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Guide to configuring the linux kernel/net

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Guide to configuring the linux kernel / net



  • Option: NET
    • bool "Networking support"
      Unless you really know what you are doing, you should say Y here. The reason is that some programs need kernel networking support even when running on a stand-alone machine that isn't connected to any other computer.
      If you are upgrading from an older kernel, you should consider updating your networking tools too because changes in the kernel and the tools often go hand in hand. The tools are contained in the package net-tools, the location and version number of which are given in <file:Documentation/Changes>.
      For a general introduction to Linux networking, it is highly recommended to read the NET-HOWTO, available from

All of the following dependent on NET

Networking optionsEdit


  • Option: INET
    • bool "TCP/IP networking"
      These are the protocols used on the Internet and on most local Ethernets. It is highly recommended to say Y here (this will enlarge your kernel by about 144 KB), since some programs (e.g. the X window system) use TCP/IP even if your machine is not connected to any other computer. You will get the so-called loopback device which allows you to ping yourself (great fun, that!).
      For an excellent introduction to Linux networking, please read the Linux Networking HOWTO, available from
      If you say Y here and also to "/proc file system support" and "Sysctl support" below, you can change various aspects of the behavior of the TCP/IP code by writing to the (virtual) files in /proc/sys/net/ipv4/*; the options are explained in the file <file:Documentation/networking/ip-sysctl.txt>.
    • Short answer: say Y.
  • Option: Guide to configuring the linux kernel/net/ipv4
  • Option: Guide to configuring the linux kernel/net/ipv6
  • Option: NETFILTER
    • bool "Network packet filtering (replaces ipchains)"
      Netfilter is a framework for filtering and mangling network packets that pass through your Linux box.
      The most common use of packet filtering is to run your Linux box as a firewall protecting a local network from the Internet. The type of firewall provided by this kernel support is called a "packet filter", which means that it can reject individual network packets based on type, source, destination etc. The other kind of firewall, a "proxy-based" one, is more secure but more intrusive and more bothersome to set up; it inspects the network traffic much more closely, modifies it and has knowledge about the higher level protocols, which a packet filter lacks. Moreover, proxy-based firewalls often require changes to the programs running on the local clients. Proxy-based firewalls don't need support by the kernel, but they are often combined with a packet filter, which only works if you say Y here.
      You should also say Y here if you intend to use your Linux box as the gateway to the Internet for a local network of machines without globally valid IP addresses. This is called "masquerading": if one of the computers on your local network wants to send something to the outside, your box can "masquerade" as that computer, i.e. it forwards the traffic to the intended outside destination, but modifies the packets to make it look like they came from the firewall box itself. It works both ways: if the outside host replies, the Linux box will silently forward the traffic to the correct local computer. This way, the computers on your local net are completely invisible to the outside world, even though they can reach the outside and can receive replies. It is even possible to run globally visible servers from within a masqueraded local network using a mechanism called portforwarding. Masquerading is also often called NAT (Network Address Translation).
      Another use of Netfilter is in transparent proxying: if a machine on the local network tries to connect to an outside host, your Linux box can transparently forward the traffic to a local server, typically a caching proxy server.
      Yet another use of Netfilter is building a bridging firewall. Using a bridge with Network packet filtering enabled makes iptables "see" the bridged traffic. For filtering on the lower network and Ethernet protocols over the bridge, use ebtables (under bridge netfilter configuration).
      Various modules exist for netfilter which replace the previous masquerading (ipmasqadm), packet filtering (ipchains), transparent proxying, and portforwarding mechanisms. Please see <file:Documentation/Changes> under "iptables" for the location of these packages.
    • Make sure to say N to "Fast switching" below if you intend to say Y here, as Fast switching currently bypasses netfilter.
    • Chances are that you should say Y here if you compile a kernel which will run as a router and N for regular hosts. If unsure, say N.

    • bool "Network packet filtering debugging"
    • depends on NETFILTER
      You can say Y here if you want to get additional messages useful in debugging the netfilter code.

    • bool "Bridged IP/ARP packets filtering"
    • depends on BRIDGE && NETFILTER && INET
    • default y
      Enabling this option will let arptables resp. iptables see bridged ARP resp. IP traffic. If you want a bridging firewall, you probably want this option enabled. Enabling or disabling this option doesn't enable or disable ebtables.
    • If unsure, say N.

  • Option: NET_DIVERT
    • bool "Frame Diverter (EXPERIMENTAL)"
    • depends on EXPERIMENTAL
      The Frame Diverter allows you to divert packets from the network, that are not aimed at the interface receiving it (in promisc. mode). Typically, a Linux box setup as an Ethernet bridge with the Frames Diverter on, can do some *really* transparent www caching using a Squid proxy for example.
      This is very useful when you don't want to change your router's config (or if you simply don't have access to it).
      The other possible usages of diverting Ethernet Frames are numerous: - reroute smtp traffic to another interface - traffic-shape certain network streams - transparently proxy smtp connections
      For more informations, please refer to: <> <>
    • If unsure, say N.

Network testing

  • Option: NET_PKTGEN
    • tristate "Packet Generator (USE WITH CAUTION)"
    • depends on PROC_FS
      This module will inject preconfigured packets, at a configurable rate, out of a given interface. It is used for network interface stress testing and performance analysis.
    • If you don't understand what was just said, you don't need it: say N.
      Documentation on how to use the packet generator can be found at <file:Documentation/networking/pktgen.txt>.
    • To compile this code as a module, choose M here: the module will be called pktgen.

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