The Berkeley Packet Filter (BPF) is a technology used in certain computer operating systems for programs that need to, among other things, analyze network traffic. It provides a raw interface to data link layers, permitting raw link-layer packets to be sent and received.[1] It is available on most Unix-like operating systems. In addition, if the driver for the network interface supports promiscuous mode, it allows the interface to be put into that mode so that all packets on the network can be received, even those destined to other hosts.
Filter ACK packets. Ask Question Asked 5 years, 10 months ago. Active 5 years, 10 months ago. Viewed 1k times 1. I have the following tcpdump -i eth0 -w pkts.pcap -n tcp port 5000 to filter every packet flowing between 2 hosts. However, one of the hosts always sends an ACK. How to filter tcpdump output based on packet length. Packets captured (this is the number of packets that tcpdump has received and processed); packets received by filter (the meaning of this depends on the OS on which you're running tcpdump, and possibly on the way the OS was configured - if a filter was specified on the command line, on some OSes it counts packets regardless of whether they were.
BPF supports filtering packets, allowing a userspaceprocess to supply a filter program that specifies which packets it wants to receive. For example, a tcpdump process may want to receive only packets that initiate a TCP connection. BPF returns only packets that pass the filter that the process supplies. This avoids copying unwanted packets from the operating systemkernel to the process, greatly improving performance.
BPF is sometimes used to refer to just the filtering mechanism, rather than to the entire interface. Some systems, such as Linux and Tru64 UNIX, provide a raw interface to the data link layer other than the BPF raw interface but use the BPF filtering mechanisms for that raw interface.
Raw interface[edit]
BPF provides pseudo-devices that can be bound to a network interface; reads from the device will read buffers full of packets received on the network interface, and writes to the device will inject packets on the network interface.
In 2007, Robert Watson and Christian Peron added zero-copy buffer extensions to the BPF implementation in the FreeBSD operating system,[2] allowing kernel packet capture in the device driver interrupt handler to write directly to user process memory in order to avoid the requirement for two copies for all packet data received via the BPF device. While one copy remains in the receipt path for user processes, this preserves the independence of different BPF device consumers, as well as allowing the packing of headers into the BPF buffer rather than copying complete packet data.[3]
Filtering[edit]
BPF's filtering capabilities are implemented as an interpreter for a machine language for the BPF virtual machine. Programs in that language can fetch data from the packet, perform arithmetic operations on data from the packet, and compare the results against constants or against data in the packet or test bits in the results, accepting or rejecting the packet based on the results of those tests.
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Traditional Unix-like BPF implementations can be used in userspace, despite being written for kernel-space. This is accomplished using preprocessor conditions.
Extensions and optimizations[edit]
Some projects use BPF instruction sets or execution techniques different from the originals.
Some platforms, including FreeBSD, NetBSD, and WinPcap, use a just-in-time (JIT) compiler to convert BPF instructions into native code in order to improve performance. Linux includes a BPF JIT compiler which is disabled by default.
Kernel-mode interpreters for that same virtual machine language are used in raw data link layer mechanisms in other operating systems, such as Tru64 Unix, and for socket filters in the Linux kernel and in the WinPcap and Npcap packet capture mechanism. Since version 3.18, the Linux kernel includes an extended BPF virtual machine, termed extended BPF (eBPF). It can be used for non-networking purposes, such as for attaching eBPF programs to various tracepoints.[4][5][6] Since kernel version 3.19, eBPF filters can be attached to sockets,[7][8] and, since kernel version 4.1, to traffic control classifiers for the ingress and egress networking data path.[9][10] The original and obsolete version has been retroactively renamed to classic BPF (cBPF). Nowadays, the Linux kernel runs eBPF only and loaded cBPF bytecode is transparently translated into an eBPF representation in the kernel before program execution.[11]
A user-mode interpreter for BPF is provided with the libpcap/WinPcap/Npcap implementation of the pcapAPI, so that, when capturing packets on systems without kernel-mode support for that filtering mechanism, packets can be filtered in user mode; code using the pcap API will work on both types of systems, although, on systems where the filtering is done in user mode, all packets, including those that will be filtered out, are copied from the kernel to user space. That interpreter can also be used when reading a file containing packets captured using pcap.
History[edit]
The original paper was written by Steven McCanne and Van Jacobson in 1992 while at Lawrence Berkeley Laboratory[1][12]
In August 2003, SCO Group publicly claimed that the Linux kernel was infringing Unix code which they owned.[13] Programmers quickly discovered that one example they gave was the Berkeley Packet Filter, which in fact SCO never owned.[14] SCO has not explained or acknowledged the mistake but the ongoing legal action may eventually force an answer.[15]
Security concerns[edit]
Spectre attack may leverage Linux kernel eBPF JIT compiler to extract data from other kernel processes and allow user-space to read it.[16]
See also[edit]
References[edit]
^ abMcCanne, Steven; Jacobson, Van (1992-12-19). 'The BSD Packet Filter: A New Architecture for User-level Packet Capture'(PDF).
^McCanne, Steven; Jacobson, Van (January 1993). 'The BSD Packet Filter: A New Architecture for User-level Packet Capture'. USENIX.
^'SCOsource update'. 15 Obfuscated Copying. Archived from the original on August 25, 2003. Retrieved September 5, 2019.
^Bruce Perens. 'Analysis of SCO's Las Vegas Slide Show'. Archived from the original on February 17, 2009.
^Moglen, Eben (November 24, 2003). 'SCO: Without Fear and Without Research'. GNU Operating System. The Free Software Foundation. Retrieved September 5, 2019.
^'Reading privileged memory with a side-channel'. Project Zero team at Google. January 3, 2018. Retrieved January 20, 2018.
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External links[edit]
bpfc, a Berkeley Packet Filter compiler, Linux BPF JIT disassembler (part of netsniff-ng)
McCanne, Steven; Jacobson, Van (1992-12-19). 'The BSD Packet Filter: A New Architecture for User-level Packet Capture'(PDF).
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Berkeley_Packet_Filter&oldid=951615661'
This release contains changes in the following areas:
Improved detection of program modification
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Little Snitch has a security mechanism that ensures rules are only applied to programs for which they were originally created. This is to prevent malware from hijacking existing rules for legitimate programs. To do that, Little Snitch must be able to detect whether a program was modified. How Little Snitch does that changes with this version.
Previous versions required a program to have a valid code signature in order to be able to detect illegitimate modifications later on. Programs without a code signature could not be validated and Little Snitch warned accordingly. The focus was therefore on a program’s code signature.
Beginning with version 4.3, Little Snitch can always check whether a program has been tampered with, even if it’s not code signed at all. The focus is now on checking for modifications with the best means available. That is usually still the code signature but for programs that are not code signed, Little Snitch now computes a secure hash over the program’s executable. (There’s still a warning if a process is not signed, but only to inform you about a possible anomaly.)
This change leads to a different terminology. When editing a rule, Little Snitch Configuration no longer shows a checkbox titled “requires valid code signature” but instead one that is titled “check process identity” (or if the rule is for any process: “apply to trusted processes only”).
Instead of a “code signature mismatch”, Little Snitch’s connection alert now informs that “the program has been modified”.
In cases where Little Snitch detects such a modification, it now also better explains the possible underlying cause and the potential consequences.
For more information see the chapter Code identity checks in the online help.
Configuration File Compatibility
This version uses a new format with speed and size improvements for the configuration file in which the current rule set and the preferences are stored. This new file format is not compatible with older versions of Little Snitch, though.When updating to Little Snitch 4.3, the old configuration file is left untouched in case you want to downgrade to a previous version of Little Snitch. All changes made in Little Snitch 4.3 or later are not included in the old file, of course.Note that backup files created using File > Create Backup… in Little Snitch Configuration use the old file format and are therefore backward-compatible with previous versions of Little Snitch.
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Improved Support for macOS Mojave
Improved appearance in Dark Mode.
Fixed backup restore from Time Machine not working in Little Snitch Configuration due to the new “Full Disk Access” security mechanism.
Fixed creating Diagnostics Reports for non-admin users (on macOS High Sierra and later). When you contact our tech support, we sometimes ask you to create these reports.
Performance Improvements
Improved overall performance for large rule sets.
Reduced CPU load of Little Snitch Daemon during DNS lookups.
Reduced CPU load of Network Monitor while inactive.
Improved performance of rule sorting in Little Snitch Configuration, which leads to better overall performance.
Fixed Little Snitch Daemon hanging while updating a rule group subscription that contains many rules.
Fixed a memory leak that occurred when closing a snapshot window in Network Monitor.
Internet Access Policy
Fixed an issue causing an app’s Internet Access Policy not being shown if that app was running in App Translocation.
Fixed clickable links not working in the “Deny Consequences” popover when creating rules in connection alert or Network Monitor.
Internet Access Policy file: Fixed large values for a connection’s “Port” being rejected.
Process Identity and Code Signature Check Improvements
Added support for detecting revoked code signing certificates when checking a process’ code signature. The connection alert and Network Monitor now treat such processes like processes without a valid code signature and show relevant information. Also, rules created will use an appropriate identity check (based on the executable’s checksum, not based on the code signature).
When showing a connection alert for a process that has no valid code signature, Little Snitch now tries to find out if loading a shared library may have caused the issue with the code signature. If so, this is pointed out in the connection alert.
Fixed handling of app updates while the app is still running: Previous versions of Little Snitch would complain that the code signature could not be checked if the running app was replaced on disk, e.g. during an update.
Fixed an issue where connection alerts would erroneously contain a warning that an application’s code signing certificate was unacceptable. This mainly happened when a process’ first connection was an incoming connection.
Improved Handling of Connection Denials and Override Rules
Improved handling of override deny-rules that were created as a consequence of a suspicious program modification (“Connection Denials”). In Network Monitor, these rules are now marked with a dedicated symbol. Clicking that symbol allows to remove that override rule, if the modification is confirmed to be legitimate.
Changed override deny-rules created for failed code identity checks to not be editable or deletable. Instead, double-clicking such a rule allows you to fix the underlying issue, which then automatically deletes the override rule.
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UI and UX Improvements
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Automatically combine rules: For improved handling of large rule sets with many similar rules that only differ in host or domain names. This is common when subscribing to blocklists, which may contain thousands of similar, individual rules denying connections to various servers. The new “Automatically combine rules” option in Little Snitch Configuration (on by default) now combines such similar rules into a single row, making it much easier to keep track of large lists of rules.
Improved appearance when Accessibility option 'Increase contrast' is active.
Improved floating window mode in Network Monitor.
When choosing File > Restore from Backup in Little Snitch Configuration, the list showing possible backup files now includes backups that Little Snitch created automatically.
Improved the map shown in the “Known Networks” window in Little Snitch Configuration.
Improved the legibility of traffic rates in the status menu on Retina displays.
Fixed data rates shown in Network Monitor to match the values shown in the status menu.
Fixed the “Duration” setting in Preferences > Alert > Preselected Options not being respected.
Fixed an issue with “undo” when unsubscribing from a rule group or when deleting a profile.
Fixed an issue in Little Snitch Configuration where the “Turn into global rule” action did not work.
Fixed an issue where an error that occurred in the course of a previous rule group subscription update was still displayed, even though the problem no longer existed.
Other Improvements and Bug Fixes
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Increased the maximum number of host names allowed in a rule group subscription to 200.000.
Fixed an issue causing XPC services inside bundled frameworks to not be recognized as XPC. This resulted in connection alerts to be shown for the XPC services themselves instead of for the app the service belongs to.
Fixed an issue causing Time Machine backups to Samba servers to stop working under some circumstances.
Fixed an issue related to VPN connections with Split DNS configuration that caused only the server’s IP address to be displayed instead of its hostname.
Reduced the snap length in PCAP files, allowing them to be analyzed not only with Wireshark but also with “tcpdump”.