Pocket Linux Guide

The Linux Documentation Project


This document takes baby steps to explains how to build a small diskette-based GNU/Linux system. This is a very good starting place for Linux newbies to understand how basic linux system works.

Get information about all the network interfaces in linux

This program lists all active network interfaces in the system with associated IP address, MAC address and the subnetmask.

#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>

#define __KERNEL__
#include <asm/types.h>
#undef __KERNEL__

#include <linux/ethtool.h>
#include <linux/sockios.h>

#include <net/if.h>
#include <sys/ioctl.h>
#include <arpa/inet.h>

/* this is straight from beej's network tutorial. It is a nice wrapper
 * for inet_ntop and helpes to make the program IPv6 ready
 */
char *get_ip_str(const struct sockaddr *sa, char *s, size_t maxlen)
{
  switch(sa->sa_family) {
    case AF_INET:
      inet_ntop(AF_INET, &(((struct sockaddr_in *)sa)->sin_addr), s, maxlen);
      break;

    case AF_INET6:
      inet_ntop(AF_INET6, &(((struct sockaddr_in6 *)sa)->sin6_addr), s, maxlen);
      break;

    default:
      strncpy(s, "Unknown AF", maxlen);
      return NULL;
  }

  return s;
}

int main (int argc, char **argv)
{
  char          buf[1024] = {0};
  struct ifconf ifc = {0};
  struct ifreq *ifr = NULL;
  int           sck = 0;
  int           nInterfaces = 0;
  int           i = 0;
  struct ifreq ifr_subnet;
  struct sockaddr *addr_subnet = NULL;

  /* Get a socket handle. */
  sck = socket(AF_INET, SOCK_DGRAM, 0);
  if(-1 == sck) {
    perror("socket");
    return 1;
  }

  /* Query available interfaces. */
  ifc.ifc_len = sizeof(buf);
  ifc.ifc_buf = buf;
  if(-1 == ioctl(sck, SIOCGIFCONF, &ifc)) {
    perror("ioctl(SIOCGIFCONF)");
    close (sck);
    return 1;
  }

  /* Iterate through the list of interfaces. */
  ifr = ifc.ifc_req;
  nInterfaces = ifc.ifc_len / sizeof(struct ifreq);
  for(i = 0; i < nInterfaces; i++)
  {
    struct ifreq *item = &ifr[i];

    /* Show the device name and IP address */
    struct sockaddr *addr = &(item->ifr_addr);
    char ip[INET6_ADDRSTRLEN];
    printf("%s: IP %s",
           item->ifr_name,
           get_ip_str(addr, ip, INET6_ADDRSTRLEN));

    /* Get the MAC address */
    if(-1 == ioctl(sck, SIOCGIFHWADDR, item)) {
      perror("ioctl(SIOCGIFHWADDR)");
      close (sck);
      return 1;
    }

    /* display result */
    printf(", MAC %.2x:%.2x:%.2x:%.2x:%.2x:%.2x ",
           (unsigned char)item->ifr_hwaddr.sa_data[0],
           (unsigned char)item->ifr_hwaddr.sa_data[1],
           (unsigned char)item->ifr_hwaddr.sa_data[2],
           (unsigned char)item->ifr_hwaddr.sa_data[3],
           (unsigned char)item->ifr_hwaddr.sa_data[4],
           (unsigned char)item->ifr_hwaddr.sa_data[5]);

    /* Reset the memory allocated for ifreq */
    memset (&ifr_subnet, 0, sizeof (struct ifreq));

    /* Update interface name into ifreq */
    strcpy (ifr_subnet.ifr_name, item->ifr_name);

    /* Call ioctl function */
    if(-1 == ioctl (sck, SIOCGIFNETMASK, &ifr_subnet ))
    {
      perror("ioctl(SIOCGIFNETMASK)");
      close (sck);
      return 1;
    }
    addr_subnet = &(ifr_subnet.ifr_addr);
    printf("SubnetMask: %s\n", get_ip_str(addr_subnet, ip, INET6_ADDRSTRLEN));
  }

  close (sck);
  return 0;
}

Reference: http://www.adamrisi.com/?p=84

Simple PING implementation in C

#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/signal.h>
#include <string.h>

#define DEFDATALEN      56
#define MAXIPLEN        60
#define MAXICMPLEN      76

static char *hostname = NULL;

static int in_cksum(unsigned short *buf, int sz)
{
  int nleft = sz;
  int sum = 0;
  unsigned short *w = buf;
  unsigned short ans = 0;
  
  while (nleft > 1) {
    sum += *w++;
    nleft -= 2;
  }
  
  if (nleft == 1) {
    *(unsigned char *) (&ans) = *(unsigned char *) w;
    sum += ans;
  }
  
  sum = (sum >> 16) + (sum & 0xFFFF);
  sum += (sum >> 16);
  ans = ˜sum;
  return (ans);
}

static void noresp(int ign)
{
  printf("No response from %s\n", hostname);
  exit(0);
}

static void ping(const char *host)
{
  struct hostent *h;
  struct sockaddr_in pingaddr;
  struct icmp *pkt;
  int pingsock, c;
  char packet[DEFDATALEN + MAXIPLEN + MAXICMPLEN];
  
  if ((pingsock = socket(AF_INET, SOCK_RAW, 1)) < 0) {       /* 1 == ICMP */
    perror("ping: creating a raw socket");
    exit(1);
  }
  
  /* drop root privs if running setuid */
  setuid(getuid());
  
  memset(&pingaddr, 0, sizeof(struct sockaddr_in));
  
  pingaddr.sin_family = AF_INET;
  if (!(h = gethostbyname(host))) {
    fprintf(stderr, "ping: unknown host %s\n", host);
    exit(1);
  }
  memcpy(&pingaddr.sin_addr, h->h_addr, sizeof(pingaddr.sin_addr));
  hostname = h->h_name;
  
  pkt = (struct icmp *) packet;
  memset(pkt, 0, sizeof(packet));
  pkt->icmp_type = ICMP_ECHO;
  pkt->icmp_cksum = in_cksum((unsigned short *) pkt, sizeof(packet));
  
  c = sendto(pingsock, packet, sizeof(packet), 0,
             (struct sockaddr *) &pingaddr, sizeof(struct sockaddr_in));
  
  if (c < 0 || c != sizeof(packet)) {
    if (c < 0)
      perror("ping: sendto");
    fprintf(stderr, "ping: write incomplete\n");
    exit(1);
  }
  
  signal(SIGALRM, noresp);
  alarm(2);                                     /* give the host 5000ms to respond */
  /* listen for replies */
  while (1) {
    struct sockaddr_in from;
    size_t fromlen = sizeof(from);
    
    if ((c = recvfrom(pingsock, packet, sizeof(packet), 0,
                      (struct sockaddr *) &from, &fromlen)) < 0) {
      if (errno == EINTR)
        continue;
      perror("ping: recvfrom");
      continue;
    }
    if (c >= 76) {                   /* ip + icmp */
      struct iphdr *iphdr = (struct iphdr *) packet;
      
      pkt = (struct icmp *) (packet + (iphdr->ihl << 2));      /* skip ip hdr */
      if (pkt->icmp_type == ICMP_ECHOREPLY)
        break;
    }
  }
  printf("%s is alive!\n", hostname);
  return;
}

int main ()
{
  ping ("192.168.1.2");

}

Reference: http://www.koders.com/c/fid30EA22902AFF5800481BBC6F65DADCDAF92D6E37.aspx

Add new option in configure script in autobuild system

Example:
To get a new option "--with-unit-test" when you run "./configure --help"

Optional Packages:
  --with-unit-test        Create binary to unit test the library package
                          (default is no)

add the following block in "configure.ac"

AC_MSG_CHECKING([--with-unit-test])
AC_ARG_WITH(unit-test,
        AC_HELP_STRING([--with-unit-test], [Create binary to unit test the library package (default is no)]),
        [unittest="yes"],
        [unittest="no"]
)
AM_CONDITIONAL(UNIT_TEST, test "$unittest" = "yes")
AC_MSG_RESULT([$unittest])

and you Makefile.am will look like this,

bindir = $(prefix)/usr/local/bin
libdir = $(prefix)/usr/local/lib

if UNIT_TEST
bin_PROGRAMS = unittest
unittest_SOURCES = source1.c \
                      Source2.c
unittest_CFLAGS = -I@top_srcdir@/include \
                     -DUNIT_TEST
unittest_LDFLAGS =
else
lib_LTLIBRARIES           = libunittest.la
libunittest_la_SOURCES = source2.c \
                         source3.c
libunittest_la_CFLAGS = -I@top_srcdir@/include
libunittest_la_LDFLAGS = -version-info 0:1:0
endif

and take care of UNIT_TEST macro check in the source code.

How to create self-Extracting script

It involves 3 steps,

1. Create Payload
2. Create Installer script
3. Packaging

Create Payload:
This is a tar ball  in which all the installable binaries, libraries, script files, configuration files, etc will be placed in the same file hierarchy of the target board considering the base directory of the tar ball is the installation directory on the target board.

periask@ubuntu:~$ mkdir -vp packaging/Payload
mkdir: created directory `packaging
mkdir: created directory `packaging/Payload'
periask@ubuntu:~$ cd packaging/Payload
periask@ubuntu:~/packaging/Payload$ cp --preserve=mode -rl <installables> .
periask@ubuntu:~/packaging/Payload$ tar zcf ../Payload.tar.gz .
periask@ubuntu:~/packaging/Payload$ cd ..
periask@ubuntu:~/packaging$

Create Installer Script:

Copy the below content and create "installer" file in "packaging" directory.

#!/bin/bash

# Get the starting point of the tar ball
SKIP=`awk '/^__ARCHIVE__/ { print NR + 1; exit 0; }' $0`

# Do pre-installation steps here.

# Extracting the tar ball Which is appended to this file
tail -n "+$SKIP" $0 >/dev/null 2>&1
if [ $? -eq 0 ]; then
    tail -n +$SKIP $0 | gunzip -c | tar -C / -xvf -
else
    tail +$SKIP $0 | gunzip -c | tar -C / -xvf - 
fi

# Do post-installation steps here.

exit 0

__ARCHIVE__

Note: There should not be any space  after "__ARCHIVE__" and should be a new line after "__ARCHIVE__".

Packaging:

Concatenate the file "installer" and "Payload.tar.gz" and create the package file.

periask@ubuntu:~/packaging$ cat installer Payload.tar.gz > Package-Version.sh
periask@ubuntu:~/packaging$ chmod +x Package-Version.sh

Installer for your package "Package-Version.sh" is ready.

Source: Linux Journal

Avahi-autoipd

It is an Zeroconf implementation of Avahi which facilitates configuration of a network element automatically on the local network in the absence of DHCP server. It uses a special address range 169.254.0.0/16 and prefix fe80::/16 for IPV4 and IPV6 link-local addressing respectively. When the autoipd daemon starts, it generates an IP address randomly using the MAC address information and announces its new IP using ARP probe to find address conflict. If the given address is being used by another network element, it generates a new IP and does ARP probe till it successfully claims the address. It stores this IP address in a file for persistence across network restart (file location on linux machine will be "/var/lib/avahi-autoipd/<mac_address>"). To configure this address to be routable on the local link, autoipd adds routing rule for this IP address in the routing table.

Source: RFC3927, Avahi