From owner-freebsd-questions Tue Jun 16 00:14:18 1998 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.8/8.8.8) id AAA08233 for freebsd-questions-outgoing; Tue, 16 Jun 1998 00:14:18 -0700 (PDT) (envelope-from owner-freebsd-questions@FreeBSD.ORG) Received: from freefall.freebsd.org (freefall.FreeBSD.ORG [204.216.27.21]) by hub.freebsd.org (8.8.8/8.8.8) with ESMTP id AAA08183 for ; Tue, 16 Jun 1998 00:14:08 -0700 (PDT) (envelope-from kuku@gilberto.physik.RWTH-Aachen.DE) Received: from gilberto.physik.RWTH-Aachen.DE (gilberto.physik.rwth-aachen.de [137.226.30.2]) by freefall.freebsd.org (8.8.8/8.8.5) with ESMTP id AAA23315 for ; Tue, 16 Jun 1998 00:13:04 -0700 (PDT) Received: (from kuku@localhost) by gilberto.physik.RWTH-Aachen.DE (8.8.8/8.8.7) id JAA03105; Tue, 16 Jun 1998 09:14:00 +0200 (MEST) (envelope-from kuku) Message-ID: <19980616091359.45134@gil.physik.rwth-aachen.de> Date: Tue, 16 Jun 1998 09:13:59 +0200 From: Christoph Kukulies To: Doug White Cc: Christoph Kukulies , freebsd-questions@freefall.cdrom.com Subject: Re: using tcpdump effectively References: <199806151447.QAA29137@gilberto.physik.RWTH-Aachen.DE> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Mailer: Mutt 0.81e In-Reply-To: ; from Doug White on Mon, Jun 15, 1998 at 08:29:47PM -0700 Sender: owner-freebsd-questions@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG On Mon, Jun 15, 1998 at 08:29:47PM -0700, Doug White wrote: > On Mon, 15 Jun 1998, Christoph Kukulies wrote: > > > > > To trace down why some network based X11 sessions are spuriously failing > > I' trying to use tcpdump. > > > > What sporadically happens is that a X session to our Mentor Design Architect > > running on HP is ceased and the connection breaks (we login via rlogin > > and start the X client with DISPLAY set to the FreeBSD machine.) > > > > When the connection breaks we see something like 'no route to host' It seems that I have found the problem. I logged (tcpdump) all packets to and from the two hosts and this morning my colleague called me up and said it happened again. I peeked into my logs and found the following interesting passage right at the time it happened: 08:28:03.140374 monk.6000 > hp.1327: P 151773:151805(32) ack 661157 win 17520 ( DF) 08:28:03.151214 monk.6000 > hp.1327: P 151805:151837(32) ack 661157 win 17520 ( DF) 08:28:03.152081 arp who-has monk tell aca402a.physik.rwth-aachen.de 08:28:03.152336 arp reply monk is-at 0:40:95:24:d5:9b 08:28:03.152780 aca402a.physik.rwth-aachen.de > monk: icmp: host hp unreachable 08:28:03.163115 monk.6000 > hp.1327: P 151837:151869(32) ack 661157 win 17520 ( DF) 08:28:03.167881 hp.1327 > monk.6000: . ack 151869 win 7776 08:28:03.172922 monk.6000 > hp.1327: P 151869:151901(32) ack 661157 win 17520 ( DF) 08:28:03.185096 monk.6000 > hp.1327: P 151901:151933(32) ack 661157 win 17520 ( DF) Two things are interesting: monk (the X Display server (FreeBSD)) received a package from a host which shouldn't be involved at all (sniper hosts). This host is telling monk via icmp that hp is unreachable. I'd bet this is an old NT system (< 3.51). The address of that host is a name which consists of only hex digits - Maybe not important but you never know. I've sent a colleague through the building to take this host from the network. I'd bet it is an NT System < 3.51 (or in the worst case, a malign program ). > > Most likely the client is loosing the network connection to the host, > either by damage to the routing tables on the client or on an intermediate > network device. Run a traceroute to the HP box when MDA crashes and see if > it fails anywhere. > > > Could that be caused by denial of service attacks? What exactly is a denial > > of service attack? > > A denial of service attack (DoS) attempts to keep a machine from being > servicable by overwhelming it with requests or by disabling a server, > rending it useless. > > Doug White | University of Oregon > Internet: dwhite@resnet.uoregon.edu | Residence Networking Assistant > http://gladstone.uoregon.edu/~dwhite | Computer Science Major > NOTICE: Make sure your mailer replies to dwhite@resnet or I won't get it! > > > To Unsubscribe: send mail to majordomo@FreeBSD.org > with "unsubscribe freebsd-questions" in the body of the message -- --Chris Christoph P. U. Kukulies kuku@gil.physik.rwth-aachen.de To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message