The Linux Socket Filter: Sniffing Bytes over the Network

Even if the BPF language is pretty simple and easy to learn, most of us would probably be more comfortable with filters written in human-readable expressions. So, instead of presenting the details and instructions of the BPF language (which you can find in the above-mentioned paper), we will discuss how to obtain the code for a working filter starting from a logic expression.

First, you will need to install the tcpdump program from LBL (see Resources). But, if you are reading this article, it is likely that you already know and use tcpdump. The first versions were written by the same people who wrote the BPF paper and its first implementation. In fact, tcpdump uses BPF, in the form of a library called libpcap, to capture and filter packets. The library is an OS-independent wrapper for the BPF engine. When used on Linux machines, BPF functions are carried out by the Linux packet filter.

One of the most useful functions provided by the libpcap is pcap_compile(), which takes a string containing a logic expression as input and outputs the BPF filter code. tcpdump uses this function to translate the command-line expression passed by the user into a working BPF filter. What is interesting for our purposes is that tcpdump has a seldomly used switch, -d, which prints the code of the filter.

For example, typing tcpdump host 192.168.9.10 will start sniffing and grab only those packets whose source or destination IP address matches 192.168.9.10. Typing tcpdump -d host 192.168.9.10 will print the BPF code that recognizes the filter, as shown in Listing 3.

Listing 3. Tcpdump -d Results

Let's briefly comment on this code; lines 0-1 and 6-7 verify that the captured frame is actually transporting IP, ARP or RARP protocols by comparing their protocol IDs (see /usr/include/linux/if_ether.h) with the value found at offset 12 in the frame (see Figure 1). If the test fails, the packet is discarded (line 13).

Lines 2-5 and 8-11 compare the source and destination IP addresses with 192.168.9.10. Note that, depending on the protocol, the offsets of these addresses are different; if the protocol is IP, they are 26 and 30, otherwise they are 28 and 38. If one of the addresses matches, the packet is accepted by the filter, and the first 68 bytes are passed to the application (line 12).

The filter code is not always optimized, since it is generated for a generic BPF machine and not tailored to the specific architecture that runs the filter engine. In the particular case of the LPF, the filter is run by the PF_PACKET processing routines, which may have already checked the Ethernet protocol. This depends on the protocol field you specified in the initial socket() call: if it is not ETH_P_ALL (which means that every Ethernet frame shall be captured), then only frames having the specified Ethernet protocol will arrive at the filter. For example, in the case of an ETH_P_IP socket, we could rewrite a faster and more compact filter as follows:

(000) ld       [26]
(001) jeq      #0xc0a8090a      jt 4    jf 2
(002) ld       [30]
(003) jeq      #0xc0a8090a      jt 4    jf 5
(004) ret      #68
(005) ret      #0

Installing an LPF is a straightforward operation: all you have to do is create a sock_filter structure containing the filter and attach it to an open socket.

The filter structure is easily obtained by substituting the -d switch to tcpdump with -dd. The filter will be printed as a C array that you can copy and paste into your code, as shown in Listing 4. Afterward, you attach the filter to the socket by simply issuing a setsockopt() call.

Listing 4. Tcpdump with --dd Switch

We will conclude this article with a complete example (see Listing 5 at ftp://ftp.linuxjournal.com/pub/lj/listings/issue86/). It is exactly like the first two examples, with the addition of the LSF code and the setsockopt() call. The filter has been configured to select only UDP packets, having either source or destination IP address 192.168.9.10 and source UDP port equal to 5000.

In order to test this listing, you will need a simple way to generate arbitrary UDP packets (such as sendip or apsend, found on http://freshmeat.net/). Also, you may want to adapt the IP address to match the ones used in your own LAN. To accomplish this, just substitute 0xc0a8090a in the filter code with the IP address of your choice in hex notation.

A final remark concerns the status of the Ethernet card when you exit the program. Since we did not reset the Ethernet flags, the card will remain in promiscuous mode. To solve this problem, all you need to do is install a Control-C (SIGINT) signal handler that resets the Ethernet flags to their previous value (which you will have saved just before ORing with IFF_PROMISC) before exiting the program.