libtorrent
Description: A feature complete BitTorrent protocol implementation as a C++ library

Version: | 2.0.8

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Table of contents

- settings_pack
  - set_str() set_bool() set_int()
  - has_val()
  - clear()
  - clear()
  - get_str() get_int() get_bool()
  - enum type_bases
  - enum mmap_write_mode_t
  - enum suggest_mode_t
  - enum choking_algorithm_t
  - enum seed_choking_algorithm_t
  - enum io_buffer_mode_t
  - enum bandwidth_mixed_algo_t
  - enum enc_policy
  - enum enc_level
  - enum proxy_type_t
- setting_by_name() name_for_setting()
- default_settings()
- min_memory_usage() high_performance_seed()
- generate_fingerprint()

You have some control over session configuration through the session::apply_settings() member function. To change one or more configuration options, create a settings_pack object and fill it with the settings to be set and pass it in to session::apply_settings().

The settings_pack object is a collection of settings updates that are applied to the session when passed to session::apply_settings(). It's empty when constructed.

You have control over proxy and authorization settings and also the user-agent that will be sent to the tracker. The user-agent will also be used to identify the client with other peers.

Each configuration option is named with an enum value inside the settings_pack class. These are the available settings:

[report issue]

# settings_pack

Declared in "libtorrent/settings_pack.hpp"

The `settings_pack` struct, contains the names of all settings as enum values. These values are passed in to the `set_str()`, `set_int()`, `set_bool()` functions, to specify the setting to change.

The `settings_pack` only stores values for settings that have been explicitly set on this object. However, it can still be queried for settings that have not been set and returns the default value for those settings.

name | type | default
--- | --- | ---
user_agent | string | libtorrent/

this is the client identification to the tracker. The recommended format of this string is: "client-name/client-version libtorrent/libtorrent-version". This name will not only be used when making HTTP requests, but also when sending extended headers to peers that support that extension. It may not contain r or n

name | type | default
--- | --- | ---
announce_ip | string | nullptr

`announce_ip` is the ip address passed along to trackers as the `&ip=` parameter. If left as the default, that parameter is omitted.

Note

This setting is only meant for very special cases where a seed is running on the same host as the tracker, and the tracker accepts the IP parameter (which normal trackers don't). Do not set this option unless you also control the tracker.

name | type | default
--- | --- | ---
handshake_client_version | string | nullptr

this is the client name and version identifier sent to peers in the handshake message. If this is an empty string, the user_agent is used instead. This string must be a UTF-8 encoded unicode string.

name | type | default
--- | --- | ---
outgoing_interfaces | string |

This controls which IP address outgoing TCP peer connections are bound to, in addition to controlling whether such connections are also bound to a specific network interface/adapter (bind-to-device).

This string is a comma-separated list of IP addresses and interface names. An empty string will not bind TCP sockets to a device, and let the network stack assign the local address.

A list of names will be used to bind outgoing TCP sockets in a round-robin fashion. An IP address will simply be used to bind() the socket. An interface name will attempt to bind the socket to that interface. If that fails, or is unsupported, one of the IP addresses configured for that interface is used to bind() the socket to. If the interface or adapter doesn't exist, the outgoing peer connection will fail with an error message suggesting the device cannot be found. Adapter names on Unix systems are of the form "eth0", "eth1", "tun0", etc. This may be useful for clients that are multi-homed. Binding an outgoing connection to a local IP does not necessarily make the connection via the associated NIC/Adapter.

When outgoing interfaces are specified, incoming connections or packets sent to a local interface or IP that's not in this list will be rejected with a peer_blocked_alert with `invalid_local_interface` as the reason.

Note that these are just interface/adapter names or IP addresses. There are no ports specified in this list. IPv6 addresses without port should be specified without enclosing `[`, `]`.

name | type | default
--- | --- | ---
listen_interfaces | string | 0.0.0.0:6881,[::]:6881

a comma-separated list of (IP or device name, port) pairs. These are the listen ports that will be opened for accepting incoming uTP and TCP peer connections. These are also used for outgoing uTP and UDP tracker connections and DHT nodes.

It is possible to listen on multiple interfaces and multiple ports. Binding to port 0 will make the operating system pick the port.

Note

There are reasons to stick to the same port across sessions, which would mean only using port 0 on the first start, and recording the port that was picked for subsequent startups. Trackers, the DHT and other peers will remember the port they see you use and hand that port out to other peers trying to connect to you, as well as trying to connect to you themselves.

A port that has an "s" suffix will accept SSL peer connections. (note that SSL sockets are only available in builds with SSL support)

A port that has an "l" suffix will be considered a local network. i.e. it's assumed to only be able to reach hosts in the same local network as the IP address (based on the netmask associated with the IP, queried from the operating system).

if binding fails, the listen_failed_alert is posted. Once a socket binding succeeds (if it does), the listen_succeeded_alert is posted. There may be multiple failures before a success.

If a device name that does not exist is configured, no listen socket will be opened for that interface. If this is the only interface configured, it will be as if no listen ports are configured.

If no listen ports are configured (e.g. listen_interfaces is an empty string), networking will be disabled. No DHT will start, no outgoing uTP or tracker connections will be made. No incoming TCP or uTP connections will be accepted. (outgoing TCP connections will still be possible, depending on settings_pack::outgoing_interfaces).

For example: `[::1]:8888` - will only accept connections on the IPv6 loopback address on port 8888.

`eth0:4444,eth1:4444` - will accept connections on port 4444 on any IP address bound to device `eth0` or `eth1`.

`[::]:0s` - will accept SSL connections on a port chosen by the OS. And not accept non-SSL connections at all.

`0.0.0.0:6881,[::]:6881` - binds to all interfaces on port 6881.

`10.0.1.13:6881l` - binds to the local IP address, port 6881, but only allow talking to peers on the same local network. The netmask is queried from the operating system. Interfaces marked `l` are not announced to trackers, unless the tracker is also on the same local network.

Windows OS network adapter device name must be specified with GUID. It can be obtained from "netsh lan show interfaces" command output. GUID must be uppercased string embraced in curly brackets. `{E4F0B674-0DFC-48BB-98A5-2AA730BDB6D6}:7777` - will accept connections on port 7777 on adapter with this GUID.

For more information, see the Multi-homed hosts section.

name | type | default
--- | --- | ---
proxy_hostname | string |

when using a proxy, this is the hostname where the proxy is running see proxy_type. Note that when using a proxy, the settings_pack::listen_interfaces setting is overridden and only a single interface is created, just to contact the proxy. This means a proxy cannot be combined with SSL torrents or multiple listen interfaces. This proxy listen interface will not accept incoming TCP connections, will not map ports with any gateway and will not enable local service discovery. All traffic is supposed to be channeled through the proxy.

name | type | default
--- | --- | ---
proxy_username | string |
proxy_password | string |

when using a proxy, these are the credentials (if any) to use when connecting to it. see proxy_type

name | type | default
--- | --- | ---
i2p_hostname | string |

sets the i2p SAM bridge to connect to. set the port with the `i2p_port` setting.

name | type | default
--- | --- | ---
peer_fingerprint | string | -LT2080-

this is the fingerprint for the client. It will be used as the prefix to the peer_id. If this is 20 bytes (or longer) it will be truncated to 20 bytes and used as the entire peer-id

There is a utility function, generate_fingerprint() that can be used to generate a standard client peer ID fingerprint prefix.

name | type | default
--- | --- | ---
dht_bootstrap_nodes | string | dht.libtorrent.org:25401

This is a comma-separated list of IP port-pairs. They will be added to the DHT node (if it's enabled) as back-up nodes in case we don't know of any.

Changing these after the DHT has been started may not have any effect until the DHT is restarted.

name | type | default
--- | --- | ---
allow_multiple_connections_per_ip | bool | false

determines if connections from the same IP address as existing connections should be rejected or not. Rejecting multiple connections from the same IP address will prevent abusive behavior by peers. The logic for determining whether connections are to the same peer is more complicated with this enabled, and more likely to fail in some edge cases. It is not recommended to enable this feature.

name | type | default
--- | --- | ---
send_redundant_have | bool | true

`send_redundant_have` controls if have messages will be sent to peers that already have the piece. This is typically not necessary, but it might be necessary for collecting statistics in some cases.

name | type | default
--- | --- | ---
use_dht_as_fallback | bool | false

`use_dht_as_fallback` determines how the DHT is used. If this is true, the DHT will only be used for torrents where all trackers in its tracker list has failed. Either by an explicit error message or a time out. If this is false, the DHT is used regardless of if the trackers fail or not.

name | type | default
--- | --- | ---
upnp_ignore_nonrouters | bool | false

`upnp_ignore_nonrouters` indicates whether or not the UPnP implementation should ignore any broadcast response from a device whose address is not on our subnet. i.e. it's a way to not talk to other people's routers by mistake.

name | type | default
--- | --- | ---
use_parole_mode | bool | true

`use_parole_mode` specifies if parole mode should be used. Parole mode means that peers that participate in pieces that fail the hash check are put in a mode where they are only allowed to download whole pieces. If the whole piece a peer in parole mode fails the hash check, it is banned. If a peer participates in a piece that passes the hash check, it is taken out of parole mode.

name | type | default
--- | --- | ---
auto_manage_prefer_seeds | bool | false

if true, prefer seeding torrents when determining which torrents to give active slots to. If false, give preference to downloading torrents

name | type | default
--- | --- | ---
dont_count_slow_torrents | bool | true

if `dont_count_slow_torrents` is true, torrents without any payload transfers are not subject to the `active_seeds` and `active_downloads` limits. This is intended to make it more likely to utilize all available bandwidth, and avoid having torrents that don't transfer anything block the active slots.

name | type | default
--- | --- | ---
close_redundant_connections | bool | true

`close_redundant_connections` specifies whether libtorrent should close connections where both ends have no utility in keeping the connection open. For instance if both ends have completed their downloads, there's no point in keeping it open.

name | type | default
--- | --- | ---
prioritize_partial_pieces | bool | false

If `prioritize_partial_pieces` is true, partial pieces are picked before pieces that are more rare. If false, rare pieces are always prioritized, unless the number of partial pieces is growing out of proportion.

name | type | default
--- | --- | ---
rate_limit_ip_overhead | bool | true

if set to true, the estimated TCP/IP overhead is drained from the rate limiters, to avoid exceeding the limits with the total traffic

name | type | default
--- | --- | ---
announce_to_all_tiers | bool | false
announce_to_all_trackers | bool | false

`announce_to_all_trackers` controls how multi tracker torrents are treated. If this is set to true, all trackers in the same tier are announced to in parallel. If all trackers in tier 0 fails, all trackers in tier 1 are announced as well. If it's set to false, the behavior is as defined by the multi tracker specification.

`announce_to_all_tiers` also controls how multi tracker torrents are treated. When this is set to true, one tracker from each tier is announced to. This is the uTorrent behavior. To be compliant with the Multi-tracker specification, set it to false.

name | type | default
--- | --- | ---
prefer_udp_trackers | bool | true

`prefer_udp_trackers`: true means that trackers may be rearranged in a way that udp trackers are always tried before http trackers for the same hostname. Setting this to false means that the tracker's tier is respected and there's no preference of one protocol over another.

name | type | default
--- | --- | ---
disable_hash_checks | bool | false

when set to true, all data downloaded from peers will be assumed to be correct, and not tested to match the hashes in the torrent this is only useful for simulation and testing purposes (typically combined with disabled_storage)

name | type | default
--- | --- | ---
allow_i2p_mixed | bool | false

if this is true, i2p torrents are allowed to also get peers from other sources than the tracker, and connect to regular IPs, not providing any anonymization. This may be useful if the user is not interested in the anonymization of i2p, but still wants to be able to connect to i2p peers.

name | type | default
--- | --- | ---
no_atime_storage | bool | true

`no_atime_storage` this is a Linux-only option and passes in the `O_NOATIME` to `open()` when opening files. This may lead to some disk performance improvements.

name | type | default
--- | --- | ---
incoming_starts_queued_torrents | bool | false

`incoming_starts_queued_torrents`. If a torrent has been paused by the auto managed feature in libtorrent, i.e. the torrent is paused and auto managed, this feature affects whether or not it is automatically started on an incoming connection. The main reason to queue torrents, is not to make them unavailable, but to save on the overhead of announcing to the trackers, the DHT and to avoid spreading one's unchoke slots too thin. If a peer managed to find us, even though we're no in the torrent anymore, this setting can make us start the torrent and serve it.

name | type | default
--- | --- | ---
report_true_downloaded | bool | false

when set to true, the downloaded counter sent to trackers will include the actual number of payload bytes downloaded including redundant bytes. If set to false, it will not include any redundancy bytes

name | type | default
--- | --- | ---
strict_end_game_mode | bool | true

`strict_end_game_mode` controls when a block may be requested twice. If this is `true`, a block may only be requested twice when there's at least one request to every piece that's left to download in the torrent. This may slow down progress on some pieces sometimes, but it may also avoid downloading a lot of redundant bytes. If this is `false`, libtorrent attempts to use each peer connection to its max, by always requesting something, even if it means requesting something that has been requested from another peer already.

name | type | default
--- | --- | ---
enable_outgoing_utp | bool | true
enable_incoming_utp | bool | true
enable_outgoing_tcp | bool | true
enable_incoming_tcp | bool | true

Enables incoming and outgoing, TCP and uTP peer connections. `false` is disabled and `true` is enabled. When outgoing connections are disabled, libtorrent will simply not make outgoing peer connections with the specific transport protocol. Disabled incoming peer connections will simply be rejected. These options only apply to peer connections, not tracker- or any other kinds of connections.

name | type | default
--- | --- | ---
no_recheck_incomplete_resume | bool | false

`no_recheck_incomplete_resume` determines if the storage should check the whole files when resume data is incomplete or missing or whether it should simply assume we don't have any of the data. If false, any existing files will be checked. By setting this setting to true, the files won't be checked, but will go straight to download mode.

name | type | default
--- | --- | ---
anonymous_mode | bool | false

`anonymous_mode`: When set to true, the client tries to hide its identity to a certain degree.

- A generic user-agent will be used for trackers (except for private torrents).
- Your local IPv4 and IPv6 address won't be sent as query string parameters to private trackers.
- If announce_ip is configured, it will not be sent to trackers
- The client version will not be sent to peers in the extension handshake.

name | type | default
--- | --- | ---
report_web_seed_downloads | bool | true

specifies whether downloads from web seeds is reported to the tracker or not. Turning it off also excludes web seed traffic from other stats and download rate reporting via the libtorrent API.

name | type | default
--- | --- | ---
seeding_outgoing_connections | bool | true

`seeding_outgoing_connections` determines if seeding (and finished) torrents should attempt to make outgoing connections or not. It may be set to false in very specific applications where the cost of making outgoing connections is high, and there are no or small benefits of doing so. For instance, if no nodes are behind a firewall or a NAT, seeds don't need to make outgoing connections.

name | type | default
--- | --- | ---
no_connect_privileged_ports | bool | false

when this is true, libtorrent will not attempt to make outgoing connections to peers whose port is < 1024. This is a safety precaution to avoid being part of a DDoS attack

name | type | default
--- | --- | ---
smooth_connects | bool | true

`smooth_connects` means the number of connection attempts per second may be limited to below the `connection_speed`, in case we're close to bump up against the limit of number of connections. The intention of this setting is to more evenly distribute our connection attempts over time, instead of attempting to connect in batches, and timing them out in batches.

name | type | default
--- | --- | ---
always_send_user_agent | bool | false

always send user-agent in every web seed request. If false, only the first request per http connection will include the user agent

name | type | default
--- | --- | ---
apply_ip_filter_to_trackers | bool | true

`apply_ip_filter_to_trackers` determines whether the IP filter applies to trackers as well as peers. If this is set to false, trackers are exempt from the IP filter (if there is one). If no IP filter is set, this setting is irrelevant.

name | type | default
--- | --- | ---
ban_web_seeds | bool | true

when true, web seeds sending bad data will be banned

name | type | default
--- | --- | ---
support_share_mode | bool | true

if false, prevents libtorrent to advertise share-mode support

name | type | default
--- | --- | ---
report_redundant_bytes | bool | true

if this is true, the number of redundant bytes is sent to the tracker

name | type | default
--- | --- | ---
listen_system_port_fallback | bool | true

if this is true, libtorrent will fall back to listening on a port chosen by the operating system (i.e. binding to port 0). If a failure is preferred, set this to false.

name | type | default
--- | --- | ---
announce_crypto_support | bool | true

when this is true, and incoming encrypted connections are enabled, &supportcrypt=1 is included in http tracker announces

name | type | default
--- | --- | ---
enable_upnp | bool | true

Starts and stops the UPnP service. When started, the listen port and the DHT port are attempted to be forwarded on local UPnP router devices.

The upnp object returned by `start_upnp()` can be used to add and remove arbitrary port mappings. Mapping status is returned through the portmap_alert and the portmap_error_alert. The object will be valid until `stop_upnp()` is called. See upnp and nat pmp.

name | type | default
--- | --- | ---
enable_natpmp | bool | true

Starts and stops the NAT-PMP service. When started, the listen port and the DHT port are attempted to be forwarded on the router through NAT-PMP.

The natpmp object returned by `start_natpmp()` can be used to add and remove arbitrary port mappings. Mapping status is returned through the portmap_alert and the portmap_error_alert. The object will be valid until `stop_natpmp()` is called. See upnp and nat pmp.

name | type | default
--- | --- | ---
enable_lsd | bool | true

Starts and stops Local Service Discovery. This service will broadcast the info-hashes of all the non-private torrents on the local network to look for peers on the same swarm within multicast reach.

name | type | default
--- | --- | ---
enable_dht | bool | true

starts the dht node and makes the trackerless service available to torrents.

name | type | default
--- | --- | ---
prefer_rc4 | bool | false

if the allowed encryption level is both, setting this to true will prefer RC4 if both methods are offered, plain text otherwise

name | type | default
--- | --- | ---
proxy_hostnames | bool | true

if true, hostname lookups are done via the configured proxy (if any). This is only supported by SOCKS5 and HTTP.

name | type | default
--- | --- | ---
proxy_peer_connections | bool | true

if true, peer connections are made (and accepted) over the configured proxy, if any. Web seeds as well as regular bittorrent peer connections are considered "peer connections". Anything transporting actual torrent payload (trackers and DHT traffic are not considered peer connections).

name | type | default
--- | --- | ---
auto_sequential | bool | true

if this setting is true, torrents with a very high availability of pieces (and seeds) are downloaded sequentially. This is more efficient for the disk I/O. With many seeds, the download order is unlikely to matter anyway

name | type | default
--- | --- | ---
proxy_tracker_connections | bool | true

if true, tracker connections are made over the configured proxy, if any.

name | type | default
--- | --- | ---
enable_ip_notifier | bool | true

Starts and stops the internal IP table route changes notifier.

The current implementation supports multiple platforms, and it is recommended to have it enable, but you may want to disable it if it's supported but unreliable, or if you have a better way to detect the changes. In the later case, you should manually call `session_handle::reopen_network_sockets` to ensure network changes are taken in consideration.

name | type | default
--- | --- | ---
dht_prefer_verified_node_ids | bool | true

when this is true, nodes whose IDs are derived from their source IP according to BEP 42 are preferred in the routing table.

name | type | default
--- | --- | ---
dht_restrict_routing_ips | bool | true

determines if the routing table entries should restrict entries to one per IP. This defaults to true, which helps mitigate some attacks on the DHT. It prevents adding multiple nodes with IPs with a very close CIDR distance.

when set, nodes whose IP address that's in the same /24 (or /64 for IPv6) range in the same routing table bucket. This is an attempt to mitigate node ID spoofing attacks also restrict any IP to only have a single entry in the whole routing table

name | type | default
--- | --- | ---
dht_restrict_search_ips | bool | true

determines if DHT searches should prevent adding nodes with IPs with very close CIDR distance. This also defaults to true and helps mitigate certain attacks on the DHT.

name | type | default
--- | --- | ---
dht_extended_routing_table | bool | true

makes the first buckets in the DHT routing table fit 128, 64, 32 and 16 nodes respectively, as opposed to the standard size of 8. All other buckets have size 8 still.

name | type | default
--- | --- | ---
dht_aggressive_lookups | bool | true

slightly changes the lookup behavior in terms of how many outstanding requests we keep. Instead of having branch factor be a hard limit, we always keep branch factor outstanding requests to the closest nodes. i.e. every time we get results back with closer nodes, we query them right away. It lowers the lookup times at the cost of more outstanding queries.

name | type | default
--- | --- | ---
dht_privacy_lookups | bool | false

when set, perform lookups in a way that is slightly more expensive, but which minimizes the amount of information leaked about you.

name | type | default
--- | --- | ---
dht_enforce_node_id | bool | false

when set, node's whose IDs that are not correctly generated based on its external IP are ignored. When a query arrives from such node, an error message is returned with a message saying "invalid node ID".

name | type | default
--- | --- | ---
dht_ignore_dark_internet | bool | true

ignore DHT messages from parts of the internet we wouldn't expect to see any traffic from

name | type | default
--- | --- | ---
dht_read_only | bool | false

when set, the other nodes won't keep this node in their routing tables, it's meant for low-power and/or ephemeral devices that cannot support the DHT, it is also useful for mobile devices which are sensitive to network traffic and battery life. this node no longer responds to 'query' messages, and will place a 'ro' key (value = 1) in the top-level message dictionary of outgoing query messages.

name | type | default
--- | --- | ---
piece_extent_affinity | bool | false

when this is true, create an affinity for downloading 4 MiB extents of adjacent pieces. This is an attempt to achieve better disk I/O throughput by downloading larger extents of bytes, for torrents with small piece sizes

name | type | default
--- | --- | ---
validate_https_trackers | bool | true

when set to true, the certificate of HTTPS trackers and HTTPS web seeds will be validated against the system's certificate store (as defined by OpenSSL). If the system does not have a certificate store, this option may have to be disabled in order to get trackers and web seeds to work).

name | type | default
--- | --- | ---
ssrf_mitigation | bool | true

when enabled, tracker and web seed requests are subject to certain restrictions.

An HTTP(s) tracker requests to localhost (loopback) must have the request path start with "/announce". This is the conventional bittorrent tracker request. Any other HTTP(S) tracker request to loopback will be rejected. This applies to trackers that redirect to loopback as well.

Web seeds that end up on the client's local network (i.e. in a private IP address range) may not include query string arguments. This applies to web seeds redirecting to the local network as well.

Web seeds on global IPs (i.e. not local network) may not redirect to a local network address

name | type | default
--- | --- | ---
allow_idna | bool | false

when disabled, any tracker or web seed with an IDNA hostname (internationalized domain name) is ignored. This is a security precaution to avoid various unicode encoding attacks that might happen at the application level.

name | type | default
--- | --- | ---
enable_set_file_valid_data | bool | false

when set to true, enables the attempt to use SetFileValidData() to pre-allocate disk space. This system call will only work when running with Administrator privileges on Windows, and so this setting is only relevant in that scenario. Using SetFileValidData() poses a security risk, as it may reveal previously deleted information from the disk.

name | type | default
--- | --- | ---
socks5_udp_send_local_ep | bool | false

When using a SOCKS5 proxy, UDP traffic is routed through the proxy by sending a UDP ASSOCIATE command. If this option is true, the UDP ASSOCIATE command will include the IP address and listen port to the local UDP socket. This indicates to the proxy which source endpoint to expect our packets from. The benefit is that incoming packets can be forwarded correctly, before any outgoing packets are sent. The risk is that if there's a NAT between the client and the proxy, the IP address specified in the protocol may not be valid from the proxy's point of view.

name | type | default
--- | --- | ---
tracker_completion_timeout | int | 30

`tracker_completion_timeout` is the number of seconds the tracker connection will wait from when it sent the request until it considers the tracker to have timed-out.

name | type | default
--- | --- | ---
tracker_receive_timeout | int | 10

`tracker_receive_timeout` is the number of seconds to wait to receive any data from the tracker. If no data is received for this number of seconds, the tracker will be considered as having timed out. If a tracker is down, this is the kind of timeout that will occur.

name | type | default
--- | --- | ---
stop_tracker_timeout | int | 5

`stop_tracker_timeout` is the number of seconds to wait when sending a stopped message before considering a tracker to have timed out. This is usually shorter, to make the client quit faster. If the value is set to 0, the connections to trackers with the stopped event are suppressed.

name | type | default
--- | --- | ---
tracker_maximum_response_length | int | 1024*1024

this is the maximum number of bytes in a tracker response. If a response size passes this number of bytes it will be rejected and the connection will be closed. On gzipped responses this size is measured on the uncompressed data. So, if you get 20 bytes of gzip response that'll expand to 2 megabytes, it will be interrupted before the entire response has been uncompressed (assuming the limit is lower than 2 MiB).

name | type | default
--- | --- | ---
piece_timeout | int | 20

the number of seconds from a request is sent until it times out if no piece response is returned.

name | type | default
--- | --- | ---
request_timeout | int | 60

the number of seconds one block (16 kiB) is expected to be received within. If it's not, the block is requested from a different peer

name | type | default
--- | --- | ---
request_queue_time | int | 3

the length of the request queue given in the number of seconds it should take for the other end to send all the pieces. i.e. the actual number of requests depends on the download rate and this number.

name | type | default
--- | --- | ---
max_allowed_in_request_queue | int | 2000

the number of outstanding block requests a peer is allowed to queue up in the client. If a peer sends more requests than this (before the first one has been sent) the last request will be dropped. the higher this is, the faster upload speeds the client can get to a single peer.

name | type | default
--- | --- | ---
max_out_request_queue | int | 500

`max_out_request_queue` is the maximum number of outstanding requests to send to a peer. This limit takes precedence over `request_queue_time`. i.e. no matter the download speed, the number of outstanding requests will never exceed this limit.

name | type | default
--- | --- | ---
whole_pieces_threshold | int | 20

if a whole piece can be downloaded in this number of seconds, or less, the peer_connection will prefer to request whole pieces at a time from this peer. The benefit of this is to better utilize disk caches by doing localized accesses and also to make it easier to identify bad peers if a piece fails the hash check.

name | type | default
--- | --- | ---
peer_timeout | int | 120

`peer_timeout` is the number of seconds the peer connection should wait (for any activity on the peer connection) before closing it due to time out. 120 seconds is specified in the protocol specification. After half the time out, a keep alive message is sent.

name | type | default
--- | --- | ---
urlseed_timeout | int | 20

same as peer_timeout, but only applies to url-seeds. this is usually set lower, because web servers are expected to be more reliable.

name | type | default
--- | --- | ---
urlseed_pipeline_size | int | 5

controls the pipelining size of url and http seeds. i.e. the number of HTTP request to keep outstanding before waiting for the first one to complete. It's common for web servers to limit this to a relatively low number, like 5

name | type | default
--- | --- | ---
urlseed_wait_retry | int | 30

number of seconds until a new retry of a url-seed takes place. Default retry value for http-seeds that don't provide a valid `retry-after` header.

name | type | default
--- | --- | ---
file_pool_size | int | 40

sets the upper limit on the total number of files this session will keep open. The reason why files are left open at all is that some anti virus software hooks on every file close, and scans the file for viruses. deferring the closing of the files will be the difference between a usable system and a completely hogged down system. Most operating systems also has a limit on the total number of file descriptors a process may have open.

name | type | default
--- | --- | ---
max_failcount | int | 3

`max_failcount` is the maximum times we try to connect to a peer before stop connecting again. If a peer succeeds, the failure counter is reset. If a peer is retrieved from a peer source (other than DHT) the failcount is decremented by one, allowing another try.

name | type | default
--- | --- | ---
min_reconnect_time | int | 60

the number of seconds to wait to reconnect to a peer. this time is multiplied with the failcount.

name | type | default
--- | --- | ---
peer_connect_timeout | int | 15

`peer_connect_timeout` the number of seconds to wait after a connection attempt is initiated to a peer until it is considered as having timed out. This setting is especially important in case the number of half-open connections are limited, since stale half-open connection may delay the connection of other peers considerably.

name | type | default
--- | --- | ---
connection_speed | int | 30

`connection_speed` is the number of connection attempts that are made per second. If a number < 0 is specified, it will default to 200 connections per second. If 0 is specified, it means don't make outgoing connections at all.

name | type | default
--- | --- | ---
inactivity_timeout | int | 600

if a peer is uninteresting and uninterested for longer than this number of seconds, it will be disconnected.

name | type | default
--- | --- | ---
unchoke_interval | int | 15

`unchoke_interval` is the number of seconds between chokes/unchokes. On this interval, peers are re-evaluated for being choked/unchoked. This is defined as 30 seconds in the protocol, and it should be significantly longer than what it takes for TCP to ramp up to it's max rate.

name | type | default
--- | --- | ---
optimistic_unchoke_interval | int | 30

`optimistic_unchoke_interval` is the number of seconds between each optimistic unchoke. On this timer, the currently optimistically unchoked peer will change.

name | type | default
--- | --- | ---
num_want | int | 200

`num_want` is the number of peers we want from each tracker request. It defines what is sent as the `&num_want=` parameter to the tracker.

name | type | default
--- | --- | ---
initial_picker_threshold | int | 4

`initial_picker_threshold` specifies the number of pieces we need before we switch to rarest first picking. The first `initial_picker_threshold` pieces in any torrent are picked at random , the following pieces are picked in rarest first order.

name | type | default
--- | --- | ---
allowed_fast_set_size | int | 5

the number of allowed pieces to send to peers that supports the fast extensions

name | type | default
--- | --- | ---
suggest_mode | int | settings_pack::no_piece_suggestions

`suggest_mode` controls whether or not libtorrent will send out suggest messages to create a bias of its peers to request certain pieces. The modes are:

- `no_piece_suggestions` which will not send out suggest messages.
- `suggest_read_cache` which will send out suggest messages for the most recent pieces that are in the read cache.

name | type | default
--- | --- | ---
max_queued_disk_bytes | int | 1024 * 1024

`max_queued_disk_bytes` is the maximum number of bytes, to be written to disk, that can wait in the disk I/O thread queue. This queue is only for waiting for the disk I/O thread to receive the job and either write it to disk or insert it in the write cache. When this limit is reached, the peer connections will stop reading data from their sockets, until the disk thread catches up. Setting this too low will severely limit your download rate.

name | type | default
--- | --- | ---
handshake_timeout | int | 10

the number of seconds to wait for a handshake response from a peer. If no response is received within this time, the peer is disconnected.

name | type | default
--- | --- | ---
send_buffer_low_watermark | int | 10 * 1024
send_buffer_watermark | int | 500 * 1024
send_buffer_watermark_factor | int | 50

`send_buffer_low_watermark` the minimum send buffer target size (send buffer includes bytes pending being read from disk). For good and snappy seeding performance, set this fairly high, to at least fit a few blocks. This is essentially the initial window size which will determine how fast we can ramp up the send rate

if the send buffer has fewer bytes than `send_buffer_watermark`, we'll read another 16 kiB block onto it. If set too small, upload rate capacity will suffer. If set too high, memory will be wasted. The actual watermark may be lower than this in case the upload rate is low, this is the upper limit.

the current upload rate to a peer is multiplied by this factor to get the send buffer watermark. The factor is specified as a percentage. i.e. 50 -> 0.5 This product is clamped to the `send_buffer_watermark` setting to not exceed the max. For high speed upload, this should be set to a greater value than 100. For high capacity connections, setting this higher can improve upload performance and disk throughput. Setting it too high may waste RAM and create a bias towards read jobs over write jobs.

name | type | default
--- | --- | ---
choking_algorithm | int | settings_pack::fixed_slots_choker
seed_choking_algorithm | int | settings_pack::round_robin

`choking_algorithm` specifies which algorithm to use to determine how many peers to unchoke. The unchoking algorithm for downloading torrents is always "tit-for-tat", i.e. the peers we download the fastest from are unchoked.

The options for choking algorithms are defined in the choking_algorithm_t enum.

`seed_choking_algorithm` controls the seeding unchoke behavior. i.e. How we select which peers to unchoke for seeding torrents. Since a seeding torrent isn't downloading anything, the tit-for-tat mechanism cannot be used. The available options are defined in the seed_choking_algorithm_t enum.

name | type | default
--- | --- | ---
disk_io_write_mode | int | DISK_WRITE_MODE
disk_io_read_mode | int | settings_pack::enable_os_cache

determines how files are opened when they're in read only mode versus read and write mode. The options are:

enable_os_cache
  Files are opened normally, with the OS caching reads and writes.
disable_os_cache
  This opens all files in no-cache mode. This corresponds to the OS not letting blocks for the files linger in the cache. This makes sense in order to avoid the bittorrent client to potentially evict all other processes' cache by simply handling high throughput and large files. If libtorrent's read cache is disabled, enabling this may reduce performance.
write_through
  flush pieces to disk as they complete validation.

One reason to disable caching is that it may help the operating system from growing its file cache indefinitely.

name | type | default
--- | --- | ---
outgoing_port | int | 0
num_outgoing_ports | int | 0

this is the first port to use for binding outgoing connections to. This is useful for users that have routers that allow QoS settings based on local port. when binding outgoing connections to specific ports, `num_outgoing_ports` is the size of the range. It should be more than a few

Warning

setting outgoing ports will limit the ability to keep multiple connections to the same client, even for different torrents. It is not recommended to change this setting. Its main purpose is to use as an escape hatch for cheap routers with QoS capability but can only classify flows based on port numbers.

It is a range instead of a single port because of the problems with failing to reconnect to peers if a previous socket to that peer and port is in `TIME_WAIT` state.

name | type | default
--- | --- | ---
peer_dscp | int | 0x04

`peer_dscp` determines the DSCP field in the IP header of every packet sent to peers (including web seeds). `0x0` means no marking, `0x04` represents Lower Effort. For more details see RFC 8622.

`peer_tos` is the backwards compatible name for this setting.

name | type | default
--- | --- | ---
active_downloads | int | 3
active_seeds | int | 5
active_checking | int | 1
active_dht_limit | int | 88
active_tracker_limit | int | 1600
active_lsd_limit | int | 60
active_limit | int | 500

for auto managed torrents, these are the limits they are subject to. If there are too many torrents some of the auto managed ones will be paused until some slots free up. `active_downloads` and `active_seeds` controls how many active seeding and downloading torrents the queuing mechanism allows. The target number of active torrents is `min(active_downloads + active_seeds, active_limit)`. `active_downloads` and `active_seeds` are upper limits on the number of downloading torrents and seeding torrents respectively. Setting the value to -1 means unlimited.

For example if there are 10 seeding torrents and 10 downloading torrents, and `active_downloads` is 4 and `active_seeds` is 4, there will be 4 seeds active and 4 downloading torrents. If the settings are `active_downloads` = 2 and `active_seeds` = 4, then there will be 2 downloading torrents and 4 seeding torrents active. Torrents that are not auto managed are not counted against these limits.

`active_checking` is the limit of number of simultaneous checking torrents.

`active_limit` is a hard limit on the number of active (auto managed) torrents. This limit also applies to slow torrents.

`active_dht_limit` is the max number of torrents to announce to the DHT.

`active_tracker_limit` is the max number of torrents to announce to their trackers.

`active_lsd_limit` is the max number of torrents to announce to the local network over the local service discovery protocol.

You can have more torrents active, even though they are not announced to the DHT, lsd or their tracker. If some peer knows about you for any reason and tries to connect, it will still be accepted, unless the torrent is paused, which means it won't accept any connections.

name | type | default
--- | --- | ---
auto_manage_interval | int | 30

`auto_manage_interval` is the number of seconds between the torrent queue is updated, and rotated.

name | type | default
--- | --- | ---
seed_time_limit | int | 24 * 60 * 60

this is the limit on the time a torrent has been an active seed (specified in seconds) before it is considered having met the seed limit criteria. See queuing.

name | type | default
--- | --- | ---
auto_scrape_interval | int | 1800
auto_scrape_min_interval | int | 300

`auto_scrape_interval` is the number of seconds between scrapes of queued torrents (auto managed and paused torrents). Auto managed torrents that are paused, are scraped regularly in order to keep track of their downloader/seed ratio. This ratio is used to determine which torrents to seed and which to pause.

`auto_scrape_min_interval` is the minimum number of seconds between any automatic scrape (regardless of torrent). In case there are a large number of paused auto managed torrents, this puts a limit on how often a scrape request is sent.

name | type | default
--- | --- | ---
max_peerlist_size | int | 3000
max_paused_peerlist_size | int | 1000

`max_peerlist_size` is the maximum number of peers in the list of known peers. These peers are not necessarily connected, so this number should be much greater than the maximum number of connected peers. Peers are evicted from the cache when the list grows passed 90% of this limit, and once the size hits the limit, peers are no longer added to the list. If this limit is set to 0, there is no limit on how many peers we'll keep in the peer list.

`max_paused_peerlist_size` is the max peer list size used for torrents that are paused. This can be used to save memory for paused torrents, since it's not as important for them to keep a large peer list.

name | type | default
--- | --- | ---
min_announce_interval | int | 5 * 60

this is the minimum allowed announce interval for a tracker. This is specified in seconds and is used as a sanity check on what is returned from a tracker. It mitigates hammering mis-configured trackers.

name | type | default
--- | --- | ---
auto_manage_startup | int | 60

this is the number of seconds a torrent is considered active after it was started, regardless of upload and download speed. This is so that newly started torrents are not considered inactive until they have a fair chance to start downloading.

name | type | default
--- | --- | ---
seeding_piece_quota | int | 20

`seeding_piece_quota` is the number of pieces to send to a peer, when seeding, before rotating in another peer to the unchoke set.

name | type | default
--- | --- | ---
max_rejects | int | 50

`max_rejects` is the number of piece requests we will reject in a row while a peer is choked before the peer is considered abusive and is disconnected.

name | type | default
--- | --- | ---
recv_socket_buffer_size | int | 0
send_socket_buffer_size | int | 0

specifies the buffer sizes set on peer sockets. 0 means the OS default (i.e. don't change the buffer sizes). The socket buffer sizes are changed using setsockopt() with SOL_SOCKET/SO_RCVBUF and SO_SNDBUFFER.

name | type | default
--- | --- | ---
max_peer_recv_buffer_size | int | 2 * 1024 * 1024

the max number of bytes a single peer connection's receive buffer is allowed to grow to.

name | type | default
--- | --- | ---
optimistic_disk_retry | int | 10 * 60

`optimistic_disk_retry` is the number of seconds from a disk write errors occur on a torrent until libtorrent will take it out of the upload mode, to test if the error condition has been fixed.

libtorrent will only do this automatically for auto managed torrents.

You can explicitly take a torrent out of upload only mode using set_upload_mode().

name | type | default
--- | --- | ---
max_suggest_pieces | int | 16

`max_suggest_pieces` is the max number of suggested piece indices received from a peer that's remembered. If a peer floods suggest messages, this limit prevents libtorrent from using too much RAM.

name | type | default
--- | --- | ---
local_service_announce_interval | int | 5 * 60

`local_service_announce_interval` is the time between local network announces for a torrent. This interval is specified in seconds.

name | type | default
--- | --- | ---
dht_announce_interval | int | 15 * 60

`dht_announce_interval` is the number of seconds between announcing torrents to the distributed hash table (DHT).

name | type | default
--- | --- | ---
udp_tracker_token_expiry | int | 60

`udp_tracker_token_expiry` is the number of seconds libtorrent will keep UDP tracker connection tokens around for. This is specified to be 60 seconds. The higher this value is, the fewer packets have to be sent to the UDP tracker. In order for higher values to work, the tracker needs to be configured to match the expiration time for tokens.

name | type | default
--- | --- | ---
num_optimistic_unchoke_slots | int | 0

`num_optimistic_unchoke_slots` is the number of optimistic unchoke slots to use. Having a higher number of optimistic unchoke slots mean you will find the good peers faster but with the trade-off to use up more bandwidth. 0 means automatic, where libtorrent opens up 20% of your allowed upload slots as optimistic unchoke slots.

name | type | default
--- | --- | ---
max_pex_peers | int | 50

the max number of peers we accept from pex messages from a single peer. this limits the number of concurrent peers any of our peers claims to be connected to. If they claim to be connected to more than this, we'll ignore any peer that exceeds this limit

name | type | default
--- | --- | ---
tick_interval | int | 500

`tick_interval` specifies the number of milliseconds between internal ticks. This is the frequency with which bandwidth quota is distributed to peers. It should not be more than one second (i.e. 1000 ms). Setting this to a low value (around 100) means higher resolution bandwidth quota distribution, setting it to a higher value saves CPU cycles.

name | type | default
--- | --- | ---
share_mode_target | int | 3

`share_mode_target` specifies the target share ratio for share mode torrents. If set to 3, we'll try to upload 3 times as much as we download. Setting this very high, will make it very conservative and you might end up not downloading anything ever (and not affecting your share ratio). It does not make any sense to set this any lower than 2. For instance, if only 3 peers need to download the rarest piece, it's impossible to download a single piece and upload it more than 3 times. If the share_mode_target is set to more than 3, nothing is downloaded.

name | type | default
--- | --- | ---
upload_rate_limit | int | 0
download_rate_limit | int | 0

`upload_rate_limit` and `download_rate_limit` sets the session-global limits of upload and download rate limits, in bytes per second. By default peers on the local network are not rate limited.

A value of 0 means unlimited.

For fine grained control over rate limits, including making them apply to local peers, see peer classes.

name | type | default
--- | --- | ---
dht_upload_rate_limit | int | 8000

the number of bytes per second (on average) the DHT is allowed to send. If the incoming requests causes to many bytes to be sent in responses, incoming requests will be dropped until the quota has been replenished.

name | type | default
--- | --- | ---
unchoke_slots_limit | int | 8

`unchoke_slots_limit` is the max number of unchoked peers in the session. The number of unchoke slots may be ignored depending on what `choking_algorithm` is set to. Setting this limit to -1 means unlimited, i.e. all peers will always be unchoked.

name | type | default
--- | --- | ---
connections_limit | int | 200

`connections_limit` sets a global limit on the number of connections opened. The number of connections is set to a hard minimum of at least two per torrent, so if you set a too low connections limit, and open too many torrents, the limit will not be met.

name | type | default
--- | --- | ---
connections_slack | int | 10

`connections_slack` is the number of incoming connections exceeding the connection limit to accept in order to potentially replace existing ones.

name | type | default
--- | --- | ---
utp_target_delay | int | 100
utp_gain_factor | int | 3000
utp_min_timeout | int | 500
utp_syn_resends | int | 2
utp_fin_resends | int | 2
utp_num_resends | int | 3
utp_connect_timeout | int | 3000
utp_loss_multiplier | int | 50

`utp_target_delay` is the target delay for uTP sockets in milliseconds. A high value will make uTP connections more aggressive and cause longer queues in the upload bottleneck. It cannot be too low, since the noise in the measurements would cause it to send too slow. `utp_gain_factor` is the number of bytes the uTP congestion window can increase at the most in one RTT. If this is set too high, the congestion controller reacts too hard to noise and will not be stable, if it's set too low, it will react slow to congestion and not back off as fast.

`utp_min_timeout` is the shortest allowed uTP socket timeout, specified in milliseconds. The timeout depends on the RTT of the connection, but is never smaller than this value. A connection times out when every packet in a window is lost, or when a packet is lost twice in a row (i.e. the resent packet is lost as well).

The shorter the timeout is, the faster the connection will recover from this situation, assuming the RTT is low enough. `utp_syn_resends` is the number of SYN packets that are sent (and timed out) before giving up and closing the socket. `utp_num_resends` is the number of times a packet is sent (and lost or timed out) before giving up and closing the connection. `utp_connect_timeout` is the number of milliseconds of timeout for the initial SYN packet for uTP connections. For each timed out packet (in a row), the timeout is doubled. `utp_loss_multiplier` controls how the congestion window is changed when a packet loss is experienced. It's specified as a percentage multiplier for `cwnd`. Do not change this value unless you know what you're doing. Never set it higher than 100.

name | type | default
--- | --- | ---
mixed_mode_algorithm | int | settings_pack::peer_proportional

The `mixed_mode_algorithm` determines how to treat TCP connections when there are uTP connections. Since uTP is designed to yield to TCP, there's an inherent problem when using swarms that have both TCP and uTP connections. If nothing is done, uTP connections would often be starved out for bandwidth by the TCP connections. This mode is `prefer_tcp`. The `peer_proportional` mode simply looks at the current throughput and rate limits all TCP connections to their proportional share based on how many of the connections are TCP. This works best if uTP connections are not rate limited by the global rate limiter (which they aren't by default).

name | type | default
--- | --- | ---
listen_queue_size | int | 5

`listen_queue_size` is the value passed in to listen() for the listen socket. It is the number of outstanding incoming connections to queue up while we're not actively waiting for a connection to be accepted. 5 should be sufficient for any normal client. If this is a high performance server which expects to receive a lot of connections, or used in a simulator or test, it might make sense to raise this number. It will not take affect until the `listen_interfaces` settings is updated.

name | type | default
--- | --- | ---
torrent_connect_boost | int | 30

`torrent_connect_boost` is the number of peers to try to connect to immediately when the first tracker response is received for a torrent. This is a boost to given to new torrents to accelerate them starting up. The normal connect scheduler is run once every second, this allows peers to be connected immediately instead of waiting for the session tick to trigger connections. This may not be set higher than 255.

name | type | default
--- | --- | ---
alert_queue_size | int | 2000

`alert_queue_size` is the maximum number of alerts queued up internally. If alerts are not popped, the queue will eventually fill up to this level. Once the alert queue is full, additional alerts will be dropped, and not delivered to the client. Once the client drains the queue, new alerts may be delivered again. In order to know that alerts have been dropped, see session_handle::dropped_alerts().

name | type | default
--- | --- | ---
max_metadata_size | int | 3 * 1024 * 10240

`max_metadata_size` is the maximum allowed size (in bytes) to be received by the metadata extension, i.e. magnet links.

name | type | default
--- | --- | ---
hashing_threads | int | 1

`hashing_threads` is the number of disk I/O threads to use for piece hash verification. These threads are in addition to the regular disk I/O threads specified by settings_pack::aio_threads. These threads are only used for full checking of torrents. The hash checking done while downloading are done by the regular disk I/O threads. The hasher threads do not only compute hashes, but also perform the read from disk. On storage optimal for sequential access, such as hard drives, this setting should be set to 1, which is also the default.

name | type | default
--- | --- | ---
checking_mem_usage | int | 256

the number of blocks to keep outstanding at any given time when checking torrents. Higher numbers give faster re-checks but uses more memory. Specified in number of 16 kiB blocks

name | type | default
--- | --- | ---
predictive_piece_announce | int | 0

if set to > 0, pieces will be announced to other peers before they are fully downloaded (and before they are hash checked). The intention is to gain 1.5 potential round trip times per downloaded piece. When non-zero, this indicates how many milliseconds in advance pieces should be announced, before they are expected to be completed.

name | type | default
--- | --- | ---
aio_threads | int | 10

for some aio back-ends, `aio_threads` specifies the number of io-threads to use.

name | type | default
--- | --- | ---
tracker_backoff | int | 250

`tracker_backoff` determines how aggressively to back off from retrying failing trackers. This value determines x in the following formula, determining the number of seconds to wait until the next retry:

> delay = 5 + 5 * x / 100 * fails^2

This setting may be useful to make libtorrent more or less aggressive in hitting trackers.

name | type | default
--- | --- | ---
share_ratio_limit | int | 200
seed_time_ratio_limit | int | 700

when a seeding torrent reaches either the share ratio (bytes up / bytes down) or the seed time ratio (seconds as seed / seconds as downloader) or the seed time limit (seconds as seed) it is considered done, and it will leave room for other torrents. These are specified as percentages. Torrents that are considered done will still be allowed to be seeded, they just won't have priority anymore. For more, see queuing.

name | type | default
--- | --- | ---
peer_turnover | int | 4
peer_turnover_cutoff | int | 90
peer_turnover_interval | int | 300

peer_turnover is the percentage of peers to disconnect every turnover peer_turnover_interval (if we're at the peer limit), this is specified in percent when we are connected to more than limit * peer_turnover_cutoff peers disconnect peer_turnover fraction of the peers. It is specified in percent peer_turnover_interval is the interval (in seconds) between optimistic disconnects if the disconnects happen and how many peers are disconnected is controlled by peer_turnover and peer_turnover_cutoff

name | type | default
--- | --- | ---
connect_seed_every_n_download | int | 10

this setting controls the priority of downloading torrents over seeding or finished torrents when it comes to making peer connections. Peer connections are throttled by the connection_speed and the half-open connection limit. This makes peer connections a limited resource. Torrents that still have pieces to download are prioritized by default, to avoid having many seeding torrents use most of the connection attempts and only give one peer every now and then to the downloading torrent. libtorrent will loop over the downloading torrents to connect a peer each, and every n:th connection attempt, a finished torrent is picked to be allowed to connect to a peer. This setting controls n.

name | type | default
--- | --- | ---
max_http_recv_buffer_size | int | 4*1024*204

the max number of bytes to allow an HTTP response to be when announcing to trackers or downloading .torrent files via the `url` provided in `add_torrent_params`.

name | type | default
--- | --- | ---
max_retry_port_bind | int | 10

if binding to a specific port fails, should the port be incremented by one and tried again? This setting specifies how many times to retry a failed port bind

name | type | default
--- | --- | ---
alert_mask | int | int

a bitmask combining flags from alert_category_t defining which kinds of alerts to receive

name | type | default
--- | --- | ---
out_enc_policy | int | settings_pack::pe_enabled
in_enc_policy | int | settings_pack::pe_enabled

control the settings for incoming and outgoing connections respectively. see enc_policy enum for the available options. Keep in mind that protocol encryption degrades performance in several respects:

1. It prevents "zero copy" disk buffers being sent to peers, since each peer needs to mutate the data (i.e. encrypt it) the data must be copied per peer connection rather than sending the same buffer to multiple peers.
2. The encryption itself requires more CPU than plain bittorrent protocol. The highest cost is the Diffie Hellman exchange on connection setup.
3. The encryption handshake adds several round-trips to the connection setup, and delays transferring data.

name | type | default
--- | --- | ---
allowed_enc_level | int | settings_pack::pe_both

determines the encryption level of the connections. This setting will adjust which encryption scheme is offered to the other peer, as well as which encryption scheme is selected by the client. See enc_level enum for options.

name | type | default
--- | --- | ---
inactive_down_rate | int | 2048
inactive_up_rate | int | 2048

the download and upload rate limits for a torrent to be considered active by the queuing mechanism. A torrent whose download rate is less than `inactive_down_rate` and whose upload rate is less than `inactive_up_rate` for `auto_manage_startup` seconds, is considered inactive, and another queued torrent may be started. This logic is disabled if `dont_count_slow_torrents` is false.

name | type | default
--- | --- | ---
proxy_type | int | settings_pack::none

proxy to use. see proxy_type_t.

name | type | default
--- | --- | ---
proxy_port | int | 0

the port of the proxy server

name | type | default
--- | --- | ---
i2p_port | int | 0

sets the i2p SAM bridge port to connect to. set the hostname with the `i2p_hostname` setting.

name | type | default
--- | --- | ---
urlseed_max_request_bytes | int | 16 * 1024 * 1024

The maximum request range of an url seed in bytes. This value defines the largest possible sequential web seed request. Lower values are possible but will be ignored if they are lower then piece size. This value should be related to your download speed to prevent libtorrent from creating too many expensive http requests per second. You can select a value as high as you want but keep in mind that libtorrent can't create parallel requests if the first request did already select the whole file. If you combine bittorrent seeds with web seeds and pick strategies like rarest first you may find your web seed requests split into smaller parts because we don't download already picked pieces twice.

name | type | default
--- | --- | ---
web_seed_name_lookup_retry | int | 1800

time to wait until a new retry of a web seed name lookup

name | type | default
--- | --- | ---
close_file_interval | int | CLOSE_FILE_INTERVAL

the number of seconds between closing the file opened the longest ago. 0 means to disable the feature. The purpose of this is to periodically close files to trigger the operating system flushing disk cache. Specifically it has been observed to be required on windows to not have the disk cache grow indefinitely. This defaults to 240 seconds on windows, and disabled on other systems.

name | type | default
--- | --- | ---
utp_cwnd_reduce_timer | int | 100

When uTP experiences packet loss, it will reduce the congestion window, and not reduce it again for this many milliseconds, even if experiencing another lost packet.

name | type | default
--- | --- | ---
max_web_seed_connections | int | 3

the max number of web seeds to have connected per torrent at any given time.

name | type | default
--- | --- | ---
resolver_cache_timeout | int | 1200

the number of seconds before the internal host name resolver considers a cache value timed out, negative values are interpreted as zero.

name | type | default
--- | --- | ---
send_not_sent_low_watermark | int | 16384

specify the not-sent low watermark for socket send buffers. This corresponds to the, Linux-specific, `TCP_NOTSENT_LOWAT` TCP socket option.

name | type | default
--- | --- | ---
rate_choker_initial_threshold | int | 1024

the rate based choker compares the upload rate to peers against a threshold that increases proportionally by its size for every peer it visits, visiting peers in decreasing upload rate. The number of upload slots is determined by the number of peers whose upload rate exceeds the threshold. This option sets the start value for this threshold. A higher value leads to fewer unchoke slots, a lower value leads to more.

name | type | default
--- | --- | ---
upnp_lease_duration | int | 3600

The expiration time of UPnP port-mappings, specified in seconds. 0 means permanent lease. Some routers do not support expiration times on port-maps (nor correctly returning an error indicating lack of support). In those cases, set this to 0. Otherwise, don't set it any lower than 5 minutes.

name | type | default
--- | --- | ---
max_concurrent_http_announces | int | 50

limits the number of concurrent HTTP tracker announces. Once the limit is hit, tracker requests are queued and issued when an outstanding announce completes.

name | type | default
--- | --- | ---
dht_max_peers_reply | int | 100

the maximum number of peers to send in a reply to `get_peers`

name | type | default
--- | --- | ---
dht_search_branching | int | 5

the number of concurrent search request the node will send when announcing and refreshing the routing table. This parameter is called alpha in the kademlia paper

name | type | default
--- | --- | ---
dht_max_fail_count | int | 20

the maximum number of failed tries to contact a node before it is removed from the routing table. If there are known working nodes that are ready to replace a failing node, it will be replaced immediately, this limit is only used to clear out nodes that don't have any node that can replace them.

name | type | default
--- | --- | ---
dht_max_torrents | int | 2000

the total number of torrents to track from the DHT. This is simply an upper limit to make sure malicious DHT nodes cannot make us allocate an unbounded amount of memory.

name | type | default
--- | --- | ---
dht_max_dht_items | int | 700

max number of items the DHT will store

name | type | default
--- | --- | ---
dht_max_peers | int | 500

the max number of peers to store per torrent (for the DHT)

name | type | default
--- | --- | ---
dht_max_torrent_search_reply | int | 20

the max number of torrents to return in a torrent search query to the DHT

name | type | default
--- | --- | ---
dht_block_timeout | int | 5 * 60

the number of seconds a DHT node is banned if it exceeds the rate limit. The rate limit is averaged over 10 seconds to allow for bursts above the limit.

name | type | default
--- | --- | ---
dht_block_ratelimit | int | 5

the max number of packets per second a DHT node is allowed to send without getting banned.

name | type | default
--- | --- | ---
dht_item_lifetime | int | 0

the number of seconds a immutable/mutable item will be expired. default is 0, means never expires.

name | type | default
--- | --- | ---
dht_sample_infohashes_interval | int | 21600

the info-hashes sample recomputation interval (in seconds). The node will precompute a subset of the tracked info-hashes and return that instead of calculating it upon each request. The permissible range is between 0 and 21600 seconds (inclusive).

name | type | default
--- | --- | ---
dht_max_infohashes_sample_count | int | 20

the maximum number of elements in the sampled subset of info-hashes. If this number is too big, expect the DHT storage implementations to clamp it in order to allow UDP packets go through

name | type | default
--- | --- | ---
max_piece_count | int | 0x200000

`max_piece_count` is the maximum allowed number of pieces in metadata received via magnet links. Loading large torrents (with more pieces than the default limit) may also require passing in a higher limit to read_resume_data() and torrent_info::parse_info_section(), if those are used.

name | type | default
--- | --- | ---
metadata_token_limit | int | 2500000

when receiving metadata (torrent file) from peers, this is the max number of bencoded tokens we're willing to parse. This limit is meant to prevent DoS attacks on peers. For very large torrents, this limit may have to be raised.

name | type | default
--- | --- | ---
disk_write_mode | int | settings_pack::mmap_write_mode_t::auto_mmap_write

controls whether disk writes will be made through a memory mapped file or via normal write calls. This only affects the mmap_disk_io. When saving to a non-local drive (network share, NFS or NAS) using memory mapped files is most likely inferior. When writing to a local SSD (especially in DAX mode) using memory mapped files likely gives the best performance. The values for this setting are specified as mmap_write_mode_t.

name | type | default
--- | --- | ---
mmap_file_size_cutoff | int | 40

when using mmap_disk_io, files smaller than this number of blocks will not be memory mapped, but will use normal pread/pwrite operations. This file size limit is specified in 16 kiB blocks.

    struct settings_pack final : settings_interface
    {
       friend  void apply_pack_impl (settings_pack const*
          , aux::session_settings_single_thread&
          , std::vector<void(aux::session_impl::*)()>*);
       void set_int (int name, flags::bitfield_flag<Type, Tag> const val);
       void set_bool (int name, bool val) override;
       void set_int (int name, int val) override;
       void set_str (int name, std::string val) override;
       bool has_val (int name) const override;
       void clear ();
       void clear (int name);
       bool get_bool (int name) const override;
       std::string const& get_str (int name) const override;
       int get_int (int name) const override;
       void for_each (Fun&& f) const;

       enum type_bases
       {
          string_type_base,
          int_type_base,
          bool_type_base,
          type_mask,
          index_mask,
       };

       enum mmap_write_mode_t
       {
          always_pwrite,
          always_mmap_write,
          auto_mmap_write,
       };

       enum suggest_mode_t
       {
          no_piece_suggestions,
          suggest_read_cache,
       };

       enum choking_algorithm_t
       {
          fixed_slots_choker,
          rate_based_choker,
          deprecated_bittyrant_choker,
       };

       enum seed_choking_algorithm_t
       {
          round_robin,
          fastest_upload,
          anti_leech,
       };

       enum io_buffer_mode_t
       {
          enable_os_cache,
          deprecated_disable_os_cache_for_aligned_files,
          disable_os_cache,
          write_through,
       };

       enum bandwidth_mixed_algo_t
       {
          prefer_tcp,
          peer_proportional,
       };

       enum enc_policy
       {
          pe_forced,
          pe_enabled,
          pe_disabled,
       };

       enum enc_level
       {
          pe_plaintext,
          pe_rc4,
          pe_both,
       };

       enum proxy_type_t
       {
          none,
          socks4,
          socks5,
          socks5_pw,
          http,
          http_pw,
          i2p_proxy,
       };
    };

[report issue]

## set_str() set_bool() set_int()

    void set_int (int name, flags::bitfield_flag<Type, Tag> const val);
    void set_bool (int name, bool val) override;
    void set_int (int name, int val) override;
    void set_str (int name, std::string val) override;

set a configuration option in the settings_pack. `name` is one of the enum values from string_types, int_types or bool_types. They must match the respective type of the set_* function.

[report issue]

## has_val()

    bool has_val (int name) const override;

queries whether the specified configuration option has a value set in this pack. `name` can be any enumeration value from string_types, int_types or bool_types.

[report issue]

## clear()

    void clear ();

clear the settings pack from all settings

[report issue]

## clear()

    void clear (int name);

clear a specific setting from the pack

[report issue]

## get_str() get_int() get_bool()

    bool get_bool (int name) const override;
    std::string const& get_str (int name) const override;
    int get_int (int name) const override;

queries the current configuration option from the settings_pack. `name` is one of the enumeration values from string_types, int_types or bool_types. The enum value must match the type of the get_* function. If the specified setting field has not been set, the default value is returned.

[report issue]

## enum type_bases

Declared in "libtorrent/settings_pack.hpp"

name | value | description
--- | --- | ---
string_type_base | 0 |
int_type_base | 16384 |
bool_type_base | 32768 |
type_mask | 49152 |
index_mask | 16383 |

[report issue]

## enum mmap_write_mode_t

Declared in "libtorrent/settings_pack.hpp"

name | value | description
--- | --- | ---
always_pwrite | 0 | disable writing to disk via mmap, always use normal write calls
always_mmap_write | 1 | prefer using memory mapped files for disk writes (at least for large files where it might make sense)
auto_mmap_write | 2 | determine whether to use pwrite or memory mapped files for disk writes depending on the kind of storage behind the save path

[report issue]

## enum suggest_mode_t

Declared in "libtorrent/settings_pack.hpp"

name | value | description
--- | --- | ---
no_piece_suggestions | 0 |
suggest_read_cache | 1 |

[report issue]

## enum choking_algorithm_t

Declared in "libtorrent/settings_pack.hpp"

name | value | description
--- | --- | ---
fixed_slots_choker | 0 | This is the traditional choker with a fixed number of unchoke slots (as specified by settings_pack::unchoke_slots_limit).
rate_based_choker | 2 | This opens up unchoke slots based on the upload rate achieved to peers. The more slots that are opened, the marginal upload rate required to open up another slot increases. Configure the initial threshold with settings_pack::rate_choker_initial_threshold. For more information, see rate based choking.
deprecated_bittyrant_choker | 3 |

[report issue]

## enum seed_choking_algorithm_t

Declared in "libtorrent/settings_pack.hpp"

name | value | description
--- | --- | ---
round_robin | 0 | which round-robins the peers that are unchoked when seeding. This distributes the upload bandwidth uniformly and fairly. It minimizes the ability for a peer to download everything without redistributing it.
fastest_upload | 1 | unchokes the peers we can send to the fastest. This might be a bit more reliable in utilizing all available capacity.
anti_leech | 2 | prioritizes peers who have just started or are just about to finish the download. The intention is to force peers in the middle of the download to trade with each other. This does not just take into account the pieces a peer is reporting having downloaded, but also the pieces we have sent to it.

[report issue]

## enum io_buffer_mode_t

Declared in "libtorrent/settings_pack.hpp"

name | value | description
--- | --- | ---
enable_os_cache | 0 |
deprecated_disable_os_cache_for_aligned_files | 1 |
disable_os_cache | 2 |
write_through | 3 |

[report issue]

## enum bandwidth_mixed_algo_t

Declared in "libtorrent/settings_pack.hpp"

name | value | description
--- | --- | ---
prefer_tcp | 0 | disables the mixed mode bandwidth balancing
peer_proportional | 1 | does not throttle uTP, throttles TCP to the same proportion of throughput as there are TCP connections

[report issue]

## enum enc_policy

Declared in "libtorrent/settings_pack.hpp"

name | value | description
--- | --- | ---
pe_forced | 0 | Only encrypted connections are allowed. Incoming connections that are not encrypted are closed and if the encrypted outgoing connection fails, a non-encrypted retry will not be made.
pe_enabled | 1 | encrypted connections are enabled, but non-encrypted connections are allowed. An incoming non-encrypted connection will be accepted, and if an outgoing encrypted connection fails, a non- encrypted connection will be tried.
pe_disabled | 2 | only non-encrypted connections are allowed.

[report issue]

## enum enc_level

Declared in "libtorrent/settings_pack.hpp"

name | value | description
--- | --- | ---
pe_plaintext | 1 | use only plain text encryption
pe_rc4 | 2 | use only RC4 encryption
pe_both | 3 | allow both

[report issue]

## enum proxy_type_t

Declared in "libtorrent/settings_pack.hpp"

name | value | description
--- | --- | ---
none | 0 | No proxy server is used and all other fields are ignored.
socks4 | 1 | The server is assumed to be a SOCKS4 server that requires a username.
socks5 | 2 | The server is assumed to be a SOCKS5 server (RFC 1928) that does not require any authentication. The username and password are ignored.
socks5_pw | 3 | The server is assumed to be a SOCKS5 server that supports plain text username and password authentication (RFC 1929). The username and password specified may be sent to the proxy if it requires.
http | 4 | The server is assumed to be an HTTP proxy. If the transport used for the connection is non-HTTP, the server is assumed to support the CONNECT method. i.e. for web seeds and HTTP trackers, a plain proxy will suffice. The proxy is assumed to not require authorization. The username and password will not be used.
http_pw | 5 | The server is assumed to be an HTTP proxy that requires user authorization. The username and password will be sent to the proxy.
i2p_proxy | 6 | route through a i2p SAM proxy

[report issue]

# setting_by_name() name_for_setting()

Declared in "libtorrent/settings_pack.hpp"

    int setting_by_name (string_view name);
    char const* name_for_setting (int s);

converts a setting integer (from the enums string_types, int_types or bool_types) to a string, and vice versa.

[report issue]

# default_settings()

Declared in "libtorrent/settings_pack.hpp"

    settings_pack default_settings ();

returns a settings_pack with every setting set to its default value

[report issue]

# min_memory_usage() high_performance_seed()

Declared in "libtorrent/session.hpp"

    settings_pack min_memory_usage ();
    settings_pack high_performance_seed ();

The default values of the session settings are set for a regular bittorrent client running on a desktop system. There are functions that can set the session settings to pre set settings for other environments. These can be used for the basis, and should be tweaked to fit your needs better.

`min_memory_usage` returns settings that will use the minimal amount of RAM, at the potential expense of upload and download performance. It adjusts the socket buffer sizes, disables the disk cache, lowers the send buffer watermarks so that each connection only has at most one block in use at any one time. It lowers the outstanding blocks send to the disk I/O thread so that connections only have one block waiting to be flushed to disk at any given time. It lowers the max number of peers in the peer list for torrents. It performs multiple smaller reads when it hashes pieces, instead of reading it all into memory before hashing.

This configuration is intended to be the starting point for embedded devices. It will significantly reduce memory usage.

`high_performance_seed` returns settings optimized for a seed box, serving many peers and that doesn't do any downloading. It has a 128 MB disk cache and has a limit of 400 files in its file pool. It support fast upload rates by allowing large send buffers.

[report issue]

# generate_fingerprint()

Declared in "libtorrent/fingerprint.hpp"

    std::string generate_fingerprint (std::string name
       , int major, int minor = 0, int revision = 0, int tag = 0);

This is a utility function to produce a client ID fingerprint formatted to the most common convention. The fingerprint can be set via the `peer_fingerprint` setting, in settings_pack.

The name string should contain exactly two characters. These are the characters unique to your client, used to identify it. Make sure not to clash with anybody else. Here are some taken id's:

id chars | client
--- | ---
LT | libtorrent (default)
UT | uTorrent
UM | uTorrent Mac
qB | qBittorrent
BP | BitTorrent Pro
BT | BitTorrent
DE | Deluge
AZ | Azureus
TL | Tribler

There's an informal directory of client id's here.

The `major`, `minor`, `revision` and `tag` parameters are used to identify the version of your client.