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ts-024-sni-blocking.md

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Specification version number

2020-11-12-001

  • status: experimental

Specification name

SNI blocking (sni_blocking)

Test preconditions

  • An internet connection

Expected impact

Understanding whether there is blocking triggered by the content of the TLS Hello's SNI field. For a given SNI/domain, this nettest uses a test helper server, rather than talking to the server for the specified SNI/domain. This design is especially beneficial when additional blocking rules may cause the DNS to return bogus responses, or the IP address for the SNI/domain to be unreachable/filtered.

Expected inputs

  • control_sni (string): a SNI to use as control (e.g. example.org)

  • testhelper (endpoint; optional): endpoint where TLS is enabled expressed as IPv4:port, [IPv6]:port, or domain:port (e.g. 1.1.1.1:443)

  • targets ([]Target) one or more SNIs to measure

If testhelper is not specified we use ${control_sni}:443.

The default implementation will use a domain such as example.org as the control_sni and the empty string as testhelper endpoint. This will effectively cause us to use example.org:443 (or whatever domain is actually used on port 443) as the testhelper endpoint.

A valid Target is a valid domain name (e.g. kernel.org), a valid IP address (e.g. 1.1.1.1), or a valid URL (e.g. http://x.org). When the input is a URL, the experiment will extract the domain from the URL and use that as target SNI, ignoring any scheme, port, path, etc.

The user should be able to specify the above parameters from the CLI.

Test description

For every target, this experiment will:

  1. if target is a URL, parse it and set target as the URL's hostname

  2. determine testhelper from control_sni if needed

  3. connect to testhelper using target as SNI

  4. connect to testhelper using control_sni as SNI

The implementation may (i) randomly delay the moment where steps 3. and 4. start such that step 3. does not strictly run before 4.; (ii) cache the result of step 4. to avoid repeating it for every input target.

Expected output

{
    "test_keys": {
        "control": {},
        "result": "",
        "target": {}
    }
}

  • control (Subresult): data collected by step 4 above

  • result (string): classification of the result

  • target (Subresult): data collected by step 3 above

A Subresult data structure looks like:

{
    "failure": null,
    "network_events": [],
    "queries": [],
    "sni": "",
    "tcp_connect": [],
    "th_address": "",
    "tls_handshakes": []
}

  • failure (string; nullable): null on success, string on error as documented in df-007-errors.md;

  • network_events ([]NetworkEvent; nullable): see df-008-netevents;

  • queries ([]Query; nullable): see df-002-dnst;

  • requests ([]Transaction; nullable): see df-001-httpt;

  • sni (string): SNI being used;

  • tcp_connect ([]TCPConnect; nullable): see df-005-tcpconnect;

  • th_address (string): address of the test helper (see above);

  • tls_handshakes ([]Handshake; nullable): see df-006-tlshandshake.

We expect requests to be null unless we're using DoH; queries to be null when testhelper is an IP.

The result string summarizes what happens during the nettest. Its value is one of the following:

  • "anomaly.test_helper_unreachable": if testhelper is a domain we could not resolve the domain, or we could not connect to testhelper, or we saw a timeout when measuring the target and also the control measurement failed with any error. This is anomaly because we need to look into the data to understand whether the test helper is down, blocked, or what.

  • "anomaly.timeout": the control measurement succeded, but we did saw an I/O timeout when measuring with the target SNI. This is anomaly because the timeout may be explained by conditions different from blocking.

  • "anomaly.unexpected_failure": when measuring the target SNI we did saw a failure other than the set of failures we expected. This is anomaly and we want to look into this measurement and improve our implementation.

  • "interference.closed": the connection was closed during the TLS handshake. We flag this as interference because we expect the test helper to return a TLS Server Hello message to us. It might also be the case that the test helper is very busy and closes incoming connections. When there is doubt, we can increase our confidence by inspecting the control measurement as well as other measurements with the same report ID.

  • "interference.reset": like "interference.closed" except that the connection is resetted rather than just being gracefully closed.

  • "interference.invalid_certificate": we got a TLS Server Hello message back, but the certificate in it is invalid (e.g. expired). This is very likely to be a sign of interference, unless the test helper has an expired certificate. A future version of this document will explain how the probe should handle this specific corner case.

  • "interference.unknown_authority": we got a TLS Server Hello message back, but we don't know the authority signing the certificate. This is a sign of man in the middle in most cases, even though there may be corner cases leading to false positives. A future version of this document may provide recommendations concerning detecting such false positives.

  • "success.got_server_hello": we were able to get a TLS Server Hello back from the server, without any of the above errors. This is what we expect to happen when there's no interference.

Any other value should be treated as "anomaly.unexpected_failure" by code that is processing the JSON measurement.

Parent data format

See the above fields description.

Required output data

Fields described above (mind that many are nullable).

Possible conclusions

We examine the failure field of control and target. Because we're performing a TLS handshake with a TLS server that may not support the specified SNIs, we consider null and ssl_invalid_hostname as indicators of success. We consider any other error as potentially an anomaly.

We cannot immediately exclude the presence of a MITM box that forwards legitimate traffic and returns invalid certificates otherwise, thus causing ssl_invalid_hostname replies. However, the sequence of network events may possibly be useful to detect these specific cases.

For this reason, it is ideal to select as test helper endpoint one that knows how to handle one the control SNI, which is what the implementation of this experiment should be doing by default.

Example output sample

{
  "data_format_version": "0.3.4",
  "input": "blocked.com",
  "measurement_start_time": "2020-01-28 15:27:18",
  "test_runtime": 0.335919812,
  "probe_asn": "AS30722",
  "probe_cc": "IT",
  "probe_ip": "127.0.0.1",
  "report_id": "20200128T152718Z_AS30722_ppk81LCnhT5Ok1P6909MX9XG8L6jmgSZkJyhx8KTmN4LAVAgGX",
  "resolver_asn": "AS30722",
  "resolver_ip": "91.80.36.88",
  "resolver_network_name": "Vodafone Italia S.p.A.",
  "software_name": "miniooni",
  "software_version": "0.1.0-dev",
  "test_keys": {
    "control": {
      "failure": null,
      "network_events": [
        {
          "address": "37.218.245.90:443",
          "conn_id": 1,
          "dial_id": 1,
          "failure": null,
          "operation": "connect",
          "proto": "tcp",
          "t": 0.249971
        },
        {
          "conn_id": 1,
          "failure": null,
          "num_bytes": 262,
          "operation": "write",
          "proto": "tcp",
          "t": 0.250393
        },
        {
          "conn_id": 1,
          "failure": null,
          "num_bytes": 517,
          "operation": "read",
          "proto": "tcp",
          "t": 0.294079
        },
        {
          "conn_id": 1,
          "failure": null,
          "num_bytes": 923,
          "operation": "read",
          "proto": "tcp",
          "t": 0.294199
        },
        {
          "conn_id": 1,
          "failure": null,
          "num_bytes": 1440,
          "operation": "read",
          "proto": "tcp",
          "t": 0.294405
        },
        {
          "conn_id": 1,
          "failure": null,
          "num_bytes": 294,
          "operation": "read",
          "proto": "tcp",
          "t": 0.295752
        },
        {
          "conn_id": 1,
          "failure": null,
          "num_bytes": 85,
          "operation": "write",
          "proto": "tcp",
          "t": 0.296475
        },
        {
          "conn_id": 1,
          "failure": null,
          "num_bytes": 43,
          "operation": "read",
          "proto": "tcp",
          "t": 0.335268
        },
        {
          "conn_id": 1,
          "failure": null,
          "num_bytes": 23,
          "operation": "write",
          "proto": "tcp",
          "t": 0.335577
        }
      ],
      "queries": [
        {
          "answers": [
            {
              "answer_type": "A",
              "ipv4": "37.218.245.90",
              "ttl": null
            }
          ],
          "dial_id": 1,
          "engine": "system",
          "failure": null,
          "hostname": "ps.ooni.io",
          "query_type": "A",
          "resolver_hostname": null,
          "resolver_port": null,
          "resolver_address": "",
          "t": 0.211333
        },
        {
          "answers": null,
          "dial_id": 1,
          "engine": "system",
          "failure": null,
          "hostname": "ps.ooni.io",
          "query_type": "AAAA",
          "resolver_hostname": null,
          "resolver_port": null,
          "resolver_address": "",
          "t": 0.211333
        }
      ],
      "requests": null,
      "sni": "ps.ooni.io",
      "tcp_connect": [
        {
          "conn_id": 1,
          "dial_id": 1,
          "ip": "37.218.245.90",
          "port": 443,
          "status": {
            "failure": null,
            "success": true
          },
          "t": 0.249971
        }
      ],
      "th_address": "ps.ooni.io:443",
      "tls_handshakes": [
        {
          "cipher_suite": "TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305",
          "conn_id": 1,
          "failure": null,
          "negotiated_protocol": "",
          "peer_certificates": [
            {
              "data": "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",
              "format": "base64"
            },
            {
              "data": "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",
              "format": "base64"
            }
          ],
          "t": 0.335446,
          "tls_version": "TLSv1.2"
        }
      ]
    },
    "target": {
      "failure": "ssl_invalid_hostname",
      "network_events": [
        {
          "address": "37.218.245.90:443",
          "conn_id": 2,
          "dial_id": 2,
          "failure": null,
          "operation": "connect",
          "proto": "tcp",
          "t": 0.249802
        },
        {
          "conn_id": 2,
          "failure": null,
          "num_bytes": 263,
          "operation": "write",
          "proto": "tcp",
          "t": 0.250285
        },
        {
          "conn_id": 2,
          "failure": null,
          "num_bytes": 517,
          "operation": "read",
          "proto": "tcp",
          "t": 0.298164
        },
        {
          "conn_id": 2,
          "failure": null,
          "num_bytes": 923,
          "operation": "read",
          "proto": "tcp",
          "t": 0.298283
        },
        {
          "conn_id": 2,
          "failure": null,
          "num_bytes": 1440,
          "operation": "read",
          "proto": "tcp",
          "t": 0.298325
        },
        {
          "conn_id": 2,
          "failure": null,
          "num_bytes": 7,
          "operation": "write",
          "proto": "tcp",
          "t": 0.299192
        }
      ],
      "queries": [
        {
          "answers": [
            {
              "answer_type": "A",
              "ipv4": "37.218.245.90",
              "ttl": null
            }
          ],
          "dial_id": 2,
          "engine": "system",
          "failure": null,
          "hostname": "ps.ooni.io",
          "query_type": "A",
          "resolver_hostname": null,
          "resolver_port": null,
          "resolver_address": "",
          "t": 0.211333
        },
        {
          "answers": null,
          "dial_id": 2,
          "engine": "system",
          "failure": null,
          "hostname": "ps.ooni.io",
          "query_type": "AAAA",
          "resolver_hostname": null,
          "resolver_port": null,
          "resolver_address": "",
          "t": 0.211333
        }
      ],
      "requests": null,
      "sni": "blocked.com",
      "tcp_connect": [
        {
          "conn_id": 2,
          "dial_id": 2,
          "ip": "37.218.245.90",
          "port": 443,
          "status": {
            "failure": null,
            "success": true
          },
          "t": 0.249802
        }
      ],
      "th_address": "ps.ooni.io:443",
      "tls_handshakes": [
        {
          "cipher_suite": "",
          "conn_id": 2,
          "failure": "ssl_invalid_hostname",
          "negotiated_protocol": "",
          "peer_certificates": null,
          "t": 0.299247,
          "tls_version": ""
        }
      ]
    }
  },
  "test_name": "sni_blocking",
  "test_start_time": "2020-01-28 15:27:18",
  "test_version": "0.0.1"
}

Privacy considerations

This nettest may be less intrusive than other nettests that measure blocking of a specific host by connecting directly to it. In particular, we are not issuing DNS queries for the sensitive domain and we are not connecting to the sensitive IP address.

Packet capture considerations

This test does not capture packets by default.

Acknowledgments

In Autumn 2019, @fortuna proposed, designed, and implemented a comprehensive domain connectivity nettest. The nettest presented here is a slightly modified version of the SNI blocking subtest of @fortuna's nettest. Researchers at CIS India used a similar methodology in November 2019 to measure SNI based blocking.

Future work

This nettest is still experimental. We need to define top-level keys, run measurements with blocking using Jafar, and further develop it.