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I run a school network with a BYOD program in place. I have a linux proxy (squid) with content filtering by Mind (fork of dansguardian). Everything works fine over HTTP, the problem is of corse when kids start using HTTPS. My biggest issue is that Apple has switched to using HTTPS when doing a google search in safari. This allows kids to do searches without MinD forcing safe search. I would like to know what I could possibly do about this. Is there anyway to stop this? Thanks in advance for any help!

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Your first step will almost certainly need to be blocking all direct HTTPS, and setting up WPAD to deliver the proxy server settings so people will be forced through the proxy. –  Zoredache Feb 21 '13 at 17:19
    
@Zoredache That would take the proxy out of transparent mode, correct? So the users would be given a gateway by way of DHCP, wouldn't they? Wouldn't the users to able to just turn off the proxy settings? Or am I completely wrong? –  lampwins Feb 21 '13 at 21:55
    
Yes, that would take it out of transparent mode. You cannot transparently intercept HTTPS. So the idea is to completely block all outgoing https except when the proxy is in use. Then setup WPAD, so clients can get the proxy config. Once you do that you could simply block SSL connections to google.com. –  Zoredache Feb 21 '13 at 23:17
    
@Zoredache You can transparently intercept HTTPS. It's not perfect due to certificates but it's possible. OP is going to have the certificate issue anyway if they want squid to filter proxied HTTPS connections –  mtm Feb 22 '13 at 15:38

3 Answers 3

up vote 1 down vote accepted

It is now possible to ask google to redirect secure searches to http - https://support.google.com/websearch/answer/186669?hl=en is the google help page on the subject. They suggest using DNS trickery, this can be hard in Windows 2008r2 - I personally perefer a connect-header rewrite to achieve the same goal, which can be done in all good web filters (and some bad ones).

I work for Smoothwall who provide a filter with just such a capability, and the ability to do some other SSL filtering too. I'm biased, but I suggest you take a look. When it comes to the tough stuff, it's a lot easier than rolling your own. In the interests of impartiality, there are other web filters out there :)

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Squid supports a feature called SslBump using Bump-Server-First. This basically means browsers will attempt to establish a secure connection between them and the host. The Squid cache gets the intercepted connection and Squid establishes the external connection. Then completes the secure connection to the user. Squid is essentially the certificate authority for user/squid portion so it can decrpyt the traffic and cache/filter it.

As this is something the people who designed SSL thought could be an attack vector there's some bumps to get over. Squid does support dynamic cert generation so cert domains match on the client end and have some of the original cert information go through to the client. All this can go fairly seamlessly if your client devices can trust your CA certificate. That bits a little harder if people are using their own devices.

The thing to note is your cache is now in control of third party trust for these devices. The information that is mimicked in the dynamic certs goes some way to mitigating this but it's possible to configure squid to blindly trust things which could be bad for the users, if someone were to do a real man in the middle attack further out from your proxy, for instance.

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There's not a lot you can do.. You could generate a self-signed SSL certificate for google.com, and MITM the traffic, and then you'd be able to inspect it and block as needed, except it depends how wiley the kids are, and whether they'd fall for the process of accepting your self-signed certificate.

It's also probably illegal. You could block HTTPS traffic entirely, but that'd probably break all sorts of things.

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Would it be possible to redirect https://www.google.com to http? –  lampwins Feb 21 '13 at 16:36
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The only way to redirect an HTTPS connection would be to get the user to accept the SSL certificate you presented. –  becomingwisest Feb 21 '13 at 16:40
    
Not sure about illegal, if these are in terms of use for the network that the users have willingly accepted. I've worked for companies that do it, they had the benefit of including their own CA certs in the desktop/laptop builds and it 'just works'. –  mtm Feb 21 '13 at 19:29
    
Illegal? That's exceedingly unlikely in the US, where schools have "in loco parentis". –  ceejayoz Feb 21 '13 at 20:19
    
The US I can't speak for, and you can't include your own CA Certs, as it's a BYOD environment.. so a bit of a grey area, IMO, IANAL, etc. –  Tom O'Connor Feb 21 '13 at 21:35

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