DRIVERS-2922: Allow valid SRV hostnames with fewer than 3 parts

source/initial-dns-seedlist-discovery/initial-dns-seedlist-discovery.md

@@ -9,9 +9,9 @@ ______________________________________________________________________
 
 Presently, seeding a driver with an initial list of ReplicaSet or MongoS addresses is somewhat cumbersome, requiring a
 comma-delimited list of host names to attempt connections to. A standardized answer to this problem exists in the form
-of SRV records, which allow administrators to configure a single domain to return a list of host names. Supporting this
-feature would assist our users by decreasing maintenance load, primarily by removing the need to maintain seed lists at
-an application level.
+of SRV records, which allow administrators to configure a single SRV record to return a list of host names. Supporting
+this feature would assist our users by decreasing maintenance load, primarily by removing the need to maintain seed
+lists at an application level.
 
 This specification builds on the [Connection String](../connection-string/connection-string-spec.md) specification. It
 adds a new protocol scheme and modifies how the
@@ -31,12 +31,36 @@ step before it considers the connection string and SDAM specifications. In this
 host names is replaced with a single host name. The format is:
 
 ```
-mongodb+srv://{hostname}.{domainname}/{options}
+mongodb+srv://{hostname}/{options} 
 ```
 
 `{options}` refers to the optional elements from the [Connection String](../connection-string/connection-string-spec.md)
 specification following the `Host Information`. This includes the `Auth database` and `Connection Options`.
 
+For the purposes of this document, `{hostname}` will be divided using the following terminology. If an SRV `{hostname}`
+has:
+
+1. Three or more `.` separated parts, then the left-most part is the `{subdomain}` and the remaining portion is the
+   `{domainname}`.
+
+   - Examples:
+     - `{hostname}` = `cluster_1.tests.mongodb.co.uk`
+
+       - `{subdomain}` = `cluster_1`
+       - `{domainname}` = `tests.mongodb.co.uk`
+
+     - `{hostname}` = `hosts_34.example.com`
+
+       - `{subdomain}` = `hosts_34`
+       - `{domainname}` = `example.com`
+
+2. One or two `.` separated part(s), then the `{hostname}` is equivalent to the `{domainname}`, and there is no
+   subdomain.
+
+   - Examples:
+     - `{hostname}` = `{domainname}` = `localhost`
+     - `{hostname}` = `{domainname}` = `mongodb.local`
+
 ### MongoClient Configuration
 
 #### srvMaxHosts
@@ -81,31 +105,32 @@ parse error and MUST NOT do DNS resolution or contact hosts.
 It is an error to specify more than one host name in a connection string with the `mongodb+srv` protocol, and the driver
 MUST raise a parse error and MUST NOT do DNS resolution or contact hosts.
 
-A driver MUST verify that in addition to the `{hostname}`, the `{domainname}` consists of at least two parts: the domain
-name, and a TLD. Drivers MUST raise an error and MUST NOT contact the DNS server to obtain SRV (or TXT records) if the
-full URI does not consist of at least three parts.
-
 If `mongodb+srv` is used, a driver MUST implicitly also enable TLS. Clients can turn this off by passing `tls=false` in
 either the Connection String, or options passed in as parameters in code to the MongoClient constructor (or equivalent
 API for each driver), but not through a TXT record (discussed in a later section).
 
 #### Querying DNS
 
-In this preprocessing step, the driver will query the DNS server for SRV records on `{hostname}.{domainname}`, prefixed
-with the SRV service name and protocol. The SRV service name is provided in the `srvServiceName` URI option and defaults
-to `mongodb`. The protocol is always `tcp`. After prefixing, the URI should look like:
-`_{srvServiceName}._tcp.{hostname}.{domainname}`. This DNS query is expected to respond with one or more SRV records.
+In this preprocessing step, the driver will query the DNS server for SRV records on the hostname, prefixed with the SRV
+service name and protocol. The SRV service name is provided in the `srvServiceName` URI option and defaults to
+`mongodb`. The protocol is always `tcp`. After prefixing, the URI should look like: `_{srvServiceName}._tcp.{hostname}`.
+This DNS query is expected to respond with one or more SRV records.
 
 The priority and weight fields in returned SRV records MUST be ignored.
 
 If the DNS result returns no SRV records, or no records at all, or a DNS error happens, an error MUST be raised
 indicating that the URI could not be used to find hostnames. The error SHALL include the reason why they could not be
 found.
 
-A driver MUST verify that the host names returned through SRV records have the same parent `{domainname}`. Drivers MUST
-raise an error and MUST NOT initiate a connection to any returned host name which does not share the same
+A driver MUST verify that the host names returned through SRV records share the original SRV's `{domainname}`. Drivers
+MUST raise an error and MUST NOT initiate a connection to any returned host name which does not share the same
 `{domainname}`.
 
+In the case that the SRV record name has less than three `.` separated parts, the returned address' `{domainname}` MUST
+be the SRV's entire `{hostname}` and MUST have at least one more domain level than the `{hostname}`. The next major
+version MUST no longer allow an SRV record, with any number of parts, to return an address that doesn't contain the
+SRVs' entire `{hostname}`. Drivers MUST document this in a prior minor release.
+
 The driver MUST NOT attempt to connect to any hosts until the DNS query has returned its results.
 
 If `srvMaxHosts` is zero or greater than or equal to the number of hosts in the DNS result, the driver MUST populate the
@@ -118,12 +143,12 @@ randomization.
 
 ### Default Connection String Options
 
-As a second preprocessing step, a Client MUST also query the DNS server for TXT records on `{hostname}.{domainname}`. If
-available, a TXT record provides default connection string options. The maximum length of a TXT record string is 255
-characters, but there can be multiple strings per TXT record. A Client MUST support multiple TXT record strings and
-concatenate them as if they were one single string in the order they are defined in each TXT record. The order of
-multiple character strings in each TXT record is guaranteed. A Client MUST NOT allow multiple TXT records for the same
-host name and MUST raise an error when multiple TXT records are encountered.
+As a second preprocessing step, a Client MUST also query the DNS server for TXT records on `{hostname}`. If available, a
+TXT record provides default connection string options. The maximum length of a TXT record string is 255 characters, but
+there can be multiple strings per TXT record. A Client MUST support multiple TXT record strings and concatenate them as
+if they were one single string in the order they are defined in each TXT record. The order of multiple character strings
+in each TXT record is guaranteed. A Client MUST NOT allow multiple TXT records for the same host name and MUST raise an
+error when multiple TXT records are encountered.
 
 Information returned within a TXT record is a simple URI string, just like the `{options}` in a connection string.
 
@@ -148,10 +173,10 @@ the Connection String spec.
 
 ### CNAME not supported
 
-The use of DNS CNAME records is not supported. Clients MUST NOT check for a CNAME record on `{hostname}.{domainname}`. A
-system's DNS resolver could transparently handle CNAME, but because of how clients validate records returned from SRV
-queries, use of CNAME could break validation. Seedlist discovery therefore does not recommend or support the use of
-CNAME records in concert with SRV or TXT records.
+The use of DNS CNAME records is not supported. Clients MUST NOT check for a CNAME record on `{hostname}`. A system's DNS
+resolver could transparently handle CNAME, but because of how clients validate records returned from SRV queries, use of
+CNAME could break validation. Seedlist discovery therefore does not recommend or support the use of CNAME records in
+concert with SRV or TXT records.
 
 ## Example
 
@@ -169,7 +194,7 @@ _mongodb._tcp.server.mongodb.com. 86400 IN SRV   0        5      27317 mongodb1.
 _mongodb._tcp.server.mongodb.com. 86400 IN SRV   0        5      27017 mongodb2.mongodb.com.
 ```
 
-The returned host names (`mongodb1.mongodb.com` and `mongodb2.mongodb.com`) must share the same parent domain name
+The returned host names (`mongodb1.mongodb.com` and `mongodb2.mongodb.com`) must share the same domain name
 (`mongodb.com`) as the provided host name (`server.mongodb.com`).
 
 The driver also needs to request the DNS server for the TXT records on `server.mongodb.com`. This could return:
@@ -200,6 +225,12 @@ mongodb://mongodb1.mongodb.com:27317,mongodb2.mongodb.com:27107/?ssl=true&replic
 
 ## Test Plan
 
+### Prose Tests
+
+See README.md in the accompanying [test directory](tests).
+
+### Spec Tests
+
 See README.md in the accompanying [test directory](tests).
 
 Additionally, see the `mongodb+srv` test `invalid-uris.yml` in the
@@ -254,6 +285,11 @@ In the future we could consider using the priority and weight fields of the SRV
 
 ## ChangeLog
 
+- 2024-08-20: Removed requirement for URI to have three '.' separated parts; these SRVs have stricter parent domain
+  matching requirements for security. SRVs with three or more parts should have stricter matching requirements in next
+  major version. Create terminology section. Remove usage of term `{TLD}`. The `{hostname}` now refers to the entire
+  hostname, not just the `{subdomain}`.
+
 - 2024-03-06: Migrated from reStructuredText to Markdown.
 
 - 2022-10-05: Revise spec front matter and reformat changelog.

source/initial-dns-seedlist-discovery/tests/README.md

@@ -3,6 +3,41 @@
 This directory contains platform-independent tests that drivers can use to prove their conformance to the Initial DNS
 Seedlist Discovery spec.
 
+## Prose Tests
+
+For the following prose tests, it is assumed drivers are be able to stub DNS results to easily test invalid DNS
+resolution results.
+
+### 1. Allow SRVs with less than 3 `.` separated parts
+
+For the following test, run each of these cases: SRVs with one and two `.` separated parts.
+
+When running validation on an SRV string before DNS resolution, do not throw a error due to number of SRV parts.
+
+For example, `mongodb+srv://localhost` is a valid SRV string with one part/one domain level, and
+`mongodb+srv://mongo.local` is a valid SRV string with two parts/two domain levels.
+
+### 2. Throw when return address does not end with SRV domain
+
+For the following test, run each of these cases: SRVs with one, two, and three `.` separated parts.
+
+When given a returned address that does NOT end with the original SRV's domain name, throw a runtime error.
+
+For example, the SRV `mongodb+srv://blogs.mongodb.com` resolving to `blogs.evil.com` should prompt an error, since the
+returned address does not end with `mongodb.com`.
+
+Importantly, the domain of an SRV with one or two parts is its entire hostname. For example, the domain of
+`mongodb+srv://mySite.com` is `mySite.com`. If this SRV resolves to `evil.com`, this should prompt an error.
+
+### 3. Throw when return address is identical to SRV hostname
+
+For the following test, run each of these cases: SRVs with one and two `.` separated parts.
+
+When given a returned address that is identical to the original hostname, throw a runtime error.
+
+For example, the SRV `mongodb+srv://mongo.local` resolving to `mongo.local` should prompt an error since the returned
+address is identical to the original hostname.
+
 ## Test Setup
 
 The tests in the `replica-set` directory MUST be executed against a three-node replica set on localhost ports 27017,