Libpcaptutorial-03
Writing a Basic Packet Capture Engine
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.
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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_loopactually 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
************
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.
************
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 tcpdumpand 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!!
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