1.\" $FreeBSD$ 2.\" 3.Dd April 13, 2000 4.Dt LIBALIAS 3 5.Os FreeBSD 6.Sh NAME 7.Nm libalias 8.Nd packet aliasing library for masquerading and network address translation 9.Sh SYNOPSIS 10.Fd #include <sys/types.h> 11.Fd #include <netinet/in.h> 12.Fd #include <alias.h> 13.Pp 14Function prototypes are given in the main body of the text. 15.Sh DESCRIPTION 16The 17.Nm 18library is a collection of functions for aliasing and de-aliasing of IP 19packets, intended for masquerading and network address translation (NAT). 20.Sh INTRODUCTION 21This library is a moderately portable set of functions designed to assist 22in the process of IP masquerading and network address translation. 23Outgoing packets from a local network with unregistered IP addresses can 24be aliased to appear as if they came from an accessible IP address. 25Incoming packets are then de-aliased so that they are sent to the correct 26machine on the local network. 27.Pp 28A certain amount of flexibility is built into the packet aliasing engine. 29In the simplest mode of operation, a many-to-one address mapping takes 30place between local network and the packet aliasing host. 31This is known as IP masquerading. 32In addition, one-to-one mappings between local and public addresses can 33also be implemented, which is known as static NAT. 34In between these extremes, different groups of private addresses can be 35linked to different public addresses, comprising several distinct 36many-to-one mappings. 37Also, a given public address and port can be statically redirected to a 38private address/port. 39.Pp 40The packet aliasing engine was designed to operate in user space outside 41of the kernel, without any access to private kernel data structure, but 42the source code can also be ported to a kernel environment. 43.Sh INITIALIZATION AND CONTROL 44Two special functions, 45.Fn PacketAliasInit 46and 47.Fn PacketAliasSetAddress , 48must always be called before any packet handling may be performed. 49In addition, the operating mode of the packet aliasing engine can be 50customized by calling 51.Fn PacketAliasSetMode . 52.Pp 53.Ft void 54.Fn PacketAliasInit void 55.Bd -ragged -offset indent 56This function has no arguments or return value and is used to initialize 57internal data structures. 58The following mode bits are always set after calling 59.Fn PacketAliasInit . 60See the description of 61.Fn PacketAliasSetMode 62below for the meaning of these mode bits. 63.Pp 64.Bl -item -offset indent -compact 65.It 66.Dv PKT_ALIAS_SAME_PORTS 67.It 68.Dv PKT_ALIAS_USE_SOCKETS 69.It 70.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE 71.El 72.Pp 73This function will always return the packet aliasing engine to the same 74initial state. 75.Fn PacketAliasSetAddress 76must be called afterwards, and any desired changes from the default mode 77bits listed above require a call to 78.Fn PacketAliasSetMode . 79.Pp 80It is mandatory that this function be called at the beginning of a program 81prior to any packet handling. 82.Ed 83.Pp 84.Ft void 85.Fn PacketAliasUninit void 86.Bd -ragged -offset indent 87This function has no arguments or return value and is used to clear any 88resources attached to internal data structures. 89.Pp 90This functions should be called when a program stops using the aliasing 91engine; it does, amongst other things, clear out any firewall holes. 92To provide backwards compatibility and extra security, it is added to 93the 94.Xr atexit 3 95chain by 96.Fn PacketAliasInit . 97Calling it multiple times is harmless. 98.Ed 99.Pp 100.Ft void 101.Fn PacketAliasSetAddress "struct in_addr addr" 102.Bd -ragged -offset indent 103This function sets the source address to which outgoing packets from the 104local area network are aliased. 105All outgoing packets are re-mapped to this address unless overridden by a 106static address mapping established by 107.Fn PacketAliasRedirectAddr . 108.Pp 109If the 110.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE 111mode bit is set (the default mode of operation), then the internal aliasing 112link tables will be reset any time the aliasing address changes. 113This is useful for interfaces such as 114.Xr ppp 8 , 115where the IP 116address may or may not change on successive dial-up attempts. 117.Pp 118If the 119.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE 120mode bit is set to zero, this function can also be used to dynamically change 121the aliasing address on a packet to packet basis (it is a low overhead call). 122.Pp 123It is mandatory that this function be called prior to any packet handling. 124.Ed 125.Pp 126.Ft unsigned int 127.Fn PacketAliasSetMode "unsigned int flags" "unsigned int mask" 128.Bd -ragged -offset indent 129This function sets or clears mode bits 130according to the value of 131.Fa flags . 132Only bits marked in 133.Fa mask 134are affected. 135The following mode bits are defined in 136.Aq Pa alias.h : 137.Bl -tag -width indent 138.It Dv PKT_ALIAS_LOG 139Enables logging into 140.Pa /var/log/alias.log . 141Each time an aliasing link is created or deleted, the log file is appended 142with the current number of ICMP, TCP and UDP links. 143Mainly useful for debugging when the log file is viewed continuously with 144.Xr tail 1 . 145.It Dv PKT_ALIAS_DENY_INCOMING 146If this mode bit is set, all incoming packets associated with new TCP 147connections or new UDP transactions will be marked for being ignored 148.Po 149.Fn PacketAliasIn 150returns 151.Dv PKT_ALIAS_IGNORED 152code 153.Pc 154by the calling program. 155Response packets to connections or transactions initiated from the packet 156aliasing host or local network will be unaffected. 157This mode bit is useful for implementing a one-way firewall. 158.It Dv PKT_ALIAS_SAME_PORTS 159If this mode bit is set, the packet aliasing engine will attempt to leave 160the alias port numbers unchanged from the actual local port numbers. 161This can be done as long as the quintuple (proto, alias addr, alias port, 162remote addr, remote port) is unique. 163If a conflict exists, a new aliasing port number is chosen even if this 164mode bit is set. 165.It Dv PKT_ALIAS_USE_SOCKETS 166This bit should be set when the packet aliasing host originates network 167traffic as well as forwards it. 168When the packet aliasing host is waiting for a connection from an unknown 169host address or unknown port number (e.g. an FTP data connection), this 170mode bit specifies that a socket be allocated as a place holder to prevent 171port conflicts. 172Once a connection is established, usually within a minute or so, the socket 173is closed. 174.It Dv PKT_ALIAS_UNREGISTERED_ONLY 175If this mode bit is set, traffic on the local network which does not 176originate from unregistered address spaces will be ignored. 177Standard Class A, B and C unregistered addresses are: 178.Bd -literal -offset indent 17910.0.0.0 -> 10.255.255.255 (Class A subnet) 180172.16.0.0 -> 172.31.255.255 (Class B subnets) 181192.168.0.0 -> 192.168.255.255 (Class C subnets) 182.Ed 183.Pp 184This option is useful in the case that packet aliasing host has both 185registered and unregistered subnets on different interfaces. 186The registered subnet is fully accessible to the outside world, so traffic 187from it does not need to be passed through the packet aliasing engine. 188.It Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE 189When this mode bit is set and 190.Fn PacketAliasSetAddress 191is called to change the aliasing address, the internal link table of the 192packet aliasing engine will be cleared. 193This operating mode is useful for 194.Xr ppp 8 195links where the interface address can sometimes change or remain the same 196between dial-up attempts. 197If this mode bit is not set, the link table will never be reset in the event 198of an address change. 199.It Dv PKT_ALIAS_PUNCH_FW 200This option makes 201.Nm 202`punch holes' in an 203.Xr ipfirewall 4 204based firewall for FTP/IRC DCC connections. 205The holes punched are bound by from/to IP address and port; it will not be 206possible to use a hole for another connection. 207A hole is removed when the connection that uses it dies. 208To cater to unexpected death of a program using 209.Nm 210(e.g. kill -9), 211changing the state of the flag will clear the entire firewall range 212allocated for holes. 213This will also happen on the initial call to 214.Fn PacketAliasSetFWBase . 215This call must happen prior to setting this flag. 216.It Dv PKT_ALIAS_REVERSE 217This option makes 218.Nm 219reverse the way it handles incoming and outgoing packets, allowing it 220to be fed with data that passes through the internal interface rather 221than the external one. 222.It Dv PKT_ALIAS_PROXY_ONLY 223This option tells 224.Nm 225to obey transparent proxy rules only. 226Normal packet aliasing is not performed. 227See 228.Fn PacketAliasProxyRule 229below for details. 230.It Dv PKT_ALIAS_DENY_PPTP 231If this mode bit is set, all PPTP packets will be marked for being ignored 232(both 233.Fn PacketAliasIn 234and 235.Fn PacketAliasOut 236return 237.Dv PKT_ALIAS_IGNORED 238code). 239.El 240.Ed 241.Pp 242.Ft void 243.Fn PacketAliasSetFWBase "unsigned int base" "unsigned int num" 244.Bd -ragged -offset indent 245Set firewall range allocated for punching firewall holes (with the 246.Dv PKT_ALIAS_PUNCH_FW 247flag). 248The range will be cleared for all rules on initialization. 249.Ed 250.Sh PACKET HANDLING 251The packet handling functions are used to modify incoming (remote to local) 252and outgoing (local to remote) packets. 253The calling program is responsible for receiving and sending packets via 254network interfaces. 255.Pp 256Along with 257.Fn PacketAliasInit 258and 259.Fn PacketAliasSetAddress , 260the two packet handling functions, 261.Fn PacketAliasIn 262and 263.Fn PacketAliasOut , 264comprise minimal set of functions needed for a basic IP masquerading 265implementation. 266.Pp 267.Ft int 268.Fn PacketAliasIn "char *buffer" "int maxpacketsize" 269.Bd -ragged -offset indent 270An incoming packet coming from a remote machine to the local network is 271de-aliased by this function. 272The IP packet is pointed to by 273.Fa buffer , 274and 275.Fa maxpacketsize 276indicates the size of the data structure containing the packet and should 277be at least as large as the actual packet size. 278.Pp 279Return codes: 280.Bl -tag -width indent 281.It Dv PKT_ALIAS_OK 282The packet aliasing process was successful. 283.It Dv PKT_ALIAS_IGNORED 284The packet was ignored and not de-aliased. 285This can happen if the protocol is unrecognized, possibly an ICMP message 286type is not handled or if incoming packets for new connections are being 287ignored (if 288.Dv PKT_ALIAS_DENY_INCOMING 289mode bit was set by 290.Fn PacketAliasSetMode ) . 291.It Dv PKT_ALIAS_UNRESOLVED_FRAGMENT 292This is returned when a fragment cannot be resolved because the header 293fragment has not been sent yet. 294In this situation, fragments must be saved with 295.Fn PacketAliasSaveFragment 296until a header fragment is found. 297.It Dv PKT_ALIAS_FOUND_HEADER_FRAGMENT 298The packet aliasing process was successful, and a header fragment was found. 299This is a signal to retrieve any unresolved fragments with 300.Fn PacketAliasGetFragment 301and de-alias them with 302.Fn PacketAliasFragmentIn . 303.It Dv PKT_ALIAS_ERROR 304An internal error within the packet aliasing engine occurred. 305.El 306.Ed 307.Pp 308.Ft int 309.Fn PacketAliasOut "char *buffer" "int maxpacketsize" 310.Bd -ragged -offset indent 311An outgoing packet coming from the local network to a remote machine is 312aliased by this function. 313The IP packet is pointed to by 314.Fa buffer , 315and 316.Fa maxpacketsize 317indicates the maximum packet size permissible should the packet length be 318changed. 319IP encoding protocols place address and port information in the encapsulated 320data stream which has to be modified and can account for changes in packet 321length. 322Well known examples of such protocols are FTP and IRC DCC. 323.Pp 324Return codes: 325.Bl -tag -width indent 326.It Dv PKT_ALIAS_OK 327The packet aliasing process was successful. 328.It Dv PKT_ALIAS_IGNORED 329The packet was ignored and not aliased. 330This can happen if the protocol is unrecognized, or possibly an ICMP message 331type is not handled. 332.It Dv PKT_ALIAS_ERROR 333An internal error within the packet aliasing engine occurred. 334.El 335.Ed 336.Sh PORT AND ADDRESS REDIRECTION 337The functions described in this section allow machines on the local network 338to be accessible in some degree to new incoming connections from the external 339network. 340Individual ports can be re-mapped or static network address translations can 341be designated. 342.Pp 343.Ft struct alias_link * 344.Fo PacketAliasRedirectPort 345.Fa "struct in_addr local_addr" 346.Fa "u_short local_port" 347.Fa "struct in_addr remote_addr" 348.Fa "u_short remote_port" 349.Fa "struct in_addr alias_addr" 350.Fa "u_short alias_port" 351.Fa "u_char proto" 352.Fc 353.Bd -ragged -offset indent 354This function specifies that traffic from a given remote address/port to 355an alias address/port be redirected to a specified local address/port. 356The parameter 357.Fa proto 358can be either 359.Dv IPPROTO_TCP 360or 361.Dv IPPROTO_UDP , 362as defined in 363.Aq Pa netinet/in.h . 364.Pp 365If 366.Fa local_addr 367or 368.Fa alias_addr 369is zero, this indicates that the packet aliasing address as established 370by 371.Fn PacketAliasSetAddress 372is to be used. 373Even if 374.Fn PacketAliasSetAddress 375is called to change the address after 376.Fn PacketAliasRedirectPort 377is called, a zero reference will track this change. 378.Pp 379If 380.Fa remote_addr 381is zero, this indicates to redirect packets from any remote address. 382Likewise, if 383.Fa remote_port 384is zero, this indicates to redirect packets originating from any remote 385port number. 386Almost always, the remote port specification will be zero, but non-zero 387remote addresses can sometimes be useful for firewalling. 388If two calls to 389.Fn PacketAliasRedirectPort 390overlap in their address/port specifications, then the most recent call 391will have precedence. 392.Pp 393This function returns a pointer which can subsequently be used by 394.Fn PacketAliasRedirectDelete . 395If 396.Dv NULL 397is returned, then the function call did not complete successfully. 398.Pp 399All port numbers should be in network address byte order, so it is necessary 400to use 401.Xr htons 3 402to convert these parameters from internally readable numbers to network byte 403order. 404Addresses are also in network byte order, which is implicit in the use of the 405.Fa struct in_addr 406data type. 407.Ed 408.Pp 409.Ft struct alias_link * 410.Fo PacketAliasRedirectAddr 411.Fa "struct in_addr local_addr" 412.Fa "struct in_addr alias_addr" 413.Fc 414.Bd -ragged -offset indent 415This function designates that all incoming traffic to 416.Fa alias_addr 417be redirected to 418.Fa local_addr . 419Similarly, all outgoing traffic from 420.Fa local_addr 421is aliased to 422.Fa alias_addr . 423.Pp 424If 425.Fa local_addr 426or 427.Fa alias_addr 428is zero, this indicates that the packet aliasing address as established by 429.Fn PacketAliasSetAddress 430is to be used. 431Even if 432.Fn PacketAliasSetAddress 433is called to change the address after 434.Fn PacketAliasRedirectAddr 435is called, a zero reference will track this change. 436.Pp 437If subsequent calls to 438.Fn PacketAliasRedirectAddr 439use the same aliasing address, all new incoming traffic to this aliasing 440address will be redirected to the local address made in the last function 441call. 442New traffic generated by any of the local machines, designated in the 443several function calls, will be aliased to the same address. 444Consider the following example: 445.Bd -literal -offset indent 446PacketAliasRedirectAddr(inet_aton("192.168.0.2"), 447 inet_aton("141.221.254.101")); 448PacketAliasRedirectAddr(inet_aton("192.168.0.3"), 449 inet_aton("141.221.254.101")); 450PacketAliasRedirectAddr(inet_aton("192.168.0.4"), 451 inet_aton("141.221.254.101")); 452.Ed 453.Pp 454Any outgoing connections such as 455.Xr telnet 1 456or 457.Xr ftp 1 458from 192.168.0.2, 192.168.0.3 and 192.168.0.4 will appear to come from 459141.221.254.101. 460Any incoming connections to 141.221.254.101 will be directed to 192.168.0.4. 461.Pp 462Any calls to 463.Fn PacketAliasRedirectPort 464will have precedence over address mappings designated by 465.Fn PacketAliasRedirectAddr . 466.Pp 467This function returns a pointer which can subsequently be used by 468.Fn PacketAliasRedirectDelete . 469If 470.Dv NULL 471is returned, then the function call did not complete successfully. 472.Ed 473.Pp 474.Ft void 475.Fn PacketAliasRedirectDelete "struct alias_link *link" 476.Bd -ragged -offset indent 477This function will delete a specific static redirect rule entered by 478.Fn PacketAliasRedirectPort 479or 480.Fn PacketAliasRedirectAddr . 481The parameter 482.Fa link 483is the pointer returned by either of the redirection functions. 484If an invalid pointer is passed to 485.Fn PacketAliasRedirectDelete , 486then a program crash or unpredictable operation could result, so it is 487necessary to be careful using this function. 488.Ed 489.Pp 490.Ft int 491.Fn PacketAliasProxyRule "const char *cmd" 492.Bd -ragged -offset indent 493The passed 494.Fa cmd 495string consists of one or more pairs of words. 496The first word in each pair is a token and the second is the value that 497should be applied for that token. 498Tokens and their argument types are as follows: 499.Bl -tag -width indent 500.It Cm type encode_ip_hdr | encode_tcp_stream | no_encode 501In order to support transparent proxying, it is necessary to somehow 502pass the original address and port information into the new destination 503server. 504If 505.Cm encode_ip_hdr 506is specified, the original address and port is passed as an extra IP 507option. 508If 509.Cm encode_tcp_stream 510is specified, the original address and port is passed as the first 511piece of data in the TCP stream in the format 512.Dq DEST Ar IP port . 513.It Cm port Ar portnum 514Only packets with the destination port 515.Ar portnum 516are proxied. 517.It Cm server Ar host Ns Xo 518.Op : Ns Ar portnum 519.Xc 520This specifies the 521.Ar host 522and 523.Ar portnum 524that the data is to be redirected to. 525.Ar host 526must be an IP address rather than a DNS host name. 527If 528.Ar portnum 529is not specified, the destination port number is not changed. 530.Pp 531The 532.Ar server 533specification is mandatory unless the 534.Cm delete 535command is being used. 536.It Cm rule Ar index 537Normally, each call to 538.Fn PacketAliasProxyRule 539inserts the next rule at the start of a linear list of rules. 540If an 541.Ar index 542is specified, the new rule will be checked after all rules with lower 543indices. 544Calls to 545.Fn PacketAliasProxyRule 546that do not specify a rule are assigned rule 0. 547.It Cm delete Ar index 548This token and its argument MUST NOT be used with any other tokens. 549When used, all existing rules with the given 550.Ar index 551are deleted. 552.It Cm proto tcp | udp 553If specified, only packets of the given protocol type are matched. 554.It Cm src Ar IP Ns Xo 555.Op / Ns Ar bits 556.Xc 557If specified, only packets with a source address matching the given 558.Ar IP 559are matched. 560If 561.Ar bits 562is also specified, then the first 563.Ar bits 564bits of 565.Ar IP 566are taken as a network specification, and all IP addresses from that 567network will be matched. 568.It Cm dst Ar IP Ns Xo 569.Op / Ns Ar bits 570.Xc 571If specified, only packets with a destination address matching the given 572.Ar IP 573are matched. 574If 575.Ar bits 576is also specified, then the first 577.Ar bits 578bits of 579.Ar IP 580are taken as a network specification, and all IP addresses from that 581network will be matched. 582.El 583.Pp 584This function is usually used to redirect outgoing connections for 585internal machines that are not permitted certain types of internet 586access, or to restrict access to certain external machines. 587.Ed 588.Pp 589.Ft struct alias_link * 590.Fo PacketAliasRedirectPptp 591.Fa "struct in_addr local_addr" 592.Fa "struct in_addr remote_addr" 593.Fa "struct in_addr alias_addr" 594.Fc 595.Bd -ragged -offset indent 596This function specifies that any Point to Point Tunneling Protocol 597(PPTP) traffic from a given remote address to an alias address be 598redirected to a specified local address. 599Currently supported PPTP protocols include: 600.Pp 601.Bl -tag -width "IPPROTO_GRE" -compact 602.It IPPROTO_GRE 603Generic Routing Encapsulation (RFC 1702) 604.It IPPROTO_ESP 605IP Encapsulating Security Payload (RFC 1827) 606.It IPPROTO_AH 607IP Authentication Header (RFC 1826) 608.El 609.Pp 610If 611.Fa local_addr 612or 613.Fa alias_addr 614is zero, this indicates that the packet aliasing address as established 615by 616.Fn PacketAliasSetAddress 617is to be used. 618Even if 619.Fn PacketAliasSetAddress 620is called to change the address after 621.Fn PacketAliasRedirectPptp 622is called, a zero reference will track this change. 623.Pp 624If 625.Fa remote_addr 626is zero, this indicates to redirect PPTP packets from any remote address. 627Non-zero remote addresses can sometimes be useful for firewalling. 628.Pp 629If two calls to 630.Fn PacketAliasRedirectPptp 631overlap in their address specifications, then the most recent call 632will have precedence. 633.Pp 634This function returns a pointer which can subsequently be used by 635.Fn PacketAliasRedirectDelete . 636If 637.Dv NULL 638is returned, then the function call did not complete successfully. 639.Ed 640.Pp 641.Ft int 642.Fn PacketAliasPptp "struct in_addr addr" 643.Bd -ragged -offset indent 644This function causes any PPTP packets to be aliased using 645.Fa addr 646rather than the address set via 647.Fn PacketAliasSetAddress . 648This allows the uses of the PPTP on a single machine on the internal network. 649.Pp 650If the passed address is 651.Dv INADDR_NONE , 652then PPTP aliasing is disabled. 653.Pp 654.Bf -symbolic 655This function is made obsolete by 656.Fn PacketAliasRedirectPptp 657and 658.Dv PKT_ALIAS_DENY_PPTP 659mode bit, and is provided only for backward compatibility. 660.Ef 661.Ed 662.Sh FRAGMENT HANDLING 663The functions in this section are used to deal with incoming fragments. 664.Pp 665Outgoing fragments are handled within 666.Fn PacketAliasOut 667by changing the address according to any applicable mapping set by 668.Fn PacketAliasRedirectAddress , 669or the default aliasing address set by 670.Fn PacketAliasSetAddress . 671.Pp 672Incoming fragments are handled in one of two ways. 673If the header of a fragmented IP packet has already been seen, then all 674subsequent fragments will be re-mapped in the same manner the header 675fragment was. 676Fragments which arrive before the header are saved and then retrieved 677once the header fragment has been resolved. 678.Pp 679.Ft int 680.Fn PacketAliasSaveFragment "char *ptr" 681.Bd -ragged -offset indent 682When 683.Fn PacketAliasIn 684returns 685.Dv PKT_ALIAS_UNRESOLVED_FRAGMENT , 686this function can be used to save the pointer to the unresolved fragment. 687.Pp 688It is implicitly assumed that 689.Fa ptr 690points to a block of memory allocated by 691.Xr malloc 3 . 692If the fragment is never resolved, the packet aliasing engine will 693automatically free the memory after a timeout period. 694[Eventually this function should be modified so that a callback function 695for freeing memory is passed as an argument.] 696.Pp 697This function returns 698.Dv PKT_ALIAS_OK 699if it was successful and 700.Dv PKT_ALIAS_ERROR 701if there was an error. 702.Ed 703.Pp 704.Ft char * 705.Fn PacketAliasGetFragment "char *buffer" 706.Bd -ragged -offset indent 707This function can be used to retrieve fragment pointers saved by 708.Fn PacketAliasSaveFragment . 709The IP header fragment pointed to by 710.Fa buffer 711is the header fragment indicated when 712.Fn PacketAliasIn 713returns 714.Dv PKT_ALIAS_FOUND_HEADER_FRAGMENT . 715Once a fragment pointer is retrieved, it becomes the calling program's 716responsibility to free the dynamically allocated memory for the fragment. 717.Pp 718.Fn PacketAliasGetFragment 719can be called sequentially until there are no more fragments available, 720at which time it returns 721.Dv NULL . 722.Ed 723.Pp 724.Ft void 725.Fn PacketAliasFragmentIn "char *header" "char *fragment" 726.Bd -ragged -offset indent 727When a fragment is retrieved with 728.Fn PacketAliasGetFragment , 729it can then be de-aliased with a call to 730.Fn PacketAliasFragmentIn . 731The 732.Fa header 733argument is the pointer to a header fragment used as a template, and 734.Fa fragment 735is the pointer to the packet to be de-aliased. 736.Ed 737.Sh MISCELLANEOUS FUNCTIONS 738.Ft void 739.Fn PacketAliasSetTarget "struct in_addr addr" 740.Bd -ragged -offset indent 741When an incoming packet not associated with any pre-existing aliasing link 742arrives at the host machine, it will be sent to the address indicated by a 743call to 744.Fn PacketAliasSetTarget . 745.Pp 746If this function is not called, or is called with an 747.Dv INADDR_NONE 748address argument, then all new incoming packets go to the address set by 749.Fn PacketAliasSetAddress . 750.Pp 751If this function is called with an 752.Dv INADDR_ANY 753address argument, then all new incoming packets go to the address specified 754in the packet. 755This allows external machines to talk directly to internal machines if they 756can route packets to the machine in question. 757.Ed 758.Pp 759.Ft int 760.Fn PacketAliasCheckNewLink void 761.Bd -ragged -offset indent 762This function returns a non-zero value when a new aliasing link is created. 763In circumstances where incoming traffic is being sequentially sent to 764different local servers, this function can be used to trigger when 765.Fn PacketAliasSetTarget 766is called to change the default target address. 767.Ed 768.Pp 769.Ft u_short 770.Fn PacketAliasInternetChecksum "u_short *buffer" "int nbytes" 771.Bd -ragged -offset indent 772This is a utility function that does not seem to be available elsewhere and 773is included as a convenience. 774It computes the internet checksum, which is used in both IP and 775protocol-specific headers (TCP, UDP, ICMP). 776.Pp 777The 778.Fa buffer 779argument points to the data block to be checksummed, and 780.Fa nbytes 781is the number of bytes. 782The 16-bit checksum field should be zeroed before computing the checksum. 783.Pp 784Checksums can also be verified by operating on a block of data including 785its checksum. 786If the checksum is valid, 787.Fn PacketAliasInternetChecksum 788will return zero. 789.Ed 790.Sh AUTHORS 791.An Charles Mott Aq cmott@scientech.com , 792versions 1.0 - 1.8, 2.0 - 2.4. 793.An Eivind Eklund Aq eivind@FreeBSD.org , 794versions 1.8b, 1.9 and 2.5. 795Added IRC DCC support as well as contributing a number of architectural 796improvements; added the firewall bypass for FTP/IRC DCC. 797.Sh ACKNOWLEDGMENTS 798Listed below, in approximate chronological order, are individuals who 799have provided valuable comments and/or debugging assistance. 800.Pp 801.Bl -item -offset indent -compact 802.It 803Gary Roberts 804.It 805Tom Torrance 806.It 807Reto Burkhalter 808.It 809Martin Renters 810.It 811Brian Somers 812.It 813Paul Traina 814.It 815Ari Suutari 816.It 817Dave Remien 818.It 819J. Fortes 820.It 821Andrzej Bialecki 822.It 823Gordon Burditt 824.El 825.Sh CONCEPTUAL BACKGROUND 826This section is intended for those who are planning to modify the source 827code or want to create somewhat esoteric applications using the packet 828aliasing functions. 829.Pp 830The conceptual framework under which the packet aliasing engine operates 831is described here. 832Central to the discussion is the idea of an 833.Em aliasing link 834which describes the relationship for a given packet transaction between 835the local machine, aliased identity and remote machine. 836It is discussed how such links come into existence and are destroyed. 837.Ss ALIASING LINKS 838There is a notion of an 839.Em aliasing link , 840which is a 7-tuple describing a specific translation: 841.Bd -literal -offset indent 842(local addr, local port, alias addr, alias port, 843 remote addr, remote port, protocol) 844.Ed 845.Pp 846Outgoing packets have the local address and port number replaced with the 847alias address and port number. 848Incoming packets undergo the reverse process. 849The packet aliasing engine attempts to match packets against an internal 850table of aliasing links to determine how to modify a given IP packet. 851Both the IP header and protocol dependent headers are modified as necessary. 852Aliasing links are created and deleted as necessary according to network 853traffic. 854.Pp 855Protocols can be TCP, UDP or even ICMP in certain circumstances. 856(Some types of ICMP packets can be aliased according to sequence or ID 857number which acts as an equivalent port number for identifying how 858individual packets should be handled.) 859.Pp 860Each aliasing link must have a unique combination of the following five 861quantities: alias address/port, remote address/port and protocol. 862This ensures that several machines on a local network can share the 863same aliasing IP address. 864In cases where conflicts might arise, the aliasing port is chosen so that 865uniqueness is maintained. 866.Ss STATIC AND DYNAMIC LINKS 867Aliasing links can either be static or dynamic. 868Static links persist indefinitely and represent fixed rules for translating 869IP packets. 870Dynamic links come into existence for a specific TCP connection or UDP 871transaction or ICMP ECHO sequence. 872For the case of TCP, the connection can be monitored to see when the 873associated aliasing link should be deleted. 874Aliasing links for UDP transactions (and ICMP ECHO and TIMESTAMP requests) 875work on a simple timeout rule. 876When no activity is observed on a dynamic link for a certain amount of time 877it is automatically deleted. 878Timeout rules also apply to TCP connections which do not open or close 879properly. 880.Ss PARTIALLY SPECIFIED ALIASING LINKS 881Aliasing links can be partially specified, meaning that the remote address 882and/or remote port are unknown. 883In this case, when a packet matching the incomplete specification is found, 884a fully specified dynamic link is created. 885If the original partially specified link is dynamic, it will be deleted 886after the fully specified link is created, otherwise it will persist. 887.Pp 888For instance, a partially specified link might be 889.Bd -literal -offset indent 890(192.168.0.4, 23, 204.228.203.215, 8066, 0, 0, tcp) 891.Ed 892.Pp 893The zeros denote unspecified components for the remote address and port. 894If this link were static it would have the effect of redirecting all 895incoming traffic from port 8066 of 204.228.203.215 to port 23 (telnet) 896of machine 192.168.0.4 on the local network. 897Each individual telnet connection would initiate the creation of a distinct 898dynamic link. 899.Ss DYNAMIC LINK CREATION 900In addition to aliasing links, there are also address mappings that can be 901stored within the internal data table of the packet aliasing mechanism. 902.Bd -literal -offset indent 903(local addr, alias addr) 904.Ed 905.Pp 906Address mappings are searched when creating new dynamic links. 907.Pp 908All outgoing packets from the local network automatically create a dynamic 909link if they do not match an already existing fully specified link. 910If an address mapping exists for the outgoing packet, this determines 911the alias address to be used. 912If no mapping exists, then a default address, usually the address of the 913packet aliasing host, is used. 914If necessary, this default address can be changed as often as each individual 915packet arrives. 916.Pp 917The aliasing port number is determined such that the new dynamic link does 918not conflict with any existing links. 919In the default operating mode, the packet aliasing engine attempts to set 920the aliasing port equal to the local port number. 921If this results in a conflict, then port numbers are randomly chosen until 922a unique aliasing link can be established. 923In an alternate operating mode, the first choice of an aliasing port is also 924random and unrelated to the local port number. 925