Question - BUG - DPI appears to be on by default

Hi Michael,

As far I understood from your explanation regardless of Zone configuration(LAN-LAN, LAN-WAN) DPI engine will be enable by default. I've tested this scenario as you mentioned in which I have configured LAN-WAN rule with NONE Web Policy and applied SSL/TLS policy with (ANY Source and destination )which in result showed me HTTPS scanning getting applied to that HTTPS website.

Now my question here is, Earlier in v17 if we set NONE policy then it means that webproxy will not be applied but now with v18 lets say if I want to do some troubleshooting without Legacy/DPI based webproxy then how is it possible to check?

I tried disabling SSL/TLS inspection option in advance settings in SSL/TLS inspection global option which in result showed me website without any proxy ( DPI/legacy ) getting involved but that is not a solution because let's say if I want to test for a specific machine without any policy(Legacy/DPI) like we try to check with v17 i.e. as NONE web policy how can I verify it?

James Panther said:

As far I understood from your explanation regardless of Zone configuration(LAN-LAN, LAN-WAN) DPI engine will be enable by default. I've tested this scenario as you mentioned in which I have configured LAN-WAN rule with NONE Web Policy and applied SSL/TLS policy with (ANY Source and destination )which in result showed me HTTPS scanning getting applied to that HTTPS website.

Now my question here is, Earlier in v17 if we set NONE policy then it means that webproxy will not be applied but now with v18 lets say if I want to do some troubleshooting without Legacy/DPI based webproxy then how is it possible to check?

I tried disabling SSL/TLS inspection option in advance settings in SSL/TLS inspection global option which in result showed me website without any proxy ( DPI/legacy ) getting involved but that is not a solution because let's say if I want to test for a specific machine without any policy(Legacy/DPI) like we try to check with v17 i.e. as NONE web policy how can I verify it? 

Hi James,

There are two different systems involved. I'll have to use some internal names.

The first is sslx. This is the new way of decrypting TLS traffic, and it is configured using the SSL/TLS Inspection Rules. For your purposes, you can say it is implemented in snort.

The second is web-in-snort, which in the UI is called DPI mode. Web in snort works on HTTP traffic, as well HTTP traffic that comes from sslx (ie HTTPS). It is configured in the Firewall Rules. It is also implemented in snort.

To go further, IPS (intrusion prevention system) is also in snort. And Application policy is also in snort. And traffic shaping is in snort. And Advanced Threat Protection is in snort.

So, if you are asking "how do I make sure that nothing is looking at the HTTP traffic" then what you want to do is disable web-in-snort or DPI mode. To do that make sure that the traffic is hitting a firewall rule that does not has web policy None and no malware scanning (eg the "use proxy" button is disabled).

If you are asking "how do I make sure that nothing is interfering with TLS connection" then what you want to do is not decrypt traffic. Make sure the traffic is hitting a SSL/TLS Inspection Rule for Do not Decrypt with Maximum Compatibility.

The confusion/complication is the HTTPS traffic is both TLS (sslx) and HTTP (web-in-snort). But in v18 just because you are doing TLS decryption does not mean you are doing web-in-snort (DPI mode) and vice versa. I know it may not make sense but you can have decrypted HTTPS traffic and be using the Certificate Authority from the XG and still not be using DPI mode.

If you don't want HTTPS to be looked at all, you need to make sure that the firewall rule does not have any web policy or malware scanning, and that the TLS rule is do not decrypt.  But be aware that even then, snort will be looking at the traffic in order to enforce all the other things that snort enforces (App control, IPS, ATP, traffic shaping).

The decoupling of TLS inspection (sslx) from HTTP filtering (web-in-snort / DPI mode) allows everything to be port and protocol agnostic. The TLS decryption rules also apply to SSH sessions, smtps, and other TLS encrypted communication.  The HTTP filtering is port agnostic.

Think, if you you will, about WebAdmin which is HTTPS going to port 4444.  In 17.5 traffic like this this would not be caught by the transparent mode web proxy.  In 18.0 DPI mode it is.

Hi Michael,

in summary what you are saying is we should build our own SSL/TLS inspection rule for each device/device group/users and disable the default rules?

I would like to suggest that the default rules be unlocked so they can be the default at the bottom instead of at the top. You cannot move your own rules above the default XG rues.

Ian

Ian, That is not what I am saying at all.

The two rules that are in there at the top are for global exclusions for SSL inspections. Unless you were specifically trying to have one of those decrypted, there is no reason to not have them at the top. It is just like Web Exceptions.

Why are you trying to do that would make you want a higher priority rule?

Ian, That is not what I am saying at all.

The two rules that are in there at the top are for global exclusions for SSL inspections. Unless you were specifically trying to have one of those decrypted, there is no reason to not have them at the top. It is just like Web Exceptions.

Why are you trying to do that would make you want a higher priority rule?

Hi Michael,

All traffic appears to pass through the top rule and there does not appear to be away of passing traffic through the second rule. The second rule (application) never shows traffic. I do have a third rule which has specific devices which does show traffic.

Ian

It works like a firewall rule - it is a top-down prioritized list and a given traffic only "hits" one rule.

Lets say you have:
Rule 1: Exclusion by website (URL groups Local/Managed TLS exclusion list)
Rule 2: Exclusion by application
Rule 3: source is your tv, do not decrypt
Rule 4: source is your computer, decrypt

So first your computer goes to adobe.com. That is in the managed TLS exclusion list which means Do not decrypt.
Then your computer goes to mylocalstore.com. That misses Rule 1,2,3 and hits Rule 4. It is decrypted.
Now your phone goes to mylocalstore.com. That misses Rule 1,2,3, and 4. It missed every rule. It is not decrypted.
Then your samsung TV goes to samsungtvapps.com, it hits rule 3 which is Do not decrypt.

Hi Michael,

thank you. I misunderstood the two default rules that only apply to URLs and applications in their exclusion lists.

With that better understanding in mind I will have another go at using the SSL/TLS rules.

Ian