Filename: 186-multiple-orports.txt
Title: Multiple addresses for one OR or bridge
Author: Nick Mathewson
Created: 19-Sep-2011
Supersedes: 118
Status: Closed
Target: 0.2.4.x+
Status:
This proposal is partially implemented to the extent needed to allow nodes
to have one IPv4 and one IPv6 address.
Overview:
This document is a proposal for servers to advertise multiple
address/port combinations for their ORPort.
It supersedes proposal 118.
Motivation:
Sometimes servers want to support multiple ports for incoming
connections, either in order to support multiple address families
(ie, to add IPv6 support), to better use multiple interfaces, or
to support a variety of FascistFirewallPorts settings. This is
easy to set up now, but there's no way to advertise it to clients.
Configuring additional addresses and ports:
In consonance with our changes to the (Socks|Trans|NATD|DNS)Port
options made in 0.2.3.x for proposal 171, I make a corresponding
change to allow multiple ORPort options and deprecate
ORListenAddress.
The new syntax will be:
"ORPort" PortDescription Option*
Option = "NoAdvertise" | "NoListen" | "AllAddrs" | "IPV4Only"
| "IPV6Only"
PortDescription = PORTLIST |
ADDRESS ":" PORTLIST |
Hostname ":" PORTLIST
(PORTLIST and ADDRESS are defined below.)
The 'NoAdvertise' option performs the function of the old
ORListenAddress option. If it is set, we bind a port, but
don't put it in our descriptor.
The 'NoListen' option tells Tor to advertise an address, but not
bind to it. The operator needs to use some other mechanism to
ensure that ports are redirected to ports that _are_ listened on.
The 'AllAddrs' option tells Tor that if no address is given in the
PortDescription part, we should bind/advertise every one of our
publicly visible unicast addresses; and that if a hostname address
is given in the PortDescription, we should bind/advertise every
publicly visible unicast address that the hostname resolves to.
(Q: Should this be on by default?) The 'IPv4Only' and 'IPv6Only'
options tell Tor to interpret such situations as applying only to
IPv4 addresses or to IPv6 addresses.
As with the client *Port options, only the old format or the new
format are allowed: either a single numeric ORPort and zero or
more ORListenAddress options, or a set of one or more
ORPorts in the new extended format.
In current operating systems (unless we get into crazy nonportable
tricks) we need to use one socket for every address:port that Tor
binds on. As a sanity check, we can limit the number of such sockets
we use to, say, something between 8 and 64. If you want to bind lots
of address:port combinations, you'll want to do it at the
firewall/routing level.
Example: We want to bind on 0.0.0.0:9001
ORPort 9001
Example: Our firewall is redirecting ports 80, 443, and 7000
on all hosts in 18.244.2.0 onto our port 2929.
ORPort 2929 noadvertise
ORPort 18.244.2.0:80,443,7000 nolisten
Example: We have a dynamic DNS provider that maps
tornode.example.com to our current external IPv4 and IPv6
addresses. Our firewall forwards port 443 on those addresses to our
port 1337.
ORPort 1337 noadvertise alladdrs
ORPort tornode.example.com:443 nobind alladdrs
Self-testing:
Right now, Tor nodes need to check every port that they advertise
before they declare themselves reachable. If a Tor has
a lot of advertised ports, that could be prohibitive.
Instead, it should try a sample of ports for each address. It should
not advertise any given ORPort line until it has tried
extending to or connecting to a sample of the address/port
combinations.
It will now be possible for a Tor node to find that some addresses
work and others do not. In this case, the node should only advertise
ORPort lines that have been checked. (As a consequence, the node
should not advertise any address unless at least one ORPort without
nolisten has been specified.)
{Until support is added for extend cells to IPv6 addresses, it
will only be possible to test IPv6 addresses by connecting
directly. We might want to just skip self-testing those until we
have IPv6 extend support.}
New descriptor syntax:
We add a new line in the router descriptor, "or-address". This line
can occur zero, one, or multiple times. Its format is:
or-address SP ADDRESS ":" PORTLIST NL
ADDRESS = IPV6ADDR | IPV4ADDR
IPV6ADDR = an ipv6 address, surrounded by square brackets.
IPV4ADDR = an ipv4 address, represented as a dotted quad.
PORTLIST = PORTSPEC | PORTSPEC "," PORTLIST
PORTSPEC = PORT
PORT = a number between 1 and 65535 inclusive.
[This is the regular format for specifying sets of addresses and
ports in Tor.]
A descriptor should not include an or-address line that does
nothing but duplicate the address:port pair from its "router"
line.
A node must not list more than 8 or-address lines.
A PORTLIST must have no more than 16 PORTSPEC entries, and its entries must
be disjoint.
(Q: Any reason to allow more than 2? Multiple interfaces, I guess.)
New authority behavior:
The same rationale applies as for self-testing. An authority
needs to test the main address:port from the router line, and
every or-address line. For or-address lines that contain
multiple ports, it needs to test all of them if they are few, or a
sample if they are not.
An authority shouldn't list a node as Running unless every
or-address line it advertises looks like it will work.
Consensus directories and microdescriptors:
We introduce a new line type for microdescriptors and consensuses,
"a". Each "a" line has the same format as an or-address line.
The "a" lines (if any) appear immediately after the "r" line for a
router in the consensus, and immediately after the "onion-key"
entry in a microdescriptor.
Clients that use microdescriptors should consider a node's
addresses to be the address:port listed in the "r" line of a
consensus, plus all "a" lines for that node in the consensus, plus
all "a" lines for that node in its microdescriptor. Clients
that use full descriptors should consider a node's addresses to be
everything listed in its descriptor.
We will have to define a new voting algorithm version; when using
this version or later, votes should include a single "a" line for
every relay that has an IPv6 address, to include the first IPv6
line in its descriptor. (If there are no IPv6 or-address lines, then
they shouldn't include any "a" lines.) The remaining or-address
lines will turn into "a" lines in the microdescriptor.
As with other data in the vote derived from the descriptor, the
consensus will include whichever set of "a" lines are given by the
most authorities who voted for the descriptor digest that will be
used for the router.
Directory authorities with more addresses:
We need a way for a client to configure a TrustedDirServer as
having multiple OR addresses, specifically so that we can give at
least one default authority an IPv6 address for bootstrapping
purposes.
(Q: Do any of the current authorities have stable IPv6 addresses?)
We will want to allow the address in a "dir-source" line in a vote
to contain an IPv6 address, and/or allow voters to list themselves
with more addresses in votes/consensuses. But right now, nothing
actually uses the addresses listed for voters in dir-source lines
for anything besides log messages.
Client behavior:
I propose that initially we shouldn't change client behavior too
much here.
(Q: Is there any advantage to having a client choose a random
address? If so we can do it later. If not, why list any more
than one IPv4 and one IPv6 address?)
Tor clients not running with bridges, and running with IPv4
support, should still use the address and ORPort as advertised in
the "router" or "r" line of the appropriate directory object.
Tor clients not running with bridges, and running without IPv4
support, should use the first listed IPv6 address for a node,
using the lowest-numbered listed port for that address. They
should only connect to nodes with an IPv6 address.
Clients should accept Bridge lines with IPv6 addresses, and
address:port sets, in addition to the lines they currently accept.
Clients, for now, should only use the address:port from the router
line when making EXTEND cells; see below.
Nodes without IPv4 addresses:
Currently Tor requires every node or bridge to have an IPv4
address. We will want to maintain this property for the
foreseeable future, but we should define how a node without an IPv4
address would advertise itself.
Right now, there's no way to do that: if anything but an IPv4
address appears in a router line of a routerdesc, or the "r" line of
a consensus, then it won't parse. If something that looks like an
IPv4 address appears there, clients will (I believe) try to
connect to it.
We can make this work, though: let's allow nodes to list themselves
with a magic IPv4 address (say, 127.1.1.1) if they have
or-address entries containing only IPv6 address. We could give
these nodes a new flag other than Running to indicate that they're
up, and not give them the Running flag. That way, old clients
would never try to use them, but new clients could know to treat
the new flag as indicating that the node is running, and know not
to connect to a node listed with address 127.1.1.1.
Interaction with EXTEND and NETINFO:
Currently, EXTEND cells only support IPv4 addresses, so we should
use only those. There is a proposal draft to support more address
types.
A server's NETINFO cells must list all configured addresses for a
server.
Why not extend DirPort this way too?
Because clients are all using BEGINDIR these days.
That is, clients tunnel their directory requests inside OR
connections, and don't generally connect to DirPorts at all.
Why not have address and port ranges?
Earlier drafts of this proposal suggested that servers should provide
ranges of addresses, specified with bitmasks. That's a neat idea for
circumvention, but if we did that, you wouldn't want to advertise
publicly that you have an entire address range.
Port ranges are out because I don't think they would actually get used
much, and they add a fair bit of complexity.
Coding impact:
In addition to the obvious changes, we need to audit everything
that looks up or compares OR connections and nodes by address:port
under the assumptions that each node has only a single address or
ORPort.
TODO:
* Make it so that authorities can vote on which addresses are working
somehow.
* Specify some way to say "I only want to connect to v4/v6 addresses".
* Come up with a better alternative to running6 for the longterm?