Merge branch 'upstream-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/jgarzi...

[linux-2.6] / Documentation / networking / ip-sysctl.txt

/proc/sys/net/ipv4/* Variables:

ip_forward - BOOLEAN
        0 - disabled (default)
        not 0 - enabled 

        Forward Packets between interfaces.

        This variable is special, its change resets all configuration
        parameters to their default state (RFC1122 for hosts, RFC1812
        for routers)

ip_default_ttl - INTEGER
        default 64

ip_no_pmtu_disc - BOOLEAN
        Disable Path MTU Discovery.
        default FALSE

min_pmtu - INTEGER
        default 562 - minimum discovered Path MTU

mtu_expires - INTEGER
        Time, in seconds, that cached PMTU information is kept.

min_adv_mss - INTEGER
        The advertised MSS depends on the first hop route MTU, but will
        never be lower than this setting.

IP Fragmentation:

ipfrag_high_thresh - INTEGER
        Maximum memory used to reassemble IP fragments. When 
        ipfrag_high_thresh bytes of memory is allocated for this purpose,
        the fragment handler will toss packets until ipfrag_low_thresh
        is reached.
        
ipfrag_low_thresh - INTEGER
        See ipfrag_high_thresh  

ipfrag_time - INTEGER
        Time in seconds to keep an IP fragment in memory.       

ipfrag_secret_interval - INTEGER
        Regeneration interval (in seconds) of the hash secret (or lifetime 
        for the hash secret) for IP fragments.
        Default: 600

ipfrag_max_dist - INTEGER
        ipfrag_max_dist is a non-negative integer value which defines the 
        maximum "disorder" which is allowed among fragments which share a 
        common IP source address. Note that reordering of packets is 
        not unusual, but if a large number of fragments arrive from a source 
        IP address while a particular fragment queue remains incomplete, it 
        probably indicates that one or more fragments belonging to that queue 
        have been lost. When ipfrag_max_dist is positive, an additional check 
        is done on fragments before they are added to a reassembly queue - if 
        ipfrag_max_dist (or more) fragments have arrived from a particular IP 
        address between additions to any IP fragment queue using that source 
        address, it's presumed that one or more fragments in the queue are 
        lost. The existing fragment queue will be dropped, and a new one 
        started. An ipfrag_max_dist value of zero disables this check.

        Using a very small value, e.g. 1 or 2, for ipfrag_max_dist can
        result in unnecessarily dropping fragment queues when normal
        reordering of packets occurs, which could lead to poor application 
        performance. Using a very large value, e.g. 50000, increases the 
        likelihood of incorrectly reassembling IP fragments that originate 
        from different IP datagrams, which could result in data corruption.
        Default: 64

INET peer storage:

inet_peer_threshold - INTEGER
        The approximate size of the storage.  Starting from this threshold      
        entries will be thrown aggressively.  This threshold also determines
        entries' time-to-live and time intervals between garbage collection
        passes.  More entries, less time-to-live, less GC interval.

inet_peer_minttl - INTEGER
        Minimum time-to-live of entries.  Should be enough to cover fragment
        time-to-live on the reassembling side.  This minimum time-to-live  is
        guaranteed if the pool size is less than inet_peer_threshold.
        Measured in jiffies(1).

inet_peer_maxttl - INTEGER
        Maximum time-to-live of entries.  Unused entries will expire after
        this period of time if there is no memory pressure on the pool (i.e.
        when the number of entries in the pool is very small).
        Measured in jiffies(1).

inet_peer_gc_mintime - INTEGER
        Minimum interval between garbage collection passes.  This interval is
        in effect under high memory pressure on the pool.
        Measured in jiffies(1).

inet_peer_gc_maxtime - INTEGER
        Minimum interval between garbage collection passes.  This interval is
        in effect under low (or absent) memory pressure on the pool.
        Measured in jiffies(1).

TCP variables: 

somaxconn - INTEGER
        Limit of socket listen() backlog, known in userspace as SOMAXCONN.
        Defaults to 128.  See also tcp_max_syn_backlog for additional tuning
        for TCP sockets.

tcp_abc - INTEGER
        Controls Appropriate Byte Count (ABC) defined in RFC3465.
        ABC is a way of increasing congestion window (cwnd) more slowly
        in response to partial acknowledgments.
        Possible values are:
                0 increase cwnd once per acknowledgment (no ABC)
                1 increase cwnd once per acknowledgment of full sized segment
                2 allow increase cwnd by two if acknowledgment is
                  of two segments to compensate for delayed acknowledgments.
        Default: 0 (off)

tcp_abort_on_overflow - BOOLEAN
        If listening service is too slow to accept new connections,
        reset them. Default state is FALSE. It means that if overflow
        occurred due to a burst, connection will recover. Enable this
        option _only_ if you are really sure that listening daemon
        cannot be tuned to accept connections faster. Enabling this
        option can harm clients of your server.

tcp_adv_win_scale - INTEGER
        Count buffering overhead as bytes/2^tcp_adv_win_scale
        (if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale),
        if it is <= 0.
        Default: 2

tcp_allowed_congestion_control - STRING
        Show/set the congestion control choices available to non-privileged
        processes. The list is a subset of those listed in
        tcp_available_congestion_control.
        Default is "reno" and the default setting (tcp_congestion_control).

tcp_app_win - INTEGER
        Reserve max(window/2^tcp_app_win, mss) of window for application
        buffer. Value 0 is special, it means that nothing is reserved.
        Default: 31

tcp_available_congestion_control - STRING
        Shows the available congestion control choices that are registered.
        More congestion control algorithms may be available as modules,
        but not loaded.

tcp_base_mss - INTEGER
        The initial value of search_low to be used by Packetization Layer
        Path MTU Discovery (MTU probing).  If MTU probing is enabled,
        this is the inital MSS used by the connection.

tcp_congestion_control - STRING
        Set the congestion control algorithm to be used for new
        connections. The algorithm "reno" is always available, but
        additional choices may be available based on kernel configuration.
        Default is set as part of kernel configuration.

tcp_dsack - BOOLEAN
        Allows TCP to send "duplicate" SACKs.

tcp_ecn - BOOLEAN
        Enable Explicit Congestion Notification in TCP.

tcp_fack - BOOLEAN
        Enable FACK congestion avoidance and fast retransmission.
        The value is not used, if tcp_sack is not enabled.

tcp_fin_timeout - INTEGER
        Time to hold socket in state FIN-WAIT-2, if it was closed
        by our side. Peer can be broken and never close its side,
        or even died unexpectedly. Default value is 60sec.
        Usual value used in 2.2 was 180 seconds, you may restore
        it, but remember that if your machine is even underloaded WEB server,
        you risk to overflow memory with kilotons of dead sockets,
        FIN-WAIT-2 sockets are less dangerous than FIN-WAIT-1,
        because they eat maximum 1.5K of memory, but they tend
        to live longer. Cf. tcp_max_orphans.

tcp_frto - INTEGER
        Enables Forward RTO-Recovery (F-RTO) defined in RFC4138.
        F-RTO is an enhanced recovery algorithm for TCP retransmission
        timeouts.  It is particularly beneficial in wireless environments
        where packet loss is typically due to random radio interference
        rather than intermediate router congestion.  F-RTO is sender-side
        only modification.  Therefore it does not require any support from
        the peer, but in a typical case, however, where wireless link is
        the local access link and most of the data flows downlink, the
        faraway servers should have F-RTO enabled to take advantage of it.
        If set to 1, basic version is enabled.  2 enables SACK enhanced
        F-RTO if flow uses SACK.  The basic version can be used also when
        SACK is in use though scenario(s) with it exists where F-RTO
        interacts badly with the packet counting of the SACK enabled TCP
        flow.

tcp_frto_response - INTEGER
        When F-RTO has detected that a TCP retransmission timeout was
        spurious (i.e, the timeout would have been avoided had TCP set a
        longer retransmission timeout), TCP has several options what to do
        next. Possible values are:
                0 Rate halving based; a smooth and conservative response,
                  results in halved cwnd and ssthresh after one RTT
                1 Very conservative response; not recommended because even
                  though being valid, it interacts poorly with the rest of
                  Linux TCP, halves cwnd and ssthresh immediately
                2 Aggressive response; undoes congestion control measures
                  that are now known to be unnecessary (ignoring the
                  possibility of a lost retransmission that would require
                  TCP to be more cautious), cwnd and ssthresh are restored
                  to the values prior timeout
        Default: 0 (rate halving based)

tcp_keepalive_time - INTEGER
        How often TCP sends out keepalive messages when keepalive is enabled.
        Default: 2hours.

tcp_keepalive_probes - INTEGER
        How many keepalive probes TCP sends out, until it decides that the
        connection is broken. Default value: 9.

tcp_keepalive_intvl - INTEGER
        How frequently the probes are send out. Multiplied by
        tcp_keepalive_probes it is time to kill not responding connection,
        after probes started. Default value: 75sec i.e. connection
        will be aborted after ~11 minutes of retries.

tcp_low_latency - BOOLEAN
        If set, the TCP stack makes decisions that prefer lower
        latency as opposed to higher throughput.  By default, this
        option is not set meaning that higher throughput is preferred.
        An example of an application where this default should be
        changed would be a Beowulf compute cluster.
        Default: 0

tcp_max_orphans - INTEGER
        Maximal number of TCP sockets not attached to any user file handle,
        held by system. If this number is exceeded orphaned connections are
        reset immediately and warning is printed. This limit exists
        only to prevent simple DoS attacks, you _must_ not rely on this
        or lower the limit artificially, but rather increase it
        (probably, after increasing installed memory),
        if network conditions require more than default value,
        and tune network services to linger and kill such states
        more aggressively. Let me to remind again: each orphan eats
        up to ~64K of unswappable memory.

tcp_max_syn_backlog - INTEGER
        Maximal number of remembered connection requests, which are
        still did not receive an acknowledgment from connecting client.
        Default value is 1024 for systems with more than 128Mb of memory,
        and 128 for low memory machines. If server suffers of overload,
        try to increase this number.

tcp_max_tw_buckets - INTEGER
        Maximal number of timewait sockets held by system simultaneously.
        If this number is exceeded time-wait socket is immediately destroyed
        and warning is printed. This limit exists only to prevent
        simple DoS attacks, you _must_ not lower the limit artificially,
        but rather increase it (probably, after increasing installed memory),
        if network conditions require more than default value.

tcp_mem - vector of 3 INTEGERs: min, pressure, max
        min: below this number of pages TCP is not bothered about its
        memory appetite.

        pressure: when amount of memory allocated by TCP exceeds this number
        of pages, TCP moderates its memory consumption and enters memory
        pressure mode, which is exited when memory consumption falls
        under "min".

        max: number of pages allowed for queueing by all TCP sockets.

        Defaults are calculated at boot time from amount of available
        memory.

tcp_moderate_rcvbuf - BOOLEAN
        If set, TCP performs receive buffer autotuning, attempting to
        automatically size the buffer (no greater than tcp_rmem[2]) to
        match the size required by the path for full throughput.  Enabled by
        default.

tcp_mtu_probing - INTEGER
        Controls TCP Packetization-Layer Path MTU Discovery.  Takes three
        values:
          0 - Disabled
          1 - Disabled by default, enabled when an ICMP black hole detected
          2 - Always enabled, use initial MSS of tcp_base_mss.

tcp_no_metrics_save - BOOLEAN
        By default, TCP saves various connection metrics in the route cache
        when the connection closes, so that connections established in the
        near future can use these to set initial conditions.  Usually, this
        increases overall performance, but may sometimes cause performance
        degradation.  If set, TCP will not cache metrics on closing
        connections.

tcp_orphan_retries - INTEGER
        How may times to retry before killing TCP connection, closed
        by our side. Default value 7 corresponds to ~50sec-16min
        depending on RTO. If you machine is loaded WEB server,
        you should think about lowering this value, such sockets
        may consume significant resources. Cf. tcp_max_orphans.

tcp_reordering - INTEGER
        Maximal reordering of packets in a TCP stream.
        Default: 3      

tcp_retrans_collapse - BOOLEAN
        Bug-to-bug compatibility with some broken printers.
        On retransmit try to send bigger packets to work around bugs in
        certain TCP stacks.

tcp_retries1 - INTEGER
        How many times to retry before deciding that something is wrong
        and it is necessary to report this suspicion to network layer.
        Minimal RFC value is 3, it is default, which corresponds
        to ~3sec-8min depending on RTO.

tcp_retries2 - INTEGER
        How may times to retry before killing alive TCP connection.
        RFC1122 says that the limit should be longer than 100 sec.
        It is too small number. Default value 15 corresponds to ~13-30min
        depending on RTO.

tcp_rfc1337 - BOOLEAN
        If set, the TCP stack behaves conforming to RFC1337. If unset,
        we are not conforming to RFC, but prevent TCP TIME_WAIT
        assassination.
        Default: 0

tcp_rmem - vector of 3 INTEGERs: min, default, max
        min: Minimal size of receive buffer used by TCP sockets.
        It is guaranteed to each TCP socket, even under moderate memory
        pressure.
        Default: 8K

        default: default size of receive buffer used by TCP sockets.
        This value overrides net.core.rmem_default used by other protocols.
        Default: 87380 bytes. This value results in window of 65535 with
        default setting of tcp_adv_win_scale and tcp_app_win:0 and a bit
        less for default tcp_app_win. See below about these variables.

        max: maximal size of receive buffer allowed for automatically
        selected receiver buffers for TCP socket. This value does not override
        net.core.rmem_max, "static" selection via SO_RCVBUF does not use this.
        Default: 87380*2 bytes.

tcp_sack - BOOLEAN
        Enable select acknowledgments (SACKS).

tcp_slow_start_after_idle - BOOLEAN
        If set, provide RFC2861 behavior and time out the congestion
        window after an idle period.  An idle period is defined at
        the current RTO.  If unset, the congestion window will not
        be timed out after an idle period.
        Default: 1

tcp_stdurg - BOOLEAN
        Use the Host requirements interpretation of the TCP urg pointer field.
        Most hosts use the older BSD interpretation, so if you turn this on
        Linux might not communicate correctly with them.
        Default: FALSE

tcp_synack_retries - INTEGER
        Number of times SYNACKs for a passive TCP connection attempt will
        be retransmitted. Should not be higher than 255. Default value
        is 5, which corresponds to ~180seconds.

tcp_syncookies - BOOLEAN
        Only valid when the kernel was compiled with CONFIG_SYNCOOKIES
        Send out syncookies when the syn backlog queue of a socket
        overflows. This is to prevent against the common 'syn flood attack'
        Default: FALSE

        Note, that syncookies is fallback facility.
        It MUST NOT be used to help highly loaded servers to stand
        against legal connection rate. If you see synflood warnings
        in your logs, but investigation shows that they occur
        because of overload with legal connections, you should tune
        another parameters until this warning disappear.
        See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.

        syncookies seriously violate TCP protocol, do not allow
        to use TCP extensions, can result in serious degradation
        of some services (f.e. SMTP relaying), visible not by you,
        but your clients and relays, contacting you. While you see
        synflood warnings in logs not being really flooded, your server
        is seriously misconfigured.

tcp_syn_retries - INTEGER
        Number of times initial SYNs for an active TCP connection attempt
        will be retransmitted. Should not be higher than 255. Default value
        is 5, which corresponds to ~180seconds.

tcp_timestamps - BOOLEAN
        Enable timestamps as defined in RFC1323.

tcp_tso_win_divisor - INTEGER
        This allows control over what percentage of the congestion window
        can be consumed by a single TSO frame.
        The setting of this parameter is a choice between burstiness and
        building larger TSO frames.
        Default: 3

tcp_tw_recycle - BOOLEAN
        Enable fast recycling TIME-WAIT sockets. Default value is 0.
        It should not be changed without advice/request of technical
        experts.

tcp_tw_reuse - BOOLEAN
        Allow to reuse TIME-WAIT sockets for new connections when it is
        safe from protocol viewpoint. Default value is 0.
        It should not be changed without advice/request of technical
        experts.

tcp_window_scaling - BOOLEAN
        Enable window scaling as defined in RFC1323.

tcp_wmem - vector of 3 INTEGERs: min, default, max
        min: Amount of memory reserved for send buffers for TCP socket.
        Each TCP socket has rights to use it due to fact of its birth.
        Default: 4K

        default: Amount of memory allowed for send buffers for TCP socket
        by default. This value overrides net.core.wmem_default used
        by other protocols, it is usually lower than net.core.wmem_default.
        Default: 16K

        max: Maximal amount of memory allowed for automatically selected
        send buffers for TCP socket. This value does not override
        net.core.wmem_max, "static" selection via SO_SNDBUF does not use this.
        Default: 128K

tcp_workaround_signed_windows - BOOLEAN
        If set, assume no receipt of a window scaling option means the
        remote TCP is broken and treats the window as a signed quantity.
        If unset, assume the remote TCP is not broken even if we do
        not receive a window scaling option from them.
        Default: 0

tcp_dma_copybreak - INTEGER
        Lower limit, in bytes, of the size of socket reads that will be
        offloaded to a DMA copy engine, if one is present in the system
        and CONFIG_NET_DMA is enabled.
        Default: 4096

CIPSOv4 Variables:

cipso_cache_enable - BOOLEAN
        If set, enable additions to and lookups from the CIPSO label mapping
        cache.  If unset, additions are ignored and lookups always result in a
        miss.  However, regardless of the setting the cache is still
        invalidated when required when means you can safely toggle this on and
        off and the cache will always be "safe".
        Default: 1

cipso_cache_bucket_size - INTEGER
        The CIPSO label cache consists of a fixed size hash table with each
        hash bucket containing a number of cache entries.  This variable limits
        the number of entries in each hash bucket; the larger the value the
        more CIPSO label mappings that can be cached.  When the number of
        entries in a given hash bucket reaches this limit adding new entries
        causes the oldest entry in the bucket to be removed to make room.
        Default: 10

cipso_rbm_optfmt - BOOLEAN
        Enable the "Optimized Tag 1 Format" as defined in section 3.4.2.6 of
        the CIPSO draft specification (see Documentation/netlabel for details).
        This means that when set the CIPSO tag will be padded with empty
        categories in order to make the packet data 32-bit aligned.
        Default: 0

cipso_rbm_structvalid - BOOLEAN
        If set, do a very strict check of the CIPSO option when
        ip_options_compile() is called.  If unset, relax the checks done during
        ip_options_compile().  Either way is "safe" as errors are caught else
        where in the CIPSO processing code but setting this to 0 (False) should
        result in less work (i.e. it should be faster) but could cause problems
        with other implementations that require strict checking.
        Default: 0

IP Variables:

ip_local_port_range - 2 INTEGERS
        Defines the local port range that is used by TCP and UDP to
        choose the local port. The first number is the first, the 
        second the last local port number. Default value depends on
        amount of memory available on the system:
        > 128Mb 32768-61000
        < 128Mb 1024-4999 or even less.
        This number defines number of active connections, which this
        system can issue simultaneously to systems not supporting
        TCP extensions (timestamps). With tcp_tw_recycle enabled
        (i.e. by default) range 1024-4999 is enough to issue up to
        2000 connections per second to systems supporting timestamps.

ip_nonlocal_bind - BOOLEAN
        If set, allows processes to bind() to non-local IP addresses,
        which can be quite useful - but may break some applications.
        Default: 0

ip_dynaddr - BOOLEAN
        If set non-zero, enables support for dynamic addresses.
        If set to a non-zero value larger than 1, a kernel log
        message will be printed when dynamic address rewriting
        occurs.
        Default: 0

icmp_echo_ignore_all - BOOLEAN
        If set non-zero, then the kernel will ignore all ICMP ECHO
        requests sent to it.
        Default: 0

icmp_echo_ignore_broadcasts - BOOLEAN
        If set non-zero, then the kernel will ignore all ICMP ECHO and
        TIMESTAMP requests sent to it via broadcast/multicast.
        Default: 1

icmp_ratelimit - INTEGER
        Limit the maximal rates for sending ICMP packets whose type matches
        icmp_ratemask (see below) to specific targets.
        0 to disable any limiting, otherwise the maximal rate in jiffies(1)
        Default: 100

icmp_ratemask - INTEGER
        Mask made of ICMP types for which rates are being limited.
        Significant bits: IHGFEDCBA9876543210
        Default mask:     0000001100000011000 (6168)

        Bit definitions (see include/linux/icmp.h):
                0 Echo Reply
                3 Destination Unreachable *
                4 Source Quench *
                5 Redirect
                8 Echo Request
                B Time Exceeded *
                C Parameter Problem *
                D Timestamp Request
                E Timestamp Reply
                F Info Request
                G Info Reply
                H Address Mask Request
                I Address Mask Reply

        * These are rate limited by default (see default mask above)

icmp_ignore_bogus_error_responses - BOOLEAN
        Some routers violate RFC1122 by sending bogus responses to broadcast
        frames.  Such violations are normally logged via a kernel warning.
        If this is set to TRUE, the kernel will not give such warnings, which
        will avoid log file clutter.
        Default: FALSE

icmp_errors_use_inbound_ifaddr - BOOLEAN

        If zero, icmp error messages are sent with the primary address of
        the exiting interface.
 
        If non-zero, the message will be sent with the primary address of
        the interface that received the packet that caused the icmp error.
        This is the behaviour network many administrators will expect from
        a router. And it can make debugging complicated network layouts
        much easier. 

        Note that if no primary address exists for the interface selected,
        then the primary address of the first non-loopback interface that
        has one will be used regardless of this setting.

        Default: 0

igmp_max_memberships - INTEGER
        Change the maximum number of multicast groups we can subscribe to.
        Default: 20

conf/interface/*  changes special settings per interface (where "interface" is 
                  the name of your network interface)
conf/all/*        is special, changes the settings for all interfaces


log_martians - BOOLEAN
        Log packets with impossible addresses to kernel log.
        log_martians for the interface will be enabled if at least one of
        conf/{all,interface}/log_martians is set to TRUE,
        it will be disabled otherwise

accept_redirects - BOOLEAN
        Accept ICMP redirect messages.
        accept_redirects for the interface will be enabled if:
        - both conf/{all,interface}/accept_redirects are TRUE in the case forwarding
          for the interface is enabled
        or
        - at least one of conf/{all,interface}/accept_redirects is TRUE in the case
          forwarding for the interface is disabled
        accept_redirects for the interface will be disabled otherwise
        default TRUE (host)
                FALSE (router)

forwarding - BOOLEAN
        Enable IP forwarding on this interface.

mc_forwarding - BOOLEAN
        Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE
        and a multicast routing daemon is required.
        conf/all/mc_forwarding must also be set to TRUE to enable multicast routing
        for the interface

medium_id - INTEGER
        Integer value used to differentiate the devices by the medium they
        are attached to. Two devices can have different id values when
        the broadcast packets are received only on one of them.
        The default value 0 means that the device is the only interface
        to its medium, value of -1 means that medium is not known.
        
        Currently, it is used to change the proxy_arp behavior:
        the proxy_arp feature is enabled for packets forwarded between
        two devices attached to different media.

proxy_arp - BOOLEAN
        Do proxy arp.
        proxy_arp for the interface will be enabled if at least one of
        conf/{all,interface}/proxy_arp is set to TRUE,
        it will be disabled otherwise

shared_media - BOOLEAN
        Send(router) or accept(host) RFC1620 shared media redirects.
        Overrides ip_secure_redirects.
        shared_media for the interface will be enabled if at least one of
        conf/{all,interface}/shared_media is set to TRUE,
        it will be disabled otherwise
        default TRUE

secure_redirects - BOOLEAN
        Accept ICMP redirect messages only for gateways,
        listed in default gateway list.
        secure_redirects for the interface will be enabled if at least one of
        conf/{all,interface}/secure_redirects is set to TRUE,
        it will be disabled otherwise
        default TRUE

send_redirects - BOOLEAN
        Send redirects, if router.
        send_redirects for the interface will be enabled if at least one of
        conf/{all,interface}/send_redirects is set to TRUE,
        it will be disabled otherwise
        Default: TRUE

bootp_relay - BOOLEAN
        Accept packets with source address 0.b.c.d destined
        not to this host as local ones. It is supposed, that
        BOOTP relay daemon will catch and forward such packets.
        conf/all/bootp_relay must also be set to TRUE to enable BOOTP relay
        for the interface
        default FALSE
        Not Implemented Yet.

accept_source_route - BOOLEAN
        Accept packets with SRR option.
        conf/all/accept_source_route must also be set to TRUE to accept packets
        with SRR option on the interface
        default TRUE (router)
                FALSE (host)

rp_filter - BOOLEAN
        1 - do source validation by reversed path, as specified in RFC1812
            Recommended option for single homed hosts and stub network
            routers. Could cause troubles for complicated (not loop free)
            networks running a slow unreliable protocol (sort of RIP),
            or using static routes.

        0 - No source validation.

        conf/all/rp_filter must also be set to TRUE to do source validation
        on the interface

        Default value is 0. Note that some distributions enable it
        in startup scripts.

arp_filter - BOOLEAN
        1 - Allows you to have multiple network interfaces on the same
        subnet, and have the ARPs for each interface be answered
        based on whether or not the kernel would route a packet from
        the ARP'd IP out that interface (therefore you must use source
        based routing for this to work). In other words it allows control
        of which cards (usually 1) will respond to an arp request.

        0 - (default) The kernel can respond to arp requests with addresses
        from other interfaces. This may seem wrong but it usually makes
        sense, because it increases the chance of successful communication.
        IP addresses are owned by the complete host on Linux, not by
        particular interfaces. Only for more complex setups like load-
        balancing, does this behaviour cause problems.

        arp_filter for the interface will be enabled if at least one of
        conf/{all,interface}/arp_filter is set to TRUE,
        it will be disabled otherwise

arp_announce - INTEGER
        Define different restriction levels for announcing the local
        source IP address from IP packets in ARP requests sent on
        interface:
        0 - (default) Use any local address, configured on any interface
        1 - Try to avoid local addresses that are not in the target's
        subnet for this interface. This mode is useful when target
        hosts reachable via this interface require the source IP
        address in ARP requests to be part of their logical network
        configured on the receiving interface. When we generate the
        request we will check all our subnets that include the
        target IP and will preserve the source address if it is from
        such subnet. If there is no such subnet we select source
        address according to the rules for level 2.
        2 - Always use the best local address for this target.
        In this mode we ignore the source address in the IP packet
        and try to select local address that we prefer for talks with
        the target host. Such local address is selected by looking
        for primary IP addresses on all our subnets on the outgoing
        interface that include the target IP address. If no suitable
        local address is found we select the first local address
        we have on the outgoing interface or on all other interfaces,
        with the hope we will receive reply for our request and
        even sometimes no matter the source IP address we announce.

        The max value from conf/{all,interface}/arp_announce is used.

        Increasing the restriction level gives more chance for
        receiving answer from the resolved target while decreasing
        the level announces more valid sender's information.

arp_ignore - INTEGER
        Define different modes for sending replies in response to
        received ARP requests that resolve local target IP addresses:
        0 - (default): reply for any local target IP address, configured
        on any interface
        1 - reply only if the target IP address is local address
        configured on the incoming interface
        2 - reply only if the target IP address is local address
        configured on the incoming interface and both with the
        sender's IP address are part from same subnet on this interface
        3 - do not reply for local addresses configured with scope host,
        only resolutions for global and link addresses are replied
        4-7 - reserved
        8 - do not reply for all local addresses

        The max value from conf/{all,interface}/arp_ignore is used
        when ARP request is received on the {interface}

arp_accept - BOOLEAN
        Define behavior when gratuitous arp replies are received:
        0 - drop gratuitous arp frames
        1 - accept gratuitous arp frames

app_solicit - INTEGER
        The maximum number of probes to send to the user space ARP daemon
        via netlink before dropping back to multicast probes (see
        mcast_solicit).  Defaults to 0.

disable_policy - BOOLEAN
        Disable IPSEC policy (SPD) for this interface

disable_xfrm - BOOLEAN
        Disable IPSEC encryption on this interface, whatever the policy



tag - INTEGER
        Allows you to write a number, which can be used as required.
        Default value is 0.

(1) Jiffie: internal timeunit for the kernel. On the i386 1/100s, on the
Alpha 1/1024s. See the HZ define in /usr/include/asm/param.h for the exact
value on your system. 

Alexey Kuznetsov.
kuznet@ms2.inr.ac.ru

Updated by:
Andi Kleen
ak@muc.de
Nicolas Delon
delon.nicolas@wanadoo.fr




/proc/sys/net/ipv6/* Variables:

IPv6 has no global variables such as tcp_*.  tcp_* settings under ipv4/ also
apply to IPv6 [XXX?].

bindv6only - BOOLEAN
        Default value for IPV6_V6ONLY socket option,
        which restricts use of the IPv6 socket to IPv6 communication 
        only.
                TRUE: disable IPv4-mapped address feature
                FALSE: enable IPv4-mapped address feature

        Default: FALSE (as specified in RFC2553bis)

IPv6 Fragmentation:

ip6frag_high_thresh - INTEGER
        Maximum memory used to reassemble IPv6 fragments. When 
        ip6frag_high_thresh bytes of memory is allocated for this purpose,
        the fragment handler will toss packets until ip6frag_low_thresh
        is reached.
        
ip6frag_low_thresh - INTEGER
        See ip6frag_high_thresh 

ip6frag_time - INTEGER
        Time in seconds to keep an IPv6 fragment in memory.

ip6frag_secret_interval - INTEGER
        Regeneration interval (in seconds) of the hash secret (or lifetime 
        for the hash secret) for IPv6 fragments.
        Default: 600

conf/default/*:
        Change the interface-specific default settings.


conf/all/*:
        Change all the interface-specific settings.  

        [XXX:  Other special features than forwarding?]

conf/all/forwarding - BOOLEAN
        Enable global IPv6 forwarding between all interfaces.  

        IPv4 and IPv6 work differently here; e.g. netfilter must be used 
        to control which interfaces may forward packets and which not.

        This also sets all interfaces' Host/Router setting 
        'forwarding' to the specified value.  See below for details.

        This referred to as global forwarding.

proxy_ndp - BOOLEAN
        Do proxy ndp.

conf/interface/*:
        Change special settings per interface.

        The functional behaviour for certain settings is different 
        depending on whether local forwarding is enabled or not.

accept_ra - BOOLEAN
        Accept Router Advertisements; autoconfigure using them.
        
        Functional default: enabled if local forwarding is disabled.
                            disabled if local forwarding is enabled.

accept_ra_defrtr - BOOLEAN
        Learn default router in Router Advertisement.

        Functional default: enabled if accept_ra is enabled.
                            disabled if accept_ra is disabled.

accept_ra_pinfo - BOOLEAN
        Learn Prefix Information in Router Advertisement.

        Functional default: enabled if accept_ra is enabled.
                            disabled if accept_ra is disabled.

accept_ra_rt_info_max_plen - INTEGER
        Maximum prefix length of Route Information in RA.

        Route Information w/ prefix larger than or equal to this
        variable shall be ignored.

        Functional default: 0 if accept_ra_rtr_pref is enabled.
                            -1 if accept_ra_rtr_pref is disabled.

accept_ra_rtr_pref - BOOLEAN
        Accept Router Preference in RA.

        Functional default: enabled if accept_ra is enabled.
                            disabled if accept_ra is disabled.

accept_redirects - BOOLEAN
        Accept Redirects.

        Functional default: enabled if local forwarding is disabled.
                            disabled if local forwarding is enabled.

accept_source_route - INTEGER
        Accept source routing (routing extension header).

        >= 0: Accept only routing header type 2.
        < 0: Do not accept routing header.

        Default: 0

autoconf - BOOLEAN
        Autoconfigure addresses using Prefix Information in Router 
        Advertisements.

        Functional default: enabled if accept_ra_pinfo is enabled.
                            disabled if accept_ra_pinfo is disabled.

dad_transmits - INTEGER
        The amount of Duplicate Address Detection probes to send.
        Default: 1
        
forwarding - BOOLEAN
        Configure interface-specific Host/Router behaviour.  

        Note: It is recommended to have the same setting on all 
        interfaces; mixed router/host scenarios are rather uncommon.

        FALSE:

        By default, Host behaviour is assumed.  This means:

        1. IsRouter flag is not set in Neighbour Advertisements.
        2. Router Solicitations are being sent when necessary.
        3. If accept_ra is TRUE (default), accept Router 
           Advertisements (and do autoconfiguration).
        4. If accept_redirects is TRUE (default), accept Redirects.

        TRUE:

        If local forwarding is enabled, Router behaviour is assumed. 
        This means exactly the reverse from the above:

        1. IsRouter flag is set in Neighbour Advertisements.
        2. Router Solicitations are not sent.
        3. Router Advertisements are ignored.
        4. Redirects are ignored.

        Default: FALSE if global forwarding is disabled (default),
                 otherwise TRUE.

hop_limit - INTEGER
        Default Hop Limit to set.
        Default: 64

mtu - INTEGER
        Default Maximum Transfer Unit
        Default: 1280 (IPv6 required minimum)

router_probe_interval - INTEGER
        Minimum interval (in seconds) between Router Probing described
        in RFC4191.

        Default: 60

router_solicitation_delay - INTEGER
        Number of seconds to wait after interface is brought up
        before sending Router Solicitations.
        Default: 1

router_solicitation_interval - INTEGER
        Number of seconds to wait between Router Solicitations.
        Default: 4

router_solicitations - INTEGER
        Number of Router Solicitations to send until assuming no 
        routers are present.
        Default: 3

use_tempaddr - INTEGER
        Preference for Privacy Extensions (RFC3041).
          <= 0 : disable Privacy Extensions
          == 1 : enable Privacy Extensions, but prefer public
                 addresses over temporary addresses.
          >  1 : enable Privacy Extensions and prefer temporary
                 addresses over public addresses.
        Default:  0 (for most devices)
                 -1 (for point-to-point devices and loopback devices)

temp_valid_lft - INTEGER
        valid lifetime (in seconds) for temporary addresses.
        Default: 604800 (7 days)

temp_prefered_lft - INTEGER
        Preferred lifetime (in seconds) for temporary addresses.
        Default: 86400 (1 day)

max_desync_factor - INTEGER
        Maximum value for DESYNC_FACTOR, which is a random value
        that ensures that clients don't synchronize with each 
        other and generate new addresses at exactly the same time.
        value is in seconds.
        Default: 600
        
regen_max_retry - INTEGER
        Number of attempts before give up attempting to generate
        valid temporary addresses.
        Default: 5

max_addresses - INTEGER
        Number of maximum addresses per interface.  0 disables limitation.
        It is recommended not set too large value (or 0) because it would 
        be too easy way to crash kernel to allow to create too much of 
        autoconfigured addresses.
        Default: 16

icmp/*:
ratelimit - INTEGER
        Limit the maximal rates for sending ICMPv6 packets.
        0 to disable any limiting, otherwise the maximal rate in jiffies(1)
        Default: 100


IPv6 Update by:
Pekka Savola <pekkas@netcore.fi>
YOSHIFUJI Hideaki / USAGI Project <yoshfuji@linux-ipv6.org>


/proc/sys/net/bridge/* Variables:

bridge-nf-call-arptables - BOOLEAN
        1 : pass bridged ARP traffic to arptables' FORWARD chain.
        0 : disable this.
        Default: 1

bridge-nf-call-iptables - BOOLEAN
        1 : pass bridged IPv4 traffic to iptables' chains.
        0 : disable this.
        Default: 1

bridge-nf-call-ip6tables - BOOLEAN
        1 : pass bridged IPv6 traffic to ip6tables' chains.
        0 : disable this.
        Default: 1

bridge-nf-filter-vlan-tagged - BOOLEAN
        1 : pass bridged vlan-tagged ARP/IP/IPv6 traffic to {arp,ip,ip6}tables.
        0 : disable this.
        Default: 1

bridge-nf-filter-pppoe-tagged - BOOLEAN
        1 : pass bridged pppoe-tagged IP/IPv6 traffic to {ip,ip6}tables.
        0 : disable this.
        Default: 1


UNDOCUMENTED:

dev_weight FIXME
discovery_slots FIXME
discovery_timeout FIXME
fast_poll_increase FIXME
ip6_queue_maxlen FIXME
lap_keepalive_time FIXME
lo_cong FIXME
max_baud_rate FIXME
max_dgram_qlen FIXME
max_noreply_time FIXME
max_tx_data_size FIXME
max_tx_window FIXME
min_tx_turn_time FIXME
mod_cong FIXME
no_cong FIXME
no_cong_thresh FIXME
slot_timeout FIXME
warn_noreply_time FIXME