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.\" $FreeBSD$ 27.\" 28.Dd December 25, 2013 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 SYNOPSIS 35.In sys/types.h 36.In netinet/in.h 37.In alias.h 38.Pp 39Function prototypes are given in the main body of the text. 40.Sh DESCRIPTION 41The 42.Nm 43library is a collection of functions for aliasing and de-aliasing of IP 44packets, intended for masquerading and network address translation (NAT). 45.Sh INTRODUCTION 46This library is a moderately portable set of functions designed to assist 47in the process of IP masquerading and network address translation. 48Outgoing packets from a local network with unregistered IP addresses can 49be aliased to appear as if they came from an accessible IP address. 50Incoming packets are then de-aliased so that they are sent to the correct 51machine on the local network. 52.Pp 53A certain amount of flexibility is built into the packet aliasing engine. 54In the simplest mode of operation, a many-to-one address mapping takes 55place between the local network and the packet aliasing host. 56This is known as IP masquerading. 57In addition, one-to-one mappings between local and public addresses can 58also be implemented, which is known as static NAT. 59In between these extremes, different groups of private addresses can be 60linked to different public addresses, comprising several distinct 61many-to-one mappings. 62Also, a given public address and port can be statically redirected to a 63private address/port. 64.Sh INITIALIZATION AND CONTROL 65One special function, 66.Fn LibAliasInit , 67must always be called before any packet handling may be performed, and 68the returned instance pointer must be passed to all the other functions. 69Normally, the 70.Fn LibAliasSetAddress 71function is called afterwards, to set the default aliasing address. 72In addition, the operating mode of the packet aliasing engine can be 73customized by calling 74.Fn LibAliasSetMode . 75.Pp 76.Ft "struct libalias *" 77.Fn LibAliasInit "struct libalias *" 78.Bd -ragged -offset indent 79This function is used to initialize 80internal data structures. 81When called the first time, a 82.Dv NULL 83pointer should be passed as an argument. 84The following mode bits are always set after calling 85.Fn LibAliasInit . 86See the description of 87.Fn LibAliasSetMode 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. 101The 102.Fn LibAliasSetAddress 103function is normally called afterwards, and any desired changes from the 104default mode bits listed above require a call to 105.Fn LibAliasSetMode . 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 LibAliasUninit "struct libalias *" 113.Bd -ragged -offset indent 114This function has no return value and is used to clear any 115resources attached to internal data structures. 116.Pp 117This function should be called when a program stops using the aliasing 118engine; amongst other things, it clears out any firewall holes. 119To provide backwards compatibility and extra security, it is added to 120the 121.Xr atexit 3 122chain by 123.Fn LibAliasInit . 124.Ed 125.Pp 126.Ft void 127.Fn LibAliasSetAddress "struct libalias *" "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 LibAliasRedirectAddr . 134If this function has not been called, and no static rules match, an outgoing 135packet retains its source address. 136.Pp 137If the 138.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE 139mode bit is set (the default mode of operation), then the internal aliasing 140link tables will be reset any time the aliasing address changes. 141This is useful for interfaces such as 142.Xr ppp 8 , 143where the IP 144address may or may not change on successive dial-up attempts. 145.Pp 146If the 147.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE 148mode bit is set to zero, this function can also be used to dynamically change 149the aliasing address on a packet-to-packet basis (it is a low overhead call). 150.Pp 151It is mandatory that this function be called prior to any packet handling. 152.Ed 153.Pp 154.Ft unsigned int 155.Fn LibAliasSetMode "struct libalias *" "unsigned int flags" "unsigned int mask" 156.Bd -ragged -offset indent 157This function sets or clears mode bits 158according to the value of 159.Fa flags . 160Only bits marked in 161.Fa mask 162are affected. 163The following mode bits are defined in 164.In alias.h : 165.Bl -tag -width indent 166.It Dv PKT_ALIAS_LOG 167Enables logging into 168.Pa /var/log/alias.log . 169Each time an aliasing link is created or deleted, the log file is appended to 170with the current number of ICMP, TCP and UDP links. 171Mainly useful for debugging when the log file is viewed continuously with 172.Xr tail 1 . 173.It Dv PKT_ALIAS_DENY_INCOMING 174If this mode bit is set, all incoming packets associated with new TCP 175connections or new UDP transactions will be marked for being ignored 176.Po 177.Fn LibAliasIn 178returns 179.Dv PKT_ALIAS_IGNORED 180code 181.Pc 182by the calling program. 183Response packets to connections or transactions initiated from the packet 184aliasing host or local network will be unaffected. 185This mode bit is useful for implementing a one-way firewall. 186.It Dv PKT_ALIAS_SAME_PORTS 187If this mode bit is set, the packet-aliasing engine will attempt to leave 188the alias port numbers unchanged from the actual local port numbers. 189This can be done as long as the quintuple (proto, alias addr, alias port, 190remote addr, remote port) is unique. 191If a conflict exists, a new aliasing port number is chosen even if this 192mode bit is set. 193.It Dv PKT_ALIAS_USE_SOCKETS 194This bit should be set when the packet aliasing host originates network 195traffic as well as forwards it. 196When the packet aliasing host is waiting for a connection from an unknown 197host address or unknown port number (e.g.\& an FTP data connection), this 198mode bit specifies that a socket be allocated as a place holder to prevent 199port conflicts. 200Once a connection is established, usually within a minute or so, the socket 201is closed. 202.It Dv PKT_ALIAS_UNREGISTERED_ONLY 203If this mode bit is set, traffic on the local network which does not 204originate from unregistered address spaces will be ignored. 205Standard Class A, B and C unregistered addresses are: 206.Pp 20710.0.0.0 -> 10.255.255.255 (Class A subnet) 208172.16.0.0 -> 172.31.255.255 (Class B subnets) 209192.168.0.0 -> 192.168.255.255 (Class C subnets) 210.Pp 211This option is useful in the case that the packet aliasing host has both 212registered and unregistered subnets on different interfaces. 213The registered subnet is fully accessible to the outside world, so traffic 214from it does not need to be passed through the packet aliasing engine. 215.It Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE 216When this mode bit is set and 217.Fn LibAliasSetAddress 218is called to change the aliasing address, the internal link table of the 219packet aliasing engine will be cleared. 220This operating mode is useful for 221.Xr ppp 8 222links where the interface address can sometimes change or remain the same 223between dial-up attempts. 224If this mode bit is not set, the link table will never be reset in the event 225of an address change. 226.It Dv PKT_ALIAS_PUNCH_FW 227This option makes 228.Nm 229.Dq punch holes 230in an 231.Xr ipfirewall 4 - 232based firewall for FTP/IRC DCC connections. 233The holes punched are bound by from/to IP address and port; it will not be 234possible to use a hole for another connection. 235A hole is removed when the connection that uses it dies. 236To cater to unexpected death of a program using 237.Nm 238(e.g.\& kill -9), 239changing the state of the flag will clear the entire firewall range 240allocated for holes. 241This clearing will also happen on the initial call to 242.Fn LibAliasSetFWBase , 243which must happen prior to setting this flag. 244.It Dv PKT_ALIAS_REVERSE 245This option makes 246.Nm 247reverse the way it handles incoming and outgoing packets, allowing it 248to be fed with data that passes through the internal interface rather 249than the external one. 250.It Dv PKT_ALIAS_PROXY_ONLY 251This option tells 252.Nm 253to obey transparent proxy rules only. 254Normal packet aliasing is not performed. 255See 256.Fn LibAliasProxyRule 257below for details. 258.It Dv PKT_ALIAS_SKIP_GLOBAL 259This option is used by 260.Pa ipfw_nat 261only. 262Specifying it as a flag to 263.Fn LibAliasSetMode 264has no effect. 265See section 266.Sx NETWORK ADDRESS TRANSLATION 267in 268.Xr ipfw 8 269for more details. 270.El 271.Ed 272.Pp 273.Ft void 274.Fn LibAliasSetFWBase "struct libalias *" "unsigned int base" "unsigned int num" 275.Bd -ragged -offset indent 276Set the firewall range allocated for punching firewall holes (with the 277.Dv PKT_ALIAS_PUNCH_FW 278flag). 279The range is cleared for all rules on initialization. 280.Ed 281.Pp 282.Ft void 283.Fn LibAliasSkinnyPort "struct libalias *" "unsigned int port" 284.Bd -ragged -offset indent 285Set the TCP port used by the Skinny Station protocol. 286Skinny is used by Cisco IP phones to communicate with 287Cisco Call Managers to set up voice over IP calls. 288If this is not set, Skinny aliasing will not be done. 289The typical port used by Skinny is 2000. 290.Ed 291.Sh PACKET HANDLING 292The packet handling functions are used to modify incoming (remote to local) 293and outgoing (local to remote) packets. 294The calling program is responsible for receiving and sending packets via 295network interfaces. 296.Pp 297Along with 298.Fn LibAliasInit 299and 300.Fn LibAliasSetAddress , 301the two packet handling functions, 302.Fn LibAliasIn 303and 304.Fn LibAliasOut , 305comprise the minimal set of functions needed for a basic IP masquerading 306implementation. 307.Pp 308.Ft int 309.Fn LibAliasIn "struct libalias *" "char *buffer" "int maxpacketsize" 310.Bd -ragged -offset indent 311An incoming packet coming from a remote machine to the local network is 312de-aliased by this function. 313The IP packet is pointed to by 314.Fa buffer , 315and 316.Fa maxpacketsize 317indicates the size of the data structure containing the packet and should 318be at least as large as the actual packet size. 319.Pp 320Return codes: 321.Bl -tag -width indent 322.It Dv PKT_ALIAS_OK 323The packet aliasing process was successful. 324.It Dv PKT_ALIAS_IGNORED 325The packet was ignored and not de-aliased. 326This can happen if the protocol is unrecognized, as for an ICMP message 327type that is not handled, or if incoming packets for new connections are being 328ignored (if the 329.Dv PKT_ALIAS_DENY_INCOMING 330mode bit was set using 331.Fn LibAliasSetMode ) . 332.It Dv PKT_ALIAS_UNRESOLVED_FRAGMENT 333This is returned when a fragment cannot be resolved because the header 334fragment has not been sent yet. 335In this situation, fragments must be saved with 336.Fn LibAliasSaveFragment 337until a header fragment is found. 338.It Dv PKT_ALIAS_FOUND_HEADER_FRAGMENT 339The packet aliasing process was successful, and a header fragment was found. 340This is a signal to retrieve any unresolved fragments with 341.Fn LibAliasGetFragment 342and de-alias them with 343.Fn LibAliasFragmentIn . 344.It Dv PKT_ALIAS_ERROR 345An internal error within the packet aliasing engine occurred. 346.El 347.Ed 348.Pp 349.Ft int 350.Fn LibAliasOut "struct libalias *" "char *buffer" "int maxpacketsize" 351.Bd -ragged -offset indent 352An outgoing packet coming from the local network to a remote machine is 353aliased by this function. 354The IP packet is pointed to by 355.Fa buffer , 356and 357.Fa maxpacketsize 358indicates the maximum packet size permissible should the packet length be 359changed. 360IP encoding protocols place address and port information in the encapsulated 361data stream which has to be modified and can account for changes in packet 362length. 363Well known examples of such protocols are FTP and IRC DCC. 364.Pp 365Return codes: 366.Bl -tag -width indent 367.It Dv PKT_ALIAS_OK 368The packet aliasing process was successful. 369.It Dv PKT_ALIAS_IGNORED 370The packet was ignored and not aliased. 371This can happen if the protocol is unrecognized, or possibly an ICMP message 372type is not handled. 373.It Dv PKT_ALIAS_ERROR 374An internal error within the packet aliasing engine occurred. 375.El 376.Ed 377.Sh PORT AND ADDRESS REDIRECTION 378The functions described in this section allow machines on the local network 379to be accessible in some degree to new incoming connections from the external 380network. 381Individual ports can be re-mapped or static network address translations can 382be designated. 383.Pp 384.Ft struct alias_link * 385.Fo LibAliasRedirectPort 386.Fa "struct libalias *" 387.Fa "struct in_addr local_addr" 388.Fa "u_short local_port" 389.Fa "struct in_addr remote_addr" 390.Fa "u_short remote_port" 391.Fa "struct in_addr alias_addr" 392.Fa "u_short alias_port" 393.Fa "u_char proto" 394.Fc 395.Bd -ragged -offset indent 396This function specifies that traffic from a given remote address/port to 397an alias address/port be redirected to a specified local address/port. 398The parameter 399.Fa proto 400can be either 401.Dv IPPROTO_TCP 402or 403.Dv IPPROTO_UDP , 404as defined in 405.In netinet/in.h . 406.Pp 407If 408.Fa local_addr 409or 410.Fa alias_addr 411is zero, this indicates that the packet aliasing address as established 412by 413.Fn LibAliasSetAddress 414is to be used. 415Even if 416.Fn LibAliasSetAddress 417is called to change the address after 418.Fn LibAliasRedirectPort 419is called, a zero reference will track this change. 420.Pp 421If the link is further set up to operate with load sharing, then 422.Fa local_addr 423and 424.Fa local_port 425are ignored, and are selected dynamically from the server pool, as described in 426.Fn LibAliasAddServer 427below. 428.Pp 429If 430.Fa remote_addr 431is zero, this indicates to redirect packets from any remote address. 432Likewise, if 433.Fa remote_port 434is zero, this indicates to redirect packets originating from any remote 435port number. 436The remote port specification will almost always be zero, but non-zero 437remote addresses can sometimes be useful for firewalling. 438If two calls to 439.Fn LibAliasRedirectPort 440overlap in their address/port specifications, then the most recent call 441will have precedence. 442.Pp 443This function returns a pointer which can subsequently be used by 444.Fn LibAliasRedirectDelete . 445If 446.Dv NULL 447is returned, then the function call did not complete successfully. 448.Pp 449All port numbers should be in network address byte order, so it is necessary 450to use 451.Xr htons 3 452to convert these parameters from internally readable numbers to network byte 453order. 454Addresses are also in network byte order, which is implicit in the use of the 455.Fa struct in_addr 456data type. 457.Ed 458.Pp 459.Ft struct alias_link * 460.Fo LibAliasRedirectAddr 461.Fa "struct libalias *" 462.Fa "struct in_addr local_addr" 463.Fa "struct in_addr alias_addr" 464.Fc 465.Bd -ragged -offset indent 466This function designates that all incoming traffic to 467.Fa alias_addr 468be redirected to 469.Fa local_addr . 470Similarly, all outgoing traffic from 471.Fa local_addr 472is aliased to 473.Fa alias_addr . 474.Pp 475If 476.Fa local_addr 477or 478.Fa alias_addr 479is zero, this indicates that the packet aliasing address as established by 480.Fn LibAliasSetAddress 481is to be used. 482Even if 483.Fn LibAliasSetAddress 484is called to change the address after 485.Fn LibAliasRedirectAddr 486is called, a zero reference will track this change. 487.Pp 488If the link is further set up to operate with load sharing, then the 489.Fa local_addr 490argument is ignored, and is selected dynamically from the server pool, 491as described in 492.Fn LibAliasAddServer 493below. 494.Pp 495If subsequent calls to 496.Fn LibAliasRedirectAddr 497use the same aliasing address, all new incoming traffic to this aliasing 498address will be redirected to the local address made in the last function 499call. 500New traffic generated by any of the local machines, designated in the 501several function calls, will be aliased to the same address. 502Consider the following example: 503.Pp 504LibAliasRedirectAddr(la, inet_aton("192.168.0.2"), 505 inet_aton("141.221.254.101")); 506LibAliasRedirectAddr(la, inet_aton("192.168.0.3"), 507 inet_aton("141.221.254.101")); 508LibAliasRedirectAddr(la, inet_aton("192.168.0.4"), 509 inet_aton("141.221.254.101")); 510.Pp 511Any outgoing connections such as 512.Xr telnet 1 513or 514.Xr ftp 1 515from 192.168.0.2, 192.168.0.3 and 192.168.0.4 will appear to come from 516141.221.254.101. 517Any incoming connections to 141.221.254.101 will be directed to 192.168.0.4. 518.Pp 519Any calls to 520.Fn LibAliasRedirectPort 521will have precedence over address mappings designated by 522.Fn LibAliasRedirectAddr . 523.Pp 524This function returns a pointer which can subsequently be used by 525.Fn LibAliasRedirectDelete . 526If 527.Dv NULL 528is returned, then the function call did not complete successfully. 529.Ed 530.Pp 531.Ft int 532.Fo LibAliasAddServer 533.Fa "struct libalias *" 534.Fa "struct alias_link *link" 535.Fa "struct in_addr addr" 536.Fa "u_short port" 537.Fc 538.Bd -ragged -offset indent 539This function sets the 540.Fa link 541up for Load Sharing using IP Network Address Translation (RFC 2391, LSNAT). 542LSNAT operates as follows. 543A client attempts to access a server by using the server virtual address. 544The LSNAT router transparently redirects the request to one of the hosts 545in the server pool, using a real-time load sharing algorithm. 546Multiple sessions may be initiated from the same client, and each session 547could be directed to a different host based on the load balance across server 548pool hosts when the sessions are initiated. 549If load sharing is desired for just a few specific services, the configuration 550on LSNAT could be defined to restrict load sharing to just the services 551desired. 552.Pp 553Currently, only the simplest selection algorithm is implemented, where a 554host is selected on a round-robin basis only, without regard to load on 555the host. 556.Pp 557First, the 558.Fa link 559is created by either 560.Fn LibAliasRedirectPort 561or 562.Fn LibAliasRedirectAddr . 563Then, 564.Fn LibAliasAddServer 565is called multiple times to add entries to the 566.Fa link Ns 's 567server pool. 568.Pp 569For links created with 570.Fn LibAliasRedirectAddr , 571the 572.Fa port 573argument is ignored and could have any value, e.g.\& htons(~0). 574.Pp 575This function returns 0 on success, \-1 otherwise. 576.Ed 577.Pp 578.Ft int 579.Fn LibAliasRedirectDynamic "struct libalias *" "struct alias_link *link" 580.Bd -ragged -offset indent 581This function marks the specified static redirect rule entered by 582.Fn LibAliasRedirectPort 583as dynamic. 584This can be used to e.g.\& dynamically redirect a single TCP connection, 585after which the rule is removed. 586Only fully specified links can be made dynamic. 587(See the 588.Sx STATIC AND DYNAMIC LINKS 589and 590.Sx PARTIALLY SPECIFIED ALIASING LINKS 591sections below for a definition of static vs.\& dynamic, 592and partially vs.\& fully specified links.) 593.Pp 594This function returns 0 on success, \-1 otherwise. 595.Ed 596.Pp 597.Ft void 598.Fn LibAliasRedirectDelete "struct libalias *" "struct alias_link *link" 599.Bd -ragged -offset indent 600This function will delete a specific static redirect rule entered by 601.Fn LibAliasRedirectPort 602or 603.Fn LibAliasRedirectAddr . 604The parameter 605.Fa link 606is the pointer returned by either of the redirection functions. 607If an invalid pointer is passed to 608.Fn LibAliasRedirectDelete , 609then a program crash or unpredictable operation could result, so 610care is needed when using this function. 611.Ed 612.Pp 613.Ft int 614.Fn LibAliasProxyRule "struct libalias *" "const char *cmd" 615.Bd -ragged -offset indent 616The passed 617.Fa cmd 618string consists of one or more pairs of words. 619The first word in each pair is a token and the second is the value that 620should be applied for that token. 621Tokens and their argument types are as follows: 622.Bl -tag -width indent 623.It Cm type encode_ip_hdr | encode_tcp_stream | no_encode 624In order to support transparent proxying, it is necessary to somehow 625pass the original address and port information into the new destination 626server. 627If 628.Cm encode_ip_hdr 629is specified, the original destination address and port are passed 630as an extra IP option. 631If 632.Cm encode_tcp_stream 633is specified, the original destination address and port are passed 634as the first piece of data in the TCP stream in the format 635.Dq Li DEST Ar IP port . 636.It Cm port Ar portnum 637Only packets with the destination port 638.Ar portnum 639are proxied. 640.It Cm server Ar host Ns Op : Ns Ar portnum 641This specifies the 642.Ar host 643and 644.Ar portnum 645that the data is to be redirected to. 646.Ar host 647must be an IP address rather than a DNS host name. 648If 649.Ar portnum 650is not specified, the destination port number is not changed. 651.Pp 652The 653.Ar server 654specification is mandatory unless the 655.Cm delete 656command is being used. 657.It Cm rule Ar index 658Normally, each call to 659.Fn LibAliasProxyRule 660inserts the next rule at the start of a linear list of rules. 661If an 662.Ar index 663is specified, the new rule will be checked after all rules with lower 664indices. 665Calls to 666.Fn LibAliasProxyRule 667that do not specify a rule are assigned rule 0. 668.It Cm delete Ar index 669This token and its argument MUST NOT be used with any other tokens. 670When used, all existing rules with the given 671.Ar index 672are deleted. 673.It Cm proto tcp | udp 674If specified, only packets of the given protocol type are matched. 675.It Cm src Ar IP Ns Op / Ns Ar bits 676If specified, only packets with a source address matching the given 677.Ar IP 678are matched. 679If 680.Ar bits 681is also specified, then the first 682.Ar bits 683bits of 684.Ar IP 685are taken as a network specification, and all IP addresses from that 686network will be matched. 687.It Cm dst Ar IP Ns Op / Ns Ar bits 688If specified, only packets with a destination address matching the given 689.Ar IP 690are matched. 691If 692.Ar bits 693is also specified, then the first 694.Ar bits 695bits of 696.Ar IP 697are taken as a network specification, and all IP addresses from that 698network will be matched. 699.El 700.Pp 701This function is usually used to redirect outgoing connections for 702internal machines that are not permitted certain types of internet 703access, or to restrict access to certain external machines. 704.Ed 705.Pp 706.Ft struct alias_link * 707.Fo LibAliasRedirectProto 708.Fa "struct libalias *" 709.Fa "struct in_addr local_addr" 710.Fa "struct in_addr remote_addr" 711.Fa "struct in_addr alias_addr" 712.Fa "u_char proto" 713.Fc 714.Bd -ragged -offset indent 715This function specifies that any IP packet with protocol number of 716.Fa proto 717from a given remote address to an alias address will be 718redirected to a specified local address. 719.Pp 720If 721.Fa local_addr 722or 723.Fa alias_addr 724is zero, this indicates that the packet aliasing address as established 725by 726.Fn LibAliasSetAddress 727is to be used. 728Even if 729.Fn LibAliasSetAddress 730is called to change the address after 731.Fn LibAliasRedirectProto 732is called, a zero reference will track this change. 733.Pp 734If 735.Fa remote_addr 736is zero, this indicates to redirect packets from any remote address. 737Non-zero remote addresses can sometimes be useful for firewalling. 738.Pp 739If two calls to 740.Fn LibAliasRedirectProto 741overlap in their address specifications, then the most recent call 742will have precedence. 743.Pp 744This function returns a pointer which can subsequently be used by 745.Fn LibAliasRedirectDelete . 746If 747.Dv NULL 748is returned, then the function call did not complete successfully. 749.Ed 750.Sh FRAGMENT HANDLING 751The functions in this section are used to deal with incoming fragments. 752.Pp 753Outgoing fragments are handled within 754.Fn LibAliasOut 755by changing the address according to any applicable mapping set by 756.Fn LibAliasRedirectAddr , 757or the default aliasing address set by 758.Fn LibAliasSetAddress . 759.Pp 760Incoming fragments are handled in one of two ways. 761If the header of a fragmented IP packet has already been seen, then all 762subsequent fragments will be re-mapped in the same manner the header 763fragment was. 764Fragments which arrive before the header are saved and then retrieved 765once the header fragment has been resolved. 766.Pp 767.Ft int 768.Fn LibAliasSaveFragment "struct libalias *" "char *ptr" 769.Bd -ragged -offset indent 770When 771.Fn LibAliasIn 772returns 773.Dv PKT_ALIAS_UNRESOLVED_FRAGMENT , 774this function can be used to save the pointer to the unresolved fragment. 775.Pp 776It is implicitly assumed that 777.Fa ptr 778points to a block of memory allocated by 779.Xr malloc 3 . 780If the fragment is never resolved, the packet aliasing engine will 781automatically free the memory after a timeout period. 782[Eventually this function should be modified so that a callback function 783for freeing memory is passed as an argument.] 784.Pp 785This function returns 786.Dv PKT_ALIAS_OK 787if it was successful and 788.Dv PKT_ALIAS_ERROR 789if there was an error. 790.Ed 791.Pp 792.Ft char * 793.Fn LibAliasGetFragment "struct libalias *" "char *buffer" 794.Bd -ragged -offset indent 795This function can be used to retrieve fragment pointers saved by 796.Fn LibAliasSaveFragment . 797The IP header fragment pointed to by 798.Fa buffer 799is the header fragment indicated when 800.Fn LibAliasIn 801returns 802.Dv PKT_ALIAS_FOUND_HEADER_FRAGMENT . 803Once a fragment pointer is retrieved, it becomes the calling program's 804responsibility to free the dynamically allocated memory for the fragment. 805.Pp 806The 807.Fn LibAliasGetFragment 808function can be called sequentially until there are no more fragments 809available, at which time it returns 810.Dv NULL . 811.Ed 812.Pp 813.Ft void 814.Fn LibAliasFragmentIn "struct libalias *" "char *header" "char *fragment" 815.Bd -ragged -offset indent 816When a fragment is retrieved with 817.Fn LibAliasGetFragment , 818it can then be de-aliased with a call to 819.Fn LibAliasFragmentIn . 820The 821.Fa header 822argument is the pointer to a header fragment used as a template, and 823.Fa fragment 824is the pointer to the packet to be de-aliased. 825.Ed 826.Sh MISCELLANEOUS FUNCTIONS 827.Ft struct alias_link * 828.Fn AddLink "struct libalias *" "struct in_addr src_addr" "struct in_addr dst_addr" \ 829"struct in_addr alias_addr" "u_short src_port" "u_short dst_port" \ 830"int alias_param" "int link_type" 831.Bd -ragged -offset indent 832This function adds new state to the instance hash table. 833The dst_address and/or dst_port may be given as zero, which 834introduces some dynamic character into the link, since 835LibAliasSetAddress can change the address that is used. 836However, in the current implementation, such links can only be used 837for inbound (ext -> int) traffic. 838.Ed 839.Pp 840.Ft void 841.Fn LibAliasSetTarget "struct libalias *" "struct in_addr addr" 842.Bd -ragged -offset indent 843When an incoming packet not associated with any pre-existing aliasing link 844arrives at the host machine, it will be sent to the address indicated by a 845call to 846.Fn LibAliasSetTarget . 847.Pp 848If this function is called with an 849.Dv INADDR_NONE 850address argument, then all new incoming packets go to the address set by 851.Fn LibAliasSetAddress . 852.Pp 853If this function is not called, or is called with an 854.Dv INADDR_ANY 855address argument, then all new incoming packets go to the address specified 856in the packet. 857This allows external machines to talk directly to internal machines if they 858can route packets to the machine in question. 859.Ed 860.Pp 861.Ft int 862.Fn LibAliasCheckNewLink "struct libalias *" 863.Bd -ragged -offset indent 864This function returns a non-zero value when a new aliasing link is created. 865In circumstances where incoming traffic is being sequentially sent to 866different local servers, this function can be used to trigger when 867.Fn LibAliasSetTarget 868is called to change the default target address. 869.Ed 870.Pp 871.Ft u_short 872.Fn LibAliasInternetChecksum "struct libalias *" "u_short *buffer" "int nbytes" 873.Bd -ragged -offset indent 874This is a utility function that does not seem to be available elsewhere and 875is included as a convenience. 876It computes the internet checksum, which is used in both IP and 877protocol-specific headers (TCP, UDP, ICMP). 878.Pp 879The 880.Fa buffer 881argument points to the data block to be checksummed, and 882.Fa nbytes 883is the number of bytes. 884The 16-bit checksum field should be zeroed before computing the checksum. 885.Pp 886Checksums can also be verified by operating on a block of data including 887its checksum. 888If the checksum is valid, 889.Fn LibAliasInternetChecksum 890will return zero. 891.Ed 892.Pp 893.Ft int 894.Fn LibAliasUnaliasOut "struct libalias *" "char *buffer" "int maxpacketsize" 895.Bd -ragged -offset indent 896An outgoing packet, which has already been aliased, 897has its private address/port information restored by this function. 898The IP packet is pointed to by 899.Fa buffer , 900and 901.Fa maxpacketsize 902is provided for error checking purposes. 903This function can be used if an already-aliased packet needs to have its 904original IP header restored for further processing (e.g.\& logging). 905.Ed 906.Sh CONCEPTUAL BACKGROUND 907This section is intended for those who are planning to modify the source 908code or want to create somewhat esoteric applications using the packet 909aliasing functions. 910.Pp 911The conceptual framework under which the packet aliasing engine operates 912is described here. 913Central to the discussion is the idea of an 914.Em aliasing link 915which describes the relationship for a given packet transaction between 916the local machine, aliased identity and remote machine. 917It is discussed how such links come into existence and are destroyed. 918.Ss ALIASING LINKS 919There is a notion of an 920.Em aliasing link , 921which is a 7-tuple describing a specific translation: 922.Bd -literal -offset indent 923(local addr, local port, alias addr, alias port, 924 remote addr, remote port, protocol) 925.Ed 926.Pp 927Outgoing packets have the local address and port number replaced with the 928alias address and port number. 929Incoming packets undergo the reverse process. 930The packet aliasing engine attempts to match packets against an internal 931table of aliasing links to determine how to modify a given IP packet. 932Both the IP header and protocol dependent headers are modified as necessary. 933Aliasing links are created and deleted as necessary according to network 934traffic. 935.Pp 936Protocols can be TCP, UDP or even ICMP in certain circumstances. 937(Some types of ICMP packets can be aliased according to sequence or ID 938number which acts as an equivalent port number for identifying how 939individual packets should be handled.) 940.Pp 941Each aliasing link must have a unique combination of the following five 942quantities: alias address/port, remote address/port and protocol. 943This ensures that several machines on a local network can share the 944same aliasing IP address. 945In cases where conflicts might arise, the aliasing port is chosen so that 946uniqueness is maintained. 947.Ss STATIC AND DYNAMIC LINKS 948Aliasing links can either be static or dynamic. 949Static links persist indefinitely and represent fixed rules for translating 950IP packets. 951Dynamic links come into existence for a specific TCP connection or UDP 952transaction or ICMP ECHO sequence. 953For the case of TCP, the connection can be monitored to see when the 954associated aliasing link should be deleted. 955Aliasing links for UDP transactions (and ICMP ECHO and TIMESTAMP requests) 956work on a simple timeout rule. 957When no activity is observed on a dynamic link for a certain amount of time 958it is automatically deleted. 959Timeout rules also apply to TCP connections which do not open or close 960properly. 961.Ss PARTIALLY SPECIFIED ALIASING LINKS 962Aliasing links can be partially specified, meaning that the remote address 963and/or remote port are unknown. 964In this case, when a packet matching the incomplete specification is found, 965a fully specified dynamic link is created. 966If the original partially specified link is dynamic, it will be deleted 967after the fully specified link is created, otherwise it will persist. 968.Pp 969For instance, a partially specified link might be 970.Bd -literal -offset indent 971(192.168.0.4, 23, 204.228.203.215, 8066, 0, 0, tcp) 972.Ed 973.Pp 974The zeros denote unspecified components for the remote address and port. 975If this link were static it would have the effect of redirecting all 976incoming traffic from port 8066 of 204.228.203.215 to port 23 (telnet) 977of machine 192.168.0.4 on the local network. 978Each individual telnet connection would initiate the creation of a distinct 979dynamic link. 980.Ss DYNAMIC LINK CREATION 981In addition to aliasing links, there are also address mappings that can be 982stored within the internal data table of the packet aliasing mechanism. 983.Bd -literal -offset indent 984(local addr, alias addr) 985.Ed 986.Pp 987Address mappings are searched when creating new dynamic links. 988.Pp 989All outgoing packets from the local network automatically create a dynamic 990link if they do not match an already existing fully specified link. 991If an address mapping exists for the outgoing packet, this determines 992the alias address to be used. 993If no mapping exists, then a default address, usually the address of the 994packet aliasing host, is used. 995If necessary, this default address can be changed as often as each individual 996packet arrives. 997.Pp 998The aliasing port number is determined such that the new dynamic link does 999not conflict with any existing links. 1000In the default operating mode, the packet aliasing engine attempts to set 1001the aliasing port equal to the local port number. 1002If this results in a conflict, then port numbers are randomly chosen until 1003a unique aliasing link can be established. 1004In an alternate operating mode, the first choice of an aliasing port is also 1005random and unrelated to the local port number. 1006.Sh MODULAR ARCHITECTURE Po AND Xr ipfw 4 SUPPORT Pc 1007One of the latest improvements to 1008.Nm 1009was to make its support 1010for new protocols independent from the rest of the library, giving it 1011the ability to load/unload support for new protocols at run-time. 1012To achieve this feature, all the code for protocol handling was moved 1013to a series of modules outside of the main library. 1014These modules are compiled from the same sources but work in 1015different ways, depending on whether they are compiled to work inside a kernel 1016or as part of the userland library. 1017.Ss LIBALIAS MODULES IN KERNEL LAND 1018When compiled for the kernel, 1019.Nm 1020modules are plain KLDs recognizable with the 1021.Pa alias_ 1022prefix. 1023.Pp 1024To add support for a new protocol, load the corresponding module. 1025For example: 1026.Pp 1027.Dl "kldload alias_ftp" 1028.Pp 1029When support for a protocol is no longer needed, its module can be unloaded: 1030.Pp 1031.Dl "kldunload alias_ftp" 1032.Ss LIBALIAS MODULES IN USERLAND 1033Due to the differences between kernel and userland (no KLD mechanism, 1034many different address spaces, etc.), we had to change a bit how to 1035handle module loading/tracking/unloading in userland. 1036.Pp 1037While compiled for a userland 1038.Nm , 1039all the modules are plain libraries, residing in 1040.Pa /usr/lib , 1041and recognizable with the 1042.Pa libalias_ 1043prefix. 1044.Pp 1045There is a configuration file, 1046.Pa /etc/libalias.conf , 1047with the following contents (by default): 1048.Bd -literal -offset indent 1049/usr/lib/libalias_cuseeme.so 1050/usr/lib/libalias_ftp.so 1051/usr/lib/libalias_irc.so 1052/usr/lib/libalias_nbt.so 1053/usr/lib/libalias_pptp.so 1054/usr/lib/libalias_skinny.so 1055/usr/lib/libalias_smedia.so 1056.Ed 1057.Pp 1058This file contains the paths to the modules that 1059.Nm 1060will load. 1061To load/unload a new module, just add its path to 1062.Pa libalias.conf 1063and call 1064.Fn LibAliasRefreshModules 1065from the program. 1066In case the application provides a 1067.Dv SIGHUP 1068signal handler, add a call to 1069.Fn LibAliasRefreshModules 1070inside the handler, and everytime you want to refresh the loaded modules, 1071send it the 1072.Dv SIGHUP 1073signal: 1074.Pp 1075.Dl "kill -HUP <process_pid>" 1076.Ss MODULAR ARCHITECURE: HOW IT WORKS 1077The modular architecture of 1078.Nm 1079works similar whether it is running inside the 1080kernel or in userland. 1081From 1082.Pa alias_mod.c : 1083.Bd -literal 1084/* Protocol and userland module handlers chains. */ 1085LIST_HEAD(handler_chain, proto_handler) handler_chain ... 1086\&... 1087SLIST_HEAD(dll_chain, dll) dll_chain ... 1088.Ed 1089.Pp 1090.Va handler_chain 1091keeps track of all the protocol handlers loaded, while 1092.Va ddl_chain 1093tracks which userland modules are loaded. 1094.Pp 1095.Va handler_chain 1096is composed of 1097.Vt "struct proto_handler" 1098entries: 1099.Bd -literal 1100struct proto_handler { 1101 u_int pri; 1102 int16_t dir; 1103 uint8_t proto; 1104 int (*fingerprint)(struct libalias *la, 1105 struct ip *pip, struct alias_data *ah); 1106 int (*protohandler)(struct libalias *la, 1107 struct ip *pip, struct alias_data *ah); 1108 TAILQ_ENTRY(proto_handler) link; 1109}; 1110.Ed 1111.Pp 1112where: 1113.Bl -inset 1114.It Va pri 1115is the priority assigned to a protocol handler; lower priority 1116is better. 1117.It Va dir 1118is the direction of packets: ingoing or outgoing. 1119.It Va proto 1120indicates to which protocol this packet belongs: IP, TCP or UDP. 1121.It Va fingerprint 1122points to the fingerprint function while protohandler points 1123to the protocol handler function. 1124.El 1125.Pp 1126The 1127.Va fingerprint 1128function has the dual role of checking if the 1129incoming packet is found, and if it belongs to any categories that this 1130module can handle. 1131.Pp 1132The 1133.Va protohandler 1134function actually manipulates 1135the packet to make 1136.Nm 1137correctly NAT it. 1138.Pp 1139When a packet enters 1140.Nm , 1141if it meets a module hook, 1142.Va handler_chain 1143is searched to see if there is an handler that matches 1144this type of a packet (it checks protocol and direction of packet). 1145Then, if more than one handler is found, it starts with the module with 1146the lowest priority number: it calls the 1147.Va fingerprint 1148function and interprets the result. 1149.Pp 1150If the result value is equal to 0 then it calls the protocol handler 1151of this handler and returns. 1152Otherwise, it proceeds to the next eligible module until the 1153.Va handler_chain 1154is exhausted. 1155.Pp 1156Inside 1157.Nm , 1158the module hook looks like this: 1159.Bd -literal -offset indent 1160struct alias_data ad = { 1161 lnk, 1162 &original_address, 1163 &alias_address, 1164 &alias_port, 1165 &ud->uh_sport, /* original source port */ 1166 &ud->uh_dport, /* original dest port */ 1167 256 /* maxpacketsize */ 1168}; 1169 1170\&... 1171 1172/* walk out chain */ 1173err = find_handler(IN, UDP, la, pip, &ad); 1174.Ed 1175.Pp 1176All data useful to a module are gathered together in an 1177.Vt alias_data 1178structure, then 1179.Fn find_handler 1180is called. 1181The 1182.Fn find_handler 1183function is responsible for walking the handler 1184chain; it receives as input parameters: 1185.Bl -tag -width indent 1186.It Fa IN 1187direction 1188.It Fa UDP 1189working protocol 1190.It Fa la 1191pointer to this instance of libalias 1192.It Fa pip 1193pointer to a 1194.Vt "struct ip" 1195.It Fa ad 1196pointer to 1197.Vt "struct alias_data" 1198(see above) 1199.El 1200.Pp 1201In this case, 1202.Fn find_handler 1203will search only for modules registered for 1204supporting INcoming UDP packets. 1205.Pp 1206As was mentioned earlier, 1207.Nm 1208in userland is a bit different, as 1209care must be taken in module handling as well (avoiding duplicate load of 1210modules, avoiding modules with same name, etc.) so 1211.Va dll_chain 1212was introduced. 1213.Pp 1214.Va dll_chain 1215contains a list of all userland 1216.Nm 1217modules loaded. 1218.Pp 1219When an application calls 1220.Fn LibAliasRefreshModules , 1221.Nm 1222first unloads all the loaded modules, then reloads all the modules listed in 1223.Pa /etc/libalias.conf : 1224for every module loaded, a new entry is added to 1225.Va dll_chain . 1226.Pp 1227.Va dll_chain 1228is composed of 1229.Vt "struct dll" 1230entries: 1231.Bd -literal 1232struct dll { 1233 /* name of module */ 1234 char name[DLL_LEN]; 1235 /* 1236 * ptr to shared obj obtained through 1237 * dlopen() - use this ptr to get access 1238 * to any symbols from a loaded module 1239 * via dlsym() 1240 */ 1241 void *handle; 1242 struct dll *next; 1243}; 1244.Ed 1245.Bl -inset 1246.It Va name 1247is the name of the module. 1248.It Va handle 1249is a pointer to the module obtained through 1250.Xr dlopen 3 . 1251.El 1252Whenever a module is loaded in userland, an entry is added to 1253.Va dll_chain , 1254then every protocol handler present in that module 1255is resolved and registered in 1256.Va handler_chain . 1257.Ss HOW TO WRITE A MODULE FOR LIBALIAS 1258There is a module (called 1259.Pa alias_dummy.[ch] ) 1260in 1261.Nm 1262that can be used as a skeleton for future work. 1263Here we analyse some parts of that module. 1264From 1265.Pa alias_dummy.c : 1266.Bd -literal 1267struct proto_handler handlers[] = { 1268 { 1269 .pri = 666, 1270 .dir = IN|OUT, 1271 .proto = UDP|TCP, 1272 .fingerprint = fingerprint, 1273 .protohandler= protohandler, 1274 }, 1275 { EOH } 1276}; 1277.Ed 1278.Pp 1279The variable 1280.Va handlers 1281is the 1282.Dq "most important thing" 1283in a module 1284since it describes the handlers present and lets the outside world use 1285it in an opaque way. 1286.Pp 1287It must ALWAYS be present in every module, and it MUST retain 1288the name 1289.Va handlers , 1290otherwise attempting to load a module in userland will fail and 1291complain about missing symbols: for more information about module 1292load/unload, please refer to 1293.Fn LibAliasRefreshModules , 1294.Fn LibAliasLoadModule 1295and 1296.Fn LibAliasUnloadModule 1297in 1298.Pa alias.c . 1299.Pp 1300.Va handlers 1301contains all the 1302.Vt proto_handler 1303structures present in a module. 1304.Bd -literal 1305static int 1306mod_handler(module_t mod, int type, void *data) 1307{ 1308 int error; 1309 1310 switch (type) { 1311 case MOD_LOAD: 1312 error = LibAliasAttachHandlers(handlers); 1313 break; 1314 case MOD_UNLOAD: 1315 error = LibAliasDetachHandlers(handlers); 1316 break; 1317 default: 1318 error = EINVAL; 1319 } 1320 return (error); 1321} 1322.Ed 1323When running as KLD, 1324.Fn mod_handler 1325registers/deregisters the module using 1326.Fn LibAliasAttachHandlers 1327and 1328.Fn LibAliasDetachHandlers , 1329respectively. 1330.Pp 1331Every module must contain at least 2 functions: one fingerprint 1332function and a protocol handler function. 1333.Bd -literal 1334#ifdef _KERNEL 1335static 1336#endif 1337int 1338fingerprint(struct libalias *la, struct ip *pip, struct alias_data *ah) 1339{ 1340 1341\&... 1342} 1343 1344#ifdef _KERNEL 1345static 1346#endif 1347int 1348protohandler(struct libalias *la, struct ip *pip, 1349 struct alias_data *ah) 1350{ 1351 1352\&... 1353} 1354.Ed 1355and they must accept exactly these input parameters. 1356.Ss PATCHING AN APPLICATION FOR USERLAND LIBALIAS MODULES 1357To add module support into an application that uses 1358.Nm , 1359the following simple steps can be followed. 1360.Bl -enum 1361.It 1362Find the main file of an application 1363(let us call it 1364.Pa main.c ) . 1365.It 1366Add this to the header section of 1367.Pa main.c , 1368if not already present: 1369.Pp 1370.Dl "#include <signal.h>" 1371.Pp 1372and this just after the header section: 1373.Pp 1374.Dl "static void signal_handler(int);" 1375.It 1376Add the following line to the init function of an application or, 1377if it does not have any init function, put it in 1378.Fn main : 1379.Pp 1380.Dl "signal(SIGHUP, signal_handler);" 1381.Pp 1382and place the 1383.Fn signal_handler 1384function somewhere in 1385.Pa main.c : 1386.Bd -literal -offset indent 1387static void 1388signal_handler(int sig) 1389{ 1390 1391 LibAliasRefreshModules(); 1392} 1393.Ed 1394.Pp 1395Otherwise, if an application already traps the 1396.Dv SIGHUP 1397signal, just add a call to 1398.Fn LibAliasRefreshModules 1399in the signal handler function. 1400.El 1401For example, to patch 1402.Xr natd 8 1403to use 1404.Nm 1405modules, just add the following line to 1406.Fn RefreshAddr "int sig __unused" : 1407.Pp 1408.Dl "LibAliasRefreshModules()" 1409.Pp 1410recompile and you are done. 1411.Ss LOGGING SUPPORT IN KERNEL LAND 1412When working as KLD, 1413.Nm 1414now has log support that 1415happens on a buffer allocated inside 1416.Vt "struct libalias" 1417(from 1418.Pa alias_local.h ) : 1419.Bd -literal 1420struct libalias { 1421 ... 1422 1423 /* log descriptor */ 1424#ifdef KERNEL_LOG 1425 char *logDesc; /* 1426 * ptr to an auto-malloced 1427 * memory buffer when libalias 1428 * works as kld 1429 */ 1430#else 1431 FILE *logDesc; /* 1432 * ptr to /var/log/alias.log 1433 * when libalias runs as a 1434 * userland lib 1435 */ 1436#endif 1437 1438 ... 1439} 1440.Ed 1441so all applications using 1442.Nm 1443will be able to handle their 1444own logs, if they want, accessing 1445.Va logDesc . 1446Moreover, every change to a log buffer is automatically added to 1447.Xr syslog 3 1448with the 1449.Dv LOG_SECURITY 1450facility and the 1451.Dv LOG_INFO 1452level. 1453.Sh AUTHORS 1454.An Charles Mott Aq cm@linktel.net , 1455versions 1.0 - 1.8, 2.0 - 2.4. 1456.An Eivind Eklund Aq eivind@FreeBSD.org , 1457versions 1.8b, 1.9 and 2.5. 1458Added IRC DCC support as well as contributing a number of architectural 1459improvements; added the firewall bypass for FTP/IRC DCC. 1460.An Erik Salander Aq erik@whistle.com 1461added support for PPTP and RTSP. 1462.An Junichi Satoh Aq junichi@junichi.org 1463added support for RTSP/PNA. 1464.An Ruslan Ermilov Aq ru@FreeBSD.org 1465added support for PPTP and LSNAT as well as general hacking. 1466.An Gleb Smirnoff Aq glebius@FreeBSD.org 1467ported the library to kernel space. 1468.An Paolo Pisati Aq piso@FreeBSD.org 1469made the library modular, moving support for all 1470protocols (except for IP, TCP and UDP) to external modules. 1471.Sh ACKNOWLEDGEMENTS 1472Listed below, in approximate chronological order, are individuals who 1473have provided valuable comments and/or debugging assistance. 1474.Bd -ragged -offset indent 1475.An -split 1476.An Gary Roberts 1477.An Tom Torrance 1478.An Reto Burkhalter 1479.An Martin Renters 1480.An Brian Somers 1481.An Paul Traina 1482.An Ari Suutari 1483.An Dave Remien 1484.An J. Fortes 1485.An Andrzej Bialecki 1486.An Gordon Burditt 1487.Ed 1488