859.Sh ACKNOWLEDGMENTS 860Listed below, in approximate chronological order, are individuals who 861have provided valuable comments and/or debugging assistance. 862.Bd -ragged -offset indent 863.An -split 864.An Gary Roberts 865.An Tom Torrance 866.An Reto Burkhalter 867.An Martin Renters 868.An Brian Somers 869.An Paul Traina 870.An Ari Suutari 871.An Dave Remien 872.An J. Fortes 873.An Andrzej Bialecki 874.An Gordon Burditt 875.Ed 876.Sh CONCEPTUAL BACKGROUND 877This section is intended for those who are planning to modify the source 878code or want to create somewhat esoteric applications using the packet 879aliasing functions. 880.Pp 881The conceptual framework under which the packet aliasing engine operates 882is described here. 883Central to the discussion is the idea of an 884.Em aliasing link 885which describes the relationship for a given packet transaction between 886the local machine, aliased identity and remote machine. 887It is discussed how such links come into existence and are destroyed. 888.Ss ALIASING LINKS 889There is a notion of an 890.Em aliasing link , 891which is a 7-tuple describing a specific translation: 892.Bd -literal -offset indent 893(local addr, local port, alias addr, alias port, 894 remote addr, remote port, protocol) 895.Ed 896.Pp 897Outgoing packets have the local address and port number replaced with the 898alias address and port number. 899Incoming packets undergo the reverse process. 900The packet aliasing engine attempts to match packets against an internal 901table of aliasing links to determine how to modify a given IP packet. 902Both the IP header and protocol dependent headers are modified as necessary. 903Aliasing links are created and deleted as necessary according to network 904traffic. 905.Pp 906Protocols can be TCP, UDP or even ICMP in certain circumstances. 907(Some types of ICMP packets can be aliased according to sequence or ID 908number which acts as an equivalent port number for identifying how 909individual packets should be handled.) 910.Pp 911Each aliasing link must have a unique combination of the following five 912quantities: alias address/port, remote address/port and protocol. 913This ensures that several machines on a local network can share the 914same aliasing IP address. 915In cases where conflicts might arise, the aliasing port is chosen so that 916uniqueness is maintained. 917.Ss STATIC AND DYNAMIC LINKS 918Aliasing links can either be static or dynamic. 919Static links persist indefinitely and represent fixed rules for translating 920IP packets. 921Dynamic links come into existence for a specific TCP connection or UDP 922transaction or ICMP ECHO sequence. 923For the case of TCP, the connection can be monitored to see when the 924associated aliasing link should be deleted. 925Aliasing links for UDP transactions (and ICMP ECHO and TIMESTAMP requests) 926work on a simple timeout rule. 927When no activity is observed on a dynamic link for a certain amount of time 928it is automatically deleted. 929Timeout rules also apply to TCP connections which do not open or close 930properly. 931.Ss PARTIALLY SPECIFIED ALIASING LINKS 932Aliasing links can be partially specified, meaning that the remote address 933and/or remote port are unknown. 934In this case, when a packet matching the incomplete specification is found, 935a fully specified dynamic link is created. 936If the original partially specified link is dynamic, it will be deleted 937after the fully specified link is created, otherwise it will persist. 938.Pp 939For instance, a partially specified link might be 940.Bd -literal -offset indent 941(192.168.0.4, 23, 204.228.203.215, 8066, 0, 0, tcp) 942.Ed 943.Pp 944The zeros denote unspecified components for the remote address and port. 945If this link were static it would have the effect of redirecting all 946incoming traffic from port 8066 of 204.228.203.215 to port 23 (telnet) 947of machine 192.168.0.4 on the local network. 948Each individual telnet connection would initiate the creation of a distinct 949dynamic link. 950.Ss DYNAMIC LINK CREATION 951In addition to aliasing links, there are also address mappings that can be 952stored within the internal data table of the packet aliasing mechanism. 953.Bd -literal -offset indent 954(local addr, alias addr) 955.Ed 956.Pp 957Address mappings are searched when creating new dynamic links. 958.Pp 959All outgoing packets from the local network automatically create a dynamic 960link if they do not match an already existing fully specified link. 961If an address mapping exists for the outgoing packet, this determines 962the alias address to be used. 963If no mapping exists, then a default address, usually the address of the 964packet aliasing host, is used. 965If necessary, this default address can be changed as often as each individual 966packet arrives. 967.Pp 968The aliasing port number is determined such that the new dynamic link does 969not conflict with any existing links. 970In the default operating mode, the packet aliasing engine attempts to set 971the aliasing port equal to the local port number. 972If this results in a conflict, then port numbers are randomly chosen until 973a unique aliasing link can be established. 974In an alternate operating mode, the first choice of an aliasing port is also 975random and unrelated to the local port number. 976.Sh BUGS 977PPTP aliasing does not work when more than one internal client 978connects to the same external server at the same time, because 979PPTP requires a single TCP control connection to be established 980between any two IP addresses. 981