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This doesn't sound particularly intriguing until you realize that you can safely put arbitrary BPF bytecode in the kernel: It can never hang or crash (by design). In other words, sniffing is closely intermingled with filtering.Ī technology called Berkeley Packet Filters (BPF) was conceived long ago just for these purposes, and libpcap implements a high-level filtering language it compiles into BPF bytecode. Modern networks may run at whopping 40Gbit/sec or more, yet you might be interested in that small VoIP session your boss is complaining about. pcap files are, well, just files, reading from them does not typically require root privileges.Īnother problem libpcap solves elegantly for you is getting only data you want. pcap file is almost indistinguishable from capturing content live, so replay is usually doable with any sniffer. The libpcap library is designed so that from the application point of view reading a. This way, you (or someone acting on your behalf) can capture traffic on one box to analyze it on another. pcap (packet capture, you guessed it) files and later you can read, or "replay" a. For example, it can save captured packets in so-called. Most rely on libpcap, a library that abstracts away platform specifics and adds some bonus features. We've discussed capabilities in the last issue of Linux Magazine, but typically it just means you need to be root.Ī rare network sniffer uses packet sockets directly. Capturing from the wire and crafting raw Layer 2 datagrams is a sensitive operation, so only processes with CAP_NET_RAW capability can do it. Packet sockets come into play early, shortly after the network card receives a frame, and well before the data goes through the Linux networking stack. An "address" of this family ( struct sockaddr_ll) tells the kernel from which interface to sniff packets, and in which Layer 2 protocols you are interested. The AF_PACKET address family operates on raw Layer 2 packets (or frames).
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In Linux, packet sockets are the standard mechanism. The exact way of doing this is platform-specific. Back to the BeginningĪny network sniffer relies on the operating system's ability to forward it to all packets the network card receives, regardless of which process (or even host) they really target. Anyway, employ common sense and don't sniff traffic that could be sensitive, even if your housemates or colleagues are careless enough to send it unencrypted. A virtual machine based dedicated test lab is the safest option. Many ISPs deem it illegal, too, so be careful when experimenting. In this Core Tech, we'll discover perhaps the most popular one (or just my favorite).Īlthough sniffing is a legitimate technique, it is still largely prohibited in corporate environments. Linux comes with many tools of this kind, both GUI and terminal based. What goes in the wire is an ultimate answer to "What you've thrown at my service?" and "How did I reply?" Debugging aside, network sniffers may collect statistics or perform security monitoring. In social relations, that's probably true, but in computing (especially networking) where this activity is known as sniffing, it's an indispensable debugging technique. We are always told that eavesdropping is bad.