Handshake violation when trying to access one website

Question

I have a TZ 190 Wireless Enhanced with SonicOS Enhanced 4.2.1.0-20e.

Yesterday, people could access without any problems a bank website wich uses HTTPS. Today, it is imposible to access only that website, every other ones works without problems.

When checking the log message filtering to my IP only, this is what appears and I suspect is the cause of this problem, because all other websites are working:

Priority: Notice
Category: Network Access
Message: TCP handshake violation detected; TCP connection dropped   
Source: X.Y.Z.3, 51997, LAN (admin) 
Destination: 200.14.232.18, 443, WAN    
Notes: Handshake Timeout

Where X.Y.Z.3 is my local IP.

I've tried to change TCP Settings under Firewall option, and activated this options with no success:

Enforce strict TCP compliance with RFC 793 and RFC 1122

and

Enable TCP checksum enforcement

I've also tried to find the MTU and at first I got:

Packet needs to be fragmented but DF set

But when I lower the value of ping -f -l to 1468 I got:

Request timeout.

Also I deactivate CFS in lan and wan zones. Nothing works.

Can you please help me? Any Ideas?

Solution

Disable the Handshake Timeout.

Go on Firewall Setings -> Flood Protection:

Uncheck the option "Enable TCP handshake timeout"

Comments

[psarossy]

Could you explain a bit why this solves the problem?

[Ramon Lucas]

I have the same problem. I do not know why gives this error "TCP handshake timeout" . But this is the only solution I found.

[Dennis Kaarsemaker]

That means that the device thinks the TCP handshake times out, so either its timeout setting is far too low, or the banking website is ridiculously slow.

[MDMoore313]

@DennisKaarsemaker the fact that this happened all of a sudden suggests the site started running slow, possibly victim of a DDOS?