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How to configure the Linux kernel/net/ipv4

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Howto configure the Linux kernel / net / ipv4


IP configuration


  • Option: IP_MULTICAST
    • Kernel Versions: 2.6.15.6 ...
    • (on/off) IP: multicasting
      This is code for addressing several networked computers at once, enlarging your kernel by about 2 KB. You need multicasting if you intend to participate in the MBONE, a high bandwidth network on top of the Internet which carries audio and video broadcasts. More information about the MBONE is on the WWW at <http://www-itg.lbl.gov/mbone/>. Information about the multicast capabilities of the various network cards is contained in <file:Documentation/networking/multicast.txt>. For most people, it's safe to say N.


  • Option: IP_ADVANCED_ROUTER
    • Kernel Versions: 2.6.15.6 ...
    • (on/off) IP: advanced router
      If you intend to run your Linux box mostly as a router, i.e. as a computer that forwards and redistributes network packets, say Y; you will then be presented with several options that allow more precise control about the routing process.
      The answer to this question won't directly affect the kernel: answering N will just cause the configurator to skip all the questions about advanced routing.
      Note that your box can only act as a router if you enable IP forwarding in your kernel; you can do that by saying Y to "/proc file system support" and "Sysctl support" below and executing the line
      echo "1" > /proc/sys/net/ipv4/ip_forward
      at boot time after the /proc file system has been mounted.
      If you turn on IP forwarding, you will also get the rp_filter, which automatically rejects incoming packets if the routing table entry for their source address doesn't match the network interface they're arriving on. This has security advantages because it prevents the so-called IP spoofing, however it can pose problems if you use asymmetric routing (packets from you to a host take a different path than packets from that host to you) or if you operate a non-routing host which has several IP addresses on different interfaces. To turn rp_filter off use:
      echo 0 > /proc/sys/net/ipv4/conf/<device>/rp_filter or echo 0 > /proc/sys/net/ipv4/conf/all/rp_filter
      If unsure, say N here.


"Choose IP: FIB lookup algorithm (choose FIB_HASH if unsure)"

    • depends on IP_ADVANCED_ROUTER
    • default ASK_IP_FIB_HASH


  • Option: ASK_IP_FIB_HASH
    • Kernel Versions: 2.6.15.6 ...
    • (on/off) FIB_HASH

FIB is very proven and good enough for most users.


  • Option: IP_FIB_TRIE
    • Kernel Versions: 2.6.15.6 ...
    • (on/off) FIB_TRIE

new experimental LC-trie as FIB lookup algorithm. This improves lookup performance if you have a large of routes.

-trie is a longest matching prefix lookup algorithm which better than FIB_HASH for large routing tables. , it consumes more memory and is more complex.

-trie is described in: -address lookup using LC-tries. Stefan Nilsson and Gunnar KarlssonIEEE Journal on Selected Areas in Communications, 17(6):1083-1092, June 1999 experimental study of compression methods for dynamic triesStefan Nilsson and Matti Tikkanen. Algorithmica, 33(1):19-33, 2002.http://www.nada.kth.se/~snilsson/public/papers/dyntrie2/



  • Option: IP_FIB_HASH
    • Kernel Versions: 2.6.15.6 ...

_bool ASK_IP_FIB_HASH || !IP_ADVANCED_ROUTER


  • Option: IP_MULTIPLE_TABLES


  • Option: IP_ROUTE_FWMARK
    • Kernel Versions: 2.6.15.6 ...
    • (on/off) IP: use netfilter MARK value as routing key
    • depends on IP_MULTIPLE_TABLES && NETFILTER
      If you say Y here, you will be able to specify different routes for packets with different mark values (see iptables(8), MARK target).


  • Option: IP_ROUTE_MULTIPATH
    • Kernel Versions: 2.6.15.6 ...
    • (on/off) IP: equal cost multipath
    • depends on IP_ADVANCED_ROUTER
      Normally, the routing tables specify a single action to be taken in a deterministic manner for a given packet. If you say Y here however, it becomes possible to attach several actions to a packet pattern, in effect specifying several alternative paths to travel for those packets. The router considers all these paths to be of equal "cost" and chooses one of them in a non-deterministic fashion if a matching packet arrives.


  • Option: IP_ROUTE_MULTIPATH_CACHED
    • Kernel Versions: 2.6.15.6 ...
    • (on/off) IP: equal cost multipath with caching support (EXPERIMENTAL)
    • depends on IP_ROUTE_MULTIPATH
      Normally, equal cost multipath routing is not supported by the routing cache. If you say Y here, alternative routes are cached and on cache lookup a route is chosen in a configurable fashion.
      If unsure, say N.


  • Option: IP_ROUTE_MULTIPATH_RR
    • Kernel Versions: 2.6.15.6 ...
    • (on/off/module) MULTIPATH: round robin algorithm
    • depends on IP_ROUTE_MULTIPATH_CACHED
      Mulitpath routes are chosen according to Round Robin


  • Option: IP_ROUTE_MULTIPATH_RANDOM
    • Kernel Versions: 2.6.15.6 ...
    • (on/off/module) MULTIPATH: random algorithm
    • depends on IP_ROUTE_MULTIPATH_CACHED
      Multipath routes are chosen in a random fashion. Actually, there is no weight for a route. The advantage of this policy is that it is implemented stateless and therefore introduces only a very small delay.


  • Option: IP_ROUTE_MULTIPATH_WRANDOM
    • Kernel Versions: 2.6.15.6 ...
    • (on/off/module) MULTIPATH: weighted random algorithm
    • depends on IP_ROUTE_MULTIPATH_CACHED
      Multipath routes are chosen in a weighted random fashion. The per route weights are the weights visible via ip route 2. As the corresponding state management introduces some overhead routing delay is increased.


  • Option: IP_ROUTE_MULTIPATH_DRR
    • Kernel Versions: 2.6.15.6 ...
    • (on/off/module) MULTIPATH: interface round robin algorithm
    • depends on IP_ROUTE_MULTIPATH_CACHED
      Connections are distributed in a round robin fashion over the available interfaces. This policy makes sense if the connections should be primarily distributed on interfaces and not on routes.


  • Option: IP_ROUTE_VERBOSE
    • Kernel Versions: 2.6.15.6 ...
    • (on/off) IP: verbose route monitoring
    • depends on IP_ADVANCED_ROUTER
      If you say Y here, which is recommended, then the kernel will print verbose messages regarding the routing, for example warnings about received packets which look strange and could be evidence of an attack or a misconfigured system somewhere. The information is handled by the klogd daemon which is responsible for kernel messages ("man klogd").


  • Option: IP_PNP
    • Kernel Versions: 2.6.15.6 ...
    • (on/off) IP: kernel level autoconfiguration
      This enables automatic configuration of IP addresses of devices and of the routing table during kernel boot, based on either information supplied on the kernel command line or by BOOTP or RARP protocols. You need to say Y only for diskless machines requiring network access to boot (in which case you want to say Y to "Root file system on NFS" as well), because all other machines configure the network in their startup scripts.


  • Option: IP_PNP_DHCP
    • Kernel Versions: 2.6.15.6 ...
    • (on/off) IP: DHCP support
    • depends on IP_PNP
      If you want your Linux box to mount its whole root file system (the one containing the directory /) from some other computer over the net via NFS and you want the IP address of your computer to be discovered automatically at boot time using the DHCP protocol (a special protocol designed for doing this job), say Y here. In case the boot ROM of your network card was designed for booting Linux and does DHCP itself, providing all necessary information on the kernel command line, you can say N here.
      If unsure, say Y. Note that if you want to use DHCP, a DHCP server must be operating on your network. Read <file:Documentation/nfsroot.txt> for details.


  • Option: IP_PNP_BOOTP
    • Kernel Versions: 2.6.15.6 ...
    • (on/off) IP: BOOTP support
    • depends on IP_PNP
      If you want your Linux box to mount its whole root file system (the one containing the directory /) from some other computer over the net via NFS and you want the IP address of your computer to be discovered automatically at boot time using the BOOTP protocol (a special protocol designed for doing this job), say Y here. In case the boot ROM of your network card was designed for booting Linux and does BOOTP itself, providing all necessary information on the kernel command line, you can say N here. If unsure, say Y. Note that if you want to use BOOTP, a BOOTP server must be operating on your network. Read <file:Documentation/nfsroot.txt> for details.


  • Option: IP_PNP_RARP
    • Kernel Versions: 2.6.15.6 ...
    • (on/off) IP: RARP support
    • depends on IP_PNP
      If you want your Linux box to mount its whole root file system (the one containing the directory /) from some other computer over the net via NFS and you want the IP address of your computer to be discovered automatically at boot time using the RARP protocol (an older protocol which is being obsoleted by BOOTP and DHCP), say Y here. Note that if you want to use RARP, a RARP server must be operating on your network. Read <file:Documentation/nfsroot.txt> for details.
not yet ready..
bool ' IP: ARP support' CONFIG_IP_PNP_ARP


  • Option: NET_IPIP
    • Kernel Versions: 2.6.15.6 ...
    • (on/off/module) IP: tunneling
      Tunneling means encapsulating data of one protocol type within another protocol and sending it over a channel that understands the encapsulating protocol. This particular tunneling driver implements encapsulation of IP within IP, which sounds kind of pointless, but can be useful if you want to make your (or some other) machine appear on a different network than it physically is, or to use mobile-IP facilities (allowing laptops to seamlessly move between networks without changing their IP addresses).
      Saying Y to this option will produce two modules ( = code which can be inserted in and removed from the running kernel whenever you want). Most people won't need this and can say N.


  • Option: NET_IPGRE
    • Kernel Versions: 2.6.15.6 ...
    • (on/off/module) IP: GRE tunnels over IP
      Tunneling means encapsulating data of one protocol type within another protocol and sending it over a channel that understands the encapsulating protocol. This particular tunneling driver implements GRE (Generic Routing Encapsulation) and at this time allows encapsulating of IPv4 or IPv6 over existing IPv4 infrastructure. This driver is useful if the other endpoint is a Cisco router: Cisco likes GRE much better than the other Linux tunneling driver ("IP tunneling" above). In addition, GRE allows multicast redistribution through the tunnel.


  • Option: NET_IPGRE_BROADCAST
    • Kernel Versions: 2.6.15.6 ...
    • (on/off) IP: broadcast GRE over IP
    • depends on IP_MULTICAST && NET_IPGRE
      One application of GRE/IP is to construct a broadcast WAN (Wide Area Network), which looks like a normal Ethernet LAN (Local Area Network), but can be distributed all over the Internet. If you want to do that, say Y here and to "IP multicast routing" below.


  • Option: IP_MROUTE
    • Kernel Versions: 2.6.15.6 ...
    • (on/off) IP: multicast routing
    • depends on IP_MULTICAST
      This is used if you want your machine to act as a router for IP packets that have several destination addresses. It is needed on the MBONE, a high bandwidth network on top of the Internet which carries audio and video broadcasts. In order to do that, you would most likely run the program mrouted. Information about the multicast capabilities of the various network cards is contained in <file:Documentation/networking/multicast.txt>. If you haven't heard about it, you don't need it.


  • Option: IP_PIMSM_V1
    • Kernel Versions: 2.6.15.6 ...
    • (on/off) IP: PIM-SM version 1 support
    • depends on IP_MROUTE
      Kernel side support for Sparse Mode PIM (Protocol Independent Multicast) version 1. This multicast routing protocol is used widely because Cisco supports it. You need special software to use it (pimd-v1). Please see <http://netweb.usc.edu/pim/> for more information about PIM.
      Say Y if you want to use PIM-SM v1. Note that you can say N here if you just want to use Dense Mode PIM.


  • Option: IP_PIMSM_V2
    • Kernel Versions: 2.6.15.6 ...
    • (on/off) IP: PIM-SM version 2 support
    • depends on IP_MROUTE
      Kernel side support for Sparse Mode PIM version 2. In order to use this, you need an experimental routing daemon supporting it (pimd or gated-5). This routing protocol is not used widely, so say N unless you want to play with it.


  • Option: ARPD
    • Kernel Versions: 2.6.15.6 ...
    • (on/off) IP: ARP daemon support (EXPERIMENTAL)
    • depends on EXPERIMENTAL
      Normally, the kernel maintains an internal cache which maps IP addresses to hardware addresses on the local network, so that Ethernet/Token Ring/ etc. frames are sent to the proper address on the physical networking layer. For small networks having a few hundred directly connected hosts or less, keeping this address resolution (ARP) cache inside the kernel works well. However, maintaining an internal ARP cache does not work well for very large switched networks, and will use a lot of kernel memory if TCP/IP connections are made to many machines on the network.
      If you say Y here, the kernel's internal ARP cache will never grow to more than 256 entries (the oldest entries are expired in a LIFO manner) and communication will be attempted with the user space ARP daemon arpd. Arpd then answers the address resolution request either from its own cache or by asking the net.
      This code is experimental and also obsolete. If you want to use it, you need to find a version of the daemon arpd on the net somewhere, and you should also say Y to "Kernel/User network link driver", below. If unsure, say N.


  • Option: SYN_COOKIES
    • Kernel Versions: 2.6.15.6 ...
    • (on/off) IP: TCP syncookie support (disabled per default)
      Normal TCP/IP networking is open to an attack known as "SYN flooding". This denial-of-service attack prevents legitimate remote users from being able to connect to your computer during an ongoing attack and requires very little work from the attacker, who can operate from anywhere on the Internet.
      SYN cookies provide protection against this type of attack. If you say Y here, the TCP/IP stack will use a cryptographic challenge protocol known as "SYN cookies" to enable legitimate users to continue to connect, even when your machine is under attack. There is no need for the legitimate users to change their TCP/IP software; SYN cookies work transparently to them. For technical information about SYN cookies, check out <http://cr.yp.to/syncookies.html>.
      If you are SYN flooded, the source address reported by the kernel is likely to have been forged by the attacker; it is only reported as an aid in tracing the packets to their actual source and should not be taken as absolute truth.
      SYN cookies may prevent correct error reporting on clients when the server is really overloaded. If this happens frequently better turn them off.
      If you say Y here, note that SYN cookies aren't enabled by default; you can enable them by saying Y to "/proc file system support" and Sysctl support below and executing the command
      echo 1 >/proc/sys/net/ipv4/tcp_syncookies
      at boot time after the /proc file system has been mounted.
      If unsure, say N.


  • Option: INET_AH
    • Kernel Versions: 2.6.15.6 ...
    • (on/off/module) IP: AH transformation
    • select XFRM
    • select CRYPTO
    • select CRYPTO_HMAC
    • select CRYPTO_MD5
    • select CRYPTO_SHA1
      Support for IPsec AH.
      If unsure, say Y.


  • Option: INET_ESP
    • Kernel Versions: 2.6.15.6 ...
    • (on/off/module) IP: ESP transformation
    • select XFRM
    • select CRYPTO
    • select CRYPTO_HMAC
    • select CRYPTO_MD5
    • select CRYPTO_SHA1
    • select CRYPTO_DES
      Support for IPsec ESP.
      If unsure, say Y.


  • Option: INET_IPCOMP
    • Kernel Versions: 2.6.15.6 ...
    • (on/off/module) IP: IPComp transformation
    • select XFRM
    • select INET_TUNNEL
    • select CRYPTO
    • select CRYPTO_DEFLATE
      Support for IP Payload Compression Protocol (IPComp) (RFC3173), typically needed for IPsec. If unsure, say Y.


  • Option: INET_TUNNEL
    • Kernel Versions: 2.6.15.6 ...
    • (on/off/module) IP: tunnel transformation
    • select XFRM
      Support for generic IP tunnel transformation, which is required by the IP tunneling module as well as tunnel mode IPComp. If unsure, say Y.


  • Option: INET_DIAG
    • Kernel Versions: 2.6.15.6 ...
    • (on/off/module) INET: socket monitoring interface
    • default y
      Support for INET (TCP, DCCP, etc.) socket monitoring interface used by native Linux tools such as ss. ss is included in iproute2, currently downloadable at <http://developer.osdl.org/dev/iproute2>. If unsure, say Y.


  • Option: INET_TCP_DIAG
    • Kernel Versions: 2.6.15.6 ...
    • depends on INET_DIAG

_tristate INET_DIAG


  • Option: TCP_CONG_ADVANCED
    • Kernel Versions: 2.6.15.6 ...
    • (on/off) TCP: advanced congestion control
      Support for selection of various TCP congestion control modules.
      Nearly all users can safely say no here, and a safe default selection will be made (BIC-TCP with new Reno as a fallback).
      If unsure, say N.
TCP Reno is builtin (required as fallback)

TCP congestion controlEdit

    • depends on TCP_CONG_ADVANCED


  • Option: TCP_CONG_BIC
    • Kernel Versions: 2.6.15.6 ...
    • (on/off/module) Binary Increase Congestion (BIC) control
    • default y

-TCP is a sender-side only change that ensures a linear RTT under large windows while offering both scalability and TCP-friendliness. The protocol combines two schemes additive increase and binary search increase. When the window is large, additive increase with a large ensures linear RTT fairness as well as good . Under small congestion windows, binary search provides TCP friendliness. http://www.csc.ncsu.edu/faculty/rhee/export/bitcp/


  • Option: TCP_CONG_WESTWOOD
    • Kernel Versions: 2.6.15.6 ...
    • (on/off/module) TCP Westwood+
    • default m

Westwood+ is a sender-side only modification of the TCP Reno stack that optimizes the performance of TCP congestion . It is based on end-to-end bandwidth estimation to set window and slow start threshold after a congestion . Using this estimation, TCP Westwood+ adaptively sets a start threshold and a congestion window which takes into the bandwidth used at the time congestion is experienced. Westwood+ significantly increases fairness wrt TCP Reno in networks and throughput over wireless links.


  • Option: TCP_CONG_HTCP
    • Kernel Versions: 2.6.15.6 ... tristate H-TCP default m

-TCP is a send-side only modifications of the TCP Reno stack that optimizes the performance of TCP control for high speed network links. It uses a to change the alpha and beta parameters of TCP Reno on network conditions and in a way so as to be fair with Reno and H-TCP flows.


  • Option: TCP_CONG_HSTCP
    • Kernel Versions: 2.6.15.6 ...
    • (on/off/module) High Speed TCP
    • depends on EXPERIMENTAL
    • default n

Floyd's High Speed TCP (RFC 3649) congestion control. modification to TCP's congestion control mechanism for use large congestion windows. A table indicates how much to the congestion window by when an ACK is received.For more detailsee http://www.icir.org/floyd/hstcp.html


  • Option: TCP_CONG_HYBLA
    • Kernel Versions: 2.6.15.6 ...
    • (on/off/module) TCP-Hybla congestion control algorithm
    • depends on EXPERIMENTAL
    • default n

-Hybla is a sender-side only change that eliminates penalization of -RTT, large-bandwidth connections, like when satellite legs are , especially when sharing a common bottleneck with normal connections.


  • Option: TCP_CONG_VEGAS
    • Kernel Versions: 2.6.15.6 ...
    • (on/off/module) TCP Vegas
    • depends on EXPERIMENTAL
    • default n

Vegas is a sender-side only change to TCP that anticipates onset of congestion by estimating the bandwidth. TCP Vegas the sending rate by modifying the congestion . TCP Vegas should provide less packet loss, but it is as aggressive as TCP Reno.


  • Option: TCP_CONG_SCALABLE
    • Kernel Versions: 2.6.15.6 ...
    • (on/off/module) Scalable TCP
    • depends on EXPERIMENTAL
    • default n

TCP is a sender-side only change to TCP which uses a congestion control algorithm which has some nice scaling , though is known to have fairness issues. http://www-lce.eng.cam.ac.uk/~ctk21/scalable/



  • Option: TCP_CONG_BIC
    • Kernel Versions: 2.6.15.6 ...
    • (on/off/module)
    • depends on !TCP_CONG_ADVANCED
    • default y



Linux Kernel Configuration

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