Date: Fri, 09 Aug 2019 18:40:03 +0000 From: bugzilla-noreply@freebsd.org To: bugs@FreeBSD.org Subject: [Bug 239749] Issues path MTU discovery on IPv6 and SSH. Message-ID: <bug-239749-227@https.bugs.freebsd.org/bugzilla/>
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D239749 Bug ID: 239749 Summary: Issues path MTU discovery on IPv6 and SSH. Product: Base System Version: CURRENT Hardware: Any OS: Any Status: New Severity: Affects Only Me Priority: --- Component: kern Assignee: bugs@FreeBSD.org Reporter: zeising@FreeBSD.org There seem to be an issue with path MTU discovery, and SSH (and possibly others). I have a FreeBSD router, running 12.0-RELEASE-p9. The router terminates an IPv6 tunnel, using gif (protocol 41). The MTU on the tunnel is 1480, becau= se of the encapsulating IPv4 header. In my internal network, the MTU is the standard 1500. >From my work station, when I try to SSH over IPv6 to, for example freefall.freebsd.org, the connection fails. I can see in package dumps that the router responds with an ICMP packet too big. I can understand that thi= s is the case for the first attempt since the local link has an MTU of 1500 but = the remote link has an MTU of 1480, but ssh does multiple attempts with the same packet size, and gets ICMP ptb in return. Eventually, the connection is re= set. I expected that something, either in our TCP stack, IP stack or in SSH, wou= ld adjust to the lower MTU, and resend a shorter packet (possibly splitting it= in multiple TCP packets). After the first SSH connection attempt has failed, if I try again, the connection succeeds, so something adjusts so that later connections are not using the too big MTU. I have a packet capture of the traffic between my work station, router and freefall.freebsd.org, that hopefully can shed a little bit of light on what= 's going on. It's available on request. --=20 You are receiving this mail because: You are the assignee for the bug.=
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