kern/103135: ipsec with ipfw divert (not NAT) encodes a packet twice breaking PMTUD

[Eugene Grosbein]

>Number:         103135
>Category:       kern
>Synopsis:       ipsec with ipfw divert (not NAT) encodes a packet twice breaking PMTUD
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Mon Sep 11 13:50:16 GMT 2006
>Closed-Date:
>Last-Modified:
>Originator:     Eugene Grosbein
>Release:        FreeBSD 6.1-STABLE i386
>Organization:
Svyaz Service JSC
>Environment:
System: FreeBSD nkz.delikates-nk.ru 6.1-STABLE FreeBSD 6.1-STABLE #1: Thu Sep 7 13:31:53 KRAST 2006 root at nkz.delikates-nk.ru:/home/obj/home/src/sys/NKZ i386
	options IPDIVERT
	options IPSEC
	options IPSEC_ESP

>Description:
	When outgoing packet encoded due to corresponding IPSEC policy
	is passed to divert socket (f.e. to ipacctd for accounting),
	it is encoded second time with IPSEC then. Besides obvious
	logic error, this also results in broken Path MTU Discovery.

>How-To-Repeat:

	Use a kernel with options IPDIVERT, IPSEC, IPSEC_ESP
	(my kernel also contains IPSEC_FILTERGIF, but this should not matter).

	Suppose there are two local nets numbered 192.168.1.0/24
	and 192.168.2.0/24, each has a FreeBSD router
	(192.168.1.1 and 192.168.2.1). Routers make gif(4) tunnel between
	and use IPSEC transport mode to encrypt its contents.
	Their external IP addresses are 1.1.1.1 and 2.2.2.2

	Here is /etc/ipsec.conf:

add 1.1.1.1  81.16.143.102 esp 1007 -m transport -E blowfish-cbc "xxx";
add 2.2.2.2 1.1.1.1 esp 2007 -m transport -E blowfish-cbc "yyy";

spdadd 1.1.1.1/32 2.2.2.2/32 any -P in  ipsec esp/transport/1.1.1.1-2.2.2.2/require;
spdadd 2.2.2.2/32 1.1.1.1/32 any -P out ipsec esp/transport/2.2.2.2-1.1.1.1/require;

	Another router has similar /etc/ipsec.conf.

	Use this script to prepare IPSEC keys for tcpdump:

#!/bin/sh

setkey -D | awk '
  /^[1-9]/  { ip=$2; }
  $1=="esp" { 
		sub(/spi=[^(]+\(/, "", $3);
		sub(/\)/, "", $3);
		printf"%s@%s ",$3,ip;
	    }
  $1=="E:"  { printf "%s:0x%s%s%s%s\n", $2, $3, $4, $5, $6; }
' > /tmp/keys.txt

	When we use one of the routers to run
	"tcpdump -s0 -n -p -i $iface -E /tmp/keys.txt esp",
	we see that traffic is encoded with ESP and decoded properly,
	still good.

	Now install ports/net-mgmt/ipacctd, run it manually with
	"ipacctd -p 4000", then command:
	"ipfw add 10 divert 4000 ip from any to any out"
	and rerun tcpdump command shown above. You'll see that
	outgoing packets are encapsulated with ESP twice.
	The same you'll see at the other side for incoming packets.

>Fix:

	Unknown. The only known workaround is to avoid diverting ESP.
	This workaroung is not always acceptable.


Eugene Grosbein
>Release-Note:
>Audit-Trail:
>Unformatted:
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