Communicating Proxy Configurations in Provisioning Domains

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Table of Contents

1. Introduction

HTTP proxies that use the CONNECT method Section 9.3.6 of [HTTP] (often referred to as "forward" proxies) allow clients to open connections to hosts via a proxy. These typically allow for TCP stream proxying, but can also support UDP proxying [CONNECT-UDP] and IP packet proxying [CONNECT-IP]. Such proxies are not just defined as hostnames and ports, but can use URI templates [URITEMPLATE].¶

In order to make use of multiple related proxies, clients need a way to understand which proxies are associated with one another.¶

Client can also benefit from learning about additional information associated with the proxy to optimize their proxy usage, such knowing that a proxy is configured to only allow access to a limited set of destinations.¶

These improvements to client behavior can be achieved through the use of Provisioning Domains. Provisioning Domains (PvDs) are defined in [PVD] as consistent sets of network configuration information, which can include proxy configuration details Section 2 of [PVD]. [PVDDATA] defines a JSON [JSON] format for describing Provisioning Domain Additional Information, which is an extensible dictionary of properties of the Provisioning Domain.¶

This document defines several mechanisms to use PvDs to help clients understand how to use proxies:¶

1. A way to fetch PvD Additional Information associated with a known proxy URI (Section 2)¶

2. A way to list one or more proxy URIs in a PvD, allowing clients to learn about other proxy options given a known proxy (Section 3).¶

3. A way to define a limited set of destinations that are accessible through the proxy (Section 4).¶

Additionally, this document partly describes how these mechanisms might be used to discover proxies associated with a network (Section 5).¶

Using this mechanism a client can learn that a legacy insecure HTTP proxy that the client is configured with is also accessible using HTTPS. In this way, clients can upgrade to a more secure connection to the proxy.¶

2. Fetching PvD Additional Information for proxies

This document defines a way to fetch PvD Additional Information associated with a proxy. This PvD describes the properties of the network accessible through the proxy.¶

In order to fetch PvD Additional Information associated with a proxy, a client issues an HTTP GET request for the well-known PvD URI (".well-known/pvd") [PVDDATA] and the host authority of the proxy. This is applicable for both proxies that are identified by a host and port only (such as SOCKS proxies and HTTP CONNECT proxies) and proxies that are identified by a URI or URI template.¶

For example, a client would issue the following request for the PvD associated with "https://proxy.example.org/masque{?target_host,target_port}":¶

:method = GET
:scheme = https
:authority = proxy.example.org
:path = /.well-known/pvd
accept = application/pvd+json

For a HTTP CONNECT proxy on "proxy.example.org:8080", the client would send the following request:¶

:method = GET
:scheme = https
:authority = proxy.example.org:8080
:path = /.well-known/pvd
accept = application/pvd+json

Note that all proxies that are colocated on the same host and port share the same PvD Additional Information. Proxy deployments that need separate PvD configuration properties SHOULD use different hosts.¶

PvD Additional Information is required to contain the "identifier", "expires", and "prefixes" keys. For proxy PvDs as defined in this document, the "identifier" MUST match the hostname of the HTTP proxy. The "prefixes" array SHOULD be empty by default.¶

3. Enumerating proxies within a PvD

This document defines a new PvD Additional Information key, proxies, that is an array of dictionaries, where each dictionary in the array defines a single proxy that is available as part of the PvD (see Section 7.1). Each proxy is defined by a proxy protocol, a proxy location (i.e., a hostname and port or a URI template [URITEMPLATE]), along with potentially other keys.¶

This document defines two mandatory keys for the sub-dictionaries in the proxies array, protocol and proxy. There are also optional keys, including alpn, and destination accessibility keys defined in Section 4. Other optional keys can be added to the dictionary to further define or restrict the use of a proxy. Clients that do not recognize or understand a key in a proxy sub-dictionary MUST ignore the entire proxy definition, since the proxy might be only applicable for particular uses. These keys are registered in an IANA registry, defined in Section 7.2.¶

The values for the protocol key are defined in the proxy protocol registry (Section 7.3), with the initial contents provided below. For consistency, any new proxy types that use HTTP Upgrade Tokens (and use the :protocol pseudo-header) SHOULD define the protocol value to match the Upgrade Token / :protocol value.¶

The value of proxy depends on the Proxy Location Format defined by proxy protocol. The types defined here either use a hostname and port, or a full URI template.¶

If the alpn key is present, it provides a hint for the Application-Layer Protocol Negotiation (ALPN) [ALPN] protocol identifiers associated with this server. For HTTP proxies, this can indicate if the proxy supports HTTP/3, HTTP/2, etc.¶

When a PvD that contains the proxies key is fetched from a known proxy using the method described in Section 2 the proxies list describes equivalent proxies (potentially supporting other protocols) that can be used in addition to the known proxy.¶

Such cases are useful for informing clients of related proxies as a discovery method, with the assumption that the client already is aware of one proxy. Many historical methods of configuring a proxy only allow configuring a single FQDN hostname for the proxy. A client can attempt to fetch the PvD information from the well-known URI to learn the list of complete URIs that support non-default protocols, such as [CONNECT-UDP] and [CONNECT-IP].¶

:method = GET
:scheme = https
:authority = proxy.example.org
:path = /.well-known/pvd
accept = application/pvd+json

:status = 200
content-type = application/pvd+json
content-length = 222

{
  "identifier": "proxy.example.org.",
  "expires": "2023-06-23T06:00:00Z",
  "prefixes": [],
  "proxies": [
    {
      "protocol": "http-connect",
      "proxy": "proxy.example.org:80"
    },
    {
      "protocol": "connect-udp",
      "proxy": "https://proxy.example.org/masque{?target_host,target_port}"
    }
  ]
}

4. Destination accessibility information for proxies

Destination accessibility information is used when only a subset of destinations is reachable through a proxy. Destination restrictions are often used in VPN tunnel configurations such as split DNS in IKEv2 [IKEV2SPLIT].¶

PvD Additional Information can be used to indicate that a proxy PvD only allows access to a limited set of destinations.¶

This document defines five optional keys for subdictionaries in the proxies array that are used to signal information about destinations available through the proxy.¶

When present in a PvD Additional Information dictionary that is retrieved for a proxy as described in Section 2, entries in the matchDomains array indicate specific FQDNs and zones that are accessible using the proxy. If a hostname does match any entry, then a client SHOULD assume that the hostname will not be accessible through the proxy. If a hostname is included in the excludeDomains array, then the client SHOULD NOT access it through the proxy. The excludeDomains parameter can be present even if matchDomains is omitted. When this is the case, the client assumes that all domains except the domains listed in the excludeDomains array are accessible through the proxy.¶

Entries listed in matchDomains MUST NOT expand the set of domains that a client is willing to send to a particular proxy. The list can only narrow the list of domains that the client is willing to send through the proxy. For example, if the client has a local policy to only send requests for "*.example.com" to a proxy "proxy.example.com", and the matchDomains array contains "internal.example.com" and "other.company.com", the client would end up only proxying "internal.example.com" through the proxy.¶

A wildcard prefix (*.) is used to indicate matching entire domains or subdomains instead of specific hostnames. Note that this can be used to match multiple levels of subdomains. For example ".example.com" matches "internal.example.com" as well as "www.public.example.com". Entries that include the wildcard prefix also SHOULD be treated as if they match an FQDN that only contains the string after the prefix, with no subdomain. So, an entry in matchDomains of ".example.com" would match the FQDN "example.com", unless "example.com" were specifically included in excludeDomains. This is done to prevent commonly needing to include both "*.example.com" and "example.com" in the matchDomains list.¶

Entries in matchSubnets correspond to IP addresses and subnets that are available through the proxy, while entries in excludeSubnets define IP addresses and subnets that SHOULD NOT be used with the proxy. Subnet-based destination information SHOULD only be used when applications are communicating with destinations identified by only an IP address and not a hostname.¶

matchPorts in a list of strings that can be used to instruct the client that only specific destination TCP or UDP ports are accessible through the proxy. The list may contain individual port numbers (such as "80") or inclusive ranges of ports. For example "1024-2048" matches all ports from 1024 to 2048, including the 1024 and 1028.¶

:method = GET
:scheme = https
:authority = proxy.example.org
:path = /.well-known/pvd
accept = application/pvd+json

:status = 200
content-type = application/pvd+json
content-length = 135

{
  "identifier": "proxy.example.org.",
  "expires": "2023-06-23T06:00:00Z",
  "prefixes": [],
  "proxies": [
    {
      "protocol": "http-connect",
      "proxy": "proxy.example.org:80"
    },
    {
      "protocol": "connect-udp",
      "proxy": "https://proxy.example.org/masque{?target_host,target_port}",
      "matchDomains": [ "*.internal.example.org" ]
    }
  ]
}

5. Discovering proxies from network PvDs

[PVDDATA] defines how PvD Additional Information is discovered based on network advertisements using Router Advertisements [RFC4861]. A network defining its configuration via PvD information can include the proxies key (Section 3) to inform clients of a list of proxies available on the network.¶

This association of proxies with the network's PvD can be used as a mechanism to discover proxies, as an alternative to PAC files. However, client systems MUST NOT automatically send traffic over proxies advertised in this way without explicit configuration, policy, or user permission. For example, a client can use this mechanism to choose between known proxies, such as if the client was already proxying traffic and has multiple options to choose between.¶

Further security and experience considerations are needed for these cases.¶

6. Security Considerations

Configuration advertised via PvD Additional Information, such DNS zones or associated proxies, can only be safely used when fetched over a secure TLS-protected connection, and the client has validated that that the hostname of the proxy, the identifier of the PvD, and the validated hostname identity on the certificate all match.¶

7. IANA Considerations

8. References

Authors' Addresses