If some authority for whatever reason decides they want to monitor your traffic on a wired DSL connection, how would they do it besides exploiting your OS? What are the other ways assuming the OS is locked down against intrusion?

Ideas I have so far:

  • At the modem. Get a different modem
  • ISP server/records. Use a VPN? Do secure VPNs like AES 256 circumvent all possibility of monitoring traffic?
  • DNS records. Use different dns servers besides the ISP's.
  • Hardware based backdoor. Get a different computer.
  • Line is tapped physically at the junction box. Would a secure VPN protect your traffic in this case?

Other ideas and solutions?

4 Answers 4


All of the wires and cables, most of which is totally out of your physical control, could be tapped. Read-only CAT-5 cables can easily be made, and work even for the most inept. I'm certain that the local loop, the circuit connecting your domicile to the phone company's Central Office, can be tapped at many, many locations. The technology to decode DMT (the most common Layer 2 DSL protocol) is unknown to me.

DSL usually uses PPOE to communicate over the local loop. That means that the phone company's end of the PPP connection would be a logical, easy place to tap all of the IP-level communication. This is totally out of your control, and even if your TCP or UDP level comm is encrypted, you'd be subject to traffic analysis.

Hardware backdoors will be almost impossible to detect. Some Cambridge University people just published a good paper on a real hardware backdoor.

One way of snooping that you haven't considered is van Eck phreaking. One countermeasure to this sort of thing is to use weird fonts that are human-readable, but not TEMPEST or van Eck readable.

Another form of monitoring that you may not have considered is Traffic Analysis. For what's possible, take a look at Traffic Mining in IP Tunnels. The abstract is in French, but the PDF appears to be in English.

  • If it is a physical tap somewhere along the connection, does a secure vpn (aes-256, camellia 256) shield you? If it's a hardware backdoor it will still have to dial out and could thus be found by monitoring packets no?
    – bigbro
    Sep 27, 2012 at 16:28
  • 2
    Ultimately, I don't know, but unless the VPN makes some effort to confuse traffic analysis, stuff can leak. See: crypto.com/papers/jbug-Usenix06-final.pdf The "jitterbug" outlined in that paper could be used by the backdoor to sneak info out. Sep 27, 2012 at 16:43

Depending on which country you are in, laws may require ISP's to provide wiretap functionality for law enforcement.

If you encrypt your traffic their next step is to get the encryption key or place a Trojan on your PC that can monitor the traffic before it gets encrypted, or by decrypting it.

Depending on how valuable you are as a target, there are also options such as installing hidden cameras, replacing hardware etc.


Essentially any point you don't have control over is a vulnerability. That said -- authorities cant tap/sort though all nodes so your best bet at anonymity is tor at a Starbucks or some such.


Do secure VPNs like AES 256 circumvent all possibility of monitoring traffic?

Absolutely not.

What is to say the VPN is not complicit in this monitoring, either for domestic, or foreign, law enforcement? They may have the same legal requirement to provide wiretaps to law enforcement as any other ISP in their country.

Secondly, VPN, which is either IPv4-in-IPv4 (GRE) (optionally encrypted) packets, or SSL (OpenVPN/SSTP) like any encryption, requires you use or obtain a key locally. You have two threats to any such setup:

  • Attacks to obtain the key. Again, the VPN provider could hand law enforcement your session key, or this could be obtained from your PC, or;
  • MITM attacks. Depending on the type of VPN, how trust in the remote endpoint is verified etc, it may be possible to simply route you to a system that decrypts your data on the fly.

As Rory says, it depends how valuable you are as a target, but no, merely using VPN does not give you any guarantee of security.

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