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Re: [tor-dev] When RFC 7686 and transparent proxies collide

The thing with this issue, and their ignorant attitude to it, is that it is this easy to patch. The if statement that does this could simply be nested in another that checks for an environment variable, giving users an option to enable .onion resolution. 

diff --git a/lib/hostip.c b/lib/hostip.c
index e7c318a..c0e2583 100644
--- a/lib/hostip.c
+++ b/lib/hostip.c
@@ -693,12 +693,12 @@ enum resolve_t Curl_resolv(struct Curl_easy *data,
   struct connectdata *conn = data->conn;
   /* We should intentionally error and not resolve .onion TLDs */
   size_t hostname_len = strlen(hostname);
-  if(hostname_len >= 7 &&
-     (curl_strequal(&hostname[hostname_len - 6], ".onion") ||
-      curl_strequal(&hostname[hostname_len - 7], ".onion."))) {
-    failf(data, "Not resolving .onion address (RFC 7686)");
-    return CURLRESOLV_ERROR;
-  }
+//   if(hostname_len >= 7 &&
+//     (curl_strequal(&hostname[hostname_len - 6], ".onion") ||
+//      curl_strequal(&hostname[hostname_len - 7], ".onion."))) {
+//    failf(data, "Not resolving .onion address (RFC 7686)");
+//    return CURLRESOLV_ERROR;
+//  }
   *entry = NULL;
 #ifndef CURL_DISABLE_DOH
   conn->bits.doh = FALSE; /* default is not */

On 2023-11-13 16:34, Alec Muffett wrote:

Hi Shawn!

On Mon, 13 Nov 2023 at 15:54, Shawn Webb <shawn.webb@xxxxxxxxxxxxxxx>
wrote:


I agree that infoleaks, especially of .onion DNS requests, is
problematic. However, I disagree that prohibiting it in broadly
monocultured libraries (libcurl) is an advisable approach.


If Curl is outright banning ".onion" at the level of the Curl source
code, I would not support that on the grounds that are described in
bullet point 2 of section 2, here, which I will requote in full:

https://datatracker.ietf.org/doc/html/rfc7686#section-2


_2. Application Software: Applications (including proxies) that
implement the Tor protocol MUST recognize .onion names as special by
either accessing them directly or using a proxy (e.g., SOCKS
[RFC1928]) to do so. Applications that do not implement the Tor
protocol SHOULD generate an error upon the use of .onion and SHOULD
NOT perform a DNS lookup._


...but I will also note that I have not (maybe I missed it?) seen
bullet point 3 being referenced in this thread:


_3. Name Resolution APIs and Libraries: Resolvers MUST either
respond to requests for .onion names by resolving them according to
[tor-rendezvous] or by responding with NXDOMAIN [RFC1035]._


I see Curl/LibCurl in the context of an application (§2) which makes
calls into name resolution apis (§3).  I regret that the text of §2
("...that do not implement the Tor protocol...") is ambiguous in its
scope, and would prefer something about the app being incapable of
dealing with and unaware of the existence of multiple possible
name-resolution namespaces, instead.

Likewise I feel that _"Applications that do not implement the Tor
protocol SHOULD generate an error"_ would benefit from being rewritten
to acknowledge that the desirable error _may_ come passively as a
consequence of the name resolution libraries that are called, rather
than via some manner of "policing" invocation of the .onion domain.

tldr: I feel it should not be up to curl/libcurl to be policing the
use of ".onion" ... but I am very content for its chosen DNS-based
name resolution backends to be doing do so.

However convenient it may be to attempt to bolt ".onion" onto the DNS
for mobile or Whonix or whatever development, there's no avoiding that
in several ways it is both risky and unspecified to do that. I can't
fix that for anyone, but I also cannot deny that it's pushing water
uphill to attempt it.

My personal sense has always been that at some point in the future
systems-level Tor onion access might need to be provided via a network
interface that presents and routes AF_ONION addresses; but until then
(and per the linked video) new directions in DNS provide us with a
secondary possible solution: Those (mobile?) people who cannot get the
benefit of a solution via /etc/nsswitch.conf should probably have
their handsets reconfigured to do "DNS" lookup via DNS-over-HTTPS[1]
to a local HTTPS service that both understands and
treats-in-isolation, all ".onion" lookups.

Of course this does not solve apps which do their own DNS resolution,
yadda yadda, but then there is no way no NSS to solve them, either;
also this points to the importance of a TCB being curated with a
"systems" perspective (including NSS integration?) rather than trying
to bolt stuff together to get to a merely "functional" solution.

Overall: long-term continuing to shoehorn Onions into DNS for
transparent-proxy name resolution is relentlessly moving towards being
actively painful. I feel that now would be a good time to embrace a
different, ideally standards-compliant / more-futureproof approach.

    -a

[1] Fun reading on a related topic:
https://github.com/alecmuffett/dohot


While I can appreciate and understand the many nuances of this
particular problem, it is one that is indeed difficult to solve.

Are there other commonalities between "infoleaky" deployments that
could be improved? It seems to me that outright prohibition should
be
a method of last resort. Are we already there?

Thanks,

--
Shawn Webb
Cofounder / Security Engineer
HardenedBSD



https://git.hardenedbsd.org/hardenedbsd/pubkeys/-/raw/master/Shawn_Webb/03A4CBEBB82EA5A67D9F3853FF2E67A277F8E1FA.pub.asc

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--

https://alecmuffett.com/about
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