Host Discovery

Host discovery is the process through which a domain owner optionally specifies which web host to interrogate during capability discovery. The host discovery process involves the creation and subsequent query of SRV DNS records.

The use of an SRV record was chosen for the following reasons:

• Domain owners may choose to use a third-party paymail service provider. Delegating authority to this provider is a one-time activity (the creation of a DNS SRV record).
• TXT records were considered, however should the paymail service be reconfigured (for example, the root URI change from .../api/v1/... to .../api/v2/...), the domain owner would have to coordinate with the service provider to ensure the TXT record (containing a full endpoint URI) was updated. With SRV records (plus the Capability Discovery protocol), the Host Discovery phase is set-and-forget.
• As an optional step, if the canonical Capability Discovery host is the same as the domain found in the paymail alias, the DNS record can be omitted entirely.
• DNS records (including SRV) feature a TTL, which clients can use for caching responses. All common DNS clients implement this caching out-of-the-box, meaning implementers do not have to roll this themselves.

Setup

A domain owner may create an SRV record with the following parameters:

The TTL parameter should be set very low for test configurations (a few seconds), whereas for production deployments this should be set higher, to allow caching to work. A value of 3600 is suggested for production deployments.

Although the DNS system allows for multiple records with a variety of priorities and weights, which allows for some level of traffic management, resilience, and load-balancing via DNS records, it is recommended by this specification that these considerations be handled by more modern infrastructure and only a single SRV record be created.

See https://en.wikipedia.org/wiki/SRV_record for more information on SRV DNS records.

SRV records must be served with a valid DNSSEC signature chain with the exception that if the SRV record points to <domain>.<tld> or www.<domain>.<tld>, then the SRV records should be served with a valid DNSSEC signature chain.

Client Queries

Given a paymail alias <alias>@<domain>.<tld>, a paymail client would perform a DNS lookup for an SRV record matching _bsvalias._tcp.<domain>.<tld>. The combination of Target and Port fields are then used for Capability Discovery. Should no record be returned, a paymail client should assume a host of <domain>.<tld> and a port of 443.

Security and SRV Records

In its legacy form, that is, without DNSSEC, DNS is not a secure mechanism. It is susceptible to a range of attacks, the most serious for the purposes of the BSV Alias protocol being an intercept or man-in-the-middle (MITM) attack. In this scenario an attacker intercepts DNS queries and responds with their own data. This would allow an attacker to direct a client to a paymail implementation of their choosing, which would further allow for them to control all further communications with a client.

Note: whilst an exception to the requirement for DNSSEC is made for cases where the SRV record points to the same target domain as the SRV record itself it is best practice to enable DNSSEC in all cases.

Clients must, therefore, resolve hosts with the following procedure.

1. Query for an SRV endpoint at _bsvalias._tcp.<domain>.<tld>..
2. If an SRV record is found and the reponse target points to <domain>.<tld> or www.<domain>.<tld>, but the response is not served with a valid DNSSEC signature chain, proceed to capability discovery using the target/port combination as specified by the SRV record and rely on the SSL certificate for the prevention of man in the middle attacks. Note that in this scenario it is possible to still receive malicious DNS information, however SSL certificates prevent further MITM attacks.
3. If an SRV record is found, but the response is not served with a valid DNSSEC signature chain and the response target does NOT point to <domain>.<tld> or www.<domain>.<tld>, ignore the SRV record completely and continue with an A record query as in step 5, relying on the SSL certificate for the prevention of man in the middle attacks. Note that in this scenario it is possible to still receive malicious DNS information, however SSL certificates prevent further MITM attacks. The worst that may happen here is that, for a domain that has delegated BSV Alias services out to a service provider, further lookups fail and the client cannot proceed at this time. However the option of falling back to an A record and relying on SSL prevents a possible denial of service vector where a malicious actor has compromised DNS.
4. If an SRV record is found, and the response is served with a valid DNSSEC signature chain, proceed to capability discovery using the target/port combination as specified by the SRV record.
5. If no SRV record is found, instead query for an A record for <domain>.<tld>. and proceed to capability discovery, verifying that the SSL certificate presented by the remote host is valid for <domain>.<tld>..

A client must fail the request in the following cases:

• The SSL certificate for the target domain does not verify as valid for that target domain regardless of whether the target domain is discovered via the SRV record or via the A record.
• An SRV record is not served; AND AND <domain>.<tld> does not host a valid capability discovery service.
• The SRV response target does NOT point <domain>.<tld> or www.<domain>.<tld>; AND the SRV record is not served with a valid DNSSEC signature chain; AND <domain>.<tld> does not host a valid capability discovery service.

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