Status of This Memo

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Table of Contents

1.Introduction

The Proxy-Status HTTP response field[PROXY-STATUS] allows intermediaries to conveyinformation about how they handled the request in HTTP responses sent to clients.It defines a set of parameters that provide information, such as the name of the nexthop.¶

[PROXY-STATUS] defines anext-hop parameter, which can contain a hostname,IP address, or alias of the next hop. This parameter can contain only one such item,so it cannot be used to communicate a chain of aliases encountered during DNS resolutionwhen connecting to the next hop.¶

Knowing the full chain of names that were used during DNS resolution via CNAME records[DNS] is particularly useful for clients of forward proxies, in which theclient is requesting to connect to a specific target hostname using the CONNECT method[HTTP] or UDP proxying[CONNECT-UDP]. CNAME records can be used to"cloak" hosts that perform tracking or malicious activity behind more innocuous hostnames,and clients such as web browsers use the chain of DNS names to influence behavior like cookieusage policies[COOKIES] or the blocking of malicious hosts.¶

This document allows clients to receive the CNAME chain of DNS names for the next hopby including the list of names in a newnext-hop-aliases Proxy-Status parameter.¶

2.next-hop-aliases Parameter

The value of thenext-hop-aliases parameter is a String[STRUCTURED-FIELDS] that containsone or more DNS names in a comma-separated list. The items in the list include all alias names andcanonical names received in CNAME records[DNS] during the course of resolving the next hop's hostname using DNS andMAY include the original requested hostname itself. The names ought to appear in the order in which they were received in DNS, for the sake of consistency. If there are multiple CNAME records in the chain, the first name in thenext-hop-aliases list wouldbe the value in the CNAME record for the original hostname, and the final name in thenext-hop-aliases list would be the name that ultimately resolved to one or more addresses.¶

The list of DNS names innext-hop-aliases parameter uses a comma (",") as a separator between names.Note that if a comma is included in a name itself, the comma must be encoded as described inSection 2.1.¶

For example, consider a proxy "proxy.example.net" that receives the following records when performing DNS resolution for the next hop "host.example.com":¶

host.example.com.           CNAME   tracker.example.com.tracker.example.com.        CNAME   service1.example.com.service1.example.com.       AAAA    2001:db8::1

The proxy could include the following proxy status in its response:¶

Proxy-Status: proxy.example.net; next-hop="2001:db8::1";    next-hop-aliases="tracker.example.com,service1.example.com"

This indicates that "proxy.example.net", which used the IP address "2001:db8::1" as the next hopfor this request, encountered the names "tracker.example.com" and "service1.example.com"in the DNS resolution chain. Note that while this example includes both thenext-hop andnext-hop-aliases parameters,next-hop-aliases can be included without includingnext-hop.¶

The proxy can also include the name of the next hop as the first item in the list. This isparticularly useful for reverse proxies when clients would not otherwise know the name of thenext hop, and thenext-hop header is used to convey an IP address.¶

For example, consider a proxy "reverseproxy.example.net" that receives the following recordswhen performing DNS resolution for the next hop "host.example.com":¶

host2.example.com.          CNAME   service2.example.com.service2.example.com.       AAAA    2001:db8::2

The proxy could include the following proxy status in its response:¶

Proxy-Status: reverseproxy.example.net; next-hop="2001:db8::2";    next-hop-aliases="host2.example.com,service2.example.com"

Thenext-hop-aliases parameter only applies when DNS was used to resolve the next hop's name, and itdoes not apply in all situations. Clients can use the information in this parameter to determine how to use the connection established through the proxy, but they need to gracefully handle situations in which this parameter is not present.¶

The proxyMAY send the empty string ("") as the value ofnext-hop-aliases parameter to indicate thatno CNAME records were encountered when resolving the next hop's name.¶

Proxy-Status: proxy.example.net; next-hop="2001:db8::1";    next-hop-aliases="comma%2Cname.example.com,service1.example.com"

Proxy-Status: proxy.example.net; next-hop="2001:db8::1";    next-hop-aliases="dot%5C.label.example.com,service1.example.com"

Proxy-Status: proxy.example.net; next-hop="2001:db8::1";    next-hop-aliases="backslash%5C%5Cname.example.com,s1.example.com"

3.Implementation Considerations

In order to include thenext-hop-aliases parameter, a proxy needs to have access to the chainof alias names and canonical names received in CNAME records.¶

Implementations ought to note that the full chain of names might not be available in common DNSresolution APIs, such asgetaddrinfo[POSIX].getaddrinfo does have an option forAI_CANONNAME([RFC3493],Section 6.1), but this will only return the last name in the chain(the canonical name), not the alias names.¶

An implementationMAY include incomplete information in thenext-hop-aliases parameter to accommodate cases where it is unable to include the full chain, such as only including the canonical name if the implementation can only usegetaddrinfo as described above.¶

4.Security Considerations

Thenext-hop-aliases parameter does not include any DNSSEC information or imply that DNSSEC was used.The information included in the parameter can only be trusted to be valid insofar as the clienttrusts the proxy to provide accurate information. This information is intended to be used asa hint andSHOULD NOT be used for making security decisions about the identity of a resource accessedthrough the proxy.¶

Inspecting CNAME chains can be used to detect cloaking of trackers or malicious hosts. However, theCNAME records could be omitted by a recursive or authoritative resolver that is trying to hide this form of cloaking.In particular, recursive or authoritative resolvers can omit these records for both clients directly performing DNS nameresolution and proxies performing DNS name resolution on behalf of a client. A malicious proxy couldalso choose to not report these CNAME chains in order to hide the cloaking. In general, clients cantrust information included (or not included) in thenext-hop-aliases parameter to the degreethat the proxy and any resolvers used by the proxy are trusted.¶

5.IANA Considerations

This document registers thenext-hop-aliases parameterin the "HTTP Proxy-Status Parameters" registry<https://www.iana.org/assignments/http-proxy-status> as shown inTable 1.¶

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