DRIVERS-3329: Configurable DNS domain validation for SRV records

source/connection-string/tests/invalid-uris.yml

@@ -223,6 +223,14 @@ tests:
         hosts: ~
         auth: ~
         options: ~
+    -
+        description: "srvAllowedHostsSuffix with non-SRV URI"
+        uri: "mongodb://localhost:27017/?srvAllowedHostsSuffix=.mongodb.net"
+        valid: false
+        warning: ~
+        hosts: ~
+        auth: ~
+        options: ~
     -
         description: "Username with password containing an unescaped percent sign"
         uri: "mongodb://alice%foo:bar@127.0.0.1"

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

@@ -37,7 +37,8 @@ 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}`
+For the purposes of this document, `{hostname}` will be divided using the following terminology. If
+`srvAllowedHostsSuffix` has been configured, then that will act as the `{domainname}`. Otherwise, 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
@@ -65,6 +66,27 @@ Only `{domainname}` is used during SRV record verification and `{subdomain}` is
 
 ### MongoClient Configuration
 
+#### srvAllowedHostsSuffix
+
+This option is used to validate hosts. If present, its value MUST be treated as the `{domainname}` for
+[DNS validation](#querying-dns) and
+[SRV polling](../polling-srv-records-for-mongos-discovery/polling-srv-records-for-mongos-discovery.md). For example,
+`srvAllowedHostsSuffix=.mydomain.net`. Drivers MUST apply the following normalization and validation to the value, in
+this order:
+
+1. Any trailing `.` MUST be stripped. For example, `srvAllowedHostsSuffix=.mydomain.net.` is treated as `.mydomain.net`.
+2. The value MUST be converted to its A-label (Punycode) form, so that it is comparable against the A-label hostnames
+    returned by DNS.
+3. The value MUST be normalized to lowercase using ASCII case folding.
+4. The resulting value, with any leading `.` removed, MUST NOT be a public suffix, per the algorithm in
+    [Public Suffix List](../public-suffix-list/public-suffix-list.md).
+5. If the value does not begin with a `.`, a `.` MUST be prepended. For example, `srvAllowedHostsSuffix=mydomain.net` is
+    treated as `.mydomain.net`.
+
+If this option is not present, the `{domainname}` MUST be inferred from the `{hostname}` (as described in
+[Connection String Format](#connection-string-format)). This option MUST only be configurable at the level of a
+`MongoClient`.
+
 #### srvMaxHosts
 
 This option is used to limit the number of mongos connections that may be created for sharded topologies. This option
@@ -84,9 +106,9 @@ requires a string value and defaults to "mongodb". This option MUST only be conf
 
 #### URI Validation
 
-The driver MUST report an error if either the `srvServiceName` or `srvMaxHosts` URI options are specified with a non-SRV
-URI (i.e. scheme other than `mongodb+srv`). The driver MUST allow specifying the `srvServiceName` and `srvMaxHosts` URI
-options with an SRV URI (i.e. `mongodb+srv` scheme).
+The driver MUST report an error if any of `srvServiceName`, `srvMaxHosts`, or `srvAllowedHostsSuffix` URI options are
+specified with a non-SRV URI (i.e. scheme other than `mongodb+srv`). The driver MUST allow specifying the
+`srvServiceName`, `srvMaxHosts`, and `srvAllowedHostsSuffix` URI options with an SRV URI (i.e. `mongodb+srv` scheme).
 
 If `srvMaxHosts` is a positive integer, the driver MUST throw an error in the following cases:
 
@@ -124,10 +146,16 @@ If the DNS result returns no SRV records, or no records at all, or a DNS error h
 indicating that the URI could not be used to find hostnames. The error SHALL include the reason why they could not be
 found.
 
+Before validating returned hostnames, drivers MUST normalize them as follows:
+
+- Trailing dots MUST be stripped (e.g. `host.example.com.` becomes `host.example.com`).
+- Hostnames MUST be normalized to lowercase using ASCII case folding.
+
 A driver MUST verify that the host names returned through SRV records share the original SRV's `{domainname}`. In
-addition, SRV records with fewer than three `.` separated parts, the returned hostname MUST have at least one more
-domain level than the SRV record hostname. Drivers MUST raise an error and MUST NOT initiate a connection to any
-returned hostname which does not fulfill these requirements.
+addition, when `srvAllowedHostsSuffix` is not configured and the SRV record hostname has fewer than three `.` separated
+parts, the returned hostname MUST have at least one more domain level than the SRV record hostname. Drivers MUST raise
+an error and MUST NOT initiate a connection to any returned hostname which does not fulfill these requirements. This
+additional requirement does not apply when `srvAllowedHostsSuffix` is configured.
 
 The driver MUST NOT attempt to connect to any hosts until the DNS query has returned its results.
 
@@ -247,6 +275,39 @@ Several of our users have asked for this through tickets:
 The design specifically calls for a pre-processing stage of the processing of connection URLs to minimize the impact on
 existing functionality.
 
+### Rationale for `srvAllowedHostsSuffix`
+
+By default, the parent domain that returned host names are checked against is inferred from the seed by treating its
+leftmost label as the `{subdomain}`. A seed of `mongodb.mydomain.net` results in `mongodb1.us-east-1.mydomain.net` being
+rejected, even when both are owned by the same user. `srvAllowedHostsSuffix` allows users to work around this limitation
+by letting the user state the parent domain directly instead, admitting such nested names and hosts in a different
+domain altogether.
+
+Note that nothing requires the `{hostname}` itself to end with the configured suffix. For example,
+`mongodb+srv://cluster.example.org/?srvAllowedHostsSuffix=.hosts.example.net` is valid, even though
+`cluster.example.org` does not end with `.hosts.example.net`. This is intentional: the option exists so the user can
+state the allowed parent domain explicitly, rather than having it inferred from the connection string. Requiring the two
+to match would restore that coupling, and would rule out arrangements such as a vanity alias in one organization's
+domain resolving to hosts in a provider's domain.
+
+Because it replaces that default requirement, this option relaxes a DNS spoofing safeguard. An attacker able to forge
+SRV responses is otherwise confined to host names under the seed's `{domainname}`; with the option set, they are
+confined to host names under the configured suffix instead. The broader that suffix, the more hosts a forged response
+can direct a driver to.
+
+Requiring that the value not be a public suffix bounds how broad it can get. It rules out values such as `.com` or
+`.co.uk`, which would place no meaningful limit on a forged response. It does not, however, make a given configuration
+safe: `.example.com` is not a public suffix, but it still admits every host in a large organization's domain.
+
+Users should be encouraged to configure the narrowest suffix that covers their deployment, and drivers should say so
+wherever the option is documented. For a seed hostname of `cluster.test.internal.example.com`, prefer the second of
+these:
+
+```text
+mongodb+srv://cluster.test.internal.example.com/?srvAllowedHostsSuffix=.example.com
+mongodb+srv://cluster.test.internal.example.com/?srvAllowedHostsSuffix=.internal.example.com
+```
+
 ## Justifications
 
 ### Why Are Multiple Key-Value Pairs Allowed in One TXT Record?
@@ -283,6 +344,8 @@ In the future we could consider using the priority and weight fields of the SRV
 
 ## ChangeLog
 
+- 2026-08-24: Add `srvAllowedHostsSuffix` MongoClient option.
+
 - 2024-09-24: Removed requirement for URI to have three '.' separated parts; these SRVs have stricter parent domain
     matching requirements for security. Create terminology section. Remove usage of term `{TLD}`. The `{hostname}` now
     refers to the entire hostname, not just the `{subdomain}`.

source/initial-dns-seedlist-discovery/tests/replica-set/srvAllowedHostsSuffix-case-insensitive.yml

@@ -0,0 +1,9 @@
+# Tests that srvAllowedHostsSuffix comparison is case-insensitive (ASCII case folding per RFC 4343).
+# The suffix .BUILD.10GEN.CC is uppercase but should match the returned host localhost.build.10gen.cc.
+uri: "mongodb+srv://test12.test.build.10gen.cc/?srvAllowedHostsSuffix=.BUILD.10GEN.CC"
+seeds:
+    - localhost.build.10gen.cc:27017
+options:
+    srvAllowedHostsSuffix: .BUILD.10GEN.CC
+    ssl: true
+ping: false

source/initial-dns-seedlist-discovery/tests/replica-set/srvAllowedHostsSuffix-mismatch.yml

@@ -0,0 +1,5 @@
+# DNS record for test12.test.build.10gen.cc returns localhost.build.10gen.cc which would not match test.build.10gen.cc
+uri: "mongodb+srv://test12.test.build.10gen.cc/?srvAllowedHostsSuffix=test.build.10gen.cc"
+seeds: []
+hosts: []
+error: true

source/initial-dns-seedlist-discovery/tests/replica-set/srvAllowedHostsSuffix-tld-only.yml

@@ -0,0 +1,9 @@
+# srvAllowedHostsSuffix MUST NOT be a public suffix. "cc" is an ordinary rule in the
+# Public Suffix List, so ".cc" is a public suffix and MUST raise an error.
+# Note: .cc is used (rather than an unrelated public suffix) because the DNS records
+# return hosts ending in .cc, so the host suffix check passes and the public suffix
+# check is the only thing that can fail here.
+uri: "mongodb+srv://test1.test.build.10gen.cc/?srvAllowedHostsSuffix=.cc"
+seeds: []
+hosts: []
+error: true

source/initial-dns-seedlist-discovery/tests/replica-set/srvAllowedHostsSuffix-trailing-dot.yml

@@ -0,0 +1,9 @@
+# A trailing dot in srvAllowedHostsSuffix must be stripped before comparison,
+# so ".build.10gen.cc." is equivalent to ".build.10gen.cc".
+uri: "mongodb+srv://test12.test.build.10gen.cc/?srvAllowedHostsSuffix=.build.10gen.cc."
+seeds:
+    - localhost.build.10gen.cc:27017
+options:
+    srvAllowedHostsSuffix: .build.10gen.cc.
+    ssl: true
+ping: false

source/initial-dns-seedlist-discovery/tests/replica-set/srvAllowedHostsSuffix-with_dot.yml

@@ -0,0 +1,9 @@
+# Also tests trailing dot normalization: DNS returns targets with a trailing dot
+# (localhost.build.10gen.cc.) which must be stripped before suffix comparison.
+uri: "mongodb+srv://test12.test.build.10gen.cc/?srvAllowedHostsSuffix=.build.10gen.cc"
+seeds:
+    - localhost.build.10gen.cc:27017
+options:
+    srvAllowedHostsSuffix: .build.10gen.cc
+    ssl: true
+ping: false

source/initial-dns-seedlist-discovery/tests/replica-set/srvAllowedHostsSuffix-without_dot_fail.yml

@@ -0,0 +1,5 @@
+# dot should be prepended to `srvAllowedHostsSuffix` causing the host to be .uild.10gen.cc which does not match any available DNS records
+uri: "mongodb+srv://test12.test.build.10gen.cc/?srvAllowedHostsSuffix=uild.10gen.cc"
+seeds: []
+hosts: []
+error: true

source/initial-dns-seedlist-discovery/tests/replica-set/srvAllowedHostsSuffix-without_dot_pass.yml

@@ -0,0 +1,7 @@
+uri: mongodb+srv://test12.test.build.10gen.cc/?srvAllowedHostsSuffix=build.10gen.cc
+seeds:
+  - localhost.build.10gen.cc:27017
+options:
+  srvAllowedHostsSuffix: build.10gen.cc
+  ssl: true
+ping: false

source/polling-srv-records-for-mongos-discovery/polling-srv-records-for-mongos-discovery.md

@@ -53,9 +53,14 @@ initial seedlist discovery. Periodic scan MUST follow these rules:
     and protocol. The SRV service name is provided in the
     [srvServiceName](../initial-dns-seedlist-discovery/initial-dns-seedlist-discovery.md#srvservicename) URI option and
     defaults to `mongodb`. The protocol is always `tcp`. After prefixing, the URI should look like:
-    `_{srvServiceName}._tcp.{hostname}.{domainname}`.
-- A driver MUST verify that the host names returned through SRV records have the same parent `{domainname}`. When this
-    verification fails, a driver:
+    `_{srvServiceName}._tcp.{hostname}.{domainname}`. Here `{hostname}.{domainname}` is the host from the connection
+    string, and is not affected by
+    [srvAllowedHostsSuffix](../initial-dns-seedlist-discovery/initial-dns-seedlist-discovery.md#srvallowedhostssuffix):
+    that option changes which parent domain returned host names are verified against, not which records are queried.
+- A driver MUST verify that the host names returned through SRV records have the same parent `{domainname}`. When
+    [srvAllowedHostsSuffix](../initial-dns-seedlist-discovery/initial-dns-seedlist-discovery.md#srvallowedhostssuffix)
+    is configured, its value is the parent domain used for this verification instead of the `{domainname}` derived from
+    the connection string. When this verification fails, a driver:
     - MUST NOT add such a non-compliant host name to the topology
     - MUST NOT raise an error
     - SHOULD log the non-compliance, including the host name
@@ -165,6 +170,8 @@ No future work is expected.
 
 ## Changelog
 
+- 2026-08-24: Account for the `srvAllowedHostsSuffix` MongoClient option when verifying returned host names.
+
 - 2024-08-22: Migrated from reStructuredText to Markdown.
 
 - 2022-10-05: Revise spec front matter and reformat changelog.