diff options
author | Mauro Carvalho Chehab <mchehab+huawei@kernel.org> | 2020-04-28 00:01:49 +0200 |
---|---|---|
committer | David S. Miller <davem@davemloft.net> | 2020-04-28 14:40:18 -0700 |
commit | 1cec2cacaaec5d53adc04dd3ecfdb687b26c0e89 (patch) | |
tree | 89e5f3347d8d28ae4b2826a12f89f4db092f7778 /Documentation/networking/ip-sysctl.rst | |
parent | 355e656e017c3b42deb57d125d86c4cbd277d6db (diff) |
docs: networking: convert ip-sysctl.txt to ReST
- add SPDX header;
- adjust titles and chapters, adding proper markups;
- mark code blocks and literals as such;
- mark lists as such;
- mark tables as such;
- use footnote markup;
- adjust identation, whitespaces and blank lines;
- add to networking/index.rst.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'Documentation/networking/ip-sysctl.rst')
-rw-r--r-- | Documentation/networking/ip-sysctl.rst | 2649 |
1 files changed, 2649 insertions, 0 deletions
diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst new file mode 100644 index 000000000000..38f811d4b2f0 --- /dev/null +++ b/Documentation/networking/ip-sysctl.rst @@ -0,0 +1,2649 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========= +IP Sysctl +========= + +/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 value of TTL field (Time To Live) for outgoing (but not + forwarded) IP packets. Should be between 1 and 255 inclusive. + Default: 64 (as recommended by RFC1700) + +ip_no_pmtu_disc - INTEGER + Disable Path MTU Discovery. If enabled in mode 1 and a + fragmentation-required ICMP is received, the PMTU to this + destination will be set to min_pmtu (see below). You will need + to raise min_pmtu to the smallest interface MTU on your system + manually if you want to avoid locally generated fragments. + + In mode 2 incoming Path MTU Discovery messages will be + discarded. Outgoing frames are handled the same as in mode 1, + implicitly setting IP_PMTUDISC_DONT on every created socket. + + Mode 3 is a hardened pmtu discover mode. The kernel will only + accept fragmentation-needed errors if the underlying protocol + can verify them besides a plain socket lookup. Current + protocols for which pmtu events will be honored are TCP, SCTP + and DCCP as they verify e.g. the sequence number or the + association. This mode should not be enabled globally but is + only intended to secure e.g. name servers in namespaces where + TCP path mtu must still work but path MTU information of other + protocols should be discarded. If enabled globally this mode + could break other protocols. + + Possible values: 0-3 + + Default: FALSE + +min_pmtu - INTEGER + default 552 - minimum discovered Path MTU + +ip_forward_use_pmtu - BOOLEAN + By default we don't trust protocol path MTUs while forwarding + because they could be easily forged and can lead to unwanted + fragmentation by the router. + You only need to enable this if you have user-space software + which tries to discover path mtus by itself and depends on the + kernel honoring this information. This is normally not the + case. + + Default: 0 (disabled) + + Possible values: + + - 0 - disabled + - 1 - enabled + +fwmark_reflect - BOOLEAN + Controls the fwmark of kernel-generated IPv4 reply packets that are not + associated with a socket for example, TCP RSTs or ICMP echo replies). + If unset, these packets have a fwmark of zero. If set, they have the + fwmark of the packet they are replying to. + + Default: 0 + +fib_multipath_use_neigh - BOOLEAN + Use status of existing neighbor entry when determining nexthop for + multipath routes. If disabled, neighbor information is not used and + packets could be directed to a failed nexthop. Only valid for kernels + built with CONFIG_IP_ROUTE_MULTIPATH enabled. + + Default: 0 (disabled) + + Possible values: + + - 0 - disabled + - 1 - enabled + +fib_multipath_hash_policy - INTEGER + Controls which hash policy to use for multipath routes. Only valid + for kernels built with CONFIG_IP_ROUTE_MULTIPATH enabled. + + Default: 0 (Layer 3) + + Possible values: + + - 0 - Layer 3 + - 1 - Layer 4 + - 2 - Layer 3 or inner Layer 3 if present + +fib_sync_mem - UNSIGNED INTEGER + Amount of dirty memory from fib entries that can be backlogged before + synchronize_rcu is forced. + + Default: 512kB Minimum: 64kB Maximum: 64MB + +ip_forward_update_priority - INTEGER + Whether to update SKB priority from "TOS" field in IPv4 header after it + is forwarded. The new SKB priority is mapped from TOS field value + according to an rt_tos2priority table (see e.g. man tc-prio). + + Default: 1 (Update priority.) + + Possible values: + + - 0 - Do not update priority. + - 1 - Update priority. + +route/max_size - INTEGER + Maximum number of routes allowed in the kernel. Increase + this when using large numbers of interfaces and/or routes. + + From linux kernel 3.6 onwards, this is deprecated for ipv4 + as route cache is no longer used. + +neigh/default/gc_thresh1 - INTEGER + Minimum number of entries to keep. Garbage collector will not + purge entries if there are fewer than this number. + + Default: 128 + +neigh/default/gc_thresh2 - INTEGER + Threshold when garbage collector becomes more aggressive about + purging entries. Entries older than 5 seconds will be cleared + when over this number. + + Default: 512 + +neigh/default/gc_thresh3 - INTEGER + Maximum number of non-PERMANENT neighbor entries allowed. Increase + this when using large numbers of interfaces and when communicating + with large numbers of directly-connected peers. + + Default: 1024 + +neigh/default/unres_qlen_bytes - INTEGER + The maximum number of bytes which may be used by packets + queued for each unresolved address by other network layers. + (added in linux 3.3) + + Setting negative value is meaningless and will return error. + + Default: SK_WMEM_MAX, (same as net.core.wmem_default). + + Exact value depends on architecture and kernel options, + but should be enough to allow queuing 256 packets + of medium size. + +neigh/default/unres_qlen - INTEGER + The maximum number of packets which may be queued for each + unresolved address by other network layers. + + (deprecated in linux 3.3) : use unres_qlen_bytes instead. + + Prior to linux 3.3, the default value is 3 which may cause + unexpected packet loss. The current default value is calculated + according to default value of unres_qlen_bytes and true size of + packet. + + Default: 101 + +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 - LONG INTEGER + Maximum memory used to reassemble IP fragments. + +ipfrag_low_thresh - LONG INTEGER + (Obsolete since linux-4.17) + Maximum memory used to reassemble IP fragments before the kernel + begins to remove incomplete fragment queues to free up resources. + The kernel still accepts new fragments for defragmentation. + +ipfrag_time - INTEGER + Time in seconds to keep an IP fragment in memory. + +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 seconds. + +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 seconds. + +TCP variables +============= + +somaxconn - INTEGER + Limit of socket listen() backlog, known in userspace as SOMAXCONN. + Defaults to 4096. (Was 128 before linux-5.4) + See also tcp_max_syn_backlog for additional tuning for TCP sockets. + +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. + + Possible values are [-31, 31], inclusive. + + Default: 1 + +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_autocorking - BOOLEAN + Enable TCP auto corking : + When applications do consecutive small write()/sendmsg() system calls, + we try to coalesce these small writes as much as possible, to lower + total amount of sent packets. This is done if at least one prior + packet for the flow is waiting in Qdisc queues or device transmit + queue. Applications can still use TCP_CORK for optimal behavior + when they know how/when to uncork their sockets. + + Default : 1 + +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 the packetization layer + Path MTU discovery (MTU probing). If MTU probing is enabled, + this is the initial MSS used by the connection. + +tcp_mtu_probe_floor - INTEGER + If MTU probing is enabled this caps the minimum MSS used for search_low + for the connection. + + Default : 48 + +tcp_min_snd_mss - INTEGER + TCP SYN and SYNACK messages usually advertise an ADVMSS option, + as described in RFC 1122 and RFC 6691. + + If this ADVMSS option is smaller than tcp_min_snd_mss, + it is silently capped to tcp_min_snd_mss. + + Default : 48 (at least 8 bytes of payload per segment) + +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. + For passive connections, the listener congestion control choice + is inherited. + + [see setsockopt(listenfd, SOL_TCP, TCP_CONGESTION, "name" ...) ] + +tcp_dsack - BOOLEAN + Allows TCP to send "duplicate" SACKs. + +tcp_early_retrans - INTEGER + Tail loss probe (TLP) converts RTOs occurring due to tail + losses into fast recovery (draft-ietf-tcpm-rack). Note that + TLP requires RACK to function properly (see tcp_recovery below) + + Possible values: + + - 0 disables TLP + - 3 or 4 enables TLP + + Default: 3 + +tcp_ecn - INTEGER + Control use of Explicit Congestion Notification (ECN) by TCP. + ECN is used only when both ends of the TCP connection indicate + support for it. This feature is useful in avoiding losses due + to congestion by allowing supporting routers to signal + congestion before having to drop packets. + + Possible values are: + + = ===================================================== + 0 Disable ECN. Neither initiate nor accept ECN. + 1 Enable ECN when requested by incoming connections and + also request ECN on outgoing connection attempts. + 2 Enable ECN when requested by incoming connections + but do not request ECN on outgoing connections. + = ===================================================== + + Default: 2 + +tcp_ecn_fallback - BOOLEAN + If the kernel detects that ECN connection misbehaves, enable fall + back to non-ECN. Currently, this knob implements the fallback + from RFC3168, section 6.1.1.1., but we reserve that in future, + additional detection mechanisms could be implemented under this + knob. The value is not used, if tcp_ecn or per route (or congestion + control) ECN settings are disabled. + + Default: 1 (fallback enabled) + +tcp_fack - BOOLEAN + This is a legacy option, it has no effect anymore. + +tcp_fin_timeout - INTEGER + The length of time an orphaned (no longer referenced by any + application) connection will remain in the FIN_WAIT_2 state + before it is aborted at the local end. While a perfectly + valid "receive only" state for an un-orphaned connection, an + orphaned connection in FIN_WAIT_2 state could otherwise wait + forever for the remote to close its end of the connection. + + Cf. tcp_max_orphans + + Default: 60 seconds + +tcp_frto - INTEGER + Enables Forward RTO-Recovery (F-RTO) defined in RFC5682. + F-RTO is an enhanced recovery algorithm for TCP retransmission + timeouts. It is particularly beneficial in networks where the + RTT fluctuates (e.g., wireless). F-RTO is sender-side only + modification. It does not require any support from the peer. + + By default it's enabled with a non-zero value. 0 disables F-RTO. + +tcp_fwmark_accept - BOOLEAN + If set, incoming connections to listening sockets that do not have a + socket mark will set the mark of the accepting socket to the fwmark of + the incoming SYN packet. This will cause all packets on that connection + (starting from the first SYNACK) to be sent with that fwmark. The + listening socket's mark is unchanged. Listening sockets that already + have a fwmark set via setsockopt(SOL_SOCKET, SO_MARK, ...) are + unaffected. + + Default: 0 + +tcp_invalid_ratelimit - INTEGER + Limit the maximal rate for sending duplicate acknowledgments + in response to incoming TCP packets that are for an existing + connection but that are invalid due to any of these reasons: + + (a) out-of-window sequence number, + (b) out-of-window acknowledgment number, or + (c) PAWS (Protection Against Wrapped Sequence numbers) check failure + + This can help mitigate simple "ack loop" DoS attacks, wherein + a buggy or malicious middlebox or man-in-the-middle can + rewrite TCP header fields in manner that causes each endpoint + to think that the other is sending invalid TCP segments, thus + causing each side to send an unterminating stream of duplicate + acknowledgments for invalid segments. + + Using 0 disables rate-limiting of dupacks in response to + invalid segments; otherwise this value specifies the minimal + space between sending such dupacks, in milliseconds. + + Default: 500 (milliseconds). + +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_l3mdev_accept - BOOLEAN + Enables child sockets to inherit the L3 master device index. + Enabling this option allows a "global" listen socket to work + across L3 master domains (e.g., VRFs) with connected sockets + derived from the listen socket to be bound to the L3 domain in + which the packets originated. Only valid when the kernel was + compiled with CONFIG_NET_L3_MASTER_DEV. + + Default: 0 (disabled) + +tcp_low_latency - BOOLEAN + This is a legacy option, it has no effect anymore. + +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 (SYN_RECV), + which have not received an acknowledgment from connecting client. + + This is a per-listener limit. + + The minimal value is 128 for low memory machines, and it will + increase in proportion to the memory of machine. + + If server suffers from overload, try increasing this number. + + Remember to also check /proc/sys/net/core/somaxconn + A SYN_RECV request socket consumes about 304 bytes of memory. + +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_min_rtt_wlen - INTEGER + The window length of the windowed min filter to track the minimum RTT. + A shorter window lets a flow more quickly pick up new (higher) + minimum RTT when it is moved to a longer path (e.g., due to traffic + engineering). A longer window makes the filter more resistant to RTT + inflations such as transient congestion. The unit is seconds. + + Possible values: 0 - 86400 (1 day) + + Default: 300 + +tcp_moderate_rcvbuf - BOOLEAN + If set, TCP performs receive buffer auto-tuning, 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_probe_interval - UNSIGNED INTEGER + Controls how often to start TCP Packetization-Layer Path MTU + Discovery reprobe. The default is reprobing every 10 minutes as + per RFC4821. + +tcp_probe_threshold - INTEGER + Controls when TCP Packetization-Layer Path MTU Discovery probing + will stop in respect to the width of search range in bytes. Default + is 8 bytes. + +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_no_ssthresh_metrics_save - BOOLEAN + Controls whether TCP saves ssthresh metrics in the route cache. + + Default is 1, which disables ssthresh metrics. + +tcp_orphan_retries - INTEGER + This value influences the timeout of a locally closed TCP connection, + when RTO retransmissions remain unacknowledged. + See tcp_retries2 for more details. + + The default value is 8. + + If your machine is a loaded WEB server, + you should think about lowering this value, such sockets + may consume significant resources. Cf. tcp_max_orphans. + +tcp_recovery - INTEGER + This value is a bitmap to enable various experimental loss recovery + features. + + ========= ============================================================= + RACK: 0x1 enables the RACK loss detection for fast detection of lost + retransmissions and tail drops. It also subsumes and disables + RFC6675 recovery for SACK connections. + + RACK: 0x2 makes RACK's reordering window static (min_rtt/4). + + RACK: 0x4 disables RACK's DUPACK threshold heuristic + ========= ============================================================= + + Default: 0x1 + +tcp_reordering - INTEGER + Initial reordering level of packets in a TCP stream. + TCP stack can then dynamically adjust flow reordering level + between this initial value and tcp_max_reordering + + Default: 3 + +tcp_max_reordering - INTEGER + Maximal reordering level of packets in a TCP stream. + 300 is a fairly conservative value, but you might increase it + if paths are using per packet load balancing (like bonding rr mode) + + Default: 300 + +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 + This value influences the time, after which TCP decides, that + something is wrong due to unacknowledged RTO retransmissions, + and reports this suspicion to the network layer. + See tcp_retries2 for more details. + + RFC 1122 recommends at least 3 retransmissions, which is the + default. + +tcp_retries2 - INTEGER + This value influences the timeout of an alive TCP connection, + when RTO retransmissions remain unacknowledged. + Given a value of N, a hypothetical TCP connection following + exponential backoff with an initial RTO of TCP_RTO_MIN would + retransmit N times before killing the connection at the (N+1)th RTO. + + The default value of 15 yields a hypothetical timeout of 924.6 + seconds and is a lower bound for the effective timeout. + TCP will effectively time out at the first RTO which exceeds the + hypothetical timeout. + + RFC 1122 recommends at least 100 seconds for the timeout, + which corresponds to a value of at least 8. + +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: 4K + + default: initial 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. Calling setsockopt() with SO_RCVBUF disables + automatic tuning of that socket's receive buffer size, in which + case this value is ignored. + Default: between 87380B and 6MB, depending on RAM size. + +tcp_sack - BOOLEAN + Enable select acknowledgments (SACKS). + +tcp_comp_sack_delay_ns - LONG INTEGER + TCP tries to reduce number of SACK sent, using a timer + based on 5% of SRTT, capped by this sysctl, in nano seconds. + The default is 1ms, based on TSO autosizing period. + + Default : 1,000,000 ns (1 ms) + +tcp_comp_sack_nr - INTEGER + Max number of SACK that can be compressed. + Using 0 disables SACK compression. + + Default : 44 + +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 urgent 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 31seconds till the last retransmission + with the current initial RTO of 1second. With this the final timeout + for a passive TCP connection will happen after 63seconds. + +tcp_syncookies - INTEGER + Only valid when the kernel was compiled with CONFIG_SYN_COOKIES + Send out syncookies when the syn backlog queue of a socket + overflows. This is to prevent against the common 'SYN flood attack' + Default: 1 + + 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 SYN flood 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 + SYN flood warnings in logs not being really flooded, your server + is seriously misconfigured. + + If you want to test which effects syncookies have to your + network connections you can set this knob to 2 to enable + unconditionally generation of syncookies. + +tcp_fastopen - INTEGER + Enable TCP Fast Open (RFC7413) to send and accept data in the opening + SYN packet. + + The client support is enabled by flag 0x1 (on by default). The client + then must use sendmsg() or sendto() with the MSG_FASTOPEN flag, + rather than connect() to send data in SYN. + + The server support is enabled by flag 0x2 (off by default). Then + either enable for all listeners with another flag (0x400) or + enable individual listeners via TCP_FASTOPEN socket option with + the option value being the length of the syn-data backlog. + + The values (bitmap) are + + ===== ======== ====================================================== + 0x1 (client) enables sending data in the opening SYN on the client. + 0x2 (server) enables the server support, i.e., allowing data in + a SYN packet to be accepted and passed to the + application before 3-way handshake finishes. + 0x4 (client) send data in the opening SYN regardless of cookie + availability and without a cookie option. + 0x200 (server) accept data-in-SYN w/o any cookie option present. + 0x400 (server) enable all listeners to support Fast Open by + default without explicit TCP_FASTOPEN socket option. + ===== ======== ====================================================== + + Default: 0x1 + + Note that that additional client or server features are only + effective if the basic support (0x1 and 0x2) are enabled respectively. + +tcp_fastopen_blackhole_timeout_sec - INTEGER + Initial time period in second to disable Fastopen on active TCP sockets + when a TFO firewall blackhole issue happens. + This time period will grow exponentially when more blackhole issues + get detected right after Fastopen is re-enabled and will reset to + initial value when the blackhole issue goes away. + 0 to disable the blackhole detection. + + By default, it is set to 1hr. + +tcp_fastopen_key - list of comma separated 32-digit hexadecimal INTEGERs + The list consists of a primary key and an optional backup key. The + primary key is used for both creating and validating cookies, while the + optional backup key is only used for validating cookies. The purpose of + the backup key is to maximize TFO validation when keys are rotated. + + A randomly chosen primary key may be configured by the kernel if + the tcp_fastopen sysctl is set to 0x400 (see above), or if the + TCP_FASTOPEN setsockopt() optname is set and a key has not been + previously configured via sysctl. If keys are configured via + setsockopt() by using the TCP_FASTOPEN_KEY optname, then those + per-socket keys will be used instead of any keys that are specified via + sysctl. + + A key is specified as 4 8-digit hexadecimal integers which are separated + by a '-' as: xxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxx. Leading zeros may be + omitted. A primary and a backup key may be specified by separating them + by a comma. If only one key is specified, it becomes the primary key and + any previously configured backup keys are removed. + +tcp_syn_retries - INTEGER + Number of times initial SYNs for an active TCP connection attempt + will be retransmitted. Should not be higher than 127. Default value + is 6, which corresponds to 63seconds till the last retransmission + with the current initial RTO of 1second. With this the final timeout + for an active TCP connection attempt will happen after 127seconds. + +tcp_timestamps - INTEGER + Enable timestamps as defined in RFC1323. + + - 0: Disabled. + - 1: Enable timestamps as defined in RFC1323 and use random offset for + each connection rather than only using the current time. + - 2: Like 1, but without random offsets. + + Default: 1 + +tcp_min_tso_segs - INTEGER + Minimal number of segments per TSO frame. + + Since linux-3.12, TCP does an automatic sizing of TSO frames, + depending on flow rate, instead of filling 64Kbytes packets. + For specific usages, it's possible to force TCP to build big + TSO frames. Note that TCP stack might split too big TSO packets + if available window is too small. + + Default: 2 + +tcp_pacing_ss_ratio - INTEGER + sk->sk_pacing_rate is set by TCP stack using a ratio applied + to current rate. (current_rate = cwnd * mss / srtt) + If TCP is in slow start, tcp_pacing_ss_ratio is applied + to let TCP probe for bigger speeds, assuming cwnd can be + doubled every other RTT. + + Default: 200 + +tcp_pacing_ca_ratio - INTEGER + sk->sk_pacing_rate is set by TCP stack using a ratio applied + to current rate. (current_rate = cwnd * mss / srtt) + If TCP is in congestion avoidance phase, tcp_pacing_ca_ratio + is applied to conservatively probe for bigger throughput. + + Default: 120 + +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_reuse - INTEGER + Enable reuse of TIME-WAIT sockets for new connections when it is + safe from protocol viewpoint. + + - 0 - disable + - 1 - global enable + - 2 - enable for loopback traffic only + + It should not be changed without advice/request of technical + experts. + + Default: 2 + +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 sockets. + Each TCP socket has rights to use it due to fact of its birth. + + Default: 4K + + default: initial size of send buffer used by TCP sockets. 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 tuned + send buffers for TCP sockets. This value does not override + net.core.wmem_max. Calling setsockopt() with SO_SNDBUF disables + automatic tuning of that socket's send buffer size, in which case + this value is ignored. + + Default: between 64K and 4MB, depending on RAM size. + +tcp_notsent_lowat - UNSIGNED INTEGER + A TCP socket can control the amount of unsent bytes in its write queue, + thanks to TCP_NOTSENT_LOWAT socket option. poll()/select()/epoll() + reports POLLOUT events if the amount of unsent bytes is below a per + socket value, and if the write queue is not full. sendmsg() will + also not add new buffers if the limit is hit. + + This global variable controls the amount of unsent data for + sockets not using TCP_NOTSENT_LOWAT. For these sockets, a change + to the global variable has immediate effect. + + Default: UINT_MAX (0xFFFFFFFF) + +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_thin_linear_timeouts - BOOLEAN + Enable dynamic triggering of linear timeouts for thin streams. + If set, a check is performed upon retransmission by timeout to + determine if the stream is thin (less than 4 packets in flight). + As long as the stream is found to be thin, up to 6 linear + timeouts may be performed before exponential backoff mode is + initiated. This improves retransmission latency for + non-aggressive thin streams, often found to be time-dependent. + For more information on thin streams, see + Documentation/networking/tcp-thin.txt + + Default: 0 + +tcp_limit_output_bytes - INTEGER + Controls TCP Small Queue limit per tcp socket. + TCP bulk sender tends to increase packets in flight until it + gets losses notifications. With SNDBUF autotuning, this can + result in a large amount of packets queued on the local machine + (e.g.: qdiscs, CPU backlog, or device) hurting latency of other + flows, for typical pfifo_fast qdiscs. tcp_limit_output_bytes + limits the number of bytes on qdisc or device to reduce artificial + RTT/cwnd and reduce bufferbloat. + + Default: 1048576 (16 * 65536) + +tcp_challenge_ack_limit - INTEGER + Limits number of Challenge ACK sent per second, as recommended + in RFC 5961 (Improving TCP's Robustness to Blind In-Window Attacks) + Default: 1000 + +tcp_rx_skb_cache - BOOLEAN + Controls a per TCP socket cache of one skb, that might help + performance of some workloads. This might be dangerous + on systems with a lot of TCP sockets, since it increases + memory usage. + + Default: 0 (disabled) + +UDP variables +============= + +udp_l3mdev_accept - BOOLEAN + Enabling this option allows a "global" bound socket to work + across L3 master domains (e.g., VRFs) with packets capable of + being received regardless of the L3 domain in which they + originated. Only valid when the kernel was compiled with + CONFIG_NET_L3_MASTER_DEV. + + Default: 0 (disabled) + +udp_mem - vector of 3 INTEGERs: min, pressure, max + Number of pages allowed for queueing by all UDP sockets. + + min: Below this number of pages UDP is not bothered about its + memory appetite. When amount of memory allocated by UDP exceeds + this number, UDP starts to moderate memory usage. + + pressure: This value was introduced to follow format of tcp_mem. + + max: Number of pages allowed for queueing by all UDP sockets. + + Default is calculated at boot time from amount of available memory. + +udp_rmem_min - INTEGER + Minimal size of receive buffer used by UDP sockets in moderation. + Each UDP socket is able to use the size for receiving data, even if + total pages of UDP sockets exceed udp_mem pressure. The unit is byte. + + Default: 4K + +udp_wmem_min - INTEGER + Minimal size of send buffer used by UDP sockets in moderation. + Each UDP socket is able to use the size for sending data, even if + total pages of UDP sockets exceed udp_mem pressure. The unit is byte. + + Default: 4K + +RAW variables +============= + +raw_l3mdev_accept - BOOLEAN + Enabling this option allows a "global" bound socket to work + across L3 master domains (e.g., VRFs) with packets capable of + being received regardless of the L3 domain in which they + originated. Only valid when the kernel was compiled with + CONFIG_NET_L3_MASTER_DEV. + |