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 31.Fx Ns 's 32packet filtering implementation and the 33.Xr ipfw 8 34program. 35By reading from and writing to a divert socket, matching packets 36can be passed through an arbitrary ``filter'' as they travel through 37the host machine, special routing tricks can be done, etc. 38.Sh READING PACKETS 39Packets are diverted either as they are ``incoming'' or ``outgoing.'' 40Incoming packets are diverted after reception on an IP interface, 41whereas outgoing packets are diverted before next hop forwarding. 42.Pp 43Diverted packets may be read unaltered via 44.Xr read 2 , 45.Xr recv 2 , 46or 47.Xr recvfrom 2 . 48In the latter case, the address returned will have its port set to 49the some tag supplied by the packet diverter, (usually the ipfw rule number) 50and the IP address set to the (first) address of 51the interface on which the packet was received (if the packet 52was incoming) or 53.Dv INADDR_ANY 54(if the packet was outgoing). In the case of an incoming packet the interface 55name will also be placed in the 8 bytes following the address, 56(assuming it fits). 57.Sh WRITING PACKETS 58Writing to a divert socket is similar to writing to a raw IP socket; 59the packet is injected ``as is'' into the normal kernel IP packet 60processing and minimal error checking is done. 61Packets are written as either incoming or outgoing: 62if 63.Xr write 2 64or 65.Xr send 2 66is used to deliver the packet, or if 67.Xr sendto 2 68is used with a destination IP address of 69.Dv INADDR_ANY , 70then the packet is treated as if it were outgoing, i.e., destined 71for a non-local address. Otherwise, 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 82). 83This is to indicate on which interface the packet ``arrived.'' 84.Pp 85Normally, packets read as incoming should be written as incoming; 86similarly for outgoing packets. When reading and then writing back 87packets, passing the same socket address supplied by 88.Xr recvfrom 2 89unmodified to 90.Xr sendto 2 91simplifies things (see below). 92.Pp 93The port part of the socket address passed to the 94.Xr sendto 2 95contains a tag that should be meaningful to the diversion module. 96In the 97case of 98.Xr ipfw 8 99the tag is interpreted as the rule number 100.Em after which 101rule processing should restart. 102.Sh LOOP AVOIDANCE 103Packets written into a divert socket 104.Po 105using 106.Xr sendto 2 107.Pc 108re-enter the packet filter at the rule number 109following the tag given in the port part of the socket address, which 110is usually already set at the rule number that caused the diversion 111(not the next rule if there are several at the same number). If the 'tag' 112is altered to indicate an alternative re-entry point, care should be taken 113to avoid loops, where the same packet is diverted more than once at the 114same rule. 115.Sh DETAILS 116To enable divert sockets, your kernel must be compiled with the option 117.Dv IPDIVERT . 118.Pp 119If a packet is diverted but no socket is bound to the 120port, or if 121.Dv IPDIVERT 122is not enabled in the kernel, the packet is dropped. 123.Pp 124Incoming packet fragments which get diverted are fully reassembled 125before delivery; the diversion of any one fragment causes the entire 126packet to get diverted. 127If different fragments divert to different ports, 128then which port ultimately gets chosen is unpredictable. 129.Pp 130Packets are received and sent unchanged, except that 131packets read as outgoing have invalid IP header checksums, and 132packets written as outgoing have their IP header checksums overwritten 133with the correct value. 134Packets written as incoming and having incorrect checksums will be dropped. 135Otherwise, all header fields are unchanged (and therefore in network order). 136.Pp 137Binding to port numbers less than 1024 requires super-user access, as does 138creating a socket of type SOCK_RAW. 139.Sh ERRORS 140Writing to a divert socket can return these errors, along with 141the usual errors possible when writing raw packets: 142.Bl -tag -width Er 143.It Bq Er EINVAL 144The packet had an invalid header, or the IP options in the packet 145and the socket options set were incompatible. 146.It Bq Er EADDRNOTAVAIL 147The destination address contained an IP address not equal to 148.Dv INADDR_ANY 149that was not associated with any interface. 150.El 151.Sh SEE ALSO 152.Xr bind 2 , 153.Xr recvfrom 2 , 154.Xr sendto 2 , 155.Xr socket 2 , 156.Xr ipfw 8 157.Sh BUGS 158This is an attempt to provide a clean way for user mode processes 159to implement various IP tricks like address translation, but it 160could be cleaner, and it's too dependent on 161.Xr ipfw 8 . 162.Pp 163It's questionable whether incoming fragments should be reassembled 164before being diverted. 165For example, if only some fragments of a 166packet destined for another machine don't get routed through the 167local machine, the packet is lost. 168This should probably be 169a settable socket option in any case. 170.Sh AUTHORS 171.An Archie Cobbs Aq archie@FreeBSD.org , 172Whistle Communications Corp. 173