Libpcap tutorial-03

Hi :-), this section consists of a discussion on how to write a simple packet capture engine. The goal is to demonstrate methods of capturing and filtering multiple packets to aid in packet analysis. All the juicy info on disecting IP packets and forging new ones are reserved for later sections.. Yes I can see your dissapointment, but you must admit that a program that captures a single packet at a time is pretty much useless.

We'll start by looking at:

• int pcap_loop(pcap_t *p, int cnt, pcap_handler callback, u_char *user)

When pcap_loop(..) is called it will grab cnt packets (it will loop infinitely when cnt is -1) and pass them to the callback function which is of type pcap_handler. and what is pcap handler?? well lets go to the handy dandy header file..

typedef void (*pcap_handler)(u_char *arg, const struct pcap_pkthdr *, const u_char *);

We are interested in arguments 2 and 3, the pcap packet header and a const u_char consisting of the packet. The first argument (arg) is a pointer to data that you passed to pcap_loop initially as the last argument. This is used to pass data to the packet processing routine without having to resort to globals.

As a primer, lets write a q&d program that will loop and getn packets, then exit.

Download testpcap2.c Here or just cut and paste from below

/*********************************************************************** file:   testpcap2.c* date:   2001-Mar-14 12:14:19 AM * Author: Martin Casado* Last Modified:2001-Mar-14 12:14:11 AM** Description: Q&D proggy to demonstrate the use of pcap_loop***********************************************************************/#include #include #include #include #include #include #include #include /* callback function that is passed to pcap_loop(..) and called each time  * a packet is recieved                                                    */void my_callback(u_char *useless,const struct pcap_pkthdr* pkthdr,const u_char*        packet){    static int count = 1;    fprintf(stdout,"%d, ",count);    if(count == 4)        fprintf(stdout,"Come on baby sayyy you love me!!! ");    if(count == 7)        fprintf(stdout,"Tiiimmmeesss!! ");    fflush(stdout);    count++;}int main(int argc,char **argv){     int i;    char *dev;     char errbuf[PCAP_ERRBUF_SIZE];    pcap_t* descr;    const u_char *packet;    struct pcap_pkthdr hdr;     /* pcap.h */    struct ether_header *eptr;  /* net/ethernet.h */    if(argc != 2){ fprintf(stdout,"Usage: %s numpackets\n",argv[0]);return 0;}    /* grab a device to peak into... */    dev = pcap_lookupdev(errbuf);    if(dev == NULL)    { printf("%s\n",errbuf); exit(1); }    /* open device for reading */    descr = pcap_open_live(dev,BUFSIZ,0,-1,errbuf);    if(descr == NULL)    { printf("pcap_open_live(): %s\n",errbuf); exit(1); }    /* allright here we call pcap_loop(..) and pass in our callback function */    /* int pcap_loop(pcap_t *p, int cnt, pcap_handler callback, u_char *user)*/    /* If you are wondering what the user argument is all about, so am I!!   */    pcap_loop(descr,atoi(argv[1]),my_callback,NULL);    fprintf(stdout,"\nDone processing packets... wheew!\n");    return 0;}

Allright then, lets give her a whirl!

[root@pepe libpcap]# gcc testpcap2.c -lpcap[root@pepe libpcap]# ./a.out 7 1, 2, 3, 4, Come on baby sayyy you love me!!! 5, 6, 7, Tiiimmmeesss!!  Done processing packets... wheew![root@pepe libpcap]#

So as you can see, my_callback(...) was actually called 7 times before exiting. If you are testing your program by pinging an external machine the packets come slow enough to see them arrive in real time.. We could certainly put all of our packet analysis code in my_callback(..) and call it is done deal. But as good little coders we certainly aren't satisfied with such an easy and straightforward solution! The first problem is that pcap_loop(..) blocks indefinatly if no packet can be read. While this may be the desired behaviour it would be nice to timeout on the reads. Remember way back when we talked aboutpcap_open_live(..)? One of the arguments you can specify is a timeout value in miliseconds. pcap_loop actually ignores this argument, but pcap_dispatch(..) doesn't! So if we want our main looping mechanism to time-out replace pcap_loop() with pcap_dispatch(). Here is a description of pcap_dispatch(..) shamelessly stripped from the man page

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pcap_dispatch() is used to collect and process packets. cnt specifies the maximum number of packets to process before returning. A cnt of -1 processes all the packets received in one buffer. A cnt of 0 processes all packets until an error occurs, EOF is reached, or the read times out (when doing live reads and a non-zero read timeout is specified). callback specifies a routine to be called with three arguments: a u_char pointer which is passed in from pcap_dispatch(), a pointer to the pcap_pkthdr struct (which precede the actual network headers and data), and a u_char pointer to the packet data. The number of packets read is returned. Zero is returned when EOF is reached in a ``savefile.'' A return of -1 indicates an error in which case pcap_perror() or pcap_geterr() may be used to display the error text.
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In many applications using packet capture, you are not going to be interested in every packet on your network. Take the following scenario. Little Johny just bought the coolest new internet game to hit the markets. Little Johny wants to be the first kid to hack up a bot for the game, but unlike all other little kiddies, Johny is going to write his own packet capture engine instead of using something canned. Little Johnny uses tcpdump and notices that when the game starts up and he connects to the server.. it is connecting to 216.148.0.87 on port 26112. What should little Johnny do to only capture packets to or from 216.148.0.87 port 26112? Enter... pcap_compile(..) and pcap_setfilter(...) !!!

Note that we could read in all packets, sort through them one by one to pick out the subset we are interested in. However, each callback into user space is (of course) a kernel crossing and could become quite expenise. Luckily libpcap provides an interface where you can specify exactly which packets you are interested as bpf (berkeley packet filter) programs. In brief, to do this you need to pass a filter program as a string to pcap_compile() and then set it as a filter.... the problem is that the pcap man page doesn't provide any detail of what the filter program should look like (at least mine doesn't). Is all lost!? No! because we have the handy dandy program tcpdump and its man page. You should have tcpdump already installed on your machine (which tcpdump) but if you don't I highly suggest you put it on. Tcpdump is pretty much a wrapper of libpcap. What is useful to us at the moment is that it accepts filter programs from the command line! Aha! a reference. The tcpdump man page explicitly describes the syntax and semantics of the filter language, which is (of course) pretty straight forward. Here are the pertinent sections from my man pages..

              The program consists of one or more primitives.  Primitives usu              ally  consist  of  an  id  (name or number) preceded by one or more              qualifiers.  There are three different kinds of qualifier:              type   qualifiers say what kind of thing  the  id  name  or  number                     refers  to.   Possible  types are host, net and port.  E.g.,                     `host foo', `net 128.3', `port 20'.  If  there  is  no  type                     qualifier, host is assumed.              dir    qualifiers specify a particular transfer direction to and/or                     from id.  Possible directions are src, dst, src or  dst  and                     src  and dst.  E.g., `src foo', `dst net 128.3', `src or dst                     port ftp-data'.  If there is no dir qualifier, src or dst is                     assumed.  For `null' link layers (i.e. point to point proto                     cols such as slip) the inbound and outbound  qualifiers  can                     be used to specify a desired direction.              proto  qualifiers  restrict  the  match  to  a particular protocol.                     Possible protos are: ether, fddi,  ip,  arp,  rarp,  decnet,                     lat, sca, moprc, mopdl, tcp and udp.  E.g., `ether src foo',                     `arp net 128.3', `tcp port 21'.  If there is no proto quali                     fier,  all  protocols  consistent with the type are assumed.                     E.g., `src foo' means `(ip or arp or rarp) src foo'  (except                     the latter is not legal syntax), `net bar' means `(ip or arp                     or rarp) net bar' and `port 53' means  `(tcp  or  udp)  port                     53'.              In  addition  to the above, there are some special `primitive' key              words that don't follow  the  pattern:  gateway,  broadcast,  less,              greater  and  arithmetic  expressions.   All of these are described              below.              More complex filter expressions are built up  by  using  the  words              and,  or  and  not  to combine primitives.  E.g., `host foo and not              port ftp and not port ftp-data'.  To save typing, identical  quali              fier  lists can be omitted.  E.g., `tcp dst port ftp or ftp-data or              domain' is exactly the same as `tcp dst port ftp or  tcp  dst  port              ftp-data or tcp dst port domain'.              Allowable primitives are:              dst host host                     True  if  the  IP  destination  field of the packet is host,                     which may be either an address or a name.              src host host                     True if the IP source field of the packet is host.              host host                     True if either the IP source or destination of the packet is                     host.   Any  of  the above host expressions can be prepended                     with the keywords, ip, arp, or rarp as in:                          ip host host                     which is equivalent to:                          ether proto \ip and host host                     If host is a name with multiple IP addresses,  each  address                     will be checked for a match.              ether dst ehost                     True  if  the  ethernet destination address is ehost.  Ehost                     may be either a name  from  /etc/ethers  or  a  number  (see                     ethers(3N) for numeric format).              ether src ehost                     True if the ethernet source address is ehost.              ether host ehost                     True if either the ethernet source or destination address is                     ehost.              gateway host                     True if the packet used host as a gateway.  I.e., the ether                     net  source  or destination address was host but neither the                     IP source nor the IP destination was host.  Host must  be  a                     name  and  must be found in both /etc/hosts and /etc/ethers.                     (An equivalent expression is                          ether host ehost and not host host                     which can be used with either names or numbers  for  host  /                     ehost.)              dst net net                     True  if the IP destination address of the packet has a net                     work number of net. Net may be either a name from  /etc/net                     works or a network number (see networks(4) for details).              src net net                     True  if  the  IP source address of the packet has a network                     number of net.              net net                     True if either the IP source or destination address  of  the                     packet has a network number of net.              net net mask mask                     True  if  the  IP address matches net with the specific net                     mask.  May be qualified with src or dst.              net net/len                     True if the IP address matches net a netmask len bits  wide.                     May be qualified with src or dst.              dst port port                     True if the packet is ip/tcp or ip/udp and has a destination                     port value of port.  The port can be a number or a name used                     in  /etc/services  (see  tcp(4P) and udp(4P)).  If a name is                     used, both the port number and protocol are checked.   If  a                     number  or  ambiguous  name is used, only the port number is                     checked (e.g., dst port 513 will print both tcp/login  traf                     fic  and  udp/who  traffic,  and port domain will print both                     tcp/domain and udp/domain traffic).              src port port                     True if the packet has a source port value of port.              port port                     True if either the source or destination port of the  packet                     is port.  Any of the above port expressions can be prepended                     with the keywords, tcp or udp, as in:                          tcp src port port                     which matches only tcp packets whose source port is port.              less length                     True if the packet has  a  length  less  than  or  equal  to                     length.  This is equivalent to:                          len <= length.              greater length                     True  if  the  packet  has a length greater than or equal to                     length.  This is equivalent to:                          len >= length.              ip proto protocol                     True if the packet is an ip packet (see ip(4P)) of  protocol                     type protocol.  Protocol can be a number or one of the names                     icmp, igrp, udp, nd, or tcp.  Note that the identifiers tcp,                     udp,  and  icmp  are  also  keywords and must be escaped via                     backslash (\), which is \\ in the C-shell.              ether broadcast                     True if the packet is an  ethernet  broadcast  packet.   The                     ether keyword is optional.              ip broadcast                     True if the packet is an IP broadcast packet.  It checks for                     both the all-zeroes and all-ones broadcast conventions,  and                     looks up the local subnet mask.              ether multicast                     True  if  the  packet  is an ethernet multicast packet.  The                     ether keyword is optional.  This is shorthand for  `ether[0]                     & 1 != 0'.              ip multicast                     True if the packet is an IP multicast packet.              ether proto protocol                     True  if the packet is of ether type protocol.  Protocol can                     be a number or a name like ip, arp,  or  rarp.   Note  these                     identifiers  are also keywords and must be escaped via back                     slash (\).  [In the  case  of  FDDI  (e.g.,  `fddi  protocol                     arp'), the protocol identification comes from the 802.2 Log                     ical Link Control (LLC) header, which is usually layered  on                     top  of the FDDI header.  Tcpdump assumes, when filtering on                     the protocol identifier, that all FDDI  packets  include  an                     LLC  header,  and  that  the LLC header is in so-called SNAP                     format.]              ip, arp, rarp, decnet                     Abbreviations for:                          ether proto p where p is one of the above protocols.              tcp, udp, icmp                     Abbreviations for:                          ip proto p                     where p is one of the above protocols.              expr relop expr                     True  if the relation holds, where relop is one of >,                     <, >=, <=, =, !=, and expr is an arithmetic                     expression composed  of integer constants (expressed in                     standard C syntax), the nor mal binary operators [+, -,                     *, /, &, |], a length  operator, and  special  packet                     data accessors.  To access data inside the packet, use the                     following syntax: proto [ expr : size ] Proto is one of                     ether, fddi, ip, arp,  rarp,  tcp,  udp,  or icmp,  and                     indicates the protocol layer for the index opera tion.                     The byte offset, relative to the  indicated  protocol                     layer, is given by expr.  Size is optional and indicates                     the number of bytes in the field of interest; it can  be                     either one,  two,  or four, and defaults to one.  The                     length opera tor, indicated by the keyword len, gives the                     length  of  the packet.                     For example, `ether[0] & 1 != 0' catches all multicast traf                     fic.  The expression `ip[0] & 0xf != 5' catches all IP pack                     ets  with  options.  The  expression  `ip[6:2] & 0x1fff = 0'                     catches only unfragmented datagrams and frag zero  of  frag                     mented  datagrams.   This check is implicitly applied to the                     tcp and udp index operations.  For instance,  tcp[0]  always                     means  the first byte of the TCP header, and never means the                     first byte of an intervening fragment.   Primitives may be combined using:                     A parenthesized group of primitives and operators (parenthe                     ses are special to the Shell and must be escaped).                     Negation (`!' or `not').                     Concatenation (`&&' or `and').                     Alternation (`||' or `or').              Negation  has  highest  precedence.   Alternation and concatenation              have equal precedence and  associate  left  to  right.   Note  that              explicit  and  tokens, not juxtaposition, are now required for con              catenation.              If an identifier is given without a keyword, the most  recent  key              word is assumed.  For example,                   not host vs and ace              is short for                   not host vs and host ace              which should not be confused with                   not ( host vs or ace )              Expression  arguments  can  be passed to tcpdump as either a single              argument or as multiple arguments, whichever  is  more  convenient.              Generally,  if  the expression contains Shell metacharacters, it is              easier to pass it as a single, quoted argument.  Multiple arguments              are concatenated with spaces before being parsed.EXAMPLES       To print all packets arriving at or departing from sundown:              tcpdump host sundown       To print traffic between helios and either hot or ace:              tcpdump host helios and \( hot or ace \)       To print all IP packets between ace and any host except helios:              tcpdump ip host ace and not helios       To print all traffic between local hosts and hosts at Berkeley:              tcpdump net ucb-ether       To  print  all  ftp  traffic through internet gateway snup: (note that the       expression is quoted to prevent  the  shell  from  (mis-)interpreting  the       parentheses):              tcpdump 'gateway snup and (port ftp or ftp-data)'       To print traffic neither sourced from nor destined for local hosts (if you       gateway to one other net, this stuff should never make it onto your  local       net).              tcpdump ip and not net localnet       To  print  the start and end packets (the SYN and FIN packets) of each TCP       conversation that involves a non-local host.              tcpdump 'tcp[13] & 3 != 0 and not src and dst net localnet'       To print IP packets longer than 576 bytes sent through gateway snup:              tcpdump 'gateway snup and ip[2:2] > 576'       To print IP broadcast or multicast packets that were not sent via ethernet       broadcast or multicast:              tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'       To  print  all  ICMP packets that are not echo requests/replies (i.e., not       ping packets):              tcpdump 'icmp[0] != 8 and icmp[0] != 0"

Ok, so that is a lot of info (and probably more than we need) but it gives us a starting point. Lets give it a shot... on my network I have a linux box and a windoze machine connected to a non switched hub. Therefore, if I place my ethernet card in promiscuous mode on my linux machine I should be able to see all traffic going to (and coming from) my windows machine. Lets see if the examples from the man page will work if directly fed to pcap_compile..

Consider the following program... (download here)

/*********************************************************************** file:   testpcap3.c* date:   Sat Apr 07 23:23:02 PDT 2001  * Author: Martin Casado* Last Modified:2001-Apr-07 11:23:05 PM** Investigate using filter programs with pcap_compile() and* pcap_setfilter()***********************************************************************/#include #include #include #include #include #include #include #include  /* just print a count every time we have a packet...                        */void my_callback(u_char *useless,const struct pcap_pkthdr* pkthdr,const u_char*        packet){    static int count = 1;    fprintf(stdout,"%d, ",count);    fflush(stdout);    count++;}int main(int argc,char **argv){     int i;    char *dev;     char errbuf[PCAP_ERRBUF_SIZE];    pcap_t* descr;    const u_char *packet;    struct pcap_pkthdr hdr;     /* pcap.h                    */    struct ether_header *eptr;  /* net/ethernet.h            */    struct bpf_program fp;      /* hold compiled program     */    bpf_u_int32 maskp;          /* subnet mask               */    bpf_u_int32 netp;           /* ip                        */    if(argc != 2){ fprintf(stdout,"Usage: %s \"filter program\"\n"            ,argv[0]);return 0;}    /* grab a device to peak into... */    dev = pcap_lookupdev(errbuf);    if(dev == NULL)    { fprintf(stderr,"%s\n",errbuf); exit(1); }    /* ask pcap for the network address and mask of the device */    pcap_lookupnet(dev,&netp,&maskp,errbuf);    /* open device for reading this time lets set it in promiscuous     * mode so we can monitor traffic to another machine             */    descr = pcap_open_live(dev,BUFSIZ,1,-1,errbuf);    if(descr == NULL)    { printf("pcap_open_live(): %s\n",errbuf); exit(1); }    /* Lets try and compile the program.. non-optimized */    if(pcap_compile(descr,&fp,argv[1],0,netp) == -1)    { fprintf(stderr,"Error calling pcap_compile\n"); exit(1); }    /* set the compiled program as the filter */    if(pcap_setfilter(descr,&fp) == -1)    { fprintf(stderr,"Error setting filter\n"); exit(1); }    /* ... and loop */     pcap_loop(descr,-1,my_callback,NULL);    return 0;}

This program accepts a string from the user, (similar to tcpdump) compiles it and sets it as a filter. Lets go ahead and try it with an example similar to the one in the tcpdump examples..

[root@localhost libpcap]# gcc testpcap3.c -lpcap[root@localhost libpcap]# ./a.out "host www.google.com"(** try and ping www.slashdot.org ... nothing **)(** try and ping www.google.com **)1, 2, 3, 4, 5, 6, (** hurray! **)

It looks like our filter program worked. Lets try to see if we can capture packets from a different machine on the same network.... how about my windows machine when it connects to battle.net..

[root@localhost libpcap]# ./a.out "src 192.168.1.104"1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,

Yes yes!!! we are getting very close to actually having some power, but first we need to find out how to look inside the packets and pull out information. At last! the next section will delve into disecting packets so we can really analyze what is going on in our networks!!