USG20-VPN NAT does not work after router substitution

FrancoCorbelli Posts: 1
First Anniversary
edited May 2021 in Security
I am having difficulty activating a simple NAT on USG20 (firmware 4.62), after a VDSL router replacement.

The router have a public internet IP (5.x.y.81), and a LAN IP (
I cannot operate on the Vodafone router and, therefore, I must "trust" that a 1:1 NAT has been configured between all its ports to the USG (as requested).

The USG WAN port is (default gateway of course, and the is LAN

On the LAN (via a switch) there are various servers, for my tests a NAS, with IP, gateway ANOTHER USG (, various service (for test say 23 telnet)

I want a NAT from the router to the NAS, telnet 5.x.y.81 (from outside) should go to =>

It sounds simple, but I doesen't work.

For testing purposes I can even keep the firewall (policy control) disabled.

It is possible to reach the https interface from Internet (https:5.x.y.81), and even FTP, SSH etc (ssh 5.x.y.81) of the USG (test configuration of course).

By verifying from the web console (of USG) it is possible to telnet and log in, therefore the USG <=> NAS traffic should work

In theory, a NAT rule should  be enough (and then the firewall rule but, as mentioned, for testing I can also leave it disabled)

I can reach the Zywall (even FTP, just for test, https etc), from Internet

USG can "speak" with the NAS, as it shoud be (telnet for test)

BUT, when I try telnet 5.x.y.81 (or whatever, ssh, https etc) => impossible.

By setting a specific policy (to be able to log) everything seems to actually arrive as expected, in this case from my external internet IP (nas1 =
The same problem occurs for services other than telnet (ssh etc).

Thanks for any helps

All Replies

  • tonygibbs16
    tonygibbs16 Posts: 841  Guru Member
    First Anniversary 10 Comments Friend Collector First Answer
    edited May 2021

    My guess is that your Vodafone router is not passing telnet through.

    A way to prove this would be to put a PC running Wireshark ( ) between your Vodafone router and your USG20-VPN device or turn on the Firewall on your USG20-VPN device to block telnet and have it log the connection.
          - if you cannot see it in the firewall log, then your Vodafone router is not passing telent connections through from the Internet.

    If the above hypothesis is true, then you may need to look at using the VPN capabilities of your USG20-VPN, see section 5 of the user guide available at

    Once you have an IPSEC tunnel set-up, then you will be able to run telnet and other protocols over the top of it.

    Also, is the Vodafone router necessary and a Vodafone requirement? If it is not, then maybe your USG20-VPN device be your broadband router connecting to Vodafone ISP.
           - it would give you more control.

    Kind regards,

  • tonygibbs16
    tonygibbs16 Posts: 841  Guru Member
    First Anniversary 10 Comments Friend Collector First Answer

    And if the public router was the last thing changed, then it is probably the current root cause of your issues.

    But telnet is not secure over the Internet, because the user name and password are sent as clear text, so it would be more secure for your NAS if you were able to set up a VPN with encryption such as SSL or IPSEC.

    Kind regards,

  • Zyxel_Cooldia
    Zyxel_Cooldia Posts: 1,454  Zyxel Employee
    First Anniversary 10 Comments Friend Collector First Answer
    @tonygibbs16, thanks for the instruction.
    Welcome to Zyxel community.  :)
    You also can capture on USG20 wan interface to check if the packets reach USG wan interface. 
    Assume you still cannot see telnet packet comes to USG20 Wan interface, then it could be blocked by ISP router.
    Router> packet-trace interface wan1 extension-filter port 23
    Web GUI
    MAINTENANCE > Diagnostics > Packets Capture

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