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Querying libiptc HOWTO

Leonardo Balliache

Version 0.1 - April 30, 2002

Revision History
Revision 0.12002-04-30Revised by: lb
Initial release.

3. Disclaimer

I took this "disclaimer" from Linux-Advance Networking Overview by Saravanan Radhakrishnan (08-1999) because it applies in my own case:

All the text in this document is completely based on my understanding of implementations of various features. I have read some documents and have seen the code myself, and I described them based on my understanding. If the reader notices any concept description which appears to be contrary to their understanding of the concept, the issue can be taken up for discussion and corrections will be made to the document as necessary. I would appreciate any suggestions and comments made in order to improve the quality of this document.

4. Credits

I want to thank the following people and organizations who had helped me, directly or not, to make this document possible:

5. Objectives

This HOWTO explains how to use the libiptc library included in the iptables package. This document can show you how to use short C or C++ programs to query the internal structure of the firewalling code, to check chains and rules, packet and byte counters, and in a second phase, if you are a little "brave", to modify them.

You can find the latest version of this document at Querying libiptc HOWTO.html.

If you have suggestions to help make this document better, please submit your ideas to me at the following address:

While I wrote this HOWTO, I developed a simple bandwith meter using user-defined chains to get the data to be measured. This idea was conceived looking at, a simple perl program for bandwith measurement, written by Stef Coene at I recommend this site to people interested in bandwidth control and measurement.

8. Previous knowledge and system requirements

You have to have some previous knowledge to follow this document:

  1. Very important: You must know how to use the iptables package as a user, such as how to create or list rules and user chains. You do not need to be a firewall expert, but you should know how to use iptables fluently.

  2. You have to have kernel sources installed in your system, in /usr/src/linux as usual.

    I am using a 2.4.16 kernel in a SuSE 7.1 Linux environment. You need 2.4.x kernel code to follow this HOWTO, preferably kernel 2.4.16. For SuSE you can get the kernel sources at

  3. You have to know how to compile the kernel if you have to update your kernel version. After activating the netfilter options using make menuconfig, you must compile and install the kernel as usual.

  4. Reboot your new kernel using init 6. Ensure that you backup a copy of your previous kernel in lilo in case you encounter a problem and need to retrace your steps.

  5. Be sure that your new 2.4.x kernel is running fine. To install iptables-1.2.6 you will need to patch the kernel again (and re-compile and install it), and it is better if you follow the previous two steps to ensure that your kernel is running right before applying new iptables patches.

9. Installing iptables + libiptc

To install libiptc follow these steps:

  1. Download iptables-1.2.6.tar.bz2 from

  2. Copy the iptables tar file into /usr/local/src:

    bash# cp iptables-1.2.6.tar.bz2 /usr/local/src
  3. Unpack:

    bash# tar xjvf iptables-1.2.6.tar.bz2
  4. Go into the iptables directory:

    bash# cd iptables-1.2.6
  5. Check to see if your kernel needs some aditional patches with:

    bash# make pending-patches KERNEL_DIR=/usr/src/linux

    If your kernel source is located somewhere other than in /usr/src/linux, replace the kernel source directory in the command line above with your source directory.

    Be careful with this option. This command invokes patch-o-matic, a new patch verification utility by Rusty Russell. The utility will show you a list of new patches (some proposed, some submitted, some accepted) available for your kernel source. As Rusty himself says, "Some of these new patches have bugs", and you do not have to apply all of them.

    Read the information showed for each patch carefully and answer with y (apply the patch) or N (skip this patch). In some cases answering y will try to apply the patch, but if the patch finds some differences between your sources, it will be skipped and the next new one presented.

    I did not apply any of the proposed patches and kept my kernel in its original state before continuing to the next step.

  6. Make the iptables package with:

    bash# make KERNEL_DIR=/usr/src/linux

    Again, if your kernel source is not at /usr/src/linux, replace the kernel source directory in the command above.

    If all goes right the compiler will finish without errors.

  7. Before the next step, check to see if you have installed iptables package by typing:

    bash# rpm -q iptables

    If the iptables rpm is installed, you will see the name and version of the package, similar to:


    In this case un-install with:

    bash# rpm -e iptables
  8. Install the new created package:

    bash# make install KERNEL_DIR=/usr/src/linux

    Again, check your kernel source directory.

    This command will install the binaries (iptables, iptables-save, iptables-restore) in /usr/local/sbin, the manuals in /usr/local/man/man8 and the modules in /usr/local/lib/iptables.

  9. Finally install the headers, development libraries and associated development man pages, with:

    bash# make install-devel

    This command will install the libiptc library in /usr/local/lib.

    I think something must be wrong with this command. It does not install all headers files properly, so you must install them yourself using:

    bash# cd /usr/local/src/iptables-1.2.6
    bash# cp include/iptables.h /usr/local/include
    bash# cp include/iptables_common.h /usr/local/include
    bash# mkdir /usr/local/include/libiptc
    bash# cp include/libiptc/libiptc.h /usr/local/include/libiptc
    bash# cp include/libiptc/ipt_kernel_headers.h /usr/local/include/libiptc
    bash# cp iptables.o /usr/local/lib

    iptables.o is needed above to compile programs to get rule information from netfilter.

    Now you are ready to create programs that can communicate directly with libiptc.

10. How to create your program(s)

Create your program(s) in /usr/local/src; this way you will not have problems with gcc looking for files in the "include" section.

Your program(s) would be something like this:

/* My program */

#include <getopt.h>
#include <sys/errno.h>
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <dlfcn.h>
#include <time.h>
#include "libiptc/libiptc.h"
#include "iptables.h"

int main(void)
  /* Use always this part for your programs .... From here ... **** */
  iptc_handle_t h;
  const char *chain = NULL;
  const char *tablename = NULL;

  program_name = "my_program";
  program_version = NETFILTER_VERSION;
  /* .... To here .... ******************************************** */

  /* From here you write your own code */
  .... your code ...

} /* main */

11. Functions to query libiptc

This section explains which functions allow you to query libiptc. We will use the header file of libiptc, file usr/local/include/libiptc/libiptc.h, containing prototypes of each function as a reference to develop our explanation.

I have also included a brief description (when available) taken from Linux netfilter Hacking HOWTO within each function explanation.

11.4. iptc_next_chain

Name: iptc_next_chain

Usage: Iterator functions to run through the chains.

Prototype: const char *iptc_next_chain(iptc_handle_t *handle)

Description: This function returns the next chain name in the table; NULL means no more chains.

Parameters: Pointer to a structure of type iptc_handle_t that was obtained by a previous call to iptc_init.

Returns: Char pointer to the name of the chain.

These two previous functions allow to us to iterate through the chains of the table getting the name of each of the chains; iptc_first_chain returns the name of the first chain of the table; iptc_next_chain returns the name of next chains and NULL when the function reaches the end.

We can create Program #1 to exercise our understanding of these previous four functions:

 * How to use libiptc- program #1
 * /usr/local/src/p1.c

#include <getopt.h>
#include <sys/errno.h>
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <dlfcn.h>
#include <time.h>
#include "libiptc/libiptc.h"
#include "iptables.h"

int main(void)
  iptc_handle_t h;
  const char *chain = NULL;
  const char *tablename = "filter";

  program_name = "p1";
  program_version = NETFILTER_VERSION;

  h = iptc_init(tablename);
  if ( !h )   {
     printf("Error initializing: %s\n", iptc_strerror(errno));

  for (chain = iptc_first_chain(&h); chain; chain = iptc_next_chain(&h))  {
    printf("%s\n", chain);


} /* main */

Write this program and save it as p1.c in /usr/local/src.

Now write this "bash" script to simplify the compiling process:


gcc -Wall -Wunused -DNETFILTER_VERSION=\"1.2.6\" -rdynamic -o $1 $1.c \
/usr/local/lib/iptables.o /usr/local/lib/libiptc.a -ldl

Save it as ipt-cc and do not forget to chmod 0700 ipt-cc.

Now compile your p1 program:

bash# ./ipt-cc p1

And run it:

bash# ./p1

You will get:


These are the three built-in iptables chains.

Now create some new chains using iptables and run your program again:

bash# iptables -N chain_1
bash# iptables -N chain_2
bash# ./p1

You will get:


Try to generate an error initializing tablename to myfilter instead of filter. When you compile and execute your program again, you will get:

Error initializing: Table does not exist (do you need to insmod?)

iptables informs you that myfilter does not exist as a table.

11.9. iptc_get_target

Name: iptc_get_target

Usage: Get a pointer to the target name of this entry.

Prototype: const char *iptc_get_target(const struct ipt_entry *e, iptc_handle_t *handle)

Description: This function gets the target of the given rule. If it is an extended target, the name of that target is returned. If it is a jump to another chain, the name of that chain is returned. If it is a verdict (eg. DROP), that name is returned. If it has no target (an accounting-style rule), then the empty string is returned. Note that this function should be used instead of using the value of the verdict field of the ipt_entry structure directly, as it offers the above further interpretations of the standard verdict.

Parameters: e is a pointer to a structure of type ipt_entry that must be obtained first by a previous call to the function iptc_first_rule or the function iptc_next_rule. handle is a pointer to a structure of type iptc_handle_t that was obtained by a previous call to iptc_init.

Returns: Returns a char pointer to the target name. See Description above for more information.

Now it is time to explain the ipt_entry structure; these pieces of code are taken from iptables package sources:

/* Internet address. */
struct in_addr {
  __u32  s_addr;

/* Yes, Virginia, you have to zero the padding. */
struct ipt_ip {
  /* Source and destination IP addr */
  struct in_addr src, dst;
  /* Mask for src and dest IP addr */
  struct in_addr smsk, dmsk;
  char iniface[IFNAMSIZ], outiface[IFNAMSIZ];
  unsigned char iniface_mask[IFNAMSIZ], outiface_mask[IFNAMSIZ];

  /* Protocol, 0 = ANY */
  u_int16_t proto;

  /* Flags word */
  u_int8_t flags;
  /* Inverse flags */
  u_int8_t invflags;

struct ipt_counters
  u_int64_t pcnt, bcnt;      /* Packet and byte counters */

/* This structure defines each of the firewall rules.  Consists of 3
   parts which are 1) general IP header stuff 2) match specific
   stuff 3) the target to perform if the rule matches */
struct ipt_entry
  struct ipt_ip ip;

  /* Mark with fields that we care about. */
  unsigned int nfcache;

  /* Size of ipt_entry + matches */
  u_int16_t target_offset;
  /* Size of ipt_entry + matches + target */
  u_int16_t next_offset;

  /* Back pointer */
  unsigned int comefrom;

  /* Packet and byte counters. */
  struct ipt_counters counters;

  /* The matches (if any), then the target. */
  unsigned char elems[0];

An ipt_entry structure contains:

A simple way to work with all this information is to borrow some functions from iptables-save.c by Paul Russell and Harald Welte.

Here is another sample program Program #2 written with a lot of help from Russell-Welte:

 * How to use libiptc- program #2
 * /usr/local/src/p1.c

#include <getopt.h>
#include <sys/errno.h>
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <dlfcn.h>
#include <time.h>
#include "libiptc/libiptc.h"
#include "iptables.h"

/* Here begins some of the code taken from iptables-save.c **************** */
#define IP_PARTS_NATIVE(n)      \
(unsigned int)((n)>>24)&0xFF,   \
(unsigned int)((n)>>16)&0xFF,   \
(unsigned int)((n)>>8)&0xFF,    \
(unsigned int)((n)&0xFF)

#define IP_PARTS(n) IP_PARTS_NATIVE(ntohl(n))

/* This assumes that mask is contiguous, and byte-bounded. */
static void
print_iface(char letter, const char *iface, const unsigned char *mask,
      int invert)
  unsigned int i;

  if (mask[0] == 0)

  printf("-%c %s", letter, invert ? "! " : "");

  for (i = 0; i < IFNAMSIZ; i++) {
    if (mask[i] != 0) {
      if (iface[i] != '\0')
        printf("%c", iface[i]);
    } else {
      /* we can access iface[i-1] here, because 
       * a few lines above we make sure that mask[0] != 0 */
      if (iface[i-1] != '\0')

  printf(" ");

/* These are hardcoded backups in iptables.c, so they are safe */
struct pprot {
  char *name;
  u_int8_t num;

/* FIXME: why don't we use /etc/protocols ? */
static const struct pprot chain_protos[] = {
  { "tcp", IPPROTO_TCP },
  { "udp", IPPROTO_UDP },
  { "icmp", IPPROTO_ICMP },
  { "esp", IPPROTO_ESP },
  { "ah", IPPROTO_AH },

static void print_proto(u_int16_t proto, int invert)
  if (proto) {
    unsigned int i;
    const char *invertstr = invert ? "! " : "";

    for (i = 0; i < sizeof(chain_protos)/sizeof(struct pprot); i++)
      if (chain_protos[i].num == proto) {
        printf("-p %s%s ",
               invertstr, chain_protos[i].name);

    printf("-p %s%u ", invertstr, proto);

static int print_match(const struct ipt_entry_match *e,
      const struct ipt_ip *ip)
  struct iptables_match *match
    = find_match(e->, TRY_LOAD);

  if (match) {
    printf("-m %s ", e->;

    /* some matches don't provide a save function */
    if (match->save)
      match->save(ip, e);
  } else {
    if (e->u.match_size) {
        "Can't find library for match `%s'\n",
  return 0;

/* print a given ip including mask if neccessary */
static void print_ip(char *prefix, u_int32_t ip, u_int32_t mask, int invert)
  if (!mask && !ip)

  printf("%s %s%u.%u.%u.%u",
    invert ? "! " : "",

  if (mask != 0xffffffff) 
    printf("/%u.%u.%u.%u ", IP_PARTS(mask));
    printf(" ");

/* We want this to be readable, so only print out neccessary fields.
 * Because that's the kind of world I want to live in.  */
static void print_rule(const struct ipt_entry *e, 
    iptc_handle_t *h, const char *chain, int counters)
  struct ipt_entry_target *t;
  const char *target_name;

  /* print counters */
  if (counters)
    printf("[%llu:%llu] ", e->counters.pcnt, e->counters.bcnt);

  /* print chain name */
  printf("-A %s ", chain);

  /* Print IP part. */
  print_ip("-s", e->ip.src.s_addr,e->ip.smsk.s_addr,
      e->ip.invflags & IPT_INV_SRCIP);  

  print_ip("-d", e->ip.dst.s_addr, e->ip.dmsk.s_addr,
      e->ip.invflags & IPT_INV_DSTIP);

  print_iface('i', e->ip.iniface, e->ip.iniface_mask,
        e->ip.invflags & IPT_INV_VIA_IN);

  print_iface('o', e->ip.outiface, e->ip.outiface_mask,
        e->ip.invflags & IPT_INV_VIA_OUT);

  print_proto(e->ip.proto, e->ip.invflags & IPT_INV_PROTO);

  if (e->ip.flags & IPT_F_FRAG)
    printf("%s-f ",
           e->ip.invflags & IPT_INV_FRAG ? "! " : "");

  /* Print matchinfo part */
  if (e->target_offset) {
    IPT_MATCH_ITERATE(e, print_match, &e->ip);

  /* Print target name */  
  target_name = iptc_get_target(e, h);
  if (target_name && (*target_name != '\0'))
    printf("-j %s ", target_name);

  /* Print targinfo part */
  t = ipt_get_target((struct ipt_entry *)e);
  if (t->[0]) {
    struct iptables_target *target
      = find_target(t->, TRY_LOAD);

    if (!target) {
      fprintf(stderr, "Can't find library for target `%s'\n",

    if (target->save)
      target->save(&e->ip, t);
    else {
      /* If the target size is greater than ipt_entry_target
       * there is something to be saved, we just don't know
       * how to print it */
      if (t->u.target_size != 
          sizeof(struct ipt_entry_target)) {
        fprintf(stderr, "Target `%s' is missing "
            "save function\n",
/* Here ends some of the code taken from iptables-save.c ****************** */

int main(void)
  iptc_handle_t h;
  const struct ipt_entry *e;
  const char *chain = NULL;
  const char *tablename = "filter";
  const int counters = 1;

  program_name = "p2";
  program_version = NETFILTER_VERSION;

  /* initialize */
  h = iptc_init(tablename);
  if ( !h )   {
     printf("Error initializing: %s\n", iptc_strerror(errno));

  /* print chains and their rules */
  for (chain = iptc_first_chain(&h); chain; chain = iptc_next_chain(&h))  {
    printf("%s\n", chain);
    for (e = iptc_first_rule(chain, &h); e; e = iptc_next_rule(e, &h))  {
      print_rule(e, &h, chain, counters);


} /* main */

The function print_rule borrowed from iptables-save.c prints the information about a rule into a readable form using:

In main we iterate through each chain and for each one we iterate through each rule printing it.

The arguments of print_rule are:

OK, compile and run program p2:

bash# ./ipt-cc p2
bash# ./p2

You will get:


Now modify the environment using iptables to add some rules:

bash# iptables -A INPUT -p tcp -i eth0 -s ! --dport 20 -j ACCEPT
bash# iptables -A chain_1 -p udp -o eth1 -s --sport 33 -j DROP

Now if you run again p2 you will get:

[0:0] -A INPUT -s ! -i eth0 -p tcp -m tcp --dport 20 -j ACCEPT
[0:0] -A chain_1 -s -o eth1 -p udp -m udp --sport 33 -j DROP

We have now rules printed for INPUT and chain_1 chains. The numbers in the brackets at left are packet and byte counters respectively.

11.10. iptc_get_policy

Name: iptc_get_policy

Usage: Get the policy of a given built-in chain.

Prototype: const char *iptc_get_policy(const char *chain, struct ipt_counters *counter, iptc_handle_t *handle)

Description: This function gets the policy of a built-in chain, and fills in the counters argument with the hit statistics on that policy.

Parameters: You have to pass as arguments the name of the built-in chain you want to get the policy to, a pointer to an ipt_counters structure to be filled by the function and the iptc_handle_t structure identifying the table we are working to. The ipt_counters structure was explained in previous section; do not forget that iptc_handle_t must be obtained by a previous call to the function iptc_init.

Returns: Returns a char pointer to the policy name.

Using pieces of programs 1 and 2 we can write program #3:

 * How to use libiptc- program #3
 * /usr/local/src/p3.c

#include <getopt.h>
#include <sys/errno.h>
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <dlfcn.h>
#include <time.h>
#include "libiptc/libiptc.h"
#include "iptables.h"

int main(void)
  iptc_handle_t h;
  const char *chain = NULL;
  const char *policy = NULL;
  const char *tablename = "filter";
  struct ipt_counters counters;

  program_name = "p3";
  program_version = NETFILTER_VERSION;

  /* initialize */
  h = iptc_init(tablename);
  if ( !h )   {
     printf("Error initializing: %s\n", iptc_strerror(errno));

  /* print built-in chains, their policies and counters */
  printf("BUILT-IN   POLICY  PKTS-BYTES\n");
  for (chain = iptc_first_chain(&h); chain; chain = iptc_next_chain(&h))  {
    if ( !iptc_builtin(chain, h) )
    if ( (policy = iptc_get_policy(chain, &counters, &h)) )
      printf("%-10s %-10s [%llu:%llu]\n", 
             chain, policy, counters.pcnt, counters.bcnt);


} /* main */

OK, compile and run program p3:

bash# ./ipt-cc p3
bash# ./p3

You will get something like this:

INPUT     ACCEPT     [0:0]
FORWARD   ACCEPT     [0:0]
OUTPUT    ACCEPT     [0:0]

12. Functions to modify firewalling rules and statistics

For those of you who are a little brave, libiptc has a group of functions to directly modify the firewalling rules and statistics (use of iptables is really the safest way).

These functions are not covered by this HOWTO and I will limit myself to presenting improved information taken from libiptc.h and the Linux netfilter Hacking HOWTO by Rusty Russell.

13. Bandwidth meter

In this chapter I am going to develop a simple bandwidth meter using the following functions from libiptc:

Also the function gettimeofday will be used to measure elapsed time and the function getopt to get options from the command line.

I don't know really if the term bandwidth meter is well used here. I interpret bandwidth as a reference to a flow capacity; perhaps a better term could be flow meter.

Here is the bandwidth meter bw.c. It's well commented to be easy followed by everyone:

 * How to use libiptc- program #4
 * /usr/local/src/bw.c
 * By Leonardo Balliache - 04.09.2002
 * e-mail:
 * --WELL COMMENTED-- to be easy followed by everyone.

/* include files required */
#include <getopt.h>
#include <sys/errno.h>
#include <sys/time.h>
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <dlfcn.h>
#include <time.h>
#include <unistd.h>
#include "libiptc/libiptc.h"
#include "iptables.h"

/* colors to differentiate chains measures */
#define RED     "\033[41m\033[37m"
#define GREEN   "\033[42m\033[30m"
#define ORANGE  "\033[43m\033[30m"
#define BLUE    "\033[44m\033[37m"
#define MAGENTA "\033[45m\033[37m"
#define CYAN    "\033[46m\033[30m"
#define WHITE   "\033[47m\033[30m"
#define BLACK   "\033[40m\033[37m"
#define RESET   "\033[00m"

/* maximum number of chains to be processed */

/* time between measures in seconds; adjust as you like */
#define SLEEPTIME 1

/* structure to count bytes per chain */
struct bwcnt  {
  int start;           /* the chain was initialized */
  u_int64_t icnt;      /* bytes through; previous measure */
  u_int64_t ocnt;      /* bytes through; current measure */
  double bw;           /* bandwitdh (flow) on this chain */

/* function to calculate differences of time in seconds.
 * micro-seconds precision.
double delta(struct timeval a, struct timeval b)
  if (a.tv_usec & b.tv_usec)  {
    a.tv_usec += 1000000;
  return a.tv_sec-b.tv_sec + (a.tv_usec-b.tv_usec)/1000000.0;

/* main function */
int main(int argc, char *argv[])
  int i, j, ok;
  double totbw;
  iptc_handle_t h;
  int c, act_bw = 0;
  const char *chain = NULL;
  const char *tablename = "filter";
  struct timeval ti, to;
  struct bwcnt bw[MAXUSERCHAINS];
  struct ipt_counters *counters;

  program_name = "bw";
  program_version = NETFILTER_VERSION;

 /* check options
  * we have 2 options: 
  *        -c = display current flow (each SLEEPTIME).
  *        -a = display average flow (from start); default option.
  while ((c = getopt (argc, argv, "ac")) != -1)
  switch (c)  {
  case 'a':
    act_bw = 0;
  case 'c':
    act_bw = 1;
  case '?':
    if (isprint(optopt))
      fprintf (stderr, "Unknown option `-%c'.\n", optopt);
      fprintf (stderr,"Unknown option character `\\x%x'.\n",optopt);

  /* initialize array of chains */
  memset(&bw, 0, MAXUSERCHAINS * sizeof(struct bwcnt));

  /* get time to start meter on variable ti */
  gettimeofday(&ti, NULL);

  /* fire meter loop */
  if ( act_bw )  
    printf("Displaying current flow values ...\n");
    printf("Displaying average flow values ...\n");

  /* forever loop; abort the program with ^C */
  while ( 1 )  {
    /* you have to initialize for each measure to be done */
    if ( !(h = iptc_init(tablename)) )  {
      printf("Error initializing: %s\n", iptc_strerror(errno));
    ok = 0;    /* we start a new loop */
    gettimeofday(&to, NULL);  /* have a time shoot */

    /* iterate through each chain of the table */
    for (chain = iptc_first_chain(&h), i = 0; 
         chain = iptc_next_chain(&h))  {
      if ( iptc_builtin(chain, h) )
        continue;    /* if it is a built-in chain, ignore it */

      /* ok, read the counters of this chain */
      if ( !(counters = iptc_read_counter(chain, 0, &h)) )  {
         printf("Error reading %s: %s\n", chain, iptc_strerror(errno));

      /* check that we do not have more chains than we can process */
      if ( i >= MAXUSERCHAINS )  {
         printf("Maximum of %d user chains exceeded!!\n", MAXUSERCHAINS);

      /* this chain counter has not been initialized; initialize it */
      if ( bw[i].start == 0 )  {
        bw[i].icnt = counters->bcnt;
        bw[i].start = 1;

      /* this chain has a previous measure; take the current one */
      else  {
        bw[i].ocnt = counters->bcnt;
        if ( bw[i].ocnt == bw[i].icnt )    /* no new bytes flowing? */
          bw[i].bw = 0;                    /* flow is zero */
         /* flow in this chain is:
          *   current bytes count (bw[i].octn)    *minus*
          *   previous bytes count (bw[i].icnt)   *divided by*
          *   128.0 to convert bytes to kbits     *and divided by*
          *   difference in times in seconds      *to get*
          *   flow in kbits/sec that is what we want.
          bw[i].bw = (bw[i].ocnt - bw[i].icnt) / (128.0 * delta(to, ti));

       /* do you want current flow of this chain? initialize previous 
        * bytes count to current bytes count; we get the flow in last 
        * SLEEPTIME elapsed time.
        if ( act_bw )
          bw[i].icnt = bw[i].ocnt;
        ok = 1;    /* ok, we have some measure to show */
      ++i;  /* next chain, please */

    /* we iterate and i == 0; we do not have user chains at all */
    if ( i == 0 )  {
       printf("No user chains to meter!!\n");

   /* do you want to measure current flow? initialize previous time 
    * to actual time; we get the time elapsed in last SLEEPTIME.
    if ( act_bw )
      ti = to;

    /* do we have something to show? ok, display it */
    if ( ok )  {
      totbw = 0;
      for ( j = 0; j < i; ++j )  { 
        totbw = totbw + bw[j].bw;   /* calculate total flow */
      printf("%s%6.1fk:%s ", col[7], totbw, col[8]);  /* display total */
      for ( j = 0; j < i; ++j )  {  /* display flow of each chain in color */
        printf("%s%6.1fk%s ", col[j], bw[j].bw, col[8]);
    sleep(SLEEPTIME);  /* rest a little; you go too fast */
  }             /* give us enough time in order to let some bytes flow */

  exit(0);  /* bye, we have our measures!! */

} /* main */

Write your program and compile as before:

bash# ./ipt-cc bw

Before using the meter we need to set our environment.

First, we have to have at least 2 PCs connected in a network. This is our diagram configuration:

+--------+ eth0       eth0 +--------+
| PC #1  +-----------------+ PC #2  |
+--------+                 +--------+
eth0=           eth0=

Second, we need to install a very nice and useful package called netcat written by Hobbit. This excellent package will help us to inject and receive a flow of bytes between 2 NICs. If you don't have the package in your system, download it from

The version that I use is 1.10-277. To install it follow these instructions:

bash# cp netcat-1.10.tar.gz /usr/local/src
bash# tar xzvf netcat-1.10.tar.gz
bash# cd netcat-1.10

My version requires to make a patch first; check yours if you have a file with a .dif extension and apply it too:

bash# patch -p0 -i netcat-1.10.dif

Next compile the package using make:

bash# make linux

Copy the binary nc to your user bin directory:

bash# cp nc /usr/bin

And also to the second PC in your network:

bash# scp nc

We are going to use netcat to "listen" to a flow of bytes from PC #2 and to "talk" from PC #1. Using tty1 to tty4 consoles on PC #2 let's start netcat to listen from this PC. Go to PC #2 and in tty1 type:

bash# nc -n -v -l -s -p 1001 >/dev/null

netcat must respond with:

listening on [] 1001 ...

This command started netcat to listen from address using port number 1001. Arguments are: -n = use numeric address identification; -v = verbose; -l = listen. All the flow that netcat receives in will be redirected to the "black hole" in /dev/null.

Repeat the command in tty2, tty3 and tty4; change to tty2 using ALT-F2 and after logging in write:

bash# nc -n -v -l -s -p 1002 >/dev/null

Now we are "listening" to the same address but port number 1002.

Go on now with tty3:

bash# nc -n -v -l -s -p 1003 >/dev/null

And tty4:

bash# nc -n -v -l -s -p 1004 >/dev/null

Now we are listening in PC #2, address in ports 1001, 1002, 1003 and 1004.

Come back to PC #1 and let's set the environment to allow iptables to help us to complete our tests:

On PC #1, type the into tty1 as follows:

bash# iptables -F
bash# iptables -X
bash# iptables -N chn_1
bash# iptables -N chn_2
bash# iptables -N chn_3
bash# iptables -N chn_4
bash# iptables -A chn_1 -j ACCEPT
bash# iptables -A chn_2 -j ACCEPT
bash# iptables -A chn_3 -j ACCEPT
bash# iptables -A chn_4 -j ACCEPT
bash# iptables -A OUTPUT -o eth0 -p tcp --dport 1001 -j chn_1
bash# iptables -A OUTPUT -o eth0 -p tcp --dport 1002 -j chn_2
bash# iptables -A OUTPUT -o eth0 -p tcp --dport 1003 -j chn_3
bash# iptables -A OUTPUT -o eth0 -p tcp --dport 1004 -j chn_4

These commands will:

  • Flush all chains in table filter.

  • Delete all user chains in table filter.

  • Create user chains chn_1, chn_2, chn_3 and chn_4.

  • Establish a target ACCEPT in each user chain.

  • Create 4 rules in chain OUTPUT that matches port numbers 1001 to 1004 and target it to user chains chn_1 to chn_4.

Now start the bw meter using current values:

bash# ./bw -c

It must respond with:

Displaying current flow values ...
   0.0k:    0.0k    0.0k    0.0k    0.0k
   0.0k:    0.0k    0.0k    0.0k    0.0k
   0.0k:    0.0k    0.0k    0.0k    0.0k
   0.0k:    0.0k    0.0k    0.0k    0.0k

It informs that measures are current flows. Every line is a measure taken each SLEEPTIME lapse (1 second in our program). First column (in black) are total flow, next columns (in red, green, orange and blue) are flows in chains chn_1, chn_2, chn_3 and chn_4 respectively. Of course we do not have any flow now. However let bw run and continue reading.

Let's start now one of our byte flows; go to tty2 in PC #1 with ALT-F2 and after logging in, type:

bash# yes 000000000000000000 | nc -n -v -s -p 2001 1001

netcat responds with:

(UNKNOWN) [] 1000 (?) open

Now we have a flow of bytes from PC #1 to PC #2. yes generates a constant flow of zeroes; this flow is piped to netcat through address, port 2001 and sends it to PC #2, address, port 1001 (where PC #2 is listening).

Check now the display of bw in tty1:

7653.2k: 7653.2k    0.0k    0.0k    0.0k
7829.5k: 7829.5k    0.0k    0.0k    0.0k
7786.7k: 7786.7k    0.0k    0.0k    0.0k
7892.1k: 7982.1k    0.0k    0.0k    0.0k

Your mileage can vary depending of the physical characteristics of your system. In mine I have a flow of aproximately 7700 kbits/sec in the first chain chn_1 which corresponds to port number 1001 in PC #2.

Let's start now the second bytes flow; go to tty3 in PC #1 with ALT-F3 and after logging in, type:

bash# yes 000000000000000000 | nc -n -v -s -p 2002 1002

netcat responds with:

(UNKNOWN) [] 1002 (?) open

Now we have 2 flows of bytes from PC #1 to PC #2; one from to and another from to

Now check the display of bw in tty1:

7819.6k: 4144.2k 3675.4k    0.0k    0.0k
8090.5k: 3923.9k 4166.6k    0.0k    0.0k
7794.7k: 3920.8k 3873.9k    0.0k    0.0k
7988.3k: 3754.6k 4233.7k    0.0k    0.0k

Now we have 2 flows; each of them is more or less 50% of the total flow going out of the computer. The Linux kernel tries to balance the bandwidth available between the 2 channels of output.

To continue, start the 2 aditional flows through channels and

In tty4 type:

bash# yes 000000000000000000 | nc -n -v -s -p 2003 1003

In tty5 type:

bash# yes 000000000000000000 | nc -n -v -s -p 2004 1004

The display of bw in tty1 will be something like:

8120.6k: 1705.3k 2354.9k 1898.6k 2161.8k
7765.3k: 1634.2k 2560.2k 2011.4k 1559.5k
7911.9k: 1699.8k 2090.3k 1768.0k 2353.8k
8309.4k: 1734.5k 1999.7k 1999.9k 2575.3k

Total bandwidth is distributed between the 4 channels of flow.

14. Controlling flows

In this chapter we are going to try to control the flows using the Linux kernel queue disciplines. Perhaps, depending on how you compiled your kernel, you will again need to run make menuconfig, re-configure your options, re-compile and re-install your kernel.

This chapter is not and does not pretend to be a tutorial about the implementation of QoS (Quality of Service) in Linux. If you don't have previous experience with QoS it's better to read some references at the end of this document to acquire the concepts required for QoS implementation.

With this advice, I'm not going to explain in detail each of the commands needed to control flows in Linux because it is not the goal of this HOWTO. However, the implementation of some of these techniques will serve us to show the bandwidth meter (based on libiptc) behaviour.

First check if you have QoS implementation options implemented in your kernel. Run make menuconfig, follow the menu to Networking options and look for last menu of this option QoS and/or fair queueing. Here use (or check if they are active) these options:

       [*] QoS and/or fair queueing
       <M> CBQ packet scheduler
       <M> CSZ packet scheduler
       [*] ATM pseudo-scheduler
       <M> The simplest PRIO pseudoscheduler
       <M> RED queue
       <M> SFQ queue
       <M> TEQL queue
       <M> TBF queue
       <M> GRED queue
       <M> Diffserv field marker
       <M> Ingress Qdisc
       [*] QoS support
       [*]   Rate estimator
       [*] Packet classifier API
       <M>   TC index classifier
       <M>   Routing table based classifier
       <M>   Firewall based classifier
       <M>   U32 classifier
       <M>   Special RSVP classifier
       <M>   Special RSVP classifier for IPv6
       [*]   Traffic policing (needed for in/egress)

Save your configuration, recompile your kernel and modules, and re-install it. We are going to use the CBQ packet scheduler to implement some queues to control bytes flow in our PC #1 NIC.

Personally I preferred the excellent HTB queueing discipline implementation by Martin Devera but actually this implementation is not in standard Linux (but it will be); for implementing it you have to patch your kernel before recompiling and it's better not to complicate things more. However I have to say that this queue discipline is a lot more simple to use than CBQ happens to be. More information on HTB queueing discipline are linked at the end of this document.

Having compiled and re-installed your kernel you have to install the iproute2 package that will be used to run the commands needed to implement the queues. Download this package from

I'm working with version 2.2.4-now-ss001007. To install it follow these instructions:

bash# cp iproute2-2.2.4-now-ss001007.tar.gz /usr/local/src
bash# tar xzvf iproute2-2.2.4-now-ss001007.tar.gz
bash# cd iproute2
bash# make

After make compiles the iproute2 package successfully the ip utility will be in iproute2/ip directory and the tc utility in iproute2/tc directory. Copy both of them to /usr/bin directory:

bash# cp ip/ip /usr/bin
bash# cp tc/tc /usr/bin

Now, using the tc utility, we are going to create a CBQ queue in the interface eth0 of the PC #1 computer. This queue will have 4 classes as children and each of these classes will be used to control the 4 flows from to through ports 1001 to 1004.

Write and run the following commands:

bash# tc qdisc add dev eth0 root handle 1:0 cbq bandwidth 10Mbit \
avpkt 1000 cell 8

This command creates the main (root) cbq queue 1:0 in the eth0 interface; the bandwidth of this queue is 10Mbit/sec corresponding to our Ethernet interface.

Now write and run:

bash# tc class add dev eth0 parent 1:0 classid 1:1 cbq bandwidth 10Mbit \
rate 1000kbit prio 8 allot 1514 cell 8 maxburst 20 avpkt 1000 bounded

This command create the main cbq class 1:1. The rate of this class will be 1000kbit/sec.

Now we are going to create 4 classes ownned by this class; the classes will have rates of 100kbit, 200kbit, 300kbit and 400kbit respectively. Write and run these commands:

bash# tc class add dev eth0 parent 1:1 classid 1:3 cbq bandwidth 10Mbit \
rate 100kbit prio 5 allot 1514 cell 8 maxburst 20 avpkt 1000

bash# tc class add dev eth0 parent 1:1 classid 1:4 cbq bandwidth 10Mbit \
rate 200kbit prio 5 allot 1514 cell 8 maxburst 20 avpkt 1000

bash# tc class add dev eth0 parent 1:1 classid 1:5 cbq bandwidth 10Mbit \
rate 300kbit prio 5 allot 1514 cell 8 maxburst 20 avpkt 1000

bash# tc class add dev eth0 parent 1:1 classid 1:6 cbq bandwidth 10Mbit \
rate 400kbit prio 5 allot 1514 cell 8 maxburst 20 avpkt 1000

Each of these classes will have a sfq queue discipline attached to them to dispatch their packets. Write and run these commands:

bash# tc qdisc add dev eth0 parent 1:3 handle 30: sfq perturb 15
bash# tc qdisc add dev eth0 parent 1:4 handle 40: sfq perturb 15
bash# tc qdisc add dev eth0 parent 1:5 handle 50: sfq perturb 15
bash# tc qdisc add dev eth0 parent 1:6 handle 60: sfq perturb 15

These commands create 4 sfq queue disciplines, one for each class. sfq queue discipline is some kind of fair controlling queue. It tries to give to each connection in an interface same oportunity to their packets to be dispatched to at all.

Finally we are going to create filters to assign flows to ports 1001, 1002, 1003 and 1004 to classes 1:3, 1:4, 1:5 and 1:6 respectively. Write and run as follows:

bash# tc filter add dev eth0 parent 1:0 protocol ip prio 1 u32 match ip \
dport 1001 0xffff flowid 1:3

bash# tc filter add dev eth0 parent 1:0 protocol ip prio 1 u32 match ip \
dport 1002 0xffff flowid 1:4

bash# tc filter add dev eth0 parent 1:0 protocol ip prio 1 u32 match ip \
dport 1003 0xffff flowid 1:5

bash# tc filter add dev eth0 parent 1:0 protocol ip prio 1 u32 match ip \
dport 1004 0xffff flowid 1:6

After running all these commands, now check your bw meter (you must be running netcat listening at ports 1001 to 1004 in PC #2 and talking in PC #1 as was explained in previous chapter and bw running in current -c mode). You will have something like this:

Current flow values ...
   1099.9k:  108.8k  196.5k  337.9k  456.8k 
   1104.2k:  115.3k  184.9k  339.9k  464.1k 
   1102.1k:  117.3k  174.7k  339.7k  470.5k 
   1114.4k:  113.6k  191.7k  340.7k  468.4k 
   1118.4k:  113.7k  194.3k  340.5k  469.9k 

bw show us how flows are controlling using queue disciplines of the Linux kernel. As you see, CBQ queue discipline is not a very precise queue but you more or less have a flow of approximately 1000=100+200+300+400 on interface eth0.

To step back, write and run as follows:

bash# tc qdisc del dev eth0 root handle 1:0 cbq

on PC #1, to delete the main (root) queue discipline and owned classes and filters.

bash# killall nc

on PC #2 and PC #1, to stop netcat.

bash# iptables -F
bash# iptables -X

on PC #1, to clear iptables rules and chains.

bash# Ctrl-C

on PC #1, tty1 to stop bw bandwidth meter.

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