Re: filtering HTTPS

On 03/13/2012 03:33 PM, Marcus Kool wrote:
> Sorry for the lengthy message. I could not address the issue with less
> words.
>
> Filtering HTTPS is more difficult than filtering HTTPS because CONNECT
> is used
> by various applications (browsers, Chat applications, tunnels, VPNs etc).
> So CONNECT is used for 'regular' HTTPS sites who use SSL+HTTP and sites
> that use other protocols, e.g. SSL+XMPP, Skype, SSH, and many others.
>
> To filter CONNECT with Squid one can use ICAP or the URL rewritor.
> I have written and published ufdbGuard, a free URL rewritor, and
> I am developing a new ICAP-based filter which is not yet published.
> The problem with filtering CONNECT/HTTPS exists for both filtering
> solutions
> and differ only in a few details.
>
> The Squid administrator can choose to use the sslBump feature.
> Although sslBump enables filters to look at content and see complete URLs
> there is still a major issue: applications that use CONNECT to
> access sites that do not use SSL+HTTP (https).
>
> Issue 1: one cannot block CONNECT in an elegant way. I.e. a CONNECT
> to an undesired site cannot be redirected or anything since the
> application (possibly browser) want do a SSL handshake and it it fails
> it displays the 'vague error' "cannot connect to site www.example.com"
> which is indeed vague for an end user who usually only understands
> messages like "you are not authorised to go to www.example.com".
>
> For true SSL+HTTP (https) sites, issue 1 can be resolved by *not* blocking
> the CONNECT and wait for the next "GET https://www.example.com/index.html"
> and block/redirect this object. Lets call this a 'postponed SSL+HTTP
> block'.
> But for sites which do not use SSL+HTTP there is not a good solution since
> Squid and the URL redirector only see a CONNECT and never see a
> GET/HEAD/POST.

I think you are describing the Bump-Server-First feature which is
currently under development:
  http://wiki.squid-cache.org/Features/BumpSslServerFirst

>
> Issue 2: Skype does not work any more with sslBump. SSH tunnels, VPNs
> and other
> chat applications also stop working with sslBump since the sslBump feature
> does its SSL certificate checking and if this fails, the CONNECT fails.
> Using the options 'sslproxy_cert_error allow all' and
> 'sslproxy_flags DONT_VERIFY_PEER' is not considered useful since thay are
> truely very unsafe and I recommend never to use them.

A ways is to use ACLs to select sites and maybe applications which can
be sslBumped or not.

>
> More background information:
> The URL redirector ufdbGuard has a feature to probe HTTPS connections.
> It does a SSL-handshake if this works it is followed by "GET / HTTP/1.0"
> If the SSL-handshake does not work it probes for SSH, Skype and other
> chat protocols to find out where the the application CONNECTs to.
> ufdbGuard can block CONNECT to IP addresses but make exceptions
> for the CONNECTs which are used by allowed chat protocols.
> SSH and VPNs are blocked by ufdbGuard if the administrator has configured
> to block proxies.
>
> HTTPS is used more and more. Even Google uses it for their search engine.
> It is necessary to have a safe HTTPS proxy and content filtering in an
> absolutely safe and efficient way.
>
> Proposal:
> To have a good combination of web proxy and content filtering combination
> I propose the following:
> A) Squid's behaviour is modified for sslBump: after an unsuccessful SSL
> handshake, the CONNECT does not fail any more by default. This is to
> ensure
> that Skype et al. remains functional.

Correct.
This behaviour partially is already implemented under the
Bump-Server-First feature.

I am saying partially because currently works only with HTTPS protocol
(requires that both client-to-squid and squid-to-server connections are
supporting SSL) but can easily extended to support other protocols.
It is easy to extend Bump-Server-First to not initiate SSL connection
with the client if the server is not an SSL server.

But again, why not using ACLs to avoid applying sslBump on applications
like skype?

> B) Squid gets a new option to define its behaviour in case the SSL
> handshake
> fails. The options could be called sslBumpForNoneSSL with values
> prohibitNoneSSL (terminate connection), passNoneSSL (always allow),
> filterNoneSSL (default value - let ICAP or URL rewritor decide).

Yep, looks that you are right here, something like that required...

> C) Squid notifies the URL rewritor and ICAP server about the result of
> the SSL handshake. This is to optimise the filters and not do things twice.
> Web servers do no like probes and may temporarily block sites that use
> Squid
> if they receive too many probes, so the least number of probes the better.
> I.e. the line sent to the URL redirector is extended with a new flag
> like "SSLhandshake=(verified|noSSL)". This should not break existing URL
> redirectors since it already has the variable length urlgroup and most URL
> redirectors will consider the new flag part of the urlgroup.
> Probably a few URL redirectors need a minor modification.
> For ICAP Squid could send a new header called X-Squid-SSLhandshakeResult.

> D) squid.conf.documented, wiki and other documentation is updated that
> 'sslproxy_flags DONT_VERIFY_PEER' and 'sslproxy_cert_error allow all' are
> unsafe and not recommended.

> E) the option 'squid-uses-ssl-bump' is introduced to ufdbGuard. If set to
> 'yes' it will not verify the use of proper SSL certificates.
> If Squid can send the new flag SSLhandshake (URL redirector) or
> X-Squid-SSLhandshakeResult (ICAP server), the URL redirector and ICAP
> servers can be optimised further.

If I am understanding well you are proposing something like the following:
 - web client send CONNECT request to squid

 - squid reads the CONNECT request and start SSL handshake to the HTTP
server

Case 1: The server is an SSL server
     1a) The squid-to-server connection fails with an SSL error (and
maybe the connection dropped, if the error can not be ignored).

     1b) Squid initiates an SSL handshake with the client and wait the
first GET (or other request, tunnelled through client-to-squid SSL
handshake)

     1c) Read the GET request from client and send it to the ICAP server
WITH the "X-Squid-SSLhandshakeResult" header.

     1d) Squid send the error page to the web client. Or any possible
error/deny page from ICAP server. Or just ignore the SSL error (eg an
error certificate expired) and continue as normal.

Case 2) The server is not an SSL server:
   2a) Squid does not initiate SSL handshake with the client, and allow
CONNECT request continue without ssl bump.

Am I correct? Is this what you need?

>
> Marcus Kool
>
>
Received on Wed Mar 14 2012 - 08:39:18 MDT