[tor-dev] Proposal 312: Automatic Relay IPv6 Addresses
teor
teor at riseup.net
Wed Jan 29 14:03:49 UTC 2020
Hi,
Here is an initial draft of Proposal 312: Automatic Relay IPv6 Addresses.
This proposal includes:
* relay auto IPv6 addresses, and
* relay auto IPv6 ORPorts.
This is the second of 3 proposals:
* Proposal 311: Relay IPv6 Reachability
* Proposal 312: Automatic Relay IPv6 Addresses
* Proposal 313: Relay IPv6 Statistics
(I haven't written the final one yet.)
I also want to make some minor changes to Proposal 306, so that bridge
IPv6 behaviour stays in sync with client IPv6 behaviour. (See section
7 of this proposal for details.)
There is still one TODO item in the proposal, about Tor's current
behaviour. If you know the answer, please let me know.
The full text is included below, and it is also available as a GitHub
pull request:
https://github.com/torproject/torspec/pull/105
The related tickets are #33073 (proposal) and #5940 (implementation):
https://trac.torproject.org/projects/tor/ticket/33073
https://trac.torproject.org/projects/tor/ticket/5940
Please feel free to reply on this list, or via GitHub pull request
comments.
Filename: 312-relay-auto-ipv6-addr.txt
Title: Tor Relays Automatically Find Their IPv6 Address
Author: teor
Created: 28-January-2020
Status: Draft
Ticket: #33073
0. Abstract
We propose that Tor relays (and bridges) should automatically find their
IPv6 address, and use it to publish an IPv6 ORPort. For some relays to find
their IPv6 address, they may need to fetch some directory documents from
directory authorities over IPv6. (For anonymity reasons, bridges are unable
to fetch directory documents over IPv6, until clients start to do so.)
1. Introduction
Tor relays (and bridges) currently find their IPv4 address, and use it as
their ORPort and DirPort address when publishing their descriptor. But
relays and bridges do not automatically find their IPv6 address.
However, relay operators can manually configure an ORPort with an IPv6
address, and that ORPort is published in their descriptor in an "or-address"
line (see [Tor Directory Protocol]).
Many relay operators don't know their relay's IPv4 or IPv6 addresses. So
they rely on Tor's IPv4 auto-detection, and don't configure an IPv6
address. When operators do configure an IPv6 address, it's easy for them to
make mistakes. IPv6 ORPort issues are a significant source of relay
operator support requests.
Implementing IPv6 address auto-detection, and IPv6 ORPort reachability
checks (see [Proposal 311: Relay IPv6 Reachability]) will increase the
number of working IPv6-capable relays in the tor network.
2. Scope
This proposal modifies Tor's behaviour as follows:
Relays, bridges, and directory authorities:
* automatically find their IPv6 address, and
* for consistency between IPv4 and IPv6 detection:
* start using IPv4 ORPort and DirPort for IPv4 address detection, and
* re-order IPv4 address detection methods.
Relays and directory authorities (but not bridges):
* fetch some directory documents over IPv6.
For anonymity reasons, bridges are unable to fetch directory documents over
IPv6, until clients start to do so. (See
[Proposal 306: Client Auto IPv6 Connections].)
This proposal makes a small, optional change to existing client behaviour:
* clients also check IPv6 addresses when rotating TLS keys for new
networks.
In addition to the changes to IPv4 address resolution, most of which won't
affect clients. (Because they do not set Address, ORPort, or DirPort.)
Throughout this proposal, "relays" includes directory authorities, except
where they are specifically excluded. "relays" does not include bridges,
except where they are specifically included. (The first mention of "relays"
in each section should specifically exclude or include these other roles.)
When this proposal describes Tor's current behaviour, it covers all
supported Tor versions (0.3.5.7 to 0.4.2.5), as of January 2020, except
where another version is specifically mentioned.
3. Finding Relay IPv6 Addresses
We propose that tor relays (and bridges) automatically find their IPv6
address, and use it to publish an IPv6 ORPort.
For some relays to find their IPv6 address, they may need to fetch some
directory documents from directory authorities over IPv6. (For anonymity
reasons, bridges are unable to fetch directory documents over IPv6, until
clients start to do so.)
3.1. Current Relay IPv4 Address Implementation
Currently, all relays (and bridges) must have an IPv4 address. IPv6
addresses are optional for relays.
Tor currently tries to find relay IPv4 addresses in this order:
1. the Address torrc option
2. the address of the hostname (resolved using DNS, if needed)
3. a local interface address
(by making a self-connected socket, if needed)
4. an address reported by a directory server (using X-Your-Address-Is)
When using the Address option, or the hostname, tor supports:
* an IPv4 address literal, or
* resolving an IPv4 address from a hostname.
If tor is running on the public network, and an address isn't globally
routable, tor ignores it. (If it was explicitly set in Address, tor logs an
error.)
If there are multiple valid addresses, tor chooses:
* the first address returned by the resolver,
* the first address returned by the local interface API, or
* the latest address returned by a directory server.
3.2. Finding Relay IPv6 Addresses
We propose that relays (and bridges) try to find their IPv6 address. For
consistency, we also propose to change the address resolution order for
IPv4 addresses.
We use the following general principles to choose the order of IP address
methods:
* Explicit is better than Implicit,
* Local Information is better than a Remote Dependency,
* Trusted is better than Untrusted, and
* Reliable is better than Unreliable.
Within these constraints, we try to find the simplest working design.
Therefore, we propose that tor tries to find relay IPv4 and IPv6 addresses
in this order:
1. the Address torrc option
2. the advertised ORPort address
3. the advertised DirPort address (IPv4 only; relays, not bridges)
4. a local interface address
(by making a self-connected socket, if needed)
5. the address of the host's own hostname (resolved using DNS, if needed)
6. an address reported by a directory server (using X-Your-Address-Is)
(Each of these address resolution steps is described in more detail, in its
own subsection.)
While making these changes, we want to preserve tor's existing behaviour:
* resolve Address using the local resolver, if needed,
* ignore private addresses on public tor networks, and
* when there are multiple valid addresses, choose the first or latest
address, as appropriate.
3.2.1. Make the Address torrc Option Support IPv6
First, we propose that relays (and bridges) use the Address torrc option
to find their IPv4 and IPv6 addresses.
There are two cases we need to cover:
1. Explicit IP addresses:
* allow the option to be specified up to two times,
* use the IPv4 address for IPv4,
* use the IPv6 address for IPv6.
Configuring two addresses in the same address family is a config error.
2. Hostnames / DNS names:
* allow the option to be specified up to two times,
* look up the configured name,
* use the first IPv4 and IPv6 address returned by the resolver, and
Resolving multiple addresses in the same address family is not a
runtime error, but only the first address from each family will be
used.
These lookups should ignore private addresses on public tor networks. If
multiple IPv4 or IPv6 addresses are returned, the first public address from
each family should be used.
We should also support the following combinations:
A. IPv4 Address / hostname (for IPv6 only),
B. IPv6 Address / hostname (for IPv4 only),
C. IPv4 Address only / try to guess IPv6, then check its reachability
(see section 4.3.1 in [Proposal 311: Relay IPv6 Reachability]), and
D. IPv6 Address only / guess IPv4, then its reachability must succeed.
There are also similar configurations where a hostname is configured, but it
only provides IPv4 or IPv6 addresses.
Combination C is the most common legacy configuration. We want to
support the following outcomes for legacy configurations:
* automatic upgrades to guessed and reachable IPv6 addresses,
* continuing to operate on IPv4 when the IPv6 address can't be guessed,
and
* continuing to operate on IPv4 when the IPv6 address has been guessed,
but it is unreachable.
At this time, we do not propose guessing multiple IPv4 or IPv6 addresses
and testing their reachability (see section 3.4.2).
It is an error to configure an Address option with a private IPv4 or IPv6
address, or with a hostname that does not resolve to any publicly routable
IPv4 or IPv6 addresses.
If the Address option is not configured for IPv4 or IPv6, or the hostname
lookups do not provide both IPv4 and IPv6 addresses, address resolution
should go to the next step.
3.2.2. Use the Advertised ORPort IPv4 and IPv6 Addresses
Next, we propose that relays (and bridges) use the first advertised ORPort
IPv4 and IPv6 addresses, as configured in their torrc.
The ORPort address may be a hostname. If it is, tor should try to use it to
resolve an IPv4 and IPv6 address, and open ORPorts on the first available
IPv4 and IPv6 address. Tor should respect the IPv4Only and IPv6Only port
flags, if specified. (Tor currently resolves IPv4 addresses in ORPort
lines. It may not look for an IPv6 address.)
Relays (and bridges) currently use the first advertised ORPort IPv6 address
as their IPv6 address. We propose to use the first advertised IPv4 ORPort
address in a similar way, for consistency.
Therefore, this change may affect existing relay IPv4 addressses. We expect
that a small number of relays may change IPv4 address, from a guessed IPv4
address, to their first advertised IPv4 ORPort address.
In rare cases, relays may have been using non-advertised ORPorts for their
addresses. This change may also change their addresses.
We propose ignoring private configured ORPort addresses on public tor
networks. (Binding to private ORPort addresses is supported, even on public
tor networks, for relays that use NAT to reach the Internet.) If an ORPort
address is private, address resolution should go to the next step.
3.2.3. Use the Advertised DirPort IPv4 Address
Next, we propose that relays use the first advertised DirPort IPv4 address,
as configured in their torrc.
The following DirPort configurations can not be used for address
resolution, because they are not supported:
* bridge DirPorts, and
* advertised IPv6 DirPorts.
The DirPort address may be a hostname. If it is, tor should try to use it to
resolve an IPv4 address, and open a DirPort on the first available IPv4
address. Tor should only look for IPv6 addresses if the IPv6Only port flag
is specified. (Since advertised IPv6 DirPorts are not supported, a
working configuration may also require the NoAdvertise flag.)
Relays currently use the first advertised ORPort IPv6 address as their IPv6
address. We propose to use the first advertised IPv4 DirPort address in a
similar way, for consistency.
Therefore, this change may affect existing relay IPv4 addressses. We expect
that a very small number of relays may change IPv4 address, from a guessed
IPv4 address, to their first advertised IPv4 DirPort address. (But we expect
that most relays that change will be using their ORPort address.)
We propose ignoring private configured DirPort addresses on public relays.
(Binding to private DirPort addresses is supported, for networks that use
NAT.) If a DirPort address is private, address resolution should go to the
next step.
3.2.4. Use Local Interface IPv6 Address
Next, we propose that relays (and bridges) use publicly routable addresses
from the OS interface addresses or routing table, as their IPv4 and IPv6
addresses.
Tor has local interface address resolution functions, which support most
major OSes. Tor uses these functions to guess its IPv4 address. We propose
using them to also guess tor's IPv6 address.
We also propose modifying the address resolution order, so interface
addresses are used before the local hostname. This decision is based
on our principles: interface addresses are local, trusted, and reliable;
hostname lookups may be remote, untrusted, and unreliable.
Some developer documentation also recommends using interface addresses,
rather than resolving the host's own hostname. For example, on recent
versions of macOS, the man pages tell developers to use interface addresses
(getifaddrs) rather than look up the host's own hostname (gethostname and
getaddrinfo). Unfortunately, these man pages don't seem to be available
online, except for short quotes (see [getaddrinfo man page] for the
relevant quote).
If the local interface addresses are unavailable, tor opens a self-connected
UDP socket to a publicly routable address, but doesn't actually send any
packets. Instead, it uses the socket APIs to discover the interface address
for the socket.
Tor already ignores private IPv4 interface addresses on public relays.
(Binding to private DirPort addresses is supported, for networks that use
NAT.) We propose to also ignore private IPv6 interface addresses. If all
IPv4 or IPv6 interface addresses are private, address resolution should go
to the next step.
3.2.5. Use Own Hostname IPv6 Addresses
Next, we propose that relays (and bridges) get their local hostname, look
up its addresses, and use them as its IPv4 and IPv6 addresses.
We propose to use the same underlying lookup functions to look up the IPv4
and IPv6 addresses for:
* the Address torrc option (see section 3.2.1), and
* the local hostname.
However, OS APIs typically only return a single hostname.
Even though the hostname lookup may use remote DNS, we propose to use it on
directory authorities, to maintain compatibility with current
configurations. Even if it is remote, we expect the configured DNS to be
somewhat trusted by the operator.
The hostname lookup should ignore private addresses on public relays. If
multiple IPv4 or IPv6 addresses are returned, the first public address from
each family should be used. If all IPv4 or IPv6 hostname addresses are
private, address resolution should go to the next step.
3.2.6. Use Directory Header IPv6 Addresses
Finally, we propose that relays get their IPv4 and IPv6 addresses from the
X-Your-Address-Is HTTP header in tor directory documents. To support this
change, we propose that relays start fetching directory documents over IPv4
and IPv6.
We propose that bridges continue to only fetch directory documents over
IPv4, because they try to imitate clients. (Most clients only fetch
directory documents over IPv4, a few clients are configured to only fetch
over IPv6.) When client behaviour changes to use both IPv4 and IPv6 for
directory fetches, bridge behaviour can also change to match. (See
section 3.4.1 and [Proposal 306: Client Auto IPv6 Connections].)
We propose that directory authorities should ignore addresses in directory
headers. Allowing other authorities (or relays?) to change a directory
authority's published IP address may lead to security issues. Instead,
if interface and hostname lookups fail, tor should stop address resolution,
and return a permanent error. (And issue a log to the operator, see below.)
We propose to use a simple load balancing scheme for IPv4 and IPv6
directory requests:
* choose between IPv4 and IPv6 directory requests at random.
We do not expect this change to have any load-balancing impact on the public
tor network, because the number of relays is much smaller than the number
of clients. However, the 6 directory authorities with IPv6 enabled may see
slightly more directory load, particularly over IPv6.
To support this change, tor should also change how it handles IPv6
directory failures on relays:
* avoid recording IPv6 directory failures as remote relay failures,
because they may actually be due to a lack of IPv6 connectivity on the
local relay, and
* issue IPv6 directory failure logs at notice level, and rate-limit them
to one per hour.
If a relay is:
* explicitly configured with an IPv6 address, or
* a publicly routable, reachable IPv6 address is discovered in an
earlier step,
tor should start issuing IPv6 directory failure logs at warning level.
(Alternately, tor could stop doing IPv6 directory requests entirely. But we
prefer designs where all relays behave in a similar way, regardless of their
internal state.)
For some more complex directory load-balancing schemes, see section 3.5.2.
Tor already ignores private IPv4 addresses in directory headers. We propose
to also ignore private IPv6 addresses in directory headers. If all IPv4 and
IPv6 addresses in directory headers are private, address resolution should
pause, and return a temporary error.
Whenever address resolution fails, tor should warn the operator to set the
Address torrc option for IPv4 and IPv6. (If IPv4 is available, and only
IPv6 is missing, the log should be at notice level.)
Address resolution should continue the next time tor receives a directory
header containing a public IPv4 or IPv6 address.
3.2.7. Disabling IPv6 Address Resolution
Relays that have a reachable IPv6 address, but that address is unsuitable
for the relay, need to be able to disable IPv6 address resolution.
Based on [Proposal 311: Relay IPv6 Reachability], and this proposal, those
relays would:
* discover their IPv6 address,
* open an IPv6 ORPort,
* find it reachable,
* publish a descriptor containing that IPv6 ORPort,
* have the directory authorities find it reachable,
* have it published in the consensus, and
* have it used by clients.
Currently, relays are required to have an IPv4 address. So if the guessed
IPv4 address is unsuitable, operators can set the Address option to a
suitable IPv4 address. But IPv6 addresses are optional, so relay operators
may need to disable IPv6 entirely.
We propose a new torrc-only option, AddressDisableIPv6. This option is set
to 0 by default. If the option is set to 1, tor disables IPv6 address
resolution, IPv6 ORPorts, IPv6 reachability checks, and publishing an IPv6
ORPort in its descriptor.
3.2.8. Automatically Enabling an IPv6 ORPort
We propose that relays that discover their IPv6 address, should open an
ORPort on that address, and test its reachability (see
[Proposal 311: Relay IPv6 Reachability], particularly section 4.3.1).
The ORPort should be opened on the port configured in the relay's ORPort
torrc option. Relay operators can use the IPv4Only and IPv6Only options
to configure different ports for IPv4 and IPv6.
If both reachability checks succeed, relays should publish their IPv4 and
IPv6 ORPorts in their descriptor.
If only the IPv4 ORPort check succeeds, and the IPv6 address was guessed
(rather than being explicitly configured), then relays should publish their
IPv4 ORPort in their descriptor.
3.3. Consequential Tor Client Changes
We do not propose any required client address resolution changes at this
time.
However, clients will use the updated address resolution functions to detect
when they are on a new connection, and therefore need to rotate their TLS
keys.
This minor client change allows us to avoid keeping an outdated version of
the address resolution functions, which is only for client use.
Clients should skip address resolution steps that don't apply to them, such
as:
* the ORPort option,
* the DirPort option, and
* the Address option, if it becomes a relay module option.
3.4. Alternative Address Resolution Designs
We briefly mention some potential address resolution designs, and the
reasons that they were not used in this proposal.
(Some designs may be proposed for future Tor versions, but are not necessary
at this time.)
3.4.1. Future Bridge IPv6 Address Resolution Behaviour
When clients automatically fetch directory documents via relay IPv4 and
IPv6 ORPorts by default, bridges should also adopt this dual-stack
behaviour. (For example, see [Proposal 306: Client Auto IPv6 Connections].)
When bridges fetch directory documents via IPv6, they will be able to find
their IPv6 address using directory headers (see 3.2.6).
3.4.2. Guessing Muliple IPv4 or IPv6 Addresses
We avoid designs which guess (or configure) multiple IPv4 or IPv6
addresses, test them all for reachability, and choose one that works.
Using multiple addresses is rare, and the code to handle it is complex. It
also requires careful design to avoid:
* conflicts between multiple relays on the same address
(tor allows up to 2 relays per IPv4 address)
* flapping / race conditions / address switching
* rare
3.4.3. Rejected Address Resolution Designs
We reject designs that try all the different address resolution methods,
score addresses, and then choose the address with the highest score.
These designs are a generalisation of designs that try different methods in
a set order (like this proposal). They are more complex than required.
Complex designs can confuse operators, particularly when they fail.
Operators should not need complex address resolution in tor: most relay
addresses are fixed, or change occasionally. And most relays can reliably
discover their address using directory headers, if all other methods fail.
If complex address resolution is required, it can be configured using a
dynamic DNS name in the Address torrc option, or via the control port.
We also avoid designs that use any addresses other than the first
(or latest) valid IPv4 and IPv6 address. These designs are more complex, and
they don't have clear benefits:
* sort addresses numerically (avoid address flipping)
* sort addresses by length, then numerically
(also minimise consensus size)
* store a list of previous addresses in the state file, and use the most
recently used address that's currently available.
Operators who want to avoid address flipping should set the Address option
in the torrc. Operators who want to minimise the size of the consensus
should use all-zero IPv6 host identifiers.
3.5. Optional Efficiency and Reliability Changes
We propose some optional changes for efficiency and reliability, and
describe their impact.
Some of these changes may be more appropriate in future releases, or
along with other proposed features.
3.5.1. Only Use Authenticated Directory Header IPv4 and IPv6 Addresses
We propose this optional change, to improve relay address accuracy and
reliability.
Relays should only use:
* authenticated directory fetches,
* to directory authorities,
to discover their own IPv4 and IPv6 addresses. (Both changes are optional,
and they can be made separately.)
Tor supports authenticated, encrypted directory fetches using BEGINDIR over
ORPorts (see the [Tor Specification] for details).
Relays currently fetch unencrypted directory documents over DirPorts. The
directory document itself is signed, but the HTTP headers are not
authenticated. (Clients and bridges only fetch directory documents using
authenticated directory fetches.)
Using authenticated directory headers for relay addresses:
* avoids caches (or other machines) mangling X-Your-Address-Is headers in
transit, and
* avoids attacks where directories are deliberately given an incorrect IP
address.
To make this change, we need to modify two different parts of tor:
* when making directory requests, relays should fetch some directory
documents using BEGINDIR over ORPorts, and
* when using the X-Your-Address-Is HTTP header to guess their own IPv4 or
IPv6 addresses, relays ignore directory documents that were not fetched
using BEGINDIR over ORPorts.
Optionally, relays should also ignore addresses from other relays:
* when using the X-Your-Address-Is HTTP header to guess their own IPv4 or
IPv6 addresses, relays ignore directory documents that were not fetched
from directory authorities.
Ideally, we would like all relays to do all their directory fetches:
* using BEGINDIR over ORPorts, and
* to directory authorities.
However, this change may be unsustainable during high network load
(see [Ticket 33018: Dir auths using an unsustainable 400+ mbit/s]).
Therefore, we propose a simple load-balancing scheme between address
resolution and non-address resolution requests:
* when relays do not know their own IP addresses, they should make as many
address resolution directory fetches as is sustainable, and
* when relays know their own IP addresses, they should make an occasional
address resolution directory fetch, to learn if their address has
changed.
We use the load-balancing criteria in the next section, to select the ratio
between:
* ORPort connections to directory authorities, and
* DirPort connections to directory mirrors.
3.5.1.1. General Load Balancing Criteria
We propose the following criteria for choosing load-balancing ratios:
The selected ratios should be chosen based on the following factors:
* the current number of directory fetches that a relay makes:
* when bootstrapping with an empty cache directory, and
* in a steady state (per hour, or per new consensus),
(these numbers aren't currently collected by tor, so we may need to
write some extra code to include them in the heartbeat logs),
* relays need to discover their IPv4 and IPv6 addresses to bootstrap,
* it only takes one successful directory fetch from one authority for a
relay to discover its IP address,
* BEGINDIR over ORPort requires and TLS connection, and some additional
tor cryptography, so it is more expensive for authorities than a
DirPort fetch (and it can not be cached by a HTTP cache),
* minimising wasted CPU (and bandwidth) on IPv4-only relays, and
* other potential changes to relay directory fetches (see
[Ticket 33018: Dir auths using an unsustainable 400+ mbit/s])
The selected ratios should allow almost all relays to update both their IPv4
and IPv6 addresses:
* at least twice when they bootstrap (to allow for fetch failures), and
* at least once per directory fetch (or per hour).
In this proposal, relays choose between IPv4 and IPv6 directory fetches
at random (see section 3.2.6 for more detail). See the next section for
an alternative load-balancing scheme.
3.5.2. Load Balancing Between IPv4 and IPv6 Directories
We propose this optional change, to improve the load-balancing between IPv4
and IPv6 directories, when used by relays to find their IPv4 and IPv6
addresses (see section 3.2.6).
This change may only be necessary if the following changes result in poor
load-balancing, or other relay issues:
* randomly selecting IPv4 or IPv6 directories (see section 3.2.6), or
* only using directory headers for addresses when they come from directory
authorities, via an authenticated connection (see section 3.5.1).
We propose a new torrc option and consensus parameter:
MaxNumIPv4DirectoryAttempts. This option limits the number of IPv4 directory
requests, before the relay makes an IPv6 directory request. It should only
apply to attempts that are expected to provide a usable IPv4 or IPv6
address in their directory header. (Based on sections 3.2.6 and 3.5.1.)
The design is similar to MaxNumIPv4BootstrapAttempts in
[Proposal 306: Client Auto IPv6 Connections].
Here is a quick sketch of the design:
* when MaxNumIPv4DirectoryAttempts is reached, select an IPv6-capable
directory, and make an IPv6 connection attempt,
* use a directory authority, or an ORPort, if required (see section 3.5.1),
* use a default value between 2 and 4:
* the ideal value for load-balancing is >= 2
(because 6/9 directory authorities are already on IPv6)
* the ideal value for minimising failures is ~4
(because relays won't waste too much CPU or bandwidth)
* choose the default value based on the load-balancing criteria in section
3.5.1.1.
Alternately, we could wait until
[Proposal 306: Client Auto IPv6 Connections] is implemented, and use the
directory fetch design from that proposal.
3.5.3. Detailed Address Resolution Logs
We propose this optional change, to help diagnose relay address resolution
issues.
Relays (and bridges) should log the address chosen using each address
resolution method, when:
* address resolution succeeds,
* address resolution fails,
* reachability checks fail, or
* publishing the descriptor fails.
These logs should be rate-limited separately for successes and failures.
The logs should tell operators to set the Address torrc option for IPv4 and
IPv6 (if available).
3.5.4. Add IPv6 Support to is_local_addr()
We propose this optional change, to improve the accuracy of IPv6 address
detection from directory documents.
Directory servers use is_local_addr() to detect if the requesting tor
instance is on the same local network. If it is, the directory server does
not include the X-Your-Address-Is HTTP header in directory documents.
Currently, is_local_addr() checks for:
* an internal IPv4 or IPv6 address, or
* the same IPv4 /24 as the directory server.
We propose also checking for:
* the same IPv6 /48 as the directory server.
We choose /48 because it is typically the smallest network in the global
IPv6 routing tables, and it was previously the recommended per-customer
network block. (See [RFC 6177: IPv6 End Site Address Assignment].)
Tor currently uses:
* IPv4 /8 and IPv6 /16 for port summaries,
* IPv4 /16 and IPv6 /32 for path selection (avoiding relays in the same
network block).
3.5.5. Add IPv6 Support to AuthDirMaxServersPerAddr
We propose this optional change, to improve the health of the network, by
rejecting too many relays on the same IPv6 address.
Modify get_possible_sybil_list() so it takes an address family argument,
and returns a list of IPv4 or IPv6 sybils.
Use the modified get_possible_sybil_list() to exclude relays from the
authority's vote, if there are more than AuthDirMaxServersPerAddr on the
same IPv4 or IPv6 address.
Since these relay exclusions happen at voting time, they do not require a
new consensus method.
3.5.6. Use a Local Interface Address on the Default Route
We propose this optional change, to improve the accuracy of local interface
IPv4 and IPv6 address detection (see section 3.2.4).
Rewrite the get_interface_address*() functions to choose an interface
address on the default route, or to sort default route addresses first in
the list of addresses. (If the platform API allows us to find the default
route.)
For more information, see [Ticket 12377: Prefer default route when checking
local interface addresses].
This change may not be necessary, because the directory header IP address
method will find the IP address of the default route, in most cases
(see section 3.2.6).
3.5.7. Add IPv6 Support Using gethostbyname2()
We propose these optional changes, to add IPv6 support to hostname
resolution on older OSes. These changes affect:
* the Address torrc option, when it is a hostname (see section 3.2.1),
and
* automatic hostname resolution (see section 3.2.5).
Use gethostbyname2() to add IPv6 support to hostname resolution on older
OSes, which don't support getaddrinfo().
But this change may be unnecessary, because:
* Linux has used getaddrinfo() by default since glibc 2.20 (2014)
* macOS has recommended getaddrinfo() since before 2006
* since macOS adopts BSD changes, most BSDs would have switched to
getaddrinfo() in a similar timeframe
* Windows has supported getaddrinfo() since Windows Vista; tor's minimum
supported Windows version is Vista.
See [Tor Supported Platforms] for more details.
When looking up hostnames using gethostbyname() or gethostbyname2(), if the
first address is a private address, we may want to look at the entire list
of addresses. Some struct hostent versions (example: current macOS) also
have a h_addr_list rather than h_addr. (They define h_addr as
h_addr_list[0], for backwards compatibility.)
However, having private and public addresses resolving from the same
hostname is a rare configuration, so we may not need to make this change.
(On OSes that support getaddrinfo(), tor searches the list of addresses for
a publicly routable address.)
As an alternative, if we believe that all supported OSes have getaddrinfo(),
we could simply remove the gethostbyname() code, rather than trying to
modify it to work with IPv6.
Most relays can reliably discover their address using directory headers,
if all other methods fail. Or operators can set the Address torrc option to
an IPv4 or IPv6 literal.
3.5.8. Change Relay OutboundBindAddress Defaults
We propose this optional change, to improve the reliability of
IP address-based filters in tor.
For example, the tor network treats relay IP addresses differently when:
* resisting denial of service, and
* selecting canonical, long-term connections.
(See [Ticket 33018: Dir auths using an unsustainable 400+ mbit/s] for the
initial motivation for this change: resisting significant bandwidth load
on directory authorities.)
Now that tor knows its own addresses, we propose that relays (and bridges)
set their IPv4 and IPv6 OutboundBindAddress to these discovered addresses,
by default. If binding fails, tor should fall back to an unbound socket.
Operators would still be able to set a custom IPv4 and IPv6
OutboundBindAddress, if needed.
Currently, tor doesn't bind to a specific address, unless
OutboundBindAddress is configured. So on relays with multiple IP addresses,
the outbound address comes from the chosen route (usually the default
route).
4. Directory Protocol Specification Changes
We propose explicitly supporting IPv6 X-Your-Address-Is HTTP headers in the
tor directory protocol.
We propose the following changes to the [Tor Directory Protocol]
specification, in section 6.1:
Servers MAY include an X-Your-Address-Is: header, whose value is the
apparent IPv4 or IPv6 address of the client connecting to them. IPv6
addresses SHOULD/MAY (TODO) be formatted enclosed in square brackets.
TODO: require brackets? What does Tor currently do?
For directory connections tunneled over a BEGIN_DIR stream, servers SHOULD
report the IP from which the circuit carrying the BEGIN_DIR stream reached
them.
Servers SHOULD disable caching of multiple network statuses or multiple
server descriptors. Servers MAY enable caching of single descriptors,
single network statuses, the list of all server descriptors, a v1
directory, or a v1 running routers document, with appropriate expiry times
(around 30 minutes). Servers SHOULD disable caching of X-Your-Address-Is
headers.
5. Test Plan
We provide a quick summary of our testing plans.
5.1. Test Find Relay IPv6 Addresses
We propose to test these changes using chutney networks. However, chutney
creates a limited number of configurations, so we also need to test these
changes with relay operators on the public network.
Therefore, we propose to test these changes on the public network with a
small number of relays and bridges.
Once these changes are merged, volunteer relay and bridge operators will be
able to test them by:
* compiling from source,
* running nightly builds, or
* running alpha releases.
5.2. Test Existing Features
We will modify and test these existing features:
* Find Relay IPv4 Addresses
We do not plan on modifying these existing features:
* relay address retries
* existing warning logs
But we will test that they continue to function correctly, and fix any bugs
triggered by the modifications in this proposal.
6. Ongoing Monitoring
To monitor the impact of these changes, relays should collect basic IPv4
and IPv6 connection and bandwidth statistics (see [Proposal 313: Relay IPv6
Statistics]).
We may also collect separate statistics on connections from:
* clients (and bridges, because they act like clients), and
* other relays (and authorities, because they act like relays).
Some of these statistics may be included in tor's heartbeat logs, making
them accessible to relay operators.
We do not propose to collect additional statistics on:
* bridges,
* address resolution,
* circuit counts, or
* failure rates.
Collecting statistics like these could impact user privacy, or relay
security.
7. Changes to Other Proposals
[Proposal 306: Client Auto IPv6 Connections] needs to be modified to keep
bridge IPv6 behaviour in sync with client IPv6 behaviour. (See section
3.2.6.)
References:
[getaddrinfo man page]: See the quoted section in https://stackoverflow.com/a/42351676
[Proposal 306: Client Auto IPv6 Connections]: One possible design for automatic client IPv4 and IPv6 connections is at https://gitweb.torproject.org/torspec.git/tree/proposals/306-ipv6-happy-eyeballs.txt (TODO: modify to include bridge changes with client changes)
[Proposal 311: Relay IPv6 Reachability]: https://gitweb.torproject.org/torspec.git/tree/proposals/311-relay-ipv6-reachability.txt
[Proposal 313: Relay IPv6 Statistics]: https://gitweb.torproject.org/torspec.git/tree/proposals/313-relay-ipv6-stats.txt (TODO)
[RFC 6177: IPv6 End Site Address Assignment]: https://tools.ietf.org/html/rfc6177#page-7
[Ticket 12377: Prefer default route when checking local interface addresses]: https://trac.torproject.org/projects/tor/ticket/12377
[Ticket 33018: Dir auths using an unsustainable 400+ mbit/s]: https://trac.torproject.org/projects/tor/ticket/33018
[Tor Directory Protocol]: (version 3) https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt
[Tor Specification]: https://gitweb.torproject.org/torspec.git/tree/tor-spec.txt
[Tor Supported Platforms]: https://trac.torproject.org/projects/tor/wiki/org/teams/NetworkTeam/SupportedPlatforms#OSSupportlevels
(End)
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