North American Network Operators Group
Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical

Re: SYN floods (was: does history repeat itself?)

  • From: Mr. Jeremy Hall
  • Date: Sun Sep 15 07:49:58 1996 
-->
-->Jeremy Hall <[email protected]> wrote:
-->
-->>In order for your idea to work, the router where you're doing the
-->>filtering must know how to get to all destinations on the Internet, must
-->>not have a default network or route, and they must be symetrical.
-->
-->Well, that is certainly not the case.  For reverse-route filters
-->to work paths do not have to be symmetrical.  In fact, in interdomain
-->routing the condition for applicability of reverse-route filters is
-->that filtered AS must not do transit.
-->
-->(Note that reverse filters i described do _not_ require that the route
-->back must be best.  It just have to be present in the RIB corresponding
-->to exterior routing session over the interface in question.)
-->
You may not have said it, but I remember someone said the route had to be 
in the routing table. I would agree with you if it looked up the source 
in the BGP table and if it considered history or dampened paths valid. If 
your asymetry runs over multiple interfaces, then the best path might not 
be on the interface the packet is arriving on.

-->>As far as your other statement, when an instability occurs, all traffic
-->>starts getting slow because the routers are trying to reroute around a
-->>flapping t3 or whatever caused the outage. Since the whole point around a
-->>denial of service attack is to deny service, by adding in the fact that
-->>we need to know how to get to the source address before we forward the
-->>packet introduces more problems.
-->
-->Ughm, i do not see the relation between flapping and more problems
-->created by reverse-route filtering.
-->
I am referring to end-user problems. When a line flaps, especially a 
backbone line, lots of destinations suddenly become unreachable. If you 
look at it in the light that one's source location will soon be one's 
destination location, then you didn't add to the confusion because, in 
theory, the returning traffic from the destination to the source would 
not know how to get to the source. That said, remember you threw in 
asymetry, meaning the return traffic might take a totally different 
interface or even router to get back to the original source. If you don't 
include dampened or history paths, then if the source gets dampened by 
the path the source packets enter your router, you will drop them because 
the path those packets arrived on "shouldn't" exist because it is dampened.

-->>I think you would find this hurts more
-->>than it helps.
-->
-->Would you elaborate?  There's certainly little hope that SYN flooding
-->(as well as most of other denial of service attacks) cannot be effectively
-->prevented w/o more robust source identification, so pros are quite obvious.
-->
-->The fact that source identification works, and works well enough even
-->to do billing is beyond any doubt.  RELCOM does that on large scale.

Do they deny packets because you aren't a relcom customer? This is a nice 
idea on a sunny day, don't get me wrong, but I am wondering how the 
routers would react when an unusual condition occurs. Are you certain 
relcom bills its customers for *EVERY* source packet, or can account for 
them at least?

 --> -->>Even if you limit this kind of lookups to when the 
packet
-->>happens to be a TCP packet with the syn option, you still have a problem
-->>in establishing a connection. This creates frustration on the part of the
-->>end user.
-->
-->I will not comment, as that corollary is based on something which is
-->not likely to be true.
-->
-->--vadim
-->

Please explain. If your router does not know how to get back to the 
source in the split second it got the packet, it might know how to get to 
the destination and could send the packet on its way. Maybe by the time 
the web server responded, traffic could resume a normal route, or perhaps 
outbound traffic for the web server is unimpared because you chose to 
implement asymetry.
-- 
              -------------------------------------------
              | Jeremy Hall      Network Engineer |
              | ISDN-Net, Inc    Office +1-615-371-1625 |
              | Nashville, TN    and the southeast USA  |
              | [email protected]   Pager  +1-615-702-0750 |
              -------------------------------------------

- - - - - - - - - - - - - - - - -