Snort mailing list archives
Re: ICMP Fragment Reassembly time exceeded
From: John Sage <jsage () finchhaven com>
Date: Tue, 15 Jan 2002 20:55:08 -0800
Paul: To paraphrase WR Stevens, "TCP/IP Illustrated vol.1" page 158:"ICMP Fragment Reassembly Time Exceeded" itself is generated when the IP layer sets a timer upon the first arrival of any fragment, not necessarily the first. If all fragement have not arrived when the timer expires, then that response is sent so long as the *first* fragment *was* received...
OK? So how could a client deliberately generate this?Well, what sort of client? Stevens implies that this is not that unusual: it simply means that all fragments were not received when your server's timer expired.
After that, if someone wanted, I suppose this could be deliberately triggered by some crafted client sending partial fragment sets...
...why bother? I don't know how current it is now, but Stevens states that Berkeley-derived IP implementations never generate this error, so I 'spose some sort of OS identification could be at work.
Or maybe not.The icmp packet sent out should contain the first 8 bytes of the original IP datagram; does that show anything interesting?
- John -- The web page you seek cannot be found here: countless others await Sheahan, Paul (PCLN-NW) wrote:
Hello, In my Snort logs I am seeing "ICMP Fragment Reassembly time exceeded" on a daily basis being sent as a response from our web servers to random clients on the Internet. I am running Snort Version 1.8.1-RELEASE (Build 78) under Red Hat Linux 7.0. Can anyone tell me or point me in the right direction on how a client is able to force a web server to respond with this ICMP message? I assume it is a means of a client gathering information from a server but want to get more information. Thanks! Paul
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Current thread:
- ICMP Fragment Reassembly time exceeded Sheahan, Paul (PCLN-NW) (Jan 15)
- Re: ICMP Fragment Reassembly time exceeded John Sage (Jan 15)