Skip to main content

Simulating failures in Arti

The main reason for simulating failures is to ensure that Arti's behavior is "generally reasonable" when the network is down or misbehaving, when the localhost is set up in a confusing way, etc.

Here "generally reasonable" means that we aren't making a huge number of connections to the network or wasting a huge amount of bandwidth. Similarly, we shouldn't be using huge amounts of CPU, or filling up the logs at level info or higher.

The primary purpose of simulating failures is to verify that Arti behaves appropriately in scenarios where the network is down or misconfigured, or when the localhost setup is confusing. This includes avoiding excessive network connections, bandwidth consumption, CPU usage, and preventing log overflow at levels info or higher.

It is an extra benefit if we can ensure that our bootstrap reporting mechanisms give us accurate feedback in these cases, and diagnose the problem accurately.

Most of the examples here will use the arti-testing tool. Some will also use a small Chutney network. In either case, you'll need an explicit client configuration, since arti-testing does not want you to use the default; we'll assume you've put it in ${ARTI_CONF}.

Note: You would not need to use chutney in these cases if Arti is in fact well-behaved. Although, it is helpful to do so if you think there might be problems in Arti's behavior because you would not want to flood the real network.

This guide assumes that you have a Linux environment, and explains how to simulate different kinds of bootstrapping and network failures in Arti.

What to look at​

The output from arti-testing will tell you whether bootstrapping succeeded or failed. If bootstrapping is not expected to succeed, try adding --timeout ${DELAY} --expect timeout to indicate that the operation isn't supposed to succeed, and should eventually time out.

If bootstrapping or connecting succeeds when it shouldn't, then the test went wrong.

When we're done, arti-testing will tell us some statistics about TCP connections and log messages. Here is an example of a not-too-bad attempt to bootstrap over 30 seconds:

TCP stats: TcpCount { n_connect_attempt: 1, n_connect_ok: 1, n_accept: 0, n_bytes_send: 17223, n_bytes_recv: 59092 }
Total events: Trace: 159, Debug: 14, Info: 16, Warn: 8, Error: 0

Whereas, this is an example of problematic behavior over a similar period:

Timeout occurred [as expected]
TCP stats: TcpCount { n_connect_attempt: 1220, n_connect_ok: 1220, n_accept: 0, n_bytes_send: 1394460, n_bytes_recv: 4267636 }
Total events: Trace: 13431, Debug: 2088, Info: 2383, Warn: 15, Error: 0

These require the faketime tool.

System clock set wrong, no directory cached​

Start with an empty cache and empty state file, then run:

faketime ${WHEN} arti-testing bootstrap -c ${ARTI_CONF} --timeout 30

Try this with different values of WHEN:

  • '4 hours ago'
  • '1 day ago'
  • '1 month ago'
  • '1 day'
  • '1 month'
  • '1 year'

System clock set wrong, live directory cached.​

Again, if you run the following command with an empty cache and state file as a starting point, it is expected to succeed.

arti-testing bootstrap -c ${ARTI_CONF}

Now run:

faketime ${WHEN} arti-testing connect -c ${ARTI_CONF} \
        --target www.torproject.org:80 \
        --timeout 30 --retry 0

Try this with different values of WHEN as above. This simulates a case where we previously bootstrapped with a reasonably live directory, but we wound up with a wrong clock when we restarted.

System clock set wrong, obsolete directory cached​

You can simulate this with a directory that you made before, then copied into your cache directory. Use faketime to set the current time to a point at which the directory was valid, or recently valid.

Note: This test won't work well with chutney, since chutney directory lifetimes are very short.

The arti-testing tool can simulate multiple kinds of errors:

  • connections fail immediately or after a little while(-tcp-failure error --tcp-failure-delay 1)
  • connections time out and never succeed (-tcp-failure timeout)
  • connections succeed, but drop all data and say nothing. (-tcp-failure blackhole)

You can arrange for these failures to start in the bootstrap phase (--tcp-failure-stage bootstrap) or in the connect stage (--tcp-failure-stage connect).

With these options, you can simulate different kinds of failures by starting with an empty directory cache (and optionally empty state). The bootstrap phase failures correspond to failures on your fallback directories, while the connect-phase failures correspond to failures on the live network.

There's an issue here where if you have open connections to the fallbacks, the TCP-failure code won't yet make them start failing when you connect to the network. As a workaround, bootstrap in a separate arti-testing call, then connect with TCP failures enabled.

For example:

To fail during bootstrapping, clear your cache and run the command:

arti-testing bootstrap -c ${ARTI_CONF} --timeout 30 --tcp-failure error

To fail after bootstrapping, clear your cache then run the commands:

# This should succeed
arti-testing bootstrap -c ${ARTI_CONF}

# This will fail.
arti-testing connect -c ${ARTI_CONF} \
        --target www.torproject.org:80 \
        --timeout 30 --retry 0 \
        --tcp-failure blackhole

Partial network blocking​

You can make the above network failures conditional, to simulate different kinds of broken local networks. Try --tcp-failure-on v4 to simulate an IPv4-only network, or --tcp-failure-on non443 to simulate a network that blocks everything but HTTPS.

These won't work with chutney networks, since a typical chutney network's relays are all on IPv4 with high ports.

Network identity mismatch​

One way to generate a diverse range of failures is to combine and experiment with the arti.toml files from two different chutney networks. Older chutney networks can be found in subdirectories of $CHUTNEY_PATH/net/, except the nodes directory.

If you use an older set of fallback directories, you'll simulate the case where the client can't actually connect to any fallback directories because its beliefs about their identities are all wrong.

If you keep the running set of fallback directories, but use the older set of authorities, you'll simulate the case where the client fetches a directory, but doesn't believe in any authorities that signed it.

For both of these cases, start with an empty cache and use the arti-testing bootstrap command.

Last updated on