Question

Treason uncloaked - how to close open relays?

Asked by: Cem

I belive my server is getting attacked by someone.
What should I do to block them automatically?

Can I use iptables to do that?

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Asked On
2009-10-29 at 08:21:07ID24854916
Topics

CPanel

,

Linux Networking

,

Linux Network Security

,

Linux Administration

,

Linux

Participating Experts
3
Points
500
Comments
15

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Answers

 

by: omarfaridPosted on 2009-10-30 at 22:43:13ID: 25708630

You should have a firewall that protects your system otherwise you should use iptables to restrict access to your system

http://rlworkman.net/howtos/iptables/iptables-tutorial.html

 

by: RedimidoPosted on 2009-10-30 at 23:32:49ID: 25708717

simply a firewall will not solve your problem

I recommend you to give a deep reading to this excellent blog entry:

http://www.informedbanking.com/wiki/TCP_Treason_Uncloaked

Looks like increasing the entropy will help, but definitely there is a bug in the TCP kernel driver.

it IS however related to attacks, but simply blocking the traffic will not help, because it can arrive from any ip or mixed with valid traffic.

maybe some of the advanced kernel variables exposed there will work, and using a rate limiter would also help.


hth

 

by: MysidiaPosted on 2009-11-01 at 00:10:32ID: 25712861

Are you measuring any performance degradation, or seeing any ill effects other than the messages?  They _can_  be related to attacks,  if you are under attack,  but the messages don't necessarily  mean  any kind of DoS  attack is being attempted.

I see about 20 of these messages show up a week. Primarily on web servers, by the way, and believe they are mainly due to broken TCP stacks on clients.

See the comment by  Simon Farnsworth,  here:  http://ubuntu-tutorials.com/2008/07/04/tcp-treason-uncloaked/
For a more thorough explanation.

I wouldn't be concerned about it,  unless I were running an old kernel or seeing a fairly large number of these.    It's more likely to be some   Windows 3.1  user or other ancient OS user with a broken TCP Stack   than directed attack.


Essentially, the messages mean that the remote side of a TCP connection  to your server  has  ACK'ed  a packet,  and  reduced their  receive window by an impossible amount.

E.g.  In an  acknowledgement message,  the remote side of the connection said earlier that they could receive 500 bytes within their congestion window, in their next acknowledgement, the remote end acknowledges some TCP segments ,  but reduces the receive window by _more bytes_ than the amount of data they have acknowledged.

It's a violation of the protocol,   it essentially means they lied   when they said "WINDOW 500",   and have now  suddenly  reduced the  congestion receive window by more bytes than received.


 

by: CemPosted on 2009-11-04 at 21:43:42ID: 25747029

Can these errors be caused by defective RAM module(s)?

 

by: RedimidoPosted on 2009-11-05 at 07:33:21ID: 25750531

No.
these are valid network subsystem log entries
it can be a defective tcp stack, however.

upgrading to a newer kernel version would help but dot bet your job on it

 

by: MysidiaPosted on 2009-11-20 at 21:23:15ID: 25876599

There was ample response to this question above, and several experts posted thorough right answers...

In short: (#25708717) no, these messages alone do not indicate an attack,  it is normal to see these messages periodically.

No, trying to block will not help.
(#25750531) No, these messages cannot result from a hardware issue.
Yes, these messages suggest a possible bug in the kernel's TCP stack.

You can upgrade your own kernel (comment #25750531), which may address the issue.
However you cannot upgrade the OS  of everyone connecting to your server, so you cannot ever  eliminate these messages entirely,  other than turning off logging on your server.

 

by: CemPosted on 2009-11-21 at 06:46:59ID: 25878137

Mysidia, nothing solved it.. still getting same messages and they arent attacks since my own IP was in that list aswell.

Unless someone solves the problem Im not gonna give the points since I cant add it to knowledge base to others who wont get it solved within these posts.

 

by: RedimidoPosted on 2009-11-21 at 18:03:38ID: 25880512

Hi Cem

a) You asked how to close open relays. you only need to have the latest patched versions and the standard configuration.
b) I did not see any answer about upgrading your kernel. If you are told there will be nothing to disappear those messages on the current kernel, then that and no other is your answer.

 

by: MysidiaPosted on 2009-11-21 at 18:08:30ID: 25880523

Question should not be deleted.

Author asked "I believe my server is getting attacked by someone. What should I do to block them automatically?"  And got several times the right answer to the specific question asked,  including details about what the messages mean...

No the messages don't indicate any attack. The messages don't indicate anything wrong with your server, or any problem to be solved.  

There might be a problem that accompanies the messages, but the messages are very common on busy Linux servers, and don't alone indicate any problem.   Author  refused to answer experts request for information if there were other messages or if performance was otherwise degraded.

Legitimate users generate the messages.  There's no way to block what causes the messages,  automatically or otherwise.  (other than hiding some messages  on another server by using an application proxy).

There are some minor bugs in the TCP stack of certain Linux that can also cause the messages.  Depending on which distribution in use,  improvements might or might not be available,  with an updated kernel.

Redimido   suggested to upgrade kernel, accordingly.


 

by: MysidiaPosted on 2009-11-28 at 20:13:06ID: 25927834

I recommend  accept as answers
#25750531,  #25708717, #25712861

20120131-EE-VQP-002

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