The Linux Socket Filter: Sniffing Bytes over the Network

If you deal with network administration or security management, or if you are merely curious about what is passing by over your local network, grabbing some packets off the network card can be a useful exercise. With a little bit of C coding and a basic knowledge of networking, you will be able to capture data even if it is not addressed to your machine. In this article, we will refer to Ethernet networks, by far the most widespread LAN technology. Also, for reasons that will be explained later, we will assume that source and destination hosts belong to the same LAN.

First off, we will briefly recall how a common Ethernet network card works. Those of you who are already skilled in this field may safely skip to the next paragraph. IP packets sourced from users' applications are encapsulated into Ethernet frames (this is the name given to packets when sent over an Ethernet segment), which are just bigger lower-level packets containing the original IP packet and some information needed to carry it to its destination (see Figure 1). In particular, the destination IP address is mapped to a 6-byte destination Ethernet address (often called MAC address) through a mechanism called ARP. Thus, the frame containing the packet travels from the source host to the destination host over the cable that connects them. It is likely that the frame will go through network devices such as hubs and switches, but since we assumed no LAN borders are crossed, no routers or gateways will be involved.

Figure 1. IP Packets as Ethernet Frames

No routing process happens at the Ethernet level. In other words, the frame sent by the source host will not be headed directly toward the destination host; instead, the frame will be copied over all the cables that make up the LAN, and all the network cards will see it passing (see Figure 2). Each network card will start reading the first six bytes of the frame (which happen to contain the above-mentioned destination MAC addresses), but only one card will recognize its own address in the destination field and will pick up the frame. At this point, the frame will be taken apart by the network driver and the original IP packet will be recovered and passed up to the receiving application through the network protocol stack.

Figure 2. Sending Ethernet Frames over the LAN

More precisely, the network driver will have a look at the Protocol Type field inside the Ethernet frame header (see Figure 1) and, based on that value, forward the packet to the appropriate protocol receiving function. Most of the time the protocol will be IP, and the receiving function will take off the IP header and pass the payload up to the UDP- or TCP-receiving functions. These protocols, in turn, will pass it to the socket-handling functions, which will eventually deliver packet data to the receiving application in userland. During this trip, the packet loses all network information related to it, such as the source addresses (IP and MAC) and port, IP options, TCP parameters and so on. Furthermore, if the destination host does not have an open socket with the correct parameters, the packet will be discarded and never make it to the application level.

As a consequence, we have two distinct issues in sniffing packets over the network. One is related to Ethernet addressing—we cannot read packets that are not destined to our host; the other is related to protocol stack processing—in order for the packet not to be discarded, we should have a listening socket for each and every port. Furthermore, part of the packet information is lost during protocol stack processing.

The first issue is not fundamental, since we may not be interested in other hosts' packets and may tend to sniff all the packets directed to our machine. The second one, however, must be solved. We will see how to address these issues separately, starting with the latter.

When you open a socket with the standard call sock = socket(domain, type, protocol) you have to specify which domain (or protocol family) you are going to use with that socket. Commonly used families are PF_UNIX, for communications bounded on the local machine, and PF_INET, for communications based on IPv4 protocols. Furthermore, you have to specify a type for your socket and possible values depend on the family you specified. Common values for type, when dealing with the PF_INET family, include SOCK_STREAM (typically associated with TCP) and SOCK_DGRAM (associated with UDP). Socket types influence how packets are handled by the kernel before being passed up to the application. Finally, you specify the protocol that will handle the packets flowing through the socket (more details on this can be found on the socket(3) man page).

In recent versions of the Linux kernel (post-2.0 releases) a new protocol family has been introduced, named PF_PACKET. This family allows an application to send and receive packets dealing directly with the network card driver, thus avoiding the usual protocol stack-handling (e.g., IP/TCP or IP/UDP processing). That is, any packet sent through the socket will be directly passed to the Ethernet interface, and any packet received through the interface will be directly passed to the application.

The PF_PACKET family supports two slightly different socket types, SOCK_DGRAM and SOCK_RAW. The former leaves to the kernel the burden of adding and removing Ethernet level headers. The latter gives the application complete control over the Ethernet header. The protocol field in the socket() call must match one of the Ethernet IDs defined in /usr/include/linux/if_ether.h, which represents the registered protocols that can be shipped in an Ethernet frame. Unless dealing with very specific protocols, you typically use ETH_P_IP, which encompasses all of the IP-suite protocols (e.g., TCP, UDP, ICMP, raw IP and so on).

Since they have pretty serious security implications (for example, you may forge a frame with a spoofed MAC address), PF_PACKET-family sockets may only be used by root.

The PF_PACKET family easily solves the problem associated with protocol stack-handling of our sniffed packets. Let's see it do so with the example in Listing 1. We open a socket belonging to the PF_PACKET family, specifying a SOCK_RAW socket type and IP-related protocol type. Then we start reading from the socket and, after a few sanity checks, we print out some information extracted from the Ethernet level and IP level headers. By cross-checking the printed addresses with the offsets in Figure 1, you will see how easy it is for the application to get access to network level data.

Listing 1. Protocol Stack-Handling Sniffed Packets

Assuming that your machine is connected to an Ethernet LAN, you can experiment with our short example by running it while generating packets directed to your host from another machine (you can ping or Telnet to your host). You will be able to see all the packets directed to you, but you will not see any packet headed toward other hosts.