Deploying Publicly Trusted TLS Servers on IoT Devices Using SNI-based End-to-End TLS Forwarding (SNIF)

1.1. Notational Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3.1. Protocol Summary

SNIF CA Proxy accepts requests from SNIF Connectors via HTTP / HTTPS.¶

SNIF CA Proxy interacts with the CA using protocols supported by the CA, such as ACME [RFC8555], not covered by this document.¶

Each SNIF Connector is configured with a specific initiation URL ({initUrl}), which is specific to the SNIF CA Proxy server the Connector intends to work with. Depending on the CA Proxy rules, {initUrl} might be unique for each Connector, or common for multiple Connectors.¶

3.2. Protocol Flow

Upon the initial start or after a hard reset, the Connector SHALL generate a Private Key, which needs to be securely permanently stored by the Connector. Any key algorithm acceptable by the CA can be used, generally RSA-4096 is recommended.¶

The Connector SHALL send a CN Allocation Request using the {initUrl}.¶

Having the canonical name {cn}, the Connector SHALL generate a CSR [RFC2986] using the Private Key, the subject containing the {cn}. The CSR subject may or may not have other fields besides {cn}, according to the specific requirements of the CA.¶

The Connector SHALL issue a CSR Submission Request to send the CSR to the CA Proxy.¶

Once the CSR is submitted, the Connector MUST permanently store the {cn} by some means - to minimize the storage compartments it might be practical to generate and store a dummy self-signed certificate with the {cn} in the subject until it gets replaced with a trusted certificate issued by the CA.¶

A this point, the Connector will normally know the SNIF hostname it will be using with the SNIF Relay - it matches the {cn} in case of a single host CN, or is a one sub-level down from a wildcard {cn}, the name being derived by the Connector in a way that is not deterministically derivable from the {cn} and the public key, e.g. a hash of the Private Key.¶

The Connector can now send a Certificate Download Request, and SHOULD verify the returned Certificate. If the Certificate is valid - the Connector MUST permanently store it.¶

If the Certificate Download Request fails - the Connector SHOULD repeat the request after certain delay. In case if the response was 401 and the {authUrl} is returned in a header, and the Connector has the means of communicating with the device user - the Connector also SHOULD alert the user and bring {authUrl} to their attention by some means, so the user can complete the required authorization steps. If the Connector has no means of alerting the user, which is often the case with IoT devices - the user MUST be provided with some external means of authorizing with the CA Proxy, not covered by this domcument.¶

Once the Certificate is validated and stored, the Connector is capable of terminating SNIF connections, and may proceed launching a SNIF Control Connection (Section 4.2). The Connector SHOULD communicate its SNIF hostname by some means to the SNIF Clients that will be accessing the Connector. The means of such communication are not covered by this document.¶

The Connector SHOULD watch for the expiration of the stored Certificate. If the Certificate is about to expire in 7 days or less, or has already expired - the Connector SHOULD send a Certificate Download Requests, and repeat with appropriate delays until the renewed Certificate is successfully downloaded and verified.¶

At any stage of the flow, if the Connector receives unexpected volume of rejections or inconsistent responses from the CA Proxy, the Connector MAY decide to hard reset the storage and start the flow over from the beginning. In such case, the Connector will have to re-send its new SNIF hostname to any concerned SNIF Clients, the means of such communication is not covered by this document.¶

3.3. CN Allocation Request

Connection from:

SNIF Connector¶

Connection to:

SNIF CA Proxy¶

Protocol:

https or http¶

   GET {initUrl}

Response 200: Canonical Name (CN) is successfully allocated. The response headers MUST include X-SNIF-CN: with the value of the allocated {cn}, either a wildcard starting with "*.", or a single hostname, depending on the CA Proxy rules. The Relay MUST NOT ever return a CN that's been previously returned by another CN Allocation Request. The Connector SHOULD ignore the response body.¶

Any other response: Error, try again later.¶

3.4. CSR Submission Request

Connection from:

SNIF Connector¶

Connection to:

SNIF CA Proxy¶

Protocol:

http¶

   PUT http://{cn_host}/snif-cert/{cn_host}.csr
   Content-Type: application/pkcs10

{cn_host} is a hostname derived from the {cn} - it is identical to {cn} in case of a single-host CN, or is the {cn} with truncated initial "*." in case of a wildcard CN.¶

The request body MUST contain a PEM encoded PKCS#10 CSR [RFC5967], the newlines are either <CR><LF> or <LF>, the length of the body SHOULD NOT exceed 16384 bytes.¶

Note that a CSR for the specific allocated CN can be submitted to the CA Proxy once in a lifetime. In case of an incorrect submission the Connector SHOULD hard reset the storage and restart the flow from the beginning, including allocating a new CN.¶

Response 201: the CSR is successfully submitted. The response headers MAY include X-SNIF-AuthUrl: with the value of an {authUrl}, that SHOULD, if possible, be communicated to the user to authorize the certificate issuance.¶

Response 403: the CSR for this CN has already been submitted, or is denied by the CA Proxy rules. If the Connector receives 403, is SHOULD hard reset the storage and restart the CA Proxy flow from the beginning.¶

Response 404: the CN was not allocated.¶

Any other response: Error, try again later.¶

3.5. Certificate Download Request

Connection from:

SNIF Connector¶

Connection to:

SNIF CA Proxy¶

Protocol:

http¶

   GET http://{cn_host}/snif-cert/{cn_host}.crt

{cn_host} is a hostname derived from the {cn} - it is identical to {cn} in case of a single-host CN, or is the {cn} with truncated initial "*." in case of a wildcard CN.¶

The CA Proxy SHOULD check for a cached previously generated Certificate chain for the {cn}. If the cached Certificate chain is found and if it expires in more that 10 days in the future - the cached Certificate chain SHOULD be returned with status 200. Otherwise, if the {cn} has a valid CSR and a proper authorization to issue a certificate - the CA Proxy SHOULD return status 503 and SHOULD launch a background process that communicates with the CA to issue or renew the certificate, and caches the issued Certificate chain for subsequent Certificate Download Requests.¶

Response 200: the Certificate chain is returned. The Content-Type of such response SHOULD be "application/x-x509-ca-cert". The response body MUST be a PEM encoded X.509 certificate chain, the issued certificate being the first member, the newlines are either <CR><LF> or <LF>, the length of the body SHOULD NOT exceed 65535 bytes.¶

Response 503: the Certificate is being issued, try later.¶

Response 401: Certificate issuance authorization is required. The response headers MAY include X-SNIF-AuthUrl: with the value of an {authUrl}, that SHOULD, if possible, be communicated to the user to authorize the certificate issuance. If the CA Proxy expects to work with Connectors that cannot communicate with the user, it MUST include external means of the authorization, not covered by this document.¶

Response 404: the CN was not allocated, or the CSR was not submitted.¶

Any other response: Error, try again later.¶

4.1. SNIF Messages

A SNIF Message consists of a 1 or more ASCII characters excluding special characters, terminated by <CR><LF>.¶

The total length of a SNIF Message, including the terminal <CR><LF>, SHOULD NOT exceed 4096 bytes.¶

8-bit characters are discouraged. If 8-bit characters are used, they SHOULD comply to UTF-8 [RFC3629].¶

The receiving party SHOULD silently ignore any invalid or malformed SNIF message.¶

4.2. SNIF Control Connection Protocol

Protocol name:

snif¶

Connection from:

SNIF Connector¶

Connection to:

SNIF Relay¶

To be able to open a SNIF Control Connection, the SNIF Connector MUST have a valid trusted TLS/SSL certificate, the CN hostname DNS pointing to the SNIF Relay or a wildcard CN having a sub-host DNS pointing to the SNIF Relay, and a Private Key that matches the Certificate. Normally, the SNIF Connector will generate the Private Key and use SNIF CA Proxy Protocol (Section 3) to obtain and maintain the Certificate, although other means can be used.¶

To initiate the Control Connection, the SNIF Connector opens a TCP connection to the hostname matching the Certificate's CN, that points to the Relay.¶

Upon accepting the incoming TCP connection, the SNIF Relay MUST initiate a reversed TLS session as a client peer.¶

The SNIF Connector MUST initiate the TLS as a server peer, using the Certificate and the Private Key.¶

Upon successful TLS negotiation, the SNIF Relay MUST validate the SNIF Connector's certificate. If the certificate is not trusted, the SNIF Relay MUST shut down the TLS session and the TCP socket immediately.¶

If the certificate is accepted, both SNIF Relay and SNIF Connector are ready to accept SNIF Messages from each other over the TLS connection, as following.¶

   SNIF LISTEN {hostname}

Sent by:

SNIF Connector¶

The SNIF LISTEN message informs the Relay that the Connector is ready to accept incoming TLS connections to {hostname} through the Relay.¶

{hostname} MUST specify a single host (no wildcards), and MUST match the CN of the Connector's TLS certificate - either match a wildcard CN, or exactly match a single host CN.¶

The SNIF LISTEN message SHOULD be sent only once per the Control Connection. The Relay SHOULD ignore any invalid or subsequent SNIF LISTEN messages.¶

   SNIF CONNECT {conn_id} {dst_host}:{dst_port} {fwd_host}:{fwd_port} {cln_addr}:{cln_port}

Sent by:

SNIF Relay¶

The SNIF CONNECT message informs the Connector of an incoming TLS connection from a Client to the Connector's {dst_host}, TCP port {dst_port}.¶

{conn_id} is a unique alphanumeric connection identifier assigned by the Relay, {cln_addr}:{cln_port} are the Client's remote IPv4/IPv6 address and TCP port, {cln_addr} is supplied in "[" brackets "]".¶

The Relay sends the SNIF CONNECT message to Connectors with {dst_host} matching the {hostname} the Connector is listening to. The Connector doesn't need to verify {dst_host}.¶

If the Connector decides to accept the connection - it MUST launch a SNIF Service Connection to {fwd_host}:{fwd_port}. It also SHOULD send any SNIF message back to the Relay over the Control Connection to update the keep-alive timer, a copy of the SNIF ACCEPT message that is sent over the Service Connection can be used.¶

In case of a rejection - the Connector SHOULD send SNIF CLOSE with matching {conn_id}.¶

   SNIF CLOSE {conn_id}

Sent by:

SNIF Connector¶

The SNIF CLOSE message instructs the Relay to terminate the Client connection with matching {conn_id}.¶

For SNIF CLOSE received from a Connector, the Relay MUST validate that the connection was targeted at the Connector's {hostname}, otherwise ignore the message.¶

   SNIF ABUSE {conn_id} {abuse_score}

Sent by:

SNIF Connector¶

The SNIF ABUSE message instructs the Relay to increase the DoS protection abuse counter for the Client that initiated the connection {conn_id} by {abuse score}.¶

{abuse score} SHOULD be an integer from 1 to 255, 1 is the score for a normal non-abusive connection.¶

For SNIF ABUSE received from a Connector, the Relay MUST validate that the connection was targeted at the Connector's {hostname}, otherwise ignore the message.¶

   SNIF MSG {hostname} {content}

Sent by:

SNIF Connector or SNIF Relay¶

The SNIF MSG message is relayed between the Connector and the SNIF Peripheral Processes attached to the Relay.¶

{content} SHOULD NOT contain whitespaces or special characters. Its semantics is specific to the targeted Peripheral Process, and is not covered by this document.¶

For SNIF MSG received by the Relay from a Connector, the Relay MUST verify that the {hostname} matches the one associated with the Connector, forward the message to all IPC FIFOs if matched, ignore otherwise.¶

For SNIF MSG received by the Relay from an IPC FIFO, the Relay SHOULD forward the message to the Connector(s) with the matching {hostname}, ignore the message if none are found.¶

Note that in certain uncommon circumstances a SNIF MSG send by a Connector might come back to the Connector through a different Control Connection. The Connector SHOULD be aware of this fact to avoid a potential message storm.¶

   NOOP

Sent by:

SNIF Connector or SNIF Relay¶

The NOOP message is not associated with any explicit action, except that the Relay receiving NOOP from the connector SHOULD promply send NOOP or any other message back to the Connector. Therefore, the Connector may use NOOP as a keep-alive ping.¶

4.3. SNIF Service Connection Protocol

Protocol name:

snif-srv¶

Connection from:

SNIF Connector¶

Connection to:

SNIF Relay¶

The SNIF Connector opens a TCP connection to the {fwd_host}:{fwd_port} in response to a SNIF CONNECT message received from the Relay over the Control Connection.¶

The Connector MUST immediately send a SNIF ACCEPT message over the Service Connection as a plain TCP:¶

   SNIF ACCEPT {conn_id}

The {conn_id} is the one that was received in the SNIF CONNECT message over the Control Connection.¶

Upon sending the SNIF ACCEPT message, the Connector MUST immediately assign further control and bi-directional traffic of the SNIF Service Connection to the matching TLS server process.¶

If the Relay decides to reject the connection, either because of invalid message or {conn_id}, or because of reaching the abuse threshold - the Relay SHOULD terminate the TCP connection immediately.¶

Otherwise, the Relay SHOULD link the Service Connection to the matched Client Connection, forward to the Service Connection all buffered TLS data previously received from the Client, and start bi-directional forwarding between the Client Connection and the Service Connection.¶

When either Client or Service Connection is shut down, or an inactivity timeout is reached, the Relay SHOULD shut down both the Client Connection and the Service Connection.¶

Once the Relay has linked the Client Connection matching the {conn_id} to the Service Connection, any further SNIF ACCEPT messages with the same {conn_id} on other Service Connections MUST be rejected.¶

4.4. SNIF Client Connection Protocol

Protocol name:

snif-cln¶

Connection from:

Any TLS enabled software, such as a web browser or an email client¶

Connection to:

SNIF Relay¶

From the Client's perspective, a SNIF Client Connection functions as a direct TLS connection to the IoT Device.¶

The ports the Relay is listening to, can be any well-known ports for services with persistent TLS, such as https or imaps, or can be any custom ports agreed among the Relay, the Connectors and the Clients.¶

The Relay accepts an incoming TCP connection, receives and buffers the incoming initial data from the client, and attempts to interpret the received data as a TLS handshake.¶

If the received data is not recognized as a TLS handshake, does not contain an SNI record in a supported format, or the SNI hostname does not meet rules defined for the Relay - the Relay SHOULD immediately reject the TLS session with an appropriate error status, and shut down the Client Connection.¶

If the SNI hostname is found acceptable - the Relay allocates a unique {conn_id}, checks if there are current Control Connections that match the SNI hostname, and sends a SNIF CONNECT message over those connections.¶

If there are no active applicable Control Connections, or if the Relay doesn't receive a response from a SNIF Connector within a specified timeframe - the Relay SHOULD forward the same SNIF CONNECT message over IPC FIFOs (if any are open) to alert cluster peer Relays and Peripheral processes of the incoming Client Connection.¶

A Service Connection with a matching SNIF ACCEPT establishes an end-to-end TLS circuit with the Client Connection. Once established, the Relay bi-directionally forwards all traffic between the Client and the Service Connection until either of the connections is closed or is timed out due to inactivity.¶

Upon receiving a matching SNIF CLOSE - the Relay MUST terminate the Client Connection. If a Service Connection has already been linked it MUST be terminated too, otherwise the Relay SHOULD attempt to gracefully reject TLS on the Client Connection with an appropriate status prior to shutting down TCP.¶

4.5. SNIF IPC FIFO Protocol

Protocol name:

snif-fifo¶

Connection from:

SNIF Relay or SNIF Peripheral Service¶

Connection to:

SNIF Relay or SNIF Peripheral Service¶

SNIF IPC FIFO is a permanent trusted connection between the SNIF Relay and a SNIF Peripheral Process, or between a pair of nodes in a SNIF Relay cluster. An IPC FIFO is usually unidirectional, but a bidirectional connection can serve as a pair of FIFOs. An IPC FIFO can be implemented as a Unix FIFO pipe, a TCP socket, an SSH tunnel or by other means. The mechanism of establishing and maintaining IPC FIFOs is implementation specific and is not covered by this document.¶

The following SNIF Messages are defined over an IPC FIFO from the perspective of a SNIF Relay:¶

   SNIF CONNECT {conn_id} {dst_host}:{dst_port} {fwd_host}:{fwd_port} {cln_addr}:{cln_port}

Direction:

Send or Receive¶

(see SNIF Control Connection, Section 4.2).¶

The SNIF CONNECT message is sent by a Relay over an IPC FIFO in case if the Relay failed to reach the respective Connector through Control Connections. SNIF CONNECT sent by a Relay MUST be followed up by one of SNIF CLEAR or SNIF CLOSE to inform the Peripheral Processes of the further outcome.¶

When a SNIF CONNECT message is received by a Relay, the Relay SHOULD forward it to any matching open Control Connections, or ignore it otherwise.¶

   SNIF CLEAR {conn_id}

Direction:

Send¶

The SNIF CLEAR message SHOULD be sent by a Relay only as a followup to SNIF CONNECT with a matching {conn_id}, in case if the Client Connection that triggered SNIF CONNECT was accepted by a Service Connection.¶

The purpose of SNIF CLEAR is to advice Peripheral Processes to cease further attempts of reaching the Connector by external means, not specified within this document.¶

   SNIF CLOSE {conn_id}

Direction:

Send or Receive¶

(see SNIF Control Connection, Section 4.2).¶

The SNIF CLOSE message SHOULD be sent by a Relay only as a followup to SNIF CONNECT with a matching {conn_id}, in case if the Client Connection that triggered SNIF CONNECT was closed without being accepted.¶

When the SNIF CLOSE is received by a Relay, the Relay SHOULD immediately close the matching Client and/or Service Connection if any found, ignore the message otherwise.¶

   SNIF ABUSE {conn_id} {abuse_score}

Direction:

Receive¶

(see SNIF Control Connection, Section 4.2).¶

   SNIF MSG {hostname} {content}

Direction:

Send or Receive¶

(see SNIF Control Connection, Section 4.2).¶

   SNIF CTL {ctl_fd} {hostname} {remote_addr}:{remote_port}
   SNIF CTL {ctl_fd}

Direction:

Send¶

The SNIF CTL message is sent by a Relay to inform Peripheral Processes about Control Connections. The first version is sent for each opening Control Connection, and is followed up by the second version with the matching {ctl_fd} when the Control Connection is closed. {ctl_fd} is a numeric descriptor which is unique for open connections, but can be reused after a connection is closed.¶

4.6. Abuse Management

SNIF Relay SHOULD implement basic protection from denial of service. A separate abuse count SHOULD be assigned to each remote address, incremented by 1 on every incoming connection from the address, incremented by a specified score on every received SNIF ABUSE message, and periodically decremented or reset at regular time intervals.¶

If the abuse counter for a certain remote address reaches a specific threshold, the Relay SHOULD drop any further TCP connections from that address until the abuse counter goes below the threshold. The Relay MAY allow some grace above the threshold to incoming SNIF Service Connections, to minimize stalled Client Connections.¶

SNIF Connector MAY implement basic protection from denial of service by limiting the number of accepted connections per period of time and/or the total number of open connections, and reject connections over the limit.¶