MD5 considered harmful

Patrick W. Gilmore patrick at ianai.net
Fri Jan 27 14:52:41 CST 2012


MD5 on BGP sessions is the canonical example of a cure worse than the disease.  There has been /infinitely/ more downtime caused by MD5 than the mythical attack it protects again.  (This is true because anything times zero is still zero.)

It is far easier to take a router out than try to calculate the number of RSTs per second you can get through to the RE without your guesses being dropped / throttled, then waiting hours or days to watch a BGP session flap.  Amazingly awesome attack, because as everyone knows BGP sessions never flap on their own, so a random session flapping every day or six will totally freak out the provider in question.  And all that ignores the fact every router vendor fixed the ephemeral port selection & window size issues half a decade ago, so those "days" it takes to reset a single BGP session are actually more like months or years.

Remember, miscreants are lazy, impatient, and frequently clueless.  Who would want to reset a BGP that will come back up in 30-90 seconds when you can packet an entire router off the 'Net easier, more quickly, and for longer a period?

Unfortunately, Network Engineers are lazy, impatient, and frequently clueless as well.  They read something from 1906 that says "$FOO IS GOOD!!1!1!" and force every peer to subscribe to their own ideal without understanding the underlying technology or rationale.


Your network, your decision.  On my network, we do not do MD5.  We do more traffic than anyone and have to be in the top 10 of total eBGP peering sessions on the planet.  Guess how many times we've seen anyone even attempt this attack?  If you guessed more than zero, guess again.

I am fully well aware saying this in a public place means someone, probably many someones, will try it now just to prove me wrong.  I still don't care.  What does that tell you?

STOP USING MD5 ON BGP.

-- 
TTFN,
patrick




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