kern/184389: libalias fails to adjust MTU from jails

Monkey crapsh at
Sat Nov 30 10:00:01 UTC 2013

>Number:         184389
>Category:       kern
>Synopsis:       libalias fails to adjust MTU from jails
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    freebsd-bugs
>State:          open
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Sat Nov 30 10:00:00 UTC 2013
>Originator:     Monkey
>Release:        FreeBSD 9.2-RELEASE-p2 amd64
System: FreeBSD papadum 9.2-RELEASE-p2 FreeBSD 9.2-RELEASE-p2 #0: Sat Nov 30 00:46:30 PST 2013 monkey at papadum:/usr/obj/usr/src/sys/GENERIC amd64

	ipfw_nat, libalias
	MTU size is not properly discovered from packets generated from jails.  libalias and/or ipfw_nat
	need to communicate with the network stack to determine MTU.
	Set up ipfw_nat to nat packets generated from a jail.
	Send those packets to a second FreeBSD box that has a GRE tunnel to third machine.
	Attempt to ssh or scp a file larger than 1500 bytes from the jail to the third machine -- it hangs.
	If you SSH from the host (outside of the jail), the ssh connects.
	If you quickly log into the jail, it will briefly work (OS remembers MTU size for that path).
	Set the MTU to 1476 on the interface connecting computer A to computer B.


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