Divert Sockets mini-HOWTO
Ilia Baldine,
[email protected]
v1.1, 27 February 2000
This document describes how to get, compile and use FreeBSD divert
sockets under Linux 2.2.12.
______________________________________________________________________
Table of Contents
1. Copyright
2. Disclaimer
3. Foreword
4. Introduction
5. Getting and Compiling the Source Code
5.1 Getting *The Source*
5.2 Compiling
5.2.1 AID CDATA comp-time
6. Using Divert Sockets
6.1 Divert sockets vs. other stuff
6.1.1 Netlink sockets
6.1.2 Raw sockets
6.1.3 libpcap
6.2 Discussion on firewall chains
6.3 Using ipchains
6.4 Plain vanilla example
6.4.1 Example program
6.5 The sky's the limit
7. Advanced issues
7.1 Packet Mangling
7.2 Injection with no interception
7.3 Fragmentation
8. Geting More Information
8.1 The website
8.2 The mailing list
9. Future work
______________________________________________________________________
1. Copyright
Copyright 1999(c) by Ilia Baldine. This document may be distributed
only subject to the terms and conditions set forth in the LDP License
at, except that this document must not be distributed in modified form
without the author's consent.
2. Disclaimer
This work has been done as part of a DARPA-funded network security
project. Neither I (Ilia Baldine), nor my employer (MCNC) nor DARPA
can be held accountable for any damage real or potential that can come
to you through the use by you or other parties of the code and/or
procedures described in this document. As many other network
mechanisms, divert sockets can be used as much for evil as for good
and its your choice!
3. Foreword
Here's an easy game to play,
Here's an easy thing to say:
If a packet hits a pocket
on a socket on a port
And the bus is interrupted
as a very last resort,
And the address of the memory
makes your floppy disk abort,
Then the socket packet pocket
has an error to report!!
If your cursor finds a menu item
followed by a dash,
And the double clicking icon puts your
window in the trash,
And your data is corrupted 'cause the
index doesn't hash,
Then the situation's hopeless, and your
system's gonna crash!
YOU CAN'T SAY THIS? WHAT A SHAME SIR!
WE'LL FIND ANOTHER GAME SIR
If the label on the cable on the table
at your house,
Says the network is connected to
the button on your mouse,
But your packets want to tunnel
on another protocol,
That's repeatedly rejected
by the printer down the hall,
And your screen is all distorted
by the side effects of gauss
So your icons in the window are
as wavy as a souse,
Then you may as well reboot and
go out with a bang,
the sucker's gonna hang!
When the copy of your floppy's
getting sloppy on the disk
And the microcode instructions cause
unnecessary risc,
Then you have to flash your memory and
you'll want to RAM your ROM
Quickly turn off your computer and
be sure to tell your mom!
-- Anonymous
4. Introduction
Ever wish you could intercept packets traveling up or down the IP
stack of your host? And I'm not talking about listening in, like raw
sockets or libpcap (tcpdump). I mean literally stop the packet from
further propagating through the IP stack and then (possibly after some
changes), reinjecting it back? Well, the time to dream is over,
because divert sockets for Linux are here!
Divert sockets do exactly that - they filter out certain packets based
on firewall specifications and bring them to you in user space. You
then have the freedom of simply reinjecting them back as if nothing
happened, mangling them first and then reinjecting them, or not
reinjecting them at all.
As the name suggests, this mechanism utilizes a special type of RAW
socket called divert (IPPROTO_DIVERT) that allow you to receive and
send on them just like regular sockets. The difference is that a
divert socket is bound to a port, into which the firewall can be
instructed to send certain packets. Anything that a firewall can
filter out can be sent into a divert socket.
Divert sockets first appeared as part of FreeBSD. Divert sockets under
Linux is a port of this mechanism that strives to be source-code
compatible in terms of user-space programs that utilize it.
5. Getting and Compiling the Source Code
In order to use divert sockets under Linux you will need two things -
the kernel source code that has been patched for divert sockets and
the source code to ipchains-1.3.9 that, also, has been patched to use
divert sockets.
5.1. Getting *The Source*
Both pieces of source code can be retrieved from the divert socket
web-site
http://www.anr.mcnc.org/~divert
<
http://www.anr.mcnc.org/~divert> You can get the source code for
divert sockets kernel in two forms - as a patch to linux-2.2.12 that
you have to apply to a fresh 2.2.12 source, or as an already patched
kernel tarball (much larger than the patch). ipchains source is
provided as complete source tarball only.
5.2. Compiling
Compiling ipchains is straightforward - simply say
make
in the ipchains-1.3.9 subdirectory.
When compiling the divert-socket kernel - use your favorite way of
configuring it:
make config
make menuconfig
make xconfig
Don't forget to enable "Prompt for development and/or incomplete
code/drivers" before proceeding. There are only three compile-time
options that affect the behavior of divert sockets and they are
explained in the following ``section''
5.2.1. Kernel compile-time options
In order to enable divert sockets in your kernel you must enable
firewalling and IP firewalling first. The three kernel compile-time
options that affect the behavior of divert sockets are:
IP: divert sockets
Enables the divert sockets in your kernel.
IP: divert pass-through
Changes the behavior of DIVERT rules: by default if a DIVERT
rule is present in a firewall and no application is listening on
the port that the rule specifies, any packet that satisfies the
rule is silently dropped, as if it were a DENY rule.
Enabling the pass-through mode results in such packets
continuing their way through the IP stack as if nothing
happened. This could be helpful if you want to have a static
rule in the firewall, but don't always want to listen on it.
IP: always defragment
Changes the way that the sockets deal with fragmentation. By
default the divert socket receives individual fragments of
packets that are larger than MTU, which it then forwards to user
space. The burden of defragmentation in this case lies with the
application listening on the divert socket. Also, an application
cannot inject any fragments that are larger than MTU, because
they will be dropped (this is the limitation of the kernel, not
the divert sockets - Linux kernels up to 2.2.x do NOT fragment
raw packets with IP_HDRINCL option set). Typically, thats OK,
since if you simply reinject the fragments the way you received
them, everything will work fine, since none of them are going to
be larger than MTU.
If you enable the always defragment option, then all the
defragmentation will be done for you in the kernel. This
severely affects the performance of the interception mechanism,
since now every large packet you want intercepted will first
have to be reassembled prior to being forwarded to you, and
then, if you choose to reinject it - it will have to be
fragmented again (the kernel with this option will be enabled to
fragment raw packets with IP_HDRINCL)
This was the only option available for divert sockets under
Linux 2.0.36 because of the way the firewall code was structured
- it only looked at the first fragment of every packet and
passed all other fragments without looking at them. This way, if
the first fragment were dropped by the firewall, the rest of
them would be eventually discarded by the defragmenter. That's
why in order for DIVERT sockets to work you were forced to
compile the always defragment option in, so that you would
always get the whole packet diverted to you and not just the
first fragment.
In 2.2.12, thanks to changes in the firewall code you now have
an option of having the kernel or yourself doing
fragmentation/defragmentation.
NOTE: the defragmentation feature has not been added as of
release 1.0.4 of divert sockets. It is in the works though.
6. Using Divert Sockets
This section will give you examples of how divert sockets can be used
and how they are different of other packet interception mechanisms out
there.
6.1. Divert sockets vs. other stuff
There are other mechanisms out there that have similar functionality.
Here is why they are different:
6.1.1. Netlink sockets
Netlink sockets can intercept packets just like divert sockets by
using firewall filter. They have a special type (AF_NETLINK) and on
the surface seem to do the same thing. Two major differences are:
o Netlink sockets have no ports, so it is difficult to have multiple
processes intercepting different things (divert sockets have a
standard 16-bit port space, which means you can have 65535
processes diverting packets independently)
o Netlink sockets have no easy way of injecting the packets that are
outbound (going on the wire) because no special precautions are
taken not to reintercept the same packet over and over again as it
is injected. Divert sockets do this automatically
To be fair, the scope of netlink sockets is wider than this. In
general, netlink mechanism is intended to allow communication
between kernel and user space. There are, for instance, netlink
routing sockets that allow you to communicate with the routing
subsystem. However, as a packet interception mechanism, they are
not as robust as divert sockets.
6.1.2. Raw sockets
RAW sockets can be a good way to listen in on traffic (especially
under Linux, where RAW sockets can listen in on TCP and UDP traffic,
although most other UNI*s do not allow that) but a RAW socket can't
stop a packet from propagating through the IP stack - it simply gives
you a copy of the packet and there is no way to inject it inbound (on
the way up the stack) - only outbound. Also, you can only filter
pockets out by the protocol number, which you specify when you open a
RAW socket. There is no link between the firewall and RAW sockets.
6.1.3. libpcap
More commonly known for the tool it facilitates - tcpdump, libpcap
lets you listen in on traffic that hits your interface (whether it be
ppp or eth or whatever). For ethernet it can also put your NIC into a
promiscuous mode, so that it will forward to IP the traffic that not
only is link-layer addressed to it, but to others on the same segment.
Of course, libpcap allows for no way of actually stopping packets from
propagating and no way to inject. In fact, libpcap is in many ways
orthogonal to divert sockets.
6.2. Discussion on firewall chains
Linux provides you with three default chains: input, output and
forward. There are also accounting chains, but they are of no
consequence here. Depending on the packet origin it traverses one or
more of these chains:
Input chain
is traversed by all packets that come into the host - packets
that are addressed to it and packets that will be forwarded by
it.
Output chain
is traversed by all packets originating in the host and by all
forwarded packets
Forward chain
is traversed only by the forwarded packets.
The order in which a forwarded packet traverses the chains is:
1. Input
2. Forward
3. Output
This may sometimes create problems for the interception if you are
interested in a certain type of packets that may or may not
originate on your host. A lot of times it is not clear which chain
to use.
As a rule of thumb, forward chain should only be used to filter
packets that are forwarded and are not originating and are not
addressed to your host. If you are interested in a combination of both
forwarded packets and packets that are originating or addressed to
your host, then use input or output chain instead. Intercepting on
forward and input or output chain for the same type of packet at the
same time will create problems in reinjection and, more importantly,
is unnecessary.
6.3. Using ipchains
The patched version of ipchains that you will need to retrieve from
the website, is the tool that allows you to modify firewall rules from
a shell (most people want that). It is also possible to set up
firewall rules programmatically. See the example code for this -
setting up a DIVERT rule would be similar to setting up a REDIRECT
rule - specify DIVERT as a target and the divert port and you are set
to go.
The ipchains syntax for setting up firewall rules remains the same. To
specify a DIVERT rule you must specify -j DIVERT <port num> as a
target, everything else remains the same. For instance
ipchains -A input -p ICMP -j DIVERT 1234
would set up a divert rule for ICMP packets to be diverted from input
chain to a port 1234.
The following section explains how to use ipchains in conjunction with
an interceptor user-space program.
6.4. Plain vanilla example
6.4.1. Example program
Here is an example program that reads packets from a divert socket,
displays them and then reinjects them back. It requires that the
divert port is specified on the command line.
#include <stdio.h>
#include <errno.h>
#include <limits.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <getopt.h>
#include <netdb.h>
#include <netinet/in.h>
#include <sys/types.h>
#include <signal.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>
#include <netinet/udp.h>
#include <net/if.h>
#include <sys/param.h>
#include <linux/types.h>
#include <linux/icmp.h>
#include <linux/ip_fw.h>
#define IPPROTO_DIVERT 254
#define BUFSIZE 65535
char *progname;
#ifdef FIREWALL
char *fw_policy="DIVERT";
char *fw_chain="output";
struct ip_fw fw;
struct ip_fwuser ipfu;
struct ip_fwchange ipfc;
int fw_sock;
/* remove the firewall rule when exit */
void intHandler (int signo) {
if (setsockopt(fw_sock, IPPROTO_IP, IP_FW_DELETE, &ipfc, sizeof(ipfc))==-1) {
fprintf(stderr, "%s: could not remove rule: %s\n", progname, strerror(errno));
exit(2);
}
close(fw_sock);
exit(0);
}
#endif
int main(int argc, char** argv) {
int fd, rawfd, fdfw, ret, n;
int on=1;
struct sockaddr_in bindPort, sin;
int sinlen;
struct iphdr *hdr;
unsigned char packet[BUFSIZE];
struct in_addr addr;
int i, direction;
struct ip_mreq mreq;
if (argc!=2) {
fprintf(stderr, "Usage: %s <port number>\n", argv[0]);
exit(1);
}
progname=argv[0];
fprintf(stderr,"%s:Creating a socket\n",argv[0]);
/* open a divert socket */
fd=socket(AF_INET, SOCK_RAW, IPPROTO_DIVERT);
if (fd==-1) {
fprintf(stderr,"%s:We could not open a divert socket\n",argv[0]);
exit(1);
}
bindPort.sin_family=AF_INET;
bindPort.sin_port=htons(atol(argv[1]));
bindPort.sin_addr.s_addr=0;
fprintf(stderr,"%s:Binding a socket\n",argv[0]);
ret=bind(fd, &bindPort, sizeof(struct sockaddr_in));
if (ret!=0) {
close(fd);
fprintf(stderr, "%s: Error bind(): %s",argv[0],strerror(ret));
exit(2);
}
#ifdef FIREWALL
/* fill in the rule first */
bzero(&fw, sizeof (struct ip_fw));
fw.fw_proto=1; /* ICMP */
fw.fw_redirpt=htons(bindPort.sin_port);
fw.fw_spts[1]=0xffff;
fw.fw_dpts[1]=0xffff;
fw.fw_outputsize=0xffff;
/* fill in the fwuser structure */
ipfu.ipfw=fw;
memcpy(ipfu.label, fw_policy, strlen(fw_policy));
/* fill in the fwchange structure */
ipfc.fwc_rule=ipfu;
memcpy(ipfc.fwc_label, fw_chain, strlen(fw_chain));
/* open a socket */
if ((fw_sock=socket(AF_INET, SOCK_RAW, IPPROTO_RAW))==-1) {
fprintf(stderr, "%s: could not create a raw socket: %s\n", argv[0], strerror(errno));
exit(2);
}
/* write a rule into it */
if (setsockopt(fw_sock, IPPROTO_IP, IP_FW_APPEND, &ipfc, sizeof(ipfc))==-1) {
fprintf(stderr, "%s could not set rule: %s\n", argv[0], strerror(errno));
exit(2);
}
/* install signal handler to delete the rule */
signal(SIGINT, intHandler);
#endif /* FIREWALL */
printf("%s: Waiting for data...\n",argv[0]);
/* read data in */
sinlen=sizeof(struct sockaddr_in);
while(1) {
n=recvfrom(fd, packet, BUFSIZE, 0, &sin, &sinlen);
hdr=(struct iphdr*)packet;
printf("%s: The packet looks like this:\n",argv[0]);
for( i=0; i<40; i++) {
printf("%02x ", (int)*(packet+i));
if (!((i+1)%16)) printf("\n");
};
printf("\n");
addr.s_addr=hdr->saddr;
printf("%s: Source address: %s\n",argv[0], inet_ntoa(addr));
addr.s_addr=hdr->daddr;
printf("%s: Destination address: %s\n", argv[0], inet_ntoa(addr));
printf("%s: Receiving IF address: %s\n", argv[0], inet_ntoa(sin.sin_addr));
printf("%s: Protocol number: %i\n", argv[0], hdr->protocol);
/* reinjection */
#ifdef MULTICAST
if (IN_MULTICAST((ntohl(hdr->daddr)))) {
printf("%s: Multicast address!\n", argv[0]);
addr.s_addr = hdr->saddr;
errno = 0;
if (sin.sin_addr.s_addr == 0)
printf("%s: set_interface returns %i with errno =%i\n", argv[0], setsockopt(fd, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr)), errno);
}
#endif
#ifdef REINJECT
printf("%s Reinjecting DIVERT %i bytes\n", argv[0], n);
n=sendto(fd, packet, n ,0, &sin, sinlen);
printf("%s: %i bytes reinjected.\n", argv[0], n);
if (n<=0)
printf("%s: Oops: errno = %i\n", argv[0], errno);
if (errno == EBADRQC)
printf("errno == EBADRQC\n");
if (errno == ENETUNREACH)
printf("errno == ENETUNREACH\n");
#endif
}
}
You can simply cut-n-paste the code and compile it with your favorite
compiler. If you want to enable reinjection - compile it with the
-DREINJECT flag, otherwise it will only do the interception.
In order to get it to work, compile the kernel and ipchains-1.3.8 as
described ``above''. Insert a rule into any of the firewall chains:
input, output or forward, then send the packets that would match the
rule and watch them as they fly through the screen - your interceptor
program will display them and then reinject them back, if
appropriately compiled.
For example:
ipchains -A output -p TCP -s 172.16.128.10 -j DIVERT 4321
interceptor 4321
will divert and display all TCP packets originating on host
172.16.128.10 (for instance if your host is a gateway). It will inter-
cept them on the output just before they go on the wire.
If you did not compile the pass through option into the kernel, then
inserting the rule effectively will create a DENY rule in the firewall
for the packets you specified until you start the interceptor program.
See more on that ``above''
If you want to set a firewall rule through your program, compile it
with -DFIREWALL option and it will divert all ICMP packets from the
output chain. It will also remove the DIVERT rule from the firewall
when you use Ctrl-C to exit the program. In this case using pass-
through vs. non-pass-through divert sockets makes virtually no
difference.
6.5. The sky's the limit
As far as what you can use divert sockets for - your imagination would
be the limiting factor. I would be interested to hear about
applications that utilize divert sockets.
So, have fun!
7. Advanced issues
7.1. Packet Mangling
After you intercept a packet, it is possible to change its header or
contents before reinjecting it back. Here are a few rules you might
need to keep in mind:
o IP header checksum is always recalculated on injection
o IP ID field is filled in for you if you leave it 0.
o The length of the packet is updated for you.
All other parts of the IP header can be modified and its up to you
to insure their sanity.
7.2. Injection with no interception
It is not necessary to intercept a packet in order to inject it. You
can form your own packets and inject them into an open and bound
divert socket. The header rules from above apply.
In addition, you need to pass to the divert socket a sockaddr_in
structure (see example program), which will tell the socket where to
inject. If you leave the structure 0-ed out or pass a NULL - the
divert socket will attempt to inject the packet in the outbound
direction (on the wire). If instead you fill the sockaddr_in structure
with the address of one of the local interfaces, the divert socket
will attempt to inject the packet inbound, as if it came from that
interface. All addresses, of course, should be in network byte order.
Injection of packets that look like they are being forwarded by your
host must include an address of the incoming interface (actually - any
valid interface address will probably work).
7.3. Fragmentation
As of this reading, the divert sockets do not handle the
defragmentation and fragmentation of diverted packets - you always get
the fragments as they are on the wire and you should not inject
fragments larger than PMTU. It is anticipated that the
fragmentation/defragmentation capability will be added in the near
future.
8. Geting More Information
8.1. The website
As mentioned above, most of the information about divert sockets can
be found on the Divert Sockets for Linux website
http://www.anr.mcnc.org/~divert <
http://www.anr.mcnc.org/~divert>.
8.2. The mailing list
There is also a mailing list, whose archive can be found at the
website. To join the mailing list send email with an empty subject
and the following line in the body:
subscribe divert
to
[email protected] <mailto:anr-major-
[email protected]>. The list address is
[email protected]
<mailto:
[email protected]>.
To unsubscribe, send mail to
[email protected]
<mailto:
[email protected]> with an empty subject and the
following line in the body:
unsubscribe divert
9. Future work
As mentioned in the disclaimer, work on divert sockets is done as part
of a DARPA-funded network security effort. We will continue to port
divert sockets to further versions of the kernel as time permits.
Given that 2.4 kernel is on the horizon, in all likelihood we will
skip 2.3.x series altogether.
