1.\" $FreeBSD$ 2.\" 3.Dd June 18, 1996 4.Dt DIVERT 4 5.Os FreeBSD 6.Sh NAME 7.Nm divert 8.Nd kernel packet diversion mechanism 9.Sh SYNOPSIS 10.Fd #include <sys/types.h> 11.Fd #include <sys/socket.h> 12.Fd #include <netinet/in.h> 13.Ft int 14.Fn socket PF_INET SOCK_RAW IPPROTO_DIVERT 15.Sh DESCRIPTION 16.Pp 17Divert sockets are similar to raw IP sockets, except that they 18can be bound to a specific 19.Nm 20port via the 21.Xr bind 2 22system call. 23The IP address in the bind is ignored; only the port 24number is significant. 25A divert socket bound to a divert port will receive all packets diverted 26to that port by some (here unspecified) kernel mechanism(s). 27Packets may also be written to a divert port, in which case they 28re-enter kernel IP packet processing. 29.Pp 30Divert sockets are normally used in conjunction with 31FreeBSD's packet 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 48the some 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. Otherwise, the packet is assumed to be 71incoming and full packet routing is done. 72.Pp 73In the latter case, the 74IP address specified must match the address of some local interface, 75or an interface name 76must be found after the IP address. 77If an interface name is found, 78that interface will be used and the value of the IP address will be 79ignored (other than the fact that it is not 80.Dv INADDR_ANY 81). 82This is to indicate on which interface the packet ``arrived.'' 83.Pp 84Normally, packets read as incoming should be written as incoming; 85similarly for outgoing packets. When reading and then writing back 86packets, passing the same socket address supplied by 87.Xr recvfrom 2 88unmodified to 89.Xr sendto 2 90simplifies things (see below). 91.Pp 92The port part of the socket address passed to the 93.Xr sendto 2 94contains a tag that should be meaningful to the diversion module. 95In the 96case of 97.Xr Ipfw 8 98the tag is interpretted as the rule number 99.Em after which 100rule processing should restart. 101.Sh LOOP AVOIDANCE 102Packets written into a divert socket 103.Po 104using 105.Xr sendto 2 106.Pc 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 written as outgoing have their IP header checksums overwritten 131with the correct value. 132Packets written as incoming and having incorrect checksums will be dropped. 133Otherwise, all header fields are unchanged (and therefore in network order). 134.Pp 135Binding to port numbers less than 1024 requires super-user access, as does 136creating a socket of type SOCK_RAW. 137.Sh ERRORS 138Writing to a divert socket can return these errors, along with 139the usual errors possible when writing raw packets: 140.Bl -tag -width Er 141.It Bq Er EINVAL 142The packet had an invalid header, or the IP options in the packet 143and the socket options set were incompatible. 144.It Bq Er EADDRNOTAVAIL 145The destination address contained an IP address not equal to 146.Dv INADDR_ANY 147that was not associated with any interface. 148.El 149.Sh SEE ALSO 150.Xr bind 2 , 151.Xr recvfrom 2 , 152.Xr sendto 2 , 153.Xr socket 2 , 154.Xr ipfw 8 155.Sh BUGS 156This is an attempt to provide a clean way for user mode processes 157to implement various IP tricks like address translation, but it 158could be cleaner, and it's too dependent on 159.Xr ipfw 8 . 160.Pp 161It's questionable whether incoming fragments should be reassembled 162before being diverted. 163For example, if only some fragments of a 164packet destined for another machine don't get routed through the 165local machine, the packet is lost. 166This should probably be 167a settable socket option in any case. 168.Sh AUTHORS 169.An Archie Cobbs Aq archie@whistle.com , 170Whistle Communications Corp. 171