IPFWPCAP(8) BSD System Manager's Manual IPFWPCAP(8)NAME
ipfwpcap -- copy diverted packets to a file in tcpdump format
SYNOPSIS
ipfwpcap [-dr] [-b maxbytes] [-p maxpkts] [-P pidfile] portnum dumpfile
DESCRIPTION
The ipfwpcap utility is used to copy diverted packets to a file in tcpdump(1) format. The interesting packets are diverted by ipfw(8) to a
port on which ipfwpcap listens. The packets are then dropped unless -r is used.
The options are as follows:
-d Turns on extra debugging messages.
-r Writes packets back to the divert(4) socket.
-rr Indicates that it is okay to quit if maxbytes or maxpkts are reached. Diverted packets will silently disappear if nothing is listen-
ing on the divert(4) socket.
-b maxbytes
Stop dumping after maxbytes bytes.
-p maxpkts
Stop dumping after maxpkt packets.
-P pidfile
File to store PID number in. Default is /var/run/ipwfpcap.portnr.pid.
The portnum argument specifies which divert(4) socket port to listen on. The dumpfile argument is the path to the file to write captured
packets to. Specify '-' to write to stdout.
EXIT STATUS
The ipfwpcap utility exits 0 on success, and >0 if an error occurs.
EXAMPLES
ipfwpcap -r 8091 divt.log &
Starts ipfwpcap as a background job listening to port 8091 and reflecting the packets back to the socket.
ipfw add 2864 divert 8091 ip from 192.0.2.101
Example ipfw(8) rule to divert all packets from 192.0.2.101 to port 8091. See ipfw(8) for details.
SEE ALSO tcpdump(1), pcap(3), divert(4), ipfw(8)HISTORY
The ipfwpcap utility first appeared in FreeBSD 7.0.
AUTHORS
ipfwpcap was written by P. Kern <pkern@cns.utoronto.ca>. This manual page was written by Niclas Zeising <zeising@FreeBSD.org>.
BSD May 22, 2006 BSD
Check Out this Related Man Page
DIVERT(4) BSD Kernel Interfaces Manual DIVERT(4)NAME
divert -- kernel packet diversion mechanism
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
int
socket(PF_INET, SOCK_RAW, IPPROTO_DIVERT);
DESCRIPTION
Divert sockets are similar to raw IP sockets, except that they can be bound to a specific divert port via the bind(2) system call. The IP
address in the bind is ignored; only the port number is significant. A divert socket bound to a divert port will receive all packets
diverted to that port by some (here unspecified) kernel mechanism(s). Packets may also be written to a divert port, in which case they re-
enter kernel IP packet processing.
Divert sockets are normally used in conjunction with FreeBSD's packet filtering implementation and the ipfw(8) program. By reading from and
writing to a divert socket, matching packets can be passed through an arbitrary ``filter'' as they travel through the host machine, special
routing tricks can be done, etc.
READING PACKETS
Packets are diverted either as they are ``incoming'' or ``outgoing.'' Incoming packets are diverted after reception on an IP interface,
whereas outgoing packets are diverted before next hop forwarding.
Diverted packets may be read unaltered via read(2), recv(2), or recvfrom(2). In the latter case, the address returned will have its port set
to the some tag supplied by the packet diverter, (usually the ipfw rule number) and the IP address set to the (first) address of the inter-
face on which the packet was received (if the packet was incoming) or INADDR_ANY (if the packet was outgoing). In the case of an incoming
packet the interface name will also be placed in the 8 bytes following the address, (assuming it fits).
WRITING PACKETS
Writing to a divert socket is similar to writing to a raw IP socket; the packet is injected ``as is'' into the normal kernel IP packet pro-
cessing and minimal error checking is done. Packets are written as either incoming or outgoing: if write(2) or send(2) is used to deliver
the packet, or if sendto(2) is used with a destination IP address of INADDR_ANY, then the packet is treated as if it were outgoing, i.e.,
destined for a non-local address. Otherwise, the packet is assumed to be incoming and full packet routing is done.
In the latter case, the IP address specified must match the address of some local interface, or an interface name must be found after the IP
address. If an interface name is found, that interface will be used and the value of the IP address will be ignored (other than the fact
that it is not INADDR_ANY). This is to indicate on which interface the packet ``arrived.''
Normally, packets read as incoming should be written as incoming; similarly for outgoing packets. When reading and then writing back pack-
ets, passing the same socket address supplied by recvfrom(2) unmodified to sendto(2) simplifies things (see below).
The port part of the socket address passed to the sendto(2) contains a tag that should be meaningful to the diversion module. In the case of
ipfw(8) the tag is interpreted as the rule number after which rule processing should restart.
LOOP AVOIDANCE
Packets written into a divert socket (using sendto(2)) re-enter the packet filter at the rule number following the tag given in the port part
of the socket address, which is usually already set at the rule number that caused the diversion (not the next rule if there are several at
the same number). If the 'tag' is altered to indicate an alternative re-entry point, care should be taken to avoid loops, where the same
packet is diverted more than once at the same rule.
DETAILS
To enable divert sockets, your kernel must be compiled with the option IPDIVERT.
If a packet is diverted but no socket is bound to the port, or if IPDIVERT is not enabled in the kernel, the packet is dropped.
Incoming packet fragments which get diverted are fully reassembled before delivery; the diversion of any one fragment causes the entire
packet to get diverted. If different fragments divert to different ports, then which port ultimately gets chosen is unpredictable.
Packets are received and sent unchanged, except that packets read as outgoing have invalid IP header checksums, and packets written as outgo-
ing have their IP header checksums overwritten with the correct value. Packets written as incoming and having incorrect checksums will be
dropped. Otherwise, all header fields are unchanged (and therefore in network order).
Binding to port numbers less than 1024 requires super-user access, as does creating a socket of type SOCK_RAW.
ERRORS
Writing to a divert socket can return these errors, along with the usual errors possible when writing raw packets:
[EINVAL] The packet had an invalid header, or the IP options in the packet and the socket options set were incompatible.
[EADDRNOTAVAIL] The destination address contained an IP address not equal to INADDR_ANY that was not associated with any interface.
SEE ALSO bind(2), recvfrom(2), sendto(2), socket(2), ipfw(8)BUGS
This is an attempt to provide a clean way for user mode processes to implement various IP tricks like address translation, but it could be
cleaner, and it's too dependent on ipfw(8).
It's questionable whether incoming fragments should be reassembled before being diverted. For example, if only some fragments of a packet
destined for another machine don't get routed through the local machine, the packet is lost. This should probably be a settable socket
option in any case.
AUTHORS
Archie Cobbs <archie@FreeBSD.org>, Whistle Communications Corp.
BSD June 18, 1996 BSD