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Mosh: Modernizing SSH With IP Roaming, Instant Local Echo
An anonymous reader writes "Launched in 1995, SSH quickly became the king of network login tools, supplanting the old insecure mainstays TELNET and RLOGIN. But 17 years later, a group of MIT hackers have come out with "mosh", which claims to modernize the most annoying parts of SSH. Mosh keeps its connection alive when clients roam among WiFi networks or switch to 3G, and gives instant feedback on typing (and deleting). No more annoying network lag on typing, the MIT boffins say, citing Bufferbloat, which has been increasing latencies."
The folks involved have a pre-press research paper with the gritty details (to be presented at USENIX later this year). Mosh itself is not particularly exciting; the new State Synchronization Protocol it is based upon might be: "This is accomplished using a new protocol called the State Synchronization Protocol, for which Mosh is the first application. SSP runs over UDP, synchronizing the state of any object from one host to another. Datagrams are encrypted and authenticated using AES-128 in OCB mode. While SSP takes care of the networking protocol, it is the implementation of the object being synchronized that defines the ultimate semantics of the protocol."
And 15 years later, LOCAL_ECHO is back in mosh!
and gives instant feedback on typing (and deleting).
That sounds like a step backwards to me. Any utility in that is lost when something doesn’t sync up properly. When I hit a key, I want to know it has been sent and received and see the result.. not see the result as my shell predicts it. Maybe I’m just having local echo flashbacks from past telnet experiences.
Everything else sounds really neat though. I don’t jump wifi often enough for re-connecting and re-attaching to screen to be a big deal.. but I can see the utility for those who do.
While neat for those who are currently in areas with spotty wireless coverage it is a neat idea but for most users I don't think it's that much of an issue, even at the moment.
Fast forward five years and I just don't see this software being all that useful. Sure, there's always gonna be that handful of people who will scream that this is extremely useful because they're always hopping between wifi hotspots but most users are using 3G/4G when they're on the move and coverage for those is already "good enough" in most civilized places and steadily improving. I've taken 5+ hour train trips several times and only had ssh connections drop once or twice on those trips (due to spotty coverage in what would quality as the middle of nowhere in northern Sweden).
This is like "solving" the IPv4 address exhaustion problem with NAT, it's a neat workaround but doesn't actually solve the problem.
Greylisting is to SMTP as NAT is to IPv4
If they implement their own TCP-like layer over UDP, there's no reason it can't be just as reliable.
It's kind of hard to do things like roaming using TCP because endpoint IPs can change.
We tried to put OCB mode in 802.11i. So IBM sent a guy to explain the 'licensing terms' for their patents on OCB mode. The next vote in 802.11i after that presentation was to replace OCB mode with CCM.
Until the patents expire or are freely licensed, OCB mode should be considered off limits for free and open projects.
I should use this sig to advertise my book ISBN-13 : 978-1501515132.
So that means it's just like GNU Screen? ctrl+a d on one connection, hop wifi, ssh and screen -x. wow. Really?
IP roaming looks nice & ought to be secure with the right steps (no reply from old IP:port, correct cryto negotiation with new IP:port).
But LOCAL ECHO is a big problem -- applications have to be aware of it. On CLI, many keystrokes are commands, not text to be entered. On vi in command-mode, G goes to the last line.
Personally, a bigger thing is traffic reduction, particularly keystoke combining. Nagel's algorithm is a start, but I've modded ssh to delay and buffer likely-text keystrokes for a short time (400ms) while letting likely commands through immediately to retain responsiveness. The delays aren't irksome, and I reduce outbound traffic by ~80%.
Then they discover there was usually a good reason for something being done the way it was in the past. Eg local echo was very useful for line buffered programs such as MUDs and chat servers or even talking to SENDMAIL or an FTP server directly. It was easier to write the server to cope with just line by line rather than character by character and it used up less network resources in the process.
I think the point this has is that it will automatically do the reconnect for you. What I'm not sold on is that this now requires an arbitrary port to be open on the server side in order to connect to the mosh server, and how irritated are the security guys who control the edge firewalls on your corporate network going to get?
You, now:
so I can ssh into every server, but only mosh into a few.
You, 1995:
so I can telnet into every server, but only ssh into a few.
Don't call me back. Give me a call back. Bye. So yeah. But bye our, well, but alright we are on a shirt this chill.
You open a SSH connection (client->server:22). This port is allowed on the firewall, it lets you through. But then the server decides to listen on UDP:(random port) and tells the client, back through the (encrypted) initial connection, which UDP port to contact. So you initiate a SSP UDP session on that port. How does the firewall knows it should let you through? Since the port number is communicated on an encrypted session, it doesn't have access to that information. So how does this work in a secure environment? The paper doesn't mention any mean for the server to communicate with the network which port its listening on.
Once something becomes widely used and stable, making drastic changes becomes next to impossible.
That's why we went CVS -> SVN -> git. Too many people were using CVS to make the changes made in SVN. Too many people are using SVN now to fix the (very old and oft complained about) problems with SVN.
See also NFS. There are issues with NFS that people have complained about for years.. and they will never be fixed for the same reason.
When you're operating on a slow wireless connection with an already high amount of traffic on it, every bit counts.
Um, then don't download SSH over a slow wireless connection? Download over a high-speed wired connection and side-load it onto your device.
"First they came for the slanderers and i said nothing."
> It's kind of hard to do things like roaming using TCP because endpoint IPs can change.
Bullshit. With UDP you have to abstract the connection so that the source IP can change. With TCP you can do the exact same fucking thing. Close the old socket when you get a connection attempt from a new client with the right handshake.
Because writing a new RFC for the SSH protocol and then improving the current ubiquitous OpenSSH is an unheard of undertaking.
Let's see...
I get reconnectability (which I already have, either by using a VPN or by using screen on the server), but now it's built-in.
But now it's automatic.
I get local echo so I have no clue whether my connection has been dropped -- but OTOH, this is great if you have the brain of a goldfish and so can't remember what you just typed for a couple seconds till it gets echoed back. I presume this is optional, so non-goldfish-brains can tell it to 'degrade' to be as useful as ssh.
It also is automatic and shows what hasn't been echoed. Further, typing while lagging by a character or two is incredibly frustrating to almost all brains in existence. It's like listening to headphones which have a half second delay in what you said. Your brain simply freezes.
I get better unicode support -- well, that one's cool, anyway.
And it needs ssh for login, but also needs a mosh server -- so I can ssh into every server, but only mosh into a few.
Am I missing some really great thing about it? It seems like a major hassle for a minor improvement.
What a pessimist.
Infuriate left and right
i can't see any idle traffic ... being specified
I looked into MOSH in detail a little while ago and the keepalive packet is every 3 seconds or 3 packets per second can't remember which.
It was often enough to make me pause... that's a lot of traffic if you're metered and paying by the K and/or powered by battery...
"Science flies us to the moon. Religion flies us into buildings." - Victor Stenger
I see the need for this all the time. It's a commonplace in large enterprises like hospitals, factories, and financial services corporations.
Example: I'm working on my hospital laptop. I get called urgently to do something elsewhere in the hospital so somebody won't die right now. I grab the lappie and run, then when I get to the theatre I plug into the malfing imager and fix it. Meanwhile all my SSH connections died because I crossed three wireless boundaries at high speed.
Example 2: I'm on the line debugging the tension loader robot while a human continuously manually corrects the tension downthread. I find the upstream data to the robot is bad and I have to backtrack all the way across the building to find the malfunctioning sensor, then come back and double-check the robot again. All my SSH connections into the DCS keep failing because the factory floor's high RFI means we have to have lots of small loud wireless zones, and I have to keep moving among them.
Example 3: I'm in a conference room lecturing junior banksters on how to fleece grandmothers and the CEO throws us out so his pet congressman can use the room to tongue-polish his shoes for him. The next conference room is two wireless zones away, so my secure SSH tunnel into Dr. Evil's antarctic lair fails and I have to sacrifice another day trader to get the blood I need for our in-house key transfer protocol.
OK, that last example was a bit contrived but I was starting to get bored.
Tmux's biggest claim is one of idealogy (BSD vs GPL) rather than any real technical merit.
Actually, tmux defaults at least are nice. When I 'tmux a' to share a session with someone else, our multi-window view is synced. Fit-to-terminal is a bit more sane too.
Screen developer/advocates really need to provide a guide on how to make screen behave like tmux default, if possible. Ctrl-A default bind is annoying as anything too.
XML is like violence. If it doesn't solve the problem, use more.
Bullshit. With UDP you have to abstract the connection so that the source IP can change. With TCP you can do the exact same fucking thing. Close the old socket when you get a connection attempt from a new client with the right handshake.
I'm a little out of my depth here, but I'd imagine it'd be much easier with UDP because UDP is connectionless. With this sort of roaming, the server isn't expected to change addresses, but the client is. So, have the client sign everything with a negotiated public key, and the server doesn't even have to care where each packet is coming from, or even open any new connections when the client moves across IPs.
Since this is an SSH replacement, I'd expect the key signing to be done already, so once you build an ordering and reliability protocol on top of UDP you essentially get the roaming for free.
Modern shells have completion, and mosh is not going to predict that.
It seems to me that for my typical usage it is going to have limited utility - I'm either in a shell where I'm leaning heavily on the tab for completion, or in vi where it would need to secondguess what vi is going to display.
Ctrl-A default bind is annoying as anything too.
Yeah, always had issues with that. I always set to CTRL-G, since the only thing I ever use that key for is bailing out of thinkos in emacs, and double-pumping that in non-screen mode is pretty harmless.
Plus, being an alarm key, it's more semantically correct.
Someone had to do it.
In the context of persistent logical connections, you have to consider that the TCP connection will get severed and must be re-established. It's not enough to have an open connection sitting around; if bits aren't coming from the other side, the open connection may have been physically severed by some dead networking gear or a backhoe. So you end up coding some application-level heartbeat-and-reconnect logic with logic to securely resume a session (forgive the hand-waving here).
At this point, if the "securely resume a session" bit is sufficiently compact, you are free to do the exact same logic per-packet, and you don't need to maintain a TCP connection. You lose TCP's reliable-ordering support, but the automatic-reconnect logic is typically sufficient to do without reliable-ordering. You also lose TCP's congestion control, so you'll probably need to add some application-level throttling mechanism to avoid DoSing yourself.
This is why TCP vs. UDP is irrelevant in the topic at hand. Neither is sufficient to maintain a persistent connection, and the extra logic required on top of one can typically be ported to the other.
And many (most?) SSH clients support auto-reconnect on short network drops.
It's not even a case of reconnect. A TCP connection lasts forever or until one side says "stop"; all the client has to do is *not* explicitly time out after N seconds. Misconfigured NAT devices tend to fuck this up though; one of many reasons NAT is evil.