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  1. #1
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    Telnet over VPN uses local IP address?

    Hi guys, hoping someone might have a flash of inspiration on this one.

    Today I had cause to telnet onto a remote server to run some smtp tests. I established a vpn as normal, opened the telnet session and found I was connected via my own ADSL IP address. This caused issues when I attempted to send smtp commands which where then interpreted as spam.

    I then forced the vpn to use the remote gateway rather than local for internet access and reconnected. Same result.

    Opened a browser and checked the IP address which then returned the IP of the remote gateway - as expected.

    Scratching my head a bit. Is it normal behaviour for a telnet client over a VPN to report the local IP address or is it a function of the OSI layer that the vpn is on compared to telnet?

    Local system: Win7 Pro virtualised running on top of a Linux host. PPTP VPN. Domain joined and logged in as Standard user with Admin rights (not Admin or Standard account). Bridged networking from Host to VM.
    Remote system: SBS 2008.

  2. #2
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    So, this morning, I ran a logmein session to a PC on the remote network. Opened a telnet session from that PC to the mail server. Note: this is all on the internal network. The remote PC has a dhcp allocated 192.168.xxx.xxx address. The mail server is has a fixed local IP.

    Guess what? The response from the mail server reports that the telnet session is using the public IP of the gateway on that network, not the dhcp allocated internal IP address of the PC. That's the same kind of response that I received when I ran the telnet over a VPN from the office here.

    Now I'm really confused.

  3. #3
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    Sounds to me like the mail server is external to the remote network and you traverse the internet to get to it. This can't be right, but it's hard to tell from your description.

    I assume you use this format "telnet 192.168.x.x 25". If you ping that address does it return the server name - "ping -a 192.168.x.x".

    Maybe your router does weird things when sending packets to the mail server? Try a tracert

    cheers, Paul

  4. The Following 2 Users Say Thank You to Paul T For This Useful Post:

    capnemo2080 (2011-12-04),Tinto Tech (2011-11-04)

  5. #4
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    I assume you use this format "telnet 192.168.x.x 25"
    Thanks Paul! It's amazing how snippet of a comment makes you see the error staring you in the face.....

    telnet mail.xyz.com 25 is not the same as telnet 192.168.x.x 25 Doh!

    I was so busy thinking about the smtp relay (the reason for the remote session in the first place) and made a silly mistake by specifying the FQDN mail server name rather than the internal IP.

    So, yes, even though it was unintentional, it was going out over the internet to get to the mail server and hence seeing the gateway IP address.

    I think it's time for a Friday night beer.

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