Practical mobile Internet access traceability

January 13th, 2010 at 00:05 UTC by Richard Clayton

In an earlier article I explained how the mobile phone companies are using Network Address Translation on a massive scale to allow hundreds of Internet access customers to share a single IP address. I pointed out how it was now necessary to record the source port as well as the IP address if you wanted to track somebody down.

Having talked in detail about this with one of the UK’s major mobile phone companies, I can now further describe some practical issues (Caveat: other companies may differ in how they’ve implemented their details, but all of them are doing something very similar).

The good news, first, is that things are not as bad as they might be!

By design, NAT systems provide a constant mapping to a single IP address for any given user (at least until they next disconnect). This means that, for example, a website that is tracking visitors by IP address will not serve the wrong content; and their log analysis program will see constant IP addresses when the user changes page or fetches an image, so that audience measurements will remain valid. From a security point of view, it means that provided you have at least one logging record with IP address + port number + timestamp, then you will have sufficient data to be able to seek to make an identification.

As a quick aside, you may be thinking that you could do an “inference attack” to identify someone without using a source port number. Suppose that you can link several bad events together over a period of time, but only have the IP address of each. Despite the telco having several hundred people using each IP address at each relevant instant, only one user might be implicated on every occasion. Viewers of the wire will recall a similar scheme being used to identify Stringer Bell’s second SIM card number!

Although this inference approach would work fine in theory, the telco I spoke with does not keep its records in a suitable form for this to be at all efficient. So, even supposing that one could draft the appropriate legal request (a s22 notice, as prescribed by the UK’s Regulation of Investigatory Powers Act), the cost of doing the searches and collating the results (and those costs are borne by the investigators), would be prohibitive.

But now it’s time for the bad news.

Traditional ISP IP address usage records (in RADIUS or similar systems) have both a “start” and “stop” record. The consistency of these records within the logging system gives considerable assurance that the data is valid and complete. The NAT logging only records an event when the source port starts to be used — so if records go missing (and classical syslog systems can lose records when network errors occur), then there is no consistency check to show that the wrong account has been identified.

The logging records that show which customer was using which IP address (and source port) are extremely large — dozens of records can be generated by viewing just one web page. They also provide sensitive information about customer habits, and so if they are retained at all, it will only be for a short period of time. This means that if you want traceability then you need to get a move on. ISPs typically keep logs of IP address usage for a few weeks. At the mobile companies (because of the volume) you will in practice have to consult the records within just a few days.

Furthermore, even when the company intends to hold the data for a short period, it turns out that under heavy load the NAT equipment struggles to do what it’s supposed to be doing, leave alone generate gigabytes of logging. So the logging is often turned off for long periods for service protection reasons.

Clearly there’s a reputational risk to not having any records at all. For an example which does not have anything to do with policing: not being able to track down the sources of email spam would demean the mobile company in the eyes of other ISPs (which in practice will be seen by ever more aggressive filtering of all of their email). However, that risk is rather long-term; keeping the system running “now” is rather more important; and there is a lot that a mobile company can do to block and detect spam within their own networks — they don’t need to rely on being able to process external abuse reports.

In the third and final article of this little series I will consider the question of “data retention”. Surely the mobile phone company has a legal duty to keep traceability records? It turns out that the regulators screwed it up — and they don’t!

Entry filed under: Legal issues, Security engineering

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