> On 9. Jun 2021, at 00:20, Rodney W. Grimes <freebsd-...@gndrsh.dnsmgr.net> > wrote: > >> >> On Thu, 3 Jun 2021 15:09:06 +0200 >> Michael Gmelin <free...@grem.de> wrote: >> >>> On Tue, 1 Jun 2021 13:47:47 +0200 >>> Michael Gmelin <free...@grem.de> wrote: >>> >>>> Hi, >>>> >>>> Since upgrading servers from 12.2 to 13.0, I get >>>> >>>> Fssh_packet_write_wait: Connection to 1.2.3.4 port 22: Broken pipe >>>> >>>> consistently, usually after about 11 idle minutes, that's with and >>>> without pf enabled. Client (11.4 in a VM) wasn't altered. >>>> >>>> Verbose logging (client and server side) doesn't show anything >>>> special when the connection breaks. In the past, QoS problems >>>> caused these disconnects, but I didn't see anything apparent >>>> changing between 12.2 and 13 in this respect. >>>> >>>> I did a test on a newly commissioned server to rule out other >>>> factors (so, same client connections, some routes, same >>>> everything). On 12.2 before the update: Connection stays open for >>>> hours. After the update (same server): connections breaks >>>> consistently after < 15 minutes (this is with unaltered >>>> configurations, no *AliveInterval configured on either side of the >>>> connection). >>> >>> I did a little bit more testing and realized that the problem goes >>> away when I disable "Proportional Rate Reduction per RFC 6937" on the >>> server side: >>> >>> sysctl net.inet.tcp.do_prr=0 >>> >>> Keeping it on and enabling net.inet.tcp.do_prr_conservative doesn't >>> fix the problem. >>> >>> This seems to be specific to Parallels. After some more digging, I >>> realized that Parallels Desktop's NAT daemon (prl_naptd) handles >>> keep-alive between the VM and the external server on its own. There is >>> no direct communication between the client and the server. This means: >>> >>> - The NAT daemon starts sending keep-alive packages right away (not >>> after the VM's net.inet.tcp.keepidle), every 75 seconds. >>> - Keep-alive packages originating in the VM never reach the server. >>> - Keep-alive originating on the server never reaches the VM. >>> - Client and server basically do keep-alive with the nat daemon, not >>> with each other. >>> >>> It also seems like Parallels is filtering the tos field (so it's >>> always 0x00), but that's unrelated. >>> >>> I configured a bhyve VM running FreeBSD 11.4 on a separate laptop on >>> the same network for comparison and is has no such issues. >>> >>> Looking at TCP dump output on the server, this is what a keep-alive >>> package sent by Parallels looks like: >>> >>> 10:14:42.449681 IP (tos 0x0, ttl 64, id 15689, offset 0, flags >>> [none], proto TCP (6), length 40) >>> 192.168.1.1.58222 > 192.168.1.2.22: Flags [.], cksum x (correct), >>> seq 2534, ack 3851, win 4096, length 0 >>> >>> While those originating from the bhyve VM (after lowering >>> net.inet.tcp.keepidle) look like this: >>> >>> 12:18:43.105460 IP (tos 0x0, ttl 62, id 0, offset 0, flags [DF], >>> proto TCP (6), length 52) >>> 192.168.1.3.57555 > 192.168.1.2.22: Flags [.], cksum x >>> (correct), seq 1780337696, ack 45831723, win 1026, options >>> [nop,nop,TS val 3003646737 ecr 3331923346], length 0 >>> >>> Like written above, once net.inet.tcp.do_prr is disabled, keepalive >>> seems to be working just fine. Otherwise, Parallel's NAT daemon kills >>> the connection, as its keep-alive requests are not answered (well, >>> that's what I think is happening): >>> >>> 10:19:43.614803 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], >>> proto TCP (6), length 40) >>> 192.168.1.1.58222 > 192.168.1.2.22: Flags [R.], cksum x (correct), >>> seq 2535, ack 3851, win 4096, length 0 >>> >>> The easiest way to work around the problem Client side is to configure >>> ServerAliveInterval in ~/.ssh/config in the Client VM. >>> >>> I'm curious though if this is basically a Parallels problem that has >>> only been exposed by PRR being more correct (which is what I suspect), >>> or if this is actually a FreeBSD problem. >>> >> >> So, PRR probably was a red herring and the real reason that's happening >> is that FreeBSD (since version 13[0]) by default discards packets >> without timestamps for connections that formally had negotiated to have >> them. This new behavior seems to be in line with RFC 7323, section >> 3.2[1]: >> >> "Once TSopt has been successfully negotiated, that is both <SYN> and >> <SYN,ACK> contain TSopt, the TSopt MUST be sent in every non-<RST> >> segment for the duration of the connection, and SHOULD be sent in an >> <RST> segment (see Section 5.2 for details)." >> >> As it turns out, macOS does exactly this - send keep-alive packets >> without a timestamp for connections that were negotiated to have them. >> >> Under normal circumstances - ssh from macOS to a server running FreeBSD >> 13 - this won't be noticed, since macOS uses the same default settings >> as FreeBSD (2 hours idle time, 75 seconds intervals), so the server >> side initiated keep-alive will save the connection before it has a >> chance to break due to eight consecutive unanswered keep-alives at the >> client side. >> >> This is different for ssh connections originating from a VM inside >> Parallels, as connections created by prl_naptd will start sending tcp >> keep-alives shortly after the connection becomes idle. As a result, >> idle connections break after about 11 minutes of idle time (60s >> + 8*75s = 660s == 11m), unless countermeasures are taken. >> >> An easy way to demonstrate the problem is to change keep-alive defaults >> on *macOS* using sysctl and sshing to a FreeBSD 13 server: >> >> $ sudo sysctl net.inet.tcp.keepidle=5000 >> $ sudo sysctl net.inet.tcp.keepintvl=5000 >> $ ssh -oTCPKeepAlive=yes myserver >> >> This way, the problem described can be reproduced quite easily: >> Disconnect due to broken pipe after 45-60 seconds of idle time, tcpdump >> confirming that keep-alive packets don't have tcp timestamps, while >> they were used when negotiating the connection. >> >> There are various ways to work around the issue. >> >> Client side workarounds: >> - Use ServerAlive* settings in ~/.ssh/config (ssh only) >> - Tune net.inet.tcp.keep* sysctls on macOS (for all services) >> >> Server side workarounds: >> - Use ClientAlive* settings in ~/.ssh/config (ssh only) >> - Tolerate missing timestamps in packets using sysctl, which makes >> FreeBSD 13 behave like previous versions did: >> >> sysctl net.inet.tcp.tolerate_missing_ts=1 >> >> The last option probably being the most practical one. >> >> rscheff@ and tuexen@ (thank you!) were able to reproduce the issue and >> reached out to Apple to see if there is something they can do to fix >> this at their end (macOS) in the future. > > Can we please have the default of tolerate_missing_ts in > current, stable/13 and an errata issued to releng_13 changing > this value to =1 and staying that way until the buggy tcp > stacks are found and eliminated. The interesting part of your statement is that we find these stacks now by using the value of 0. But seriously, I thought that reasonable modern stacks would follow the RFC, which is not the case. I'll change the value in current and stable/13. But I have no idea about an errata. Will bring it up at the next transport call.
Best regards Michael > >> >> Best >> Michael >> >> [0]https://cgit.freebsd.org/src/commit/?id=283c76c7c3f2f634f19f303a771a3f81fe890cab >> [1]https://datatracker.ietf.org/doc/html/rfc7323#section-3.2 >> >> -- >> Michael Gmelin >> >> > > -- > Rod Grimes rgri...@freebsd.org >
