I intended to post in defense of manpages. I love manpages. I think most open-source manpages are high-quality, and a few are really outstanding.

"It's easier to ask an AI" can be true without implying that manpages are bad.

However, "man" the tool does have issues, and one of them bit me just now.

So, I didn't know about openssh client escapes like ~?. I thought, "surely that's in the manpage?" I opened the manpage (in less) and searched for "\~\?". No hits.

Of course, escape characters are documented in the manpage, and the string "~?" does appear. Why didn't search find it? Because man, in its infinite wisdom, decided to render every instance of "~" as some bizarre unicode not-tilde, which is visually similar but totally impossible to grep for.

This has also bitten me in the past with dash. DASH. A character that is critically important when documenting invocation options. man loves to convert it into something that looks like dash, prints like dash, but doesn't come up in search.

I'm sure there is a way to turn this "feature" off, and I'm about to spend a bunch of time figuring out what it is. But this is documentation for command-line tools. Silently destroying our ability to grep it should NOT be the default.

  /<ctrl-r>~?

I've been using the ~. shortcut for a while, but somehow escaped learning about the help menu.

Another neat thing I noticed while playing with it just now: there's an option to enter ~ twice to send a literal ~, but usually you don't have to do this when typing something like 'ls ~' in a regular session. Not only does the ~ have to be the first character on a line to start an escape sequence, but typing on a line, backspacing all the way to the start and then typing ~ also sends a literal tilde. It only triggers the escape sequence if the ~ is the chronologically first character after a newline (or first in the session), which is an unlikely thing to type into a shell in a normal session. Good choice of UI, both the character and the state machine.

I've been using SSH for ~15 years and never knew about these escape sequences. I'm eagerly awaiting my next hung session so that I can test `~.`. It's much nicer than my current approach of having to close that terminal window.

I've used ~. for a long time but did not know about others. I know, should have read man page.

Anyway, if you try it from shell prompt it is likely will not work as pressing ENTER shows the next prompt. Try `cat` followed by ENTER and then ~?

Those aren’t “secret”, they’re obviously borrowed from rsh — oh that’s right, I’m old.

Yep the menu is handy for ssh tunneling. Maybe not a lot of people doing that these days though with stuff like dev tunnels and Tailscale.

As secret as the ssh manual.

You think that one is cool, go check out "~?" in IPMI "sol activate". From there you can deliver a serial break to the kernel, which then gets you to a third rarely seen menu from the kernel's console, which allows you to do kernel debugging of various sorts.

I use it when I need to crash a kernel on purpose to test kdump over the network.

You can also send commands to the simulated console of a VM under libvirt with "virsh connect". But I don't think you can send a break to the kernel with that.

The ~ (tilde) escape character in UNIX cu (call UNIX) did the same thing in the 1970s, when the remote system was on the other side of a telephone modem or other serial port. I think it's safe to say that ssh is a descendant of cu. (I don't mean that cu printed a help menu, I mean it had the tilde escape.)

My gosh... I've spent decades closing a connection by killing the terminal.

~.

it starts with a pretty common char, but almost never gets in the way to the point I forget it exists. Meanwhile docker -t uses ^P which I use all the time for history instead of arrow keys. It's possible to configure it, but it's not worth the hassle on servers. Really, really annoying.

I've had to use [Enter] ~. in SSH sessions before. I've had SSH/network get stuck and using that was the only way to kill the session (and recover).

I guess you could call it a "secret" or at least "not super-well known (to people who aren't Linux 'experts')."

This is not specific to ssh. Telnet and rlogin have similar things with ~ as the escape character. Back in the day it was common to send BREAK and other escape sequences when you were hardwired.

Wow, never knew this... That said I'm not sure if I'll remember using it as my muscle memory is already trained to kill hung processes via ctrl-b s, ctrl-k, ctrl-b x (in tmux).

  alias ssh="echo 'dont forget: ~? for SSH escapes!'; /usr/bin/ssh "

It's like Ctrl + ] on telnet.

The good old times!

Don't tell people this. In a minute you'll realize that this menu can only mean that ssh runs multiple channels, and so you can start up other things over an established ssh connection. Then the firewall guys will realize that they really can't allow ssh ... let's just not go there. Working at a bank is annoying enough already.

This might be a good place to mention: https://linux.die.net/man/1/autossh

Secret Menu -> Escape Characters

I really hate it when people just rename terms. It made it harder to search properly for better answers.

BRB requesting access to my remote server "animal style"

I have set up Ctrl+k to kill (SIGKILL) the front process in my terminal. I just use that for hanging connections and other hanging processes.

What I thought I wanted: a way for Konsole to send SIGHUP

What I now have: ~B

What I really need: a way to stop long-running SSH connections from freezing

This is a secret? I've had this baked into my muscle memory for decades when an SSH session hangs.

FYI, you can kill forwarding tunnels with -KD portnum in that commandline too.

If hung SSH connections are common it's likely due to CGNAT which use aggressively low TCP timeouts. e.g. I've found all UK mobile carriers set their TCP timeout as low as 5 minutes. The "default" is supposed to be 2 hours, you could literally sleep your computer, zero packets, and an SSH connection would continue to work an hour later, and generally speaking this is still true unless CGNAT is in the way.

If you are interested there are a few ways you can fix this:

Easiest is to use a VPN, because the VPN's exit node becomes the effective NAT they usually have normal TCP timeouts due to being less resource constrained. Another nice benefit of this method is you can move between physical networks and your connection doesn't die... If you use Tailscale then you already have this in a more direct way.

Another is to tune the tcp_keepalive kernel parameters. Lowering the keepalive timeout to be less than the CGNAT timeout will cause keepalive probes to prevent CGNAT from dropping the connection even while your SSH connection is technically idle. For Linux I pop these into /etc/sysctl.d/z.conf, I have no idea for Windows or Mac:

  # Keepalive frequently to survive CGNAT
  net.ipv4.tcp_keepalive_time   = 240 
  net.ipv4.tcp_keepalive_intvl  = 60
  net.ipv4.tcp_keepalive_probes = 120

This is really a misuse of these settings, they are supposed to be for checking TCP connections are still alive and clearing them up from the local routing table. Instead the idea is to exploit the probes by sending them more frequently to force idle connections to stay alive in a CGNAT environment (dont worry the probes are tiny and still very infrequent).

_time=240 will send a probe after 4 mins of idle connection instead of the default 2 hours, undercutting the CGNAT timeout. _intvl=60 and _probes=120 mean it will send 120 probes 60 seconds apart (2 hours worth) before considering the connection dead. This will keep it alive for at least 2 hours, but also allows us to have the best of both worlds so that under a nice NAT it keeps the old behaviour, e.g if I temporarily lose my network the SSH connection is still valid after 2 hours, but under CGNAT it will at least not drop the connection after 5 mins so long as I keep my computer on and don't lose the network.

There are also some SSH client keepalive settings but I'm less familiar with them.

    Host *
        ServerAliveInterval 25

Have been using that weekly since probably 20 years. Will change your life :)

My other favourite is I very often SSH with -v to figure out why the connection is hanging, you rapidly figure out if DNS is failing, the TCP connection doesn't open, it does open but no traffic flows at all or it opens and SSH negotiation starts but never finishes. You can learn a lot just from this about what is wrong.

You can even chain them if you have deep ssh connections (i.e. ssh from one instance to another). I think it would be ~~. to terminate the 2nd hop.

Edit: it's already explained in the OP

Be sure to hit enter before you start typing `~.`. It only works on a new line

Probably getting closer to 30 than 25 for me. And I used rsh before that where guess what was used to escape the input.

https://en.wikipedia.org/wiki/Berkeley_r-commands

> It's much nicer than my current approach of having to close that terminal window.

You can also just kill the ssh process (say from another terminal). That way you get to keep your terminal window. And this works with everything "blocking" your terminal, not just ssh.

You don't need to actually open the menu either. Just hit enter, tilde, ., enter.

I last used this menu about 20 years ago when a dialup modem was the only way to roll, and have pretty much forgotten about it since the days of always-on direct to the desktop TCP/IP ..

If you regularly have to deal with hung connections or slow/unreliable links, I suggest trying out mosh.

https://mosh.org/

I've been using ~. on hung ssh connections for a while.

Just ssh to funky.nondeterministic.computer to test it out!

I use that every day but it's the only one I know by heart lol