http://www.news.com/8301-10784_3-9878655-7.html
By Declan McCullagh
News.com
February 25, 2008
A high-profile incident this weekend in which Pakistan's state-owned
telecommunications company managed to cut YouTube off the global Web
highlights a long-standing security weakness in the way the Internet is
managed.
After receiving a censorship order from the telecommunications ministry
directing that YouTube.com be blocked, Pakistan Telecom went even
further. By accident or design, the company broadcast instructions
worldwide claiming to be the legitimate destination for anyone trying to
reach YouTube's range of Internet addresses.
The security weakness lies in why those false instructions, which took
YouTube offline for two hours on Sunday, were believed by routers around
the globe. That's because Hong Kong-based PCCW, which provides the
Internet link to Pakistan Telecom, did not stop the misleading
broadcast--which is what most large providers in the United States and
Europe do.
This is not a new problem. A network provider in Turkey once pretended
to be the entire Internet, snarling traffic and making many Web sites
unreachable. Con Edison accidentally hijacked the Internet addresses for
Panix customers including Martha Stuart Living Omnimedia and the New
York Daily News. Problems with errant broadcasts go back as far as 1997.
It's also not an infrequent problem. An automatically-updated list of
suspicious broadcasts created by Josh Karlin of the University of New
Mexico shows apparent mischief--in the form of dubious claims to be the
true destination for certain Internet addresses--taking place on an
hourly basis.
So why hasn't anyone done something about it? False broadcasts can
amount to a denial-of-service attack and, if done with malicious intent,
can send unsuspecting users to a fake bank, merchant, or credit card
site.
To understand why this is both a serious Internet vulnerability and also
difficult to fix requires delving into the technical details a little.
How to pretend to be YouTube.com
When you type a domain like "news.com" into your Web browser, it uses
the Domain Name System to cough up a numeric Internet address, which in
our case is 216.239.113.101. That IP address is handed to your router,
which uses a table of addresses to figure out the next hop toward the
news.com server.
Network providers--called autonomous systems, or ASs--broadcast the
ranges of IP addresses to which they'll provide access. One of the
functions of the Internet Corporation for Assigned Names and Numbers is
managing the master list of AS numbers, which it does by allocating
large blocks of 1,000 or so at a time to regional address registries.
Kim Davies, ICANN's manager of route zone services, says ICANN isn't
able to revoke the AS number of a misbehaving network provider. "It's
best to think of them as similar to post codes or ZIP codes," Davies
said. "We maintain a registry of them to ensure that they aren't
conflicting."
If the address information provided by AS is reliable, all is well. But
if an AS makes a false broadcast, because of a configuration mistake or
for malicious reasons, all hell can break loose.
This is what happened with YouTube, which Pakistan's government ordered
blocked because of offensive material, apparently a video depicting the
cartoons about Muhammad that had been posted in a Danish newspaper. Some
reports have said the video featured several minutes of a film made by
Dutch politician Geert Wilders, an outspoken critic of Islam.
A spokesman for the Pakistani embassy said on Monday that the order to
block access to YouTube came from the highest levels of the government.
It would have been passed along to Pakistan's Electronic Media
Regulatory Authority and then to Pakistan's telecom authority, the
spokesman said, which in turn would have issued the formal order to the
Internet providers.
Pakistan Telecom responded by broadcasting the false claim that it was
the correct route for 256 addresses in YouTube's 208.65.153.0 network
space. Because that was a more specific destination than the true
broadcast from YouTube saying it was home to 1,024 computers, within a
few minutes traffic started flowing to the wrong place.
A timeline created by Renesys, which provides real-time monitoring
services, says that it took about 15 seconds for large Pacific-rim
providers to direct YouTube.com traffic to the Pakistan ISP, and about
45 seconds for the central routers on much of the rest of the Internet
to follow suit.
YouTube took countermeasures within minutes, first trying to reclaim its
network by narrowing its 1,024 broadcast to 256 addresses. Eleven
minutes later, YouTube added an even more specific additional broadcast
claiming just 64 addresses--which, under the Border Gateway Protocol, is
more specific and therefore should overrule the Pakistani one. Over two
hours after the initial false broadcast, Pakistan Telecom finally
stopped.
How could this have been prevented? First, Pakistan Telecom shouldn't
have broadcast to the entire world that it was hosting YouTube's IP
addresses. Second, Hong Kong-based PCCW could have recognized the
broadcast as false and filtered it out.
An employee of PCCW, who wished to remain anonymous because he is not
authorized to speak for the company, said that as soon as the false
broadcast occurred, PCCW started receiving a flurry of phone calls from
global ISPs wondering what had gone wrong. A YouTube representative also
called.
Even Pakistan Telecom contacted PCCW. "I don't think they understood
what was going on," the employee said. A spokesman for PCCW's U.S.
operations, based in Herndon, Va., declined to provide details.
At the moment, large network providers tend to trust that other network
providers are behaving reasonably--and aren't intentionally trying to
hijack someone else's Internet addresses. And errors that do arise tend
to be fixed quickly by manual intervention.
But as the number of suspicious broadcasts grows, and the potential for
fraud increases, so does the justification for more aggressive
countermeasures. (Besides, some government will eventually order its
network providers to broadcast false information about the Internet
addresses of "offensive" Web sites. We've already seen domain name
blocking in Finland and Web page blocking in the United States, both
supposedly enlightened Western democracies.)
One way to handle this is for network providers to be automatically
notified when the virtual location of an Internet address changes, which
is what some researchers have suggested in the form of a "hijack alert
system." Another is to treat broadcasts with changes of addresses as
suspicious for 24 hours and then accept them as normal. Simple filtering
of broadcasts may not always work because some networks provide
connectivity to customers with thousands of different routes.
Probably the most extensive countermeasure would be a technology like
Secure BGP, which uses encryption to verify which network providers own
Internet addresses and are authorized to broadcast changes. But Secure
BGP has been around in one form or another form since 1998, and is still
not a widely-used standard, mostly because it adds complexity and
routers that understand will add additional cost.
At least that's been the conventional view. A high-profile incident like
YouTube being knocked offline may accelerate this process, said Steven
Bellovin of Columbia University. "I know there are serious deployment
and operational issues," Bellovin said. "The question is this: When is
the pain from routing incidents great enough that we're forced to act?
It would have been nice to have done something before this, since now
all the world's script kiddies have seen what can be done."
News.com's Greg Sandoval contributed to this report.
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