1.\" $FreeBSD$ 2.\" 3.Dd June 18, 1996 4.Dt DIVERT 4 5.Os 6.Sh NAME 7.Nm divert 8.Nd kernel packet diversion mechanism 9.Sh SYNOPSIS 10.In sys/types.h 11.In sys/socket.h 12.In netinet/in.h 13.Ft int 14.Fn socket PF_INET SOCK_RAW IPPROTO_DIVERT 15.Sh DESCRIPTION 16Divert sockets are similar to raw IP sockets, except that they 17can be bound to a specific 18.Nm 19port via the 20.Xr bind 2 21system call. 22The IP address in the bind is ignored; only the port 23number is significant. 24A divert socket bound to a divert port will receive all packets diverted 25to that port by some (here unspecified) kernel mechanism(s). 26Packets may also be written to a divert port, in which case they 27re-enter kernel IP packet processing. 28.Pp 29Divert sockets are normally used in conjunction with 30.Fx Ns 's 31packet filtering implementation and the 32.Xr ipfw 8 33program. 34By reading from and writing to a divert socket, matching packets 35can be passed through an arbitrary ``filter'' as they travel through 36the host machine, special routing tricks can be done, etc. 37.Sh READING PACKETS 38Packets are diverted either as they are ``incoming'' or ``outgoing.'' 39Incoming packets are diverted after reception on an IP interface, 40whereas outgoing packets are diverted before next hop forwarding. 41.Pp 42Diverted packets may be read unaltered via 43.Xr read 2 , 44.Xr recv 2 , 45or 46.Xr recvfrom 2 . 47In the latter case, the address returned will have its port set to 48some tag supplied by the packet diverter, (usually the ipfw rule number) 49and the IP address set to the (first) address of 50the interface on which the packet was received (if the packet 51was incoming) or 52.Dv INADDR_ANY 53(if the packet was outgoing). In the case of an incoming packet the interface 54name will also be placed in the 8 bytes following the address, 55(assuming it fits). 56.Sh WRITING PACKETS 57Writing to a divert socket is similar to writing to a raw IP socket; 58the packet is injected ``as is'' into the normal kernel IP packet 59processing and minimal error checking is done. 60Packets are written as either incoming or outgoing: 61if 62.Xr write 2 63or 64.Xr send 2 65is used to deliver the packet, or if 66.Xr sendto 2 67is used with a destination IP address of 68.Dv INADDR_ANY , 69then the packet is treated as if it were outgoing, i.e., destined 70for a non-local address. 71Otherwise, the packet is assumed to be 72incoming and full packet routing is done. 73.Pp 74In the latter case, the 75IP address specified must match the address of some local interface, 76or an interface name 77must be found after the IP address. 78If an interface name is found, 79that interface will be used and the value of the IP address will be 80ignored (other than the fact that it is not 81.Dv INADDR_ANY ) . 82This is to indicate on which interface the packet 83.Dq arrived . 84.Pp 85Normally, packets read as incoming should be written as incoming; 86similarly for outgoing packets. 87When reading and then writing back 88packets, passing the same socket address supplied by 89.Xr recvfrom 2 90unmodified to 91.Xr sendto 2 92simplifies things (see below). 93.Pp 94The port part of the socket address passed to the 95.Xr sendto 2 96contains a tag that should be meaningful to the diversion module. 97In the 98case of 99.Xr ipfw 8 100the tag is interpreted as the rule number 101.Em after which 102rule processing should restart. 103.Sh LOOP AVOIDANCE 104Packets written into a divert socket 105(using 106.Xr sendto 2 ) 107re-enter the packet filter at the rule number 108following the tag given in the port part of the socket address, which 109is usually already set at the rule number that caused the diversion 110(not the next rule if there are several at the same number). If the 'tag' 111is altered to indicate an alternative re-entry point, care should be taken 112to avoid loops, where the same packet is diverted more than once at the 113same rule. 114.Sh DETAILS 115To enable divert sockets, your kernel must be compiled with the option 116.Dv IPDIVERT . 117.Pp 118If a packet is diverted but no socket is bound to the 119port, or if 120.Dv IPDIVERT 121is not enabled in the kernel, the packet is dropped. 122.Pp 123Incoming packet fragments which get diverted are fully reassembled 124before delivery; the diversion of any one fragment causes the entire 125packet to get diverted. 126If different fragments divert to different ports, 127then which port ultimately gets chosen is unpredictable. 128.Pp 129Packets are received and sent unchanged, except that 130packets read as outgoing have invalid IP header checksums, and 131packets written as outgoing have their IP header checksums overwritten 132with the correct value. 133Packets written as incoming and having incorrect checksums will be dropped. 134Otherwise, all header fields are unchanged (and therefore in network order). 135.Pp 136Binding to port numbers less than 1024 requires super-user access, as does 137creating a socket of type SOCK_RAW. 138.Sh ERRORS 139Writing to a divert socket can return these errors, along with 140the usual errors possible when writing raw packets: 141.Bl -tag -width Er 142.It Bq Er EINVAL 143The packet had an invalid header, or the IP options in the packet 144and the socket options set were incompatible. 145.It Bq Er EADDRNOTAVAIL 146The destination address contained an IP address not equal to 147.Dv INADDR_ANY 148that was not associated with any interface. 149.El 150.Sh SEE ALSO 151.Xr bind 2 , 152.Xr recvfrom 2 , 153.Xr sendto 2 , 154.Xr socket 2 , 155.Xr ipfw 8 156.Sh BUGS 157This is an attempt to provide a clean way for user mode processes 158to implement various IP tricks like address translation, but it 159could be cleaner, and it's too dependent on 160.Xr ipfw 8 . 161.Pp 162It's questionable whether incoming fragments should be reassembled 163before being diverted. 164For example, if only some fragments of a 165packet destined for another machine don't get routed through the 166local machine, the packet is lost. 167This should probably be 168a settable socket option in any case. 169.Sh AUTHORS 170.An Archie Cobbs Aq archie@FreeBSD.org , 171Whistle Communications Corp. 172