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