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/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

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.       

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: 

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_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_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_retries1 - INTEGER
        How many times to retry before deciding that something is wrong
        and it is necessary to report this suspection 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_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_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_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_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_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_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_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_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_max_syn_backlog - INTEGER
        Maximal number of remembered connection requests, which are
        still did not receive an acknowledgement 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_window_scaling - BOOLEAN
        Enable window scaling as defined in RFC1323.

tcp_timestamps - BOOLEAN
        Enable timestamps as defined in RFC1323.

tcp_sack - BOOLEAN
        Enable select acknowledgments (SACKS).

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

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

tcp_ecn - BOOLEAN
        Enable Explicit Congestion Notification in TCP.

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_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_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_mem - vector of 3 INTEGERs: min, pressure, max
        low: 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 consumtion falls
        under "low".

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

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

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_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_rfc1337 - BOOLEAN
        If set, the TCP stack behaves conforming to RFC1337. If unset,
        we are not conforming to RFC, but prevent TCP TIME_WAIT
        asassination.   
        Default: 0

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

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 adresses,
        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
icmp_echo_ignore_broadcasts - BOOLEAN
        If either is set to true, then the kernel will ignore either all
        ICMP ECHO requests sent to it or just those to broadcast/multicast
        addresses, respectively.

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

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}

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)

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.

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_redirects - BOOLEAN
        Accept Redirects.

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

autoconf - BOOLEAN
        Configure link-local addresses using L2 hardware addresses.

        Default: TRUE

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_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

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>

$Id: ip-sysctl.txt,v 1.1.1.1 2004-04-15 02:33:46 phoenix Exp $