I was never able to properly parse large man pages, I'm so happy that llms can now prepare half a usable command without spending an hour reading a time without a single usage example.
Honest question, why is ProxyCommand `fun`? What do I get out of ProxyCommand that i do not get out of setting the correct order for ProxyJump and doing an ssh finalhost -- domy --bidding?
ProxyJump is a newer functionality. There used to be only ProxyCommand. ProxyJump is a shortcut for the usual way to use ProxyCommand to connect through a bastion host but ProxyCommand is more flexible. For example with ProxyCommand you can run any command to connect to the remote host. ProxyJump only connects over ssh. I think I replaced all my ProxyCommand with ProxyJump because I don't need much else than the normal use case.
You can get a lot more out of ProxyCommand. For example, you can run SSH over non-IP protocols, such as serial, Bluetooth RFCOMM for embedded boards, or vsock for virtual machines without networking set up at all. The latter is built into and setup up automatically by systemd:
ProxyCommand allows you to use any command to setup a connection. Not necessarily an ssh command, like ProxyJump. It can be any command, as long as it receives on stdin and produces on stdout, it can act like a TCP connection.
ProxyJump is a special case of `ProxyCommand ssh -p <port> <user>@<host>`. Can't replace the `ssh` in there when using ProxyJump.
If the ssh man page were any lazier it would just be a list of arguments with no context at all, so I get the frustation. The tricky bit is that much of the "secret menu" knowledge about ssh only lives in blog posts and random issue comments, which makes the doc gap feel intentional at times. If you want real examples or usage patterns you pretty much have to scrape Stack Overflow or crawl through dotfiles in public repos hoping someone else already fought the same fight.
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.
The drawback is that if you think your session is hanging and want to bail with ~., you have to press enter, which might actually make it to the server and execute something.
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.
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:
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.
Well, for different reasons, but you have similar issues with IPv6 as well. If your client uses temporary addresses (most likely since they're enabled by default on most OS), OpenSSH will pick one of them over the stable address and when they're rotated the connection breaks.
For some reason, OpenSSH devs refuse to fix this issue, so I have to patch it myself:
--- a/sshconnect.c
+++ b/sshconnect.c
@@ -26,6 +26,7 @@
#include <net/if.h>
#include <netinet/in.h>
#include <arpa/inet.h>
+#include <linux/ipv6.h>
#include <ctype.h>
#include <errno.h>
@@ -370,6 +371,11 @@ ssh_create_socket(struct addrinfo *ai)
if (options.ip_qos_interactive != INT_MAX)
set_sock_tos(sock, options.ip_qos_interactive);
+ if (ai->ai_family == AF_INET6 && options.bind_address == NULL) {
+ int val = IPV6_PREFER_SRC_PUBLIC;
+ setsockopt(sock, IPPROTO_IPV6, IPV6_ADDR_PREFERENCES, &val, sizeof(val));
+ }
+
/* Bind the socket to an alternative local IP address */
if (options.bind_address == NULL && options.bind_interface == NULL)
return sock;
This is a very common misconception. The issue is not IPv4 or CGNAT, it's stateful middleboxes... of which IPv6 has plenty.
The largest IPv6 deployments in the world are mobile carriers, which are full of stateful firewalls, DPI, and mid-path translation. The difference is that when connections drop it gets blamed on the wireless rather than the network infrastructure.
Also, fun fact: net.ipv4.tcp_keepalive_* applies to IPv6 too. The "ipv4" is just a naming artifact.
Check Mosh. It supports these kind of cuts and it will reconnect seamlessly. It will use far less bandwidth too.
I successfully tried it with a 2.7 KBPS connection.
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.
Also helps with auth failures, I've used it several times with co-workers who can't figure out why their ssh key isn't working. It lists the keys out and some extra information.
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 ..
> 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.
Same with me, I'll still instinctively go for ~. when a connection has hung / dropped (usually because of a NAT via a rebooted firewall), but never even considered how ~ doesn't normally cause an issue. Never knew it had to be immediately following a newline. Also never knew about the other options, ~^Z in particular looks useful.
I wonder if anyone still remembers the ctrl-[ sequence in telnet. I think I only ever used the quit command in that though.
Does not for me, not even with busybox sh and no funky escape codes in PS1 at all. It does with cat or yes running, so just something being output is not the problem… Hm.
I typically just create a "new" connection in a separate tab when I want to add tunneling.
I put new in quotes because I use another little-known feature, "ControlMaster". Multiplexes multiple connections into one, it makes making " new" sessions instant (can also be configured to persist a bit after disconnecting). Also useful for tab-completing remote paths. It does not prompt for authentication again, though. And it's a bit annoying when the connection hands (can be solved with ssh -o close, IIRC).
> I use another little-known feature, "ControlMaster". Multiplexes multiple connections into one, it makes making " new" sessions instant
Is this what secureCRT used as well? I remember this being all the rage back when I used windows, and it allowed this spawn new session by reusing the main one.
I'm using that as well but had issues with tunneling where it creates the tunnel in the background and terminates and so you might not know the random port it assigned or I couldn't figure out how to un-tunnel it and tunnel again to the same port. Just bypassed the control master then.
"secret" not in the sense that it's hidden, but that most people won't know about it. Because approximately nobody actually reads man pages in their entirety, they just get in to find out how a specific flag works and then get out.
Unlike CTRL ], at least ~. doesn't require that I press two modifiers at the same time ... CTRL ALTGR $. Because people who define those kinds of shortcuts never consider how they might work on non-QWERTY layouts.
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).
Hidden or undocumented features like this always have a strange appeal. Part of it is nostalgia for older software where small Easter eggs or experimental features would sometimes ship in production builds.
I have noticed it while running ~/bin/some_command. The ~ doesn't echo until I also type the /. It doesn't cause any misbehavior because there is no binding for ~/ but can be slightly surprising.
I find it odd that you would have commands in ~/bin but not have it be the highest priority in your PATH. I use ~/.local/bin, but would never type it because i wouldn't have bins that overlap shell commands and no other path would have priority.
Usually, it is. IIRC, this was when I was just setting up my environment on a new host, after I had populated ~/bin but before I restarted my shell to pick up PATH modifications.
I'd guess this is because it only works in ssh PTY sessions. So it would have no effect on tunneling or when piping arbitrary data through ssh to a non-interactive remote command (unless you use the -t switch to force PTY allocation even when stdin is not a TTY).
Find the HIDDEN SECRETS that THEY DON'T WANT YOU TO KNOW!
Many Linux man pages have the thoroughness of a fortune cookie, so I can understand the skepticism.
Jackpot if they're just a pointer to an 'info' page.
I was never able to properly parse large man pages, I'm so happy that llms can now prepare half a usable command without spending an hour reading a time without a single usage example.
But its also true that many, many man pages have extremely valuable information that no enterprising hacker should overlook, too ..
Do we still have those? I think it was common in late '90s, due to GNU trying to get `info` gain moment but nowadays?
Mostly just the bash builtins have (had) this problem.
I blew a few minds a week ago when I told my younger coworkers about man pages. I hit ‘em with the `man man` and one dude was like “whoa!”
Further blow their minds by showing them `apropos` https://manpages.debian.org/testing/man-db/apropos.1.en.html
I remember my first days with Linux/Unix and being taught about man and apropos and info. But somehow I could never find anything useful via apropos!
With many years of insight, I think I probably never updated the database.
Reminds me of the old canard of 'man woman'
man, cat, lynx... We're all just creatures in a vast universe
$ man -k <whatever>
Who doesn’t want you to know? Well, obviously, the man. That’s why you type man ssh, you’re forcing the man to tell you what he knows.
man ssh_config is even more interesting and hidden
ProxyCommand is fun
Honest question, why is ProxyCommand `fun`? What do I get out of ProxyCommand that i do not get out of setting the correct order for ProxyJump and doing an ssh finalhost -- domy --bidding?
ProxyJump is a newer functionality. There used to be only ProxyCommand. ProxyJump is a shortcut for the usual way to use ProxyCommand to connect through a bastion host but ProxyCommand is more flexible. For example with ProxyCommand you can run any command to connect to the remote host. ProxyJump only connects over ssh. I think I replaced all my ProxyCommand with ProxyJump because I don't need much else than the normal use case.
You can get a lot more out of ProxyCommand. For example, you can run SSH over non-IP protocols, such as serial, Bluetooth RFCOMM for embedded boards, or vsock for virtual machines without networking set up at all. The latter is built into and setup up automatically by systemd:
https://www.freedesktop.org/software/systemd/man/257/systemd...
ProxyCommand allows you to use any command to setup a connection. Not necessarily an ssh command, like ProxyJump. It can be any command, as long as it receives on stdin and produces on stdout, it can act like a TCP connection.
ProxyJump is a special case of `ProxyCommand ssh -p <port> <user>@<host>`. Can't replace the `ssh` in there when using ProxyJump.
I use ProxyCommand to run spipe tunnels for SSH.
HN as a tabloid ... After all enquiring minds want to know!
It’s been a while since I’ve heard RTFM
hahaha made me laugh, thanks :-D
How's that supposed to help? The ssh man page is about as close as you can get to a Platonic example of "uninformative pretend-documentation".
If the ssh man page were any lazier it would just be a list of arguments with no context at all, so I get the frustation. The tricky bit is that much of the "secret menu" knowledge about ssh only lives in blog posts and random issue comments, which makes the doc gap feel intentional at times. If you want real examples or usage patterns you pretty much have to scrape Stack Overflow or crawl through dotfiles in public repos hoping someone else already fought the same fight.
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.
[delayed]
The drawback is that if you think your session is hanging and want to bail with ~., you have to press enter, which might actually make it to the server and execute something.
[delayed]
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.
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:
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.
Note this is only an issue if not using IPv6.
CGNAT is for access to legacy IPv4 only.
Well, for different reasons, but you have similar issues with IPv6 as well. If your client uses temporary addresses (most likely since they're enabled by default on most OS), OpenSSH will pick one of them over the stable address and when they're rotated the connection breaks.
For some reason, OpenSSH devs refuse to fix this issue, so I have to patch it myself:
Interesting! Is there anywhere a discussion around their refusal to include your fix?
This is a very common misconception. The issue is not IPv4 or CGNAT, it's stateful middleboxes... of which IPv6 has plenty.
The largest IPv6 deployments in the world are mobile carriers, which are full of stateful firewalls, DPI, and mid-path translation. The difference is that when connections drop it gets blamed on the wireless rather than the network infrastructure.
Also, fun fact: net.ipv4.tcp_keepalive_* applies to IPv6 too. The "ipv4" is just a naming artifact.
Check Mosh. It supports these kind of cuts and it will reconnect seamlessly. It will use far less bandwidth too. I successfully tried it with a 2.7 KBPS connection.
putty is sending packets for network up since like forever
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.
Also helps with auth failures, I've used it several times with co-workers who can't figure out why their ssh key isn't working. It lists the keys out and some extra information.
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
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 ..
Just ssh to funky.nondeterministic.computer to test it out!
> 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.
I use that every day but it's the only one I know by heart lol
I've been using ~. on hung ssh connections for a while.
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 ~?
Same with me, I'll still instinctively go for ~. when a connection has hung / dropped (usually because of a NAT via a rebooted firewall), but never even considered how ~ doesn't normally cause an issue. Never knew it had to be immediately following a newline. Also never knew about the other options, ~^Z in particular looks useful.
I wonder if anyone still remembers the ctrl-[ sequence in telnet. I think I only ever used the quit command in that though.
It'll still work. OpenSSH doesn't care about output (for ~ stuff), only input, so if you type <enter>~. it will close the connection.
Does not for me, not even with busybox sh and no funky escape codes in PS1 at all. It does with cat or yes running, so just something being output is not the problem… Hm.
It does not. open ssh linux to mac, typing ~ just types it on fish shell prompt. It works after`cat` followed by ENTER
Just type <enter> without cat, your shell will show you another prompt, and the ssh escape command will also work.
In newer versions, it's disabled by default and you have to do something like this to enable in ~/.ssh/config:
Those aren’t “secret”, they’re obviously borrowed from rsh — oh that’s right, I’m old.
Laughs in uucp!bangpath.
And these days you need to pass "-O" to scp to have it behave like rcp.
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.
I typically just create a "new" connection in a separate tab when I want to add tunneling.
I put new in quotes because I use another little-known feature, "ControlMaster". Multiplexes multiple connections into one, it makes making " new" sessions instant (can also be configured to persist a bit after disconnecting). Also useful for tab-completing remote paths. It does not prompt for authentication again, though. And it's a bit annoying when the connection hands (can be solved with ssh -o close, IIRC).
> I use another little-known feature, "ControlMaster". Multiplexes multiple connections into one, it makes making " new" sessions instant
Is this what secureCRT used as well? I remember this being all the rage back when I used windows, and it allowed this spawn new session by reusing the main one.
I'm using that as well but had issues with tunneling where it creates the tunnel in the background and terminates and so you might not know the random port it assigned or I couldn't figure out how to un-tunnel it and tunnel again to the same port. Just bypassed the control master then.
TIL; thanks, that's interesting (and somehow escaped my 20+ years of using ssh)! As usual the gold is in the comments :-)
My gosh... I've spent decades closing a connection by killing the terminal.
~.
As secret as the ssh manual.
"secret" not in the sense that it's hidden, but that most people won't know about it. Because approximately nobody actually reads man pages in their entirety, they just get in to find out how a specific flag works and then get out.
Sometimes things feel so simple that i dont even read the manual. cool
It's like Ctrl + ] on telnet.
The good old times!
Unlike CTRL ], at least ~. doesn't require that I press two modifiers at the same time ... CTRL ALTGR $. Because people who define those kinds of shortcuts never consider how they might work on non-QWERTY layouts.
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).
Hidden or undocumented features like this always have a strange appeal. Part of it is nostalgia for older software where small Easter eggs or experimental features would sometimes ship in production builds.
It's not hidden or undocumented; it's in the man page.
Here's 15-year old HN link about it: http://grack.com/blog/2011/02/23/ssh-escape-sequences-or-don...
Secret Menu -> Escape Characters
I really hate it when people just rename terms. It made it harder to search properly for better answers.
FYI, you can kill forwarding tunnels with -KD portnum in that commandline too.
https://xkcd.com/1053/
That doesn't do much good if you set `EscapeChar` to `none` in `.ssh/config`.
I find it convenient not to have to worry about accidentally entering escape characters. YMMV.
Note that it only works after pressing enter, so the odds are slim. In practice, I don't think I ever hit it by accident.
I have noticed it while running ~/bin/some_command. The ~ doesn't echo until I also type the /. It doesn't cause any misbehavior because there is no binding for ~/ but can be slightly surprising.
I find it odd that you would have commands in ~/bin but not have it be the highest priority in your PATH. I use ~/.local/bin, but would never type it because i wouldn't have bins that overlap shell commands and no other path would have priority.
Usually, it is. IIRC, this was when I was just setting up my environment on a new host, after I had populated ~/bin but before I restarted my shell to pick up PATH modifications.
SSH does it pretty well though. Never once have I done it by mistake.
I'd guess this is because it only works in ssh PTY sessions. So it would have no effect on tunneling or when piping arbitrary data through ssh to a non-interactive remote command (unless you use the -t switch to force PTY allocation even when stdin is not a TTY).
No I don't think so. I mainly and pretty much constantly use SSH for logging in. I'm not one of those 'cattle not pets' guys lol.
And when I port forward I usually don't even tunnel it over SSH because all my stuff is on tailscale so it's also encrypted.