Date: Tue, 7 Jan 2020 21:51:34 -0500 From: Ryan Rawdon <ryan@u13.net> To: tkhemili78@outlook.com Cc: freebsd-net@freebsd.org Subject: Re: ssh command hang Message-ID: <D20EE8E6-E3E5-4F6A-A6C6-92F2E20331CE@u13.net> In-Reply-To: <CAF9L14HWeCyRRyWUMkxkMVGTb4e73q-qnAjAP3jB44a-z6Q%2B0w@mail.gmail.com> References: <CAF9L14HWeCyRRyWUMkxkMVGTb4e73q-qnAjAP3jB44a-z6Q%2B0w@mail.gmail.com>
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> On Jan 7, 2020, at 3:30 PM, Bejiita78 . <tkhemili78@gmail.com> wrote: >=20 > has anyone ever noticed that locally a system may respond just fine, = but > running a command like port make install or top would cause the ssh = session > to hang indefinitely? This is a common sign of a MTU mismatch on a network segment somewhere = between your client and the server (large segments/packets/frames go = into a black hole and nobody knows); or the path has a = properly-configured reduced MTU, but the server is sending the traffic = with the Don=E2=80=99t Fragment bit set (IP header); but the device in = the path dropping it due to a smaller MTU is not successfully having = Packet Too Big ICMP errors get back to the server. =20 If you perform a packet capture on the server, you will likely see it = retransmitting one or more segments over and over - but not see those = arriving to the client. =20 The approach to diagnosing the point of the issue being introduced (MTU = mismatch, ICMP filtering, or the server not utilizing ICMP PTB responses = properly) depends largely on the network topology between your client = and server; and your ability to investigate or reproduce the symptoms in = systems along that path. There are plenty of other potential causes for this behavior, but this = is the first one I would investigate if experiencing this issue. Have = there been any network changes near your client or server that might = have meddled with MTU sizes or ICMP blocking?
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