path: root/spec/param-spec.md

# Tor network parameters

This file lists the recognized parameters that can appear on the "params"
line of a directory consensus.

<a id="param-spec.txt-1"></a>

## Network protocol parameters {#network-protocol}

"circwindow" -- the default package window that circuits should be
established with. It started out at 1000 DATA-bearing relay cells,
but some research
indicates that a lower value would mean fewer cells in transit in the
network at any given time.
Min: 100, Max: 1000, Default: 1000
First-appeared: Tor 0.2.1.20

"UseOptimisticData" -- If set to zero, clients by default shouldn't try
to send optimistic data to servers until they have received a
CONNECTED message.
Min: 0, Max: 1, Default: 1
First-appeared: 0.2.3.3-alpha
Default was 0 before: 0.2.9.1-alpha
Removed in 0.4.5.1-alpha; now always on.

"usecreatefast" -- Used to control whether clients use the CREATE_FAST
handshake on the first hop of their circuits.
Min: 0, Max: 1. Default: 1.
First-appeared: 0.2.4.23, 0.2.5.2-alpha
Removed in 0.4.5.1-alpha; now always off.

"min_paths_for_circs_pct" -- A percentage threshold that determines
whether clients believe they have enough directory information to
build circuits.  This value applies to the total fraction of
bandwidth-weighted paths that the client could build; see
path-spec.txt for more information.
Min: 25, Max: 95, Default: 60
First-appeared: 0.2.4

"ExtendByEd25519ID" -- If true, clients should include Ed25519
identities for relays when generating EXTEND2 messages.
Min: 0. Max: 1. Default: 0.
First-appeared: 0.3.0

"sendme_emit_min_version" -- Minimum SENDME version that can be sent.
Min: 0. Max: 255. Default 0.
First appeared: 0.4.1.1-alpha.

"sendme_accept_min_version" -- Minimum SENDME version that is accepted.
Min: 0. Max: 255. Default 0.
First appeared: 0.4.1.1-alpha.

"allow-network-reentry" -- If true, the Exit relays allow connections that
are exiting the network to re-enter. If false, any exit connections going
to a relay ORPort or an authority ORPort and DirPort is denied and the
stream is terminated.
Min: 0. Max: 1. Default: 0
First appeared: 0.4.5.1-alpha.

<a id="param-spec.txt-2"></a>

## Performance-tuning parameters {#performance-tuning}

"CircuitPriorityHalflifeMsec" -- the halflife parameter used when
weighting which circuit will send the next relay cell. Obeyed by Tor
0.2.2.10-alpha and later.  (Versions of Tor between 0.2.2.7-alpha and
0.2.2.10-alpha recognized a "CircPriorityHalflifeMsec" parameter, but
mishandled it badly.)
Min: 1, Max: 2147483647 (INT32_MAX), Default: 30000.
First-appeared: Tor 0.2.2.11-alpha

```text
    "perconnbwrate" and "perconnbwburst" -- if set, each relay sets up a
    separate token bucket for every client OR connection, and rate limits
    that connection independently. Typically left unset, except when used for
    performance experiments around trac entry 1750. Only honored by relays
    running Tor 0.2.2.16-alpha and later. (Note that relays running
    0.2.2.7-alpha through 0.2.2.14-alpha looked for bwconnrate and
    bwconnburst, but then did the wrong thing with them; see bug 1830 for
    details.)
    Min: 1, Max: 2147483647 (INT32_MAX), Default: (user setting of
        BandwidthRate/BandwidthBurst).
    First-appeared: 0.2.2.7-alpha
    Removed-in: 0.2.2.16-alpha
```

"NumNTorsPerTAP" -- When balancing ntor and TAP requests at relays,
how many ntor handshakes should we perform for each TAP handshake?
Min: 1. Max: 100000. Default: 10.
First-appeared: 0.2.4.17-rc

"circ_max_cell_queue_size" -- This parameter determines the maximum
number of relay cells allowed per circuit queue.
Min: 1000. Max: 2147483647 (INT32_MAX). Default: 50000.
First-appeared: 0.3.3.6-rc.

"KISTSchedRunInterval" -- How frequently should the "KIST" scheduler
run in order to decide which data to write to the network? Value in
units of milliseconds.
Min: 2. Max: 100. Default: 2
First appeared: 0.3.2

"KISTSchedRunIntervalClient" -- How frequently should the "KIST" scheduler
run in order to decide which data to write to the network, on clients? Value
in units of milliseconds. The client value needs to be much lower than
the relay value.
Min: 2. Max: 100. Default: 2.
First appeared: 0.4.8.2

<a id="param-spec.txt-3"></a>

## Voting-related parameters {#voting}

"bwweightscale" -- Value that bandwidth-weights are divided by. If not
present then this defaults to 10000.
Min: 1
First-appeared: 0.2.2.10-alpha

"maxunmeasuredbw" -- Used by authorities during voting with method 17 or
later. The maximum value to give for any Bandwidth= entry for a router
that isn't based on at least three measurements.

(Note: starting in version 0.4.6.1-alpha
there was a bug where Tor authorities would instead look at
a parameter called "maxunmeasurdbw", without the "e".
This bug was fixed in 0.4.9.1-alpha and in 0.4.8.8.
Until all relays are running a fixed version, then either this parameter
must not be set, or it must be set to the same value for both
spellings.)

First-appeared: 0.2.4.11-alpha

"FastFlagMinThreshold", "FastFlagMaxThreshold" -- lowest and highest
allowable values for the cutoff for routers that should get the Fast
flag.  This is used during voting to prevent the threshold for getting
the Fast flag from being too low or too high.
FastFlagMinThreshold: Min: 4. Max: INT32_MAX: Default: 4.
FastFlagMaxThreshold: Min: -. Max: INT32_MAX: Default: INT32_MAX
First-appeared: 0.2.3.11-alpha

"AuthDirNumSRVAgreements" -- Minimum number of agreeing directory
authority votes required for a fresh shared random value to be written in
the consensus (this rule only applies on the first commit round of the
shared randomness protocol).
Min: 1. Max: INT32_MAX. Default: 2/3 of the total number of
dirauth.

<a id="param-spec.txt-4"></a>

## Circuit-build-timeout parameters {#cbt}

"cbtdisabled", "cbtnummodes", "cbtrecentcount", "cbtmaxtimeouts",
"cbtmincircs", "cbtquantile", "cbtclosequantile", "cbttestfreq",
"cbtmintimeout", "cbtlearntimeout", "cbtmaxopencircs", and
"cbtinitialtimeout" -- see "2.4.5. Consensus parameters governing
behavior" in path-spec.txt for a series of circuit build time related
consensus parameters.

<a id="param-spec.txt-5"></a>

## Directory-related parameters {#directory}

"max-consensus-age-to-cache-for-diff" -- Determines how much
consensus history (in hours) relays should try to cache in order to
serve diffs.  (min 0, max 8192, default 72)

"try-diff-for-consensus-newer-than" -- This parameter determines how
old a consensus can be (in hours) before a client should no longer
try to find a diff for it.  (min 0, max 8192, default 72)

<a id="param-spec.txt-6"></a>

## Pathbias parameters {#pathbias}

"pb_mincircs", "pb_noticepct", "pb_warnpct", "pb_extremepct",
"pb_dropguards", "pb_scalecircs", "pb_scalefactor",
"pb_multfactor", "pb_minuse", "pb_noticeusepct",
"pb_extremeusepct", "pb_scaleuse" -- DOCDOC

<a id="param-spec.txt-7"></a>

## Relay behavior parameters {#relay-behavior}

"refuseunknownexits" -- if set to one, exit relays look at the previous
hop of circuits that ask to open an exit stream, and refuse to exit if
they don't recognize it as a relay. The goal is to make it harder for
people to use them as one-hop proxies. See trac entry 1751 for details.
Min: 0, Max: 1
First-appeared: 0.2.2.17-alpha

<a id="onion-key-rotation-days"></a>

"onion-key-rotation-days" -- (min 1, max 90, default 28)

"onion-key-grace-period-days" -- (min 1, max
onion-key-rotation-days, default 7)

Every relay should list each onion key it generates for
onion-key-rotation-days days after generating it, and then
replace it.  Relays should continue to accept their most recent
previous onion key for an additional onion-key-grace-period-days
days after it is replaced.  (Introduced in 0.3.1.1-alpha;
prior versions of tor hardcoded both of these values to 7 days.)

"AllowNonearlyExtend" -- If true, permit EXTEND/EXTEND2 requests that are not inside
RELAY_EARLY cells.
Min: 0. Max: 1. Default: 0.
First-appeared: 0.2.3.11-alpha

"overload_dns_timeout_scale_percent" -- This value is a percentage of how
many DNS timeout over N seconds we accept before reporting the overload
general state. It is scaled by a factor of 1000 in order to be able to
represent decimal point. As an example, a value of 1000 means 1%.
Min: 0. Max: 100000. Default: 1000.
First-appeared: 0.4.6.8
Deprecated: 0.4.7.3-alpha-dev

"overload_dns_timeout_period_secs" -- This value is the period in seconds
of the DNS timeout measurements (the N in the
"overload_dns_timeout_scale_percent" parameter). For this amount of
seconds, we will gather DNS statistics and at the end, we'll do an
assessment on the overload general signal with regards to DNS timeouts.
Min: 0. Max: 2147483647. Default: 600
First-appeared: 0.4.6.8
Deprecated: 0.4.7.3-alpha-dev

"overload_onionskin_ntor_scale_percent" -- This value is a percentage of
how many onionskin ntor drop over N seconds we accept before reporting the
overload general state. It is scaled by a factor of 1000 in order to be
able to represent decimal point. As an example, a value of 1000 means 1%.
Min: 0. Max: 100000. Default: 1000.
First-appeared: 0.4.7.5-alpha

"overload_onionskin_ntor_period_secs" -- This value is the period in
seconds of the onionskin ntor overload measurements (the N in the
"overload_onionskin_ntor_scale_percent" parameter). For this amount of
seconds, we will gather onionskin ntor statistics and at the end, we'll do
an assessment on the overload general signal.
Min: 0. Max: 2147483647. Default: 21600 (6 hours)
First-appeared: 0.4.7.5-alpha

"assume-reachable" -- If true, relays should publish descriptors
even when they cannot make a connection to their IPv4 ORPort.
Min: 0. Max: 1. Default: 0.
First appeared: 0.4.5.1-alpha.

"assume-reachable-ipv6" -- If true, relays should publish
descriptors even when they cannot make a connection to their IPv6
ORPort.
Min: 0. Max: 1. Default: 0.
First appeared: 0.4.5.1-alpha.

"exit_dns_timeout" -- The time in milliseconds an Exit sets libevent to
wait before it considers the DNS timed out. The corresponding libevent
option is "timeout:".
Min: 1. Max: 120000. Default: 1000 (1sec)
First appeared: 0.4.7.5-alpha.

"exit_dns_num_attempts" -- How many attempts _after the first_ should an
Exit should try a timing-out DNS query before calling it hopeless? (Each of
these attempts will wait for "exit_dns_timeout" independently). The
corresponding libevent option is "attempts:".
Min: 0. Max: 255. Default: 2
First appeared: 0.4.7.5-alpha.

<a id="param-spec.txt-8"></a>

## V3 onion service parameters {#onion-service}

"hs_intro_min_introduce2", "hs_intro_max_introduce2" --
Minimum/maximum amount of INTRODUCE2 messages allowed per circuit
before rotation (actual amount picked at random between these two
values).
Min: 0. Max: INT32_MAX. Defaults: 16384, 32768.

"hs_intro_min_lifetime", "hs_intro_max_lifetime" -- Minimum/maximum
lifetime in seconds that a service should keep an intro point for
(actual lifetime picked at random between these two values).
Min: 0. Max: INT32_MAX. Defaults: 18 hours, 24 hours.

"hs_intro_num_extra" -- Number of extra intro points a service is
allowed to open.  This concept comes from proposal #155.
Min: 0. Max: 128. Default: 2.

"hsdir_interval" -- The length of a time period, _in minutes_. See
rend-spec-v3.txt section \[TIME-PERIODS\].
Min: 30. Max: 14400. Default: 1440.

"hsdir_n_replicas" -- Number of HS descriptor replicas.
Min: 1. Max: 16. Default: 2.

"hsdir_spread_fetch" -- Total number of HSDirs per replica a tor
client should select to try to fetch a descriptor.
Min: 1. Max: 128. Default: 3.

"hsdir_spread_store" -- Total number of HSDirs per replica a service
will upload its descriptor to.
Min: 1. Max: 128. Default: 4

"HSV3MaxDescriptorSize" -- Maximum descriptor size (in bytes).
Min: 1. Max: INT32_MAX. Default: 50000

"hs_service_max_rdv_failures" -- This parameter determines the
maximum number of rendezvous attempt an HS service can make per
introduction.
Min 1. Max 10. Default 2.
First-appeared: 0.3.3.0-alpha.

<a id="HiddenServiceEnableIntroDoSDefense"></a>
"HiddenServiceEnableIntroDoSDefense" -- This parameter makes
introduction points
start using a rate-limiting defense if they support it.
Introduction points use this value when no
[`DOS_PARAMS` extension] is sent in the ESTABLISH_INTRO message.
Min: 0. Max: 1. Default: 0.
First appeared: 0.4.2.1-alpha.

<a id="HiddenServiceEnableIntroDoSBurstPerSec"></a>
"HiddenServiceEnableIntroDoSBurstPerSec" -- Default maximum burst
rate to be used
for token bucket for the introduction point rate-limiting.
Introduction points use this value when no
[`DOS_PARAMS` extension] is sent in the ESTABLISH_INTRO message.
Min: 0. Max: INT32_MAX. Default: 200
First appeared: 0.4.2.1-alpha.

> Note that the above parameter is slightly misnamed:
> a burst is not meaningfully "per second".

<a id="HiddenServiceEnableIntroDoSRatePerSec"></a>
"HiddenServiceEnableIntroDoSRatePerSec" -- Default maximum rate per second
to be used
for token bucket for the introduction point rate-limiting.
Introduction points use this value when no
[`DOS_PARAMS` extension] is sent.
Min: 0. Max: INT32_MAX. Default: 25
First appeared: 0.4.2.1-alpha.

[`DOS_PARAMS` extension]: ./rend-spec/introduction-protocol.md#DOS_PARAMS

<a id="param-spec.txt-9"></a>

## Denial-of-service parameters {#dos}

Denial of Service mitigation parameters. Introduced in 0.3.3.2-alpha:

"DoSCircuitCreationEnabled" -- Enable the circuit creation DoS
mitigation.

"DoSCircuitCreationMinConnections" -- Minimum threshold of
concurrent connections before a client address can be flagged as
executing a circuit creation DoS

"DoSCircuitCreationRate" -- Allowed circuit creation rate per second
per client IP address once the minimum concurrent connection
threshold is reached.

"DoSCircuitCreationBurst" -- The allowed circuit creation burst per
client IP address once the minimum concurrent connection threshold
is reached.

```text
    "DoSCircuitCreationDefenseType" -- Defense type applied to a
    detected client address for the circuit creation mitigation.
        1: No defense.
        2: Refuse circuit creation for the length of
          "DoSCircuitCreationDefenseTimePeriod".
```

"DoSCircuitCreationDefenseTimePeriod" -- The base time period that
the DoS defense is activated for.

"DoSConnectionEnabled" -- Enable the connection DoS mitigation.

"DoSConnectionMaxConcurrentCount" -- The maximum threshold of
concurrent connection from a client IP address.

```text
    "DoSConnectionDefenseType" -- Defense type applied to a detected
    client address for the connection mitigation. Possible values are:
        1: No defense.
        2: Immediately close new connections.
```

"DoSRefuseSingleHopClientRendezvous" -- Refuse establishment of
rendezvous points for single hop clients.

"DoSStreamCreationEnabled" -- Enable the stream creation DoS
mitigation.
First appeared: 0.4.9.0-alpha-dev.

"DoSStreamCreationRate" -- Allowed stream creation rate per second
per circuit.
First appeared: 0.4.9.0-alpha-dev.

"DoSStreamCreationBurst" -- The allowed stream creation burst per
circuit.
First appeared: 0.4.9.0-alpha-dev.

```text
    "DoSStreamCreationDefenseType" -- Defense type applied to a
    stream for the stream creation mitigation.
        1: No defense.
        2: Reject the stream or resolve request.
        3: Close the underlying circuit.
    First appeared: 0.4.9.0-alpha-dev.
```

<a id="param-spec.txt-10"></a>

## Padding-related parameters {#padding}

"circpad_max_circ_queued_cells" -- The circuitpadding module will
stop sending more padding relay cells if more than this many cells are in
the circuit queue a given circuit.
Min: 0. Max: 50000. Default 1000.
First appeared: 0.4.0.3-alpha.

"circpad_global_allowed_cells" -- This is the number of padding relay cells
that must be sent before the 'circpad_global_max_padding_percent'
parameter is applied.
Min: 0. Max: 65535. Default: 0

"circpad_global_max_padding_pct" -- This is the maximum ratio of
padding relay cells to total relay cells, specified as a percent. If the global
ratio of padding cells to total cells across all circuits exceeds
this percent value, no more padding is sent until the ratio becomes
lower. 0 means no limit.
Min: 0. Max: 100. Default: 0

"circpad_padding_disabled" -- If set to 1, no circuit padding machines
will negotiate, and all current padding machines will cease padding
immediately.
Min: 0. Max: 1. Default: 0

"circpad_padding_reduced" -- If set to 1, only circuit padding
machines marked as "reduced"/"low overhead" will be used.
(Currently no such machines are marked as "reduced overhead").
Min: 0. Max: 1. Default: 0

"nf_conntimeout_clients"
  - The number of seconds to keep never-used circuits opened and
    available for clients to use. Note that the actual client timeout is
    randomized uniformly from this value to twice this value.
  - The number of seconds to keep idle (not currently used) canonical
    channels are open and available. (We do this to ensure a sufficient
    time duration of padding, which is the ultimate goal.)
  - This value is also used to determine how long, after a port has been
    used, we should attempt to keep building predicted circuits for that
    port. (See path-spec.txt section 2.1.1.)  This behavior was
    originally added to work around implementation limitations, but it
    serves as a reasonable default regardless of implementation.
  - For all use cases, reduced padding clients use half the consensus
    value.
  - Implementations MAY mark circuits held open past the reduced padding
    quantity (half the consensus value) as "not to be used for streams",
    to prevent their use from becoming a distinguisher.
Min: 60. Max: 86400. Default: 1800

"nf_conntimeout_relays" -- The number of seconds that idle
relay-to-relay connections are kept open.
Min: 60. Max: 604800. Default: 3600

"nf_ito_low" -- The low end of the range to send padding when
inactive, in ms.
Min: 0. Max: 60000. Default: 1500

"nf_ito_high" -- The high end of the range to send padding, in ms.
If nf_ito_low == nf_ito_high == 0, padding will be disabled.
Min: nf_ito_low. Max: 60000. Default: 9500

"nf_ito_low_reduced" -- For reduced padding clients: the low
end of the range to send padding when inactive, in ms.
Min: 0. Max: 60000. Default: 9000

"nf_ito_high_reduced" -- For reduced padding clients: the high
end of the range to send padding, in ms.
Min: nf_ito_low_reduced. Max: 60000. Default: 14000

"nf_pad_before_usage" -- If set to 1, OR connections are padded
before the client uses them for any application traffic. If 0,
OR connections are not padded until application data begins.
Min: 0. Max: 1. Default: 1

"nf_pad_relays" -- If set to 1, we also pad inactive
relay-to-relay connections.
Min: 0. Max: 1. Default: 0

"nf_pad_single_onion" -- DOCDOC

<a id="param-spec.txt-11"></a>

## Guard-related parameters

(See guard-spec.txt for more information on the vocabulary used here.)

"UseGuardFraction" -- If true, clients use `GuardFraction`
information from the consensus in order to decide how to weight
guards when picking them.
Min: 0. Max: 1. Default: 0.
First appeared: 0.2.6

"guard-lifetime-days" -- Controls guard lifetime. If an unconfirmed
guard has been sampled more than this many days ago, it should be
removed from the guard sample.
Min: 1. Max: 3650. Default: 120.
First appeared: 0.3.0

"guard-confirmed-min-lifetime-days" -- Controls confirmed guard
lifetime: if a guard was confirmed more than this many days ago, it
should be removed from the guard sample.
Min: 1. Max: 3650. Default: 60.
First appeared: 0.3.0

"guard-internet-likely-down-interval" -- If Tor has been unable to
build a circuit for this long (in seconds), assume that the internet
connection is down, and treat guard failures as unproven.
Min: 1. Max: INT32_MAX. Default: 600.
First appeared: 0.3.0

"guard-max-sample-size" -- Largest number of guards that clients
should try to collect in their sample.
Min: 1. Max: INT32_MAX. Default: 60.
First appeared: 0.3.0

"guard-max-sample-threshold-percent" -- Largest bandwidth-weighted
fraction of guards that clients should try to collect in their
sample.
Min: 1. Max: 100. Default: 20.
First appeared: 0.3.0

"guard-meaningful-restriction-percent" -- If the client has
configured tor to exclude so many guards that the available guard
bandwidth is less than this percentage of the total, treat the guard
sample as "restricted", and keep it in a separate sample.
Min: 1. Max: 100. Default: 20.
First appeared: 0.3.0

"guard-extreme-restriction-percent" -- Warn the user if they have
configured tor to exclude so many guards that the available guard
bandwidth is less than this percentage of the total.
Min: 1. Max: 100. Default: 1.
First appeared: 0.3.0.  MAX was INT32_MAX, which would have no meaningful
effect.  MAX lowered to 100 in 0.4.7.

"guard-min-filtered-sample-size" -- If fewer than this number of
guards is available in the sample after filtering out unusable
guards, the client should try to add more guards to the sample (if
allowed).
Min: 1. Max: INT32_MAX. Default: 20.
First appeared: 0.3.0

"guard-n-primary-guards" -- The number of confirmed guards that the
client should treat as "primary guards".
Min: 1. Max: INT32_MAX. Default: 3.
First appeared: 0.3.0

```text
    "guard-n-primary-guards-to-use", "guard-n-primary-dir-guards-to-use"
    -- number of primary guards and primary directory guards that the
    client should be willing to use in parallel.  Other primary guards
    won't get used unless the earlier ones are down.
    "guard-n-primary-guards-to-use":
       Min 1, Max INT32_MAX: Default: 1.
    "guard-n-primary-dir-guards-to-use"
       Min 1, Max INT32_MAX: Default: 3.
    First appeared: 0.3.0
```

"guard-nonprimary-guard-connect-timeout" -- When trying to confirm
nonprimary guards, if a guard doesn't answer for more than this long
in seconds, treat lower-priority guards as usable.
Min: 1. Max: INT32_MAX. Default: 15
First appeared: 0.3.0

"guard-nonprimary-guard-idle-timeout" -- When trying to confirm
nonprimary guards, if a guard doesn't answer for more than this long
in seconds, treat it as down.
Min: 1. Max: INT32_MAX. Default: 600
First appeared: 0.3.0

"guard-remove-unlisted-guards-after-days" -- If a guard has been
unlisted in the consensus for at least this many days, remove it
from the sample.
Min: 1. Max: 3650. Default: 20.
First appeared: 0.3.0

<a id="param-spec.txt-X"></a>

## Obsolete parameters {#obsolete}

"NumDirectoryGuards", "NumEntryGuards" -- Number of guard nodes
clients should use by default.  If NumDirectoryGuards is 0, we
default to NumEntryGuards.
NumDirectoryGuards: Min: 0. Max: 10. Default: 0
NumEntryGuards:     Min: 1. Max: 10. Default: 3
First-appeared: 0.2.4.23, 0.2.5.6-alpha
Removed in: 0.3.0

"GuardLifetime" -- Duration for which clients should choose guard
nodes, in seconds.
Min: 30 days.  Max: 1826 days.  Default: 60 days.
First-appeared: 0.2.4.12-alpha
Removed in: 0.3.0.

"UseNTorHandshake" -- If true, then versions of Tor that support
NTor will prefer to use it by default.
Min: 0,  Max: 1. Default: 1.
First-appeared: 0.2.4.8-alpha
Removed in: 0.2.9.

"Support022HiddenServices" -- Used to implement a mass switch-over
from sending timestamps to hidden services by default to sending no
timestamps at all.  If this option is absent, or is set to 1,
clients with the default configuration send timestamps; otherwise,
they do not.
Min: 0, Max: 1. Default: 1.
First-appeared: 0.2.4.18-rc
Removed in: 0.2.6