Date: Wed, 28 Aug 2002 12:18:54 -0700 (PDT) From: Jeff Behl <jeff@expertcity.com> To: freebsd-gnats-submit@FreeBSD.org Subject: kern/42137: Path MTU broken - initial too-large packet continuously resent Message-ID: <200208281918.g7SJIsHp081709@www.freebsd.org>
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>Number: 42137 >Category: kern >Synopsis: Path MTU broken - initial too-large packet continuously resent >Confidential: no >Severity: serious >Priority: medium >Responsible: freebsd-bugs >State: open >Quarter: >Keywords: >Date-Required: >Class: sw-bug >Submitter-Id: current-users >Arrival-Date: Wed Aug 28 12:21:23 PDT 2002 >Closed-Date: >Last-Modified: >Originator: Jeff Behl >Release: 4.6.1-RELEASE-p10 >Organization: Expertcity >Environment: FreeBSD dell350-13.snv 4.6.1-RELEASE-p10 FreeBSD 4.6.1-RELEASE-p10 #1: Fri Aug 16 06:50:36 PDT 2002 root@dell350-13.snv:/usr/src/sys/compile/COMMS44-2 i386 >Description: FBSD keeps resending the packet that generated an ICMP type 3 'needs fragmentation but DF bit set' even though it contains the correct MTU in the route table for the host notice the mtu of 1420: dell350-12.snv#netstat -rnal | grep 10.4.1.134 10.4.1.134 63.xxx.224.129 UGHW 1 2268 1420 fxp0 but tcpdump shows: 17:21:43.497275 63.xxx.224.154.80 > 10.4.1.134.2314: . 1:1461(1460) ack 248 win 17520 (DF) 17:21:51.497212 63.xxx.224.154.80 > 10.4.1.134.2314: . 1:1461(1460) ack 248 win 17520 (DF) 17:22:07.497065 63.xxx.224.154.80 > 10.4.1.134.2314: . 1:1461(1460) ack 248 win 17520 (DF) Once some sort of timeout mechanism occurs and PMTUD turns off (DF bit not set), further packets seem to obey the correct MTU. Howerver, the inital packet that sparked the ICMP from the router will keep being resent until this timeout is reached. We observed this as web requests to a web server of ours running apache would sometimes hang on various images on a web page. We have also seen this same behavior on a 4.4-RELEASE system >How-To-Repeat: run some server (apache) and have a client make requests that result in large packets (images). >Fix: turn of path mtu discoverty via sysctl. >Release-Note: >Audit-Trail: >Unformatted: To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-bugs" in the body of the message
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