Mosh: the mobile shell

http://mosh.mit.edu/#about

by ColinWright • 14 years ago

257 89 14 years ago

mosh.mit.edu

rshm• 14 years ago

Previous discussion http://news.ycombinator.com/item?id=3819382

niggler• 14 years ago

Relevant discussion from an hour ago: http://news.ycombinator.com/item?id=5016357

tkahn6• 14 years ago

I would love to use it but last time this came up (almost a year ago), people voiced security concerns.

Any different perspectives on this now?

ColinWright• 14 years ago

In part, this is why I've submitted this again. There have been previous discussions, but comments there are closed, and many people aren't aware of it, so I thought it was a good opportunity to allow those "in the know" to tell us if anything has changed, and if it's fit for purpose.

zobzu6• 14 years ago

One of them, besides the elite crypto issues, is that mosh requires having a wide range of UDP ports open to function properly. Many admins are not willing to do that.

geofft16• 14 years ago

You can pick a single UDP port and use `mosh -p`.

Absent a single privileged moshd, though, every user's mosh-server process needs to have its own UDP port, hence the default behavior. If you want to assign yourself a UDP port and open that single port on the firewall, you can, though.

geofft16• 14 years ago

Well, it's been a while with no security issues, hasn't it? That's mostly how software ends up being considered secure.

prewett• 14 years ago

No, I think having a high quality secure design and a security audit by examining code is how software is considered secure. OpenBSD audits every line of code that it ships, and the last remote root exploit I could find was 2007. DJB's stuff (qmail, djbdns) are designed to be secure from the ground up, and I believe are considered to be secure. (Opinions vary about other aspects of his stuff, though.) "It's been a while since I heard of a virus on Mac OS" is how Mac OS is often considered secure, yet the list of remote exploits is rather large [2], compared to OpenBSD's (exploit-db.com doesn't even know of the 2007 one, but Mac OS has more remote exploits than OpenBSD has local explots [3]!)

[1] http://www.coresecurity.com/content/open-bsd-advisorie [2] http://www.exploit-db.com/remote/?p=osX [3] http://www.exploit-db.com/platform/?p=openbsd

kami8845• 14 years ago

I love mosh, I've been using it since it was released and it's been working flawlessly.

koffiezet• 14 years ago

While a nice idea, licensing issues are a problem for this project... I don't understand why they didn't use the standard MIT license, but instead went for GPL3, but still chose a proprietary encryption method.

iSSH on iOS now also experimentally supports it with an own independent implementation, with an in-app purchase, since the encryption uses a proprietary encryption method only freely licensed for opensource projects.

eli• 14 years ago

I definitely get the concern over proprietary encryption, but why does the license matter for an end user like me?

daenney2• 14 years ago

Because due to the license the code can not be included on certain platforms or in applications which is preventing wider adoption.

offbyone• 14 years ago

Because depending on the policies of your workplace, GPLv3 may be blacklisted; the nature of that license makes its use in certain classes of business very, very dangerous.

michael_miller2• 14 years ago

Many paranoid companies will often prohibit all use of GPLv3 software. The justification I've heard is that the terms of the license make it possible that anything you produce using GPLv3 software is also GPLv3. So, for example, if you use a GPLv3 licensed version of emacs, and you write some code, the code you wrote will also become GPLv3. It's a very conservative reading of the terms of the license, but understandable for companies which have most of their net worth tied up in source code.

michael_miller2• 14 years ago

geofft16• 14 years ago

I forgot to point out while the discussion was still active that "proprietary encryption" is a bit of a misnomer. 90% of people who use that phrase mean "we use a super-secret algorithm and if we tell you how the algorithm works the bad guys will break it", aka, a weak and unaudited security-by-obscurity algorithm.

OCB is no more proprietary than RSA was when it was still under patent protection (throughout the '90s, I believe), the reference implementation is public, and Mosh incorporates the reference implementation into its GPL source. The algorithm is well-documented in the research literature, and the implementation is available under a free software (though strong copyleft) license.

geofft16• 14 years ago

OCB has a clean, short, embeddable reference implemention, and there's a patent grant permitting usage in GPL projects. (Calling it "proprietary" is a stretch, but I understand what you mean.)

GCM didn't have any great implementations. There's starting to be an implementation in OpenSSL 1.0, but at the time mosh was written, that was difficult to count on, and even now that's still shaky.

It's important that the crypto algorithm implementation be known to be correct and safe (avoid side channels, etc.).

As far as I know there's no particular reason that iSSH does not use the standard mosh implementation. One thing to note is that iSSH is not free software (everything it embeds -- PuTTY, Xorg, dwm, etc. -- is under an MIT-style license), so embedding a GPL project would have forced them to open up the iSSH source code.

It would be great if someone wanted to write a free software terminal emulator for iOS, and at that point you can absolutely include mosh. (I'd probably encourage _building_ on mosh, which happens to include a rigorous terminal implementation.) But it's true that you can't embed GPL software inside proprietary software.

philjohn• 14 years ago

I was under the impression that you can't include GPL stuff in App Store apps full stop.

nicoritschel• 14 years ago

http://www.zinger-soft.com/iSSH_features.html

iSSH includes the mosh protocol as a 99c in-app purchase. Use mosh on your mobile device, just where it should be!

daenney2• 14 years ago

Mosh has been brilliant so far. I travel a lot by train but even SSH'ing over the trains WiFi can be problematic. Connecting through Mosh solves all the typical issues and with local echo you can continue to type as if the connection was still up.

HorizonXP• 14 years ago

I just installed this on my Chromebook with Ubuntu, and on my EC2 instance. I've been very frustrated with the lag I was experiencing while typing in the shell, due to high/variable latency. In my very preliminary testing over the last hour, this has solved my problems almost completely.

Security is less of a concern because I'm running OpenVPN to my instances, so the UDP traffic mosh uses is encrypted anyway, and not open to the public.

One issue I did notice is that irssi isn't refreshing quite as quickly. Seems to lag behind the conversation a bit. I'll have to troubleshoot. I wonder if running byobu/screen is part of the problem.

carbocation• 14 years ago

I also run a similar setup and am trying out mosh now. I agree re: the lag in irssi. Additionally, if I start typing into irssi and then rapidly delete the text, I can delete the channel name temporarily:

    [#startups] I was thiknin^H^H^H^H^H^H
    [#start

Maakuth• 14 years ago

Yeah, this is due to their clever predictive local echo not being that clever all the time. When the client sees that sending ^H actually results in character disappearing, it imitates that behaviour locally for next ^H:s making you experience less lag. Most of the time it works well but there's these edge cases with funny results.

DoubleMalt• 14 years ago

I simply could not work without mosh. I do not experience significant lag and it allows me to have connections open even if I switch from WiFi over to 3G and back which is often the case.

Karunamon• 14 years ago

Are there any versions of this that work with Windows GUI apps? (Along the lines of puTTY instead of Cygwin)?

visarga3• 14 years ago

So, er, does the [mo]bile[sh]ell have a mobile app for iPhone?

geofft16• 14 years ago

iSSH has an independent proprietary implementation of the mosh protocol as an in-app purchase. (mosh itself has license terms explicitly permitting redistribution on the App Store, but nobody has done the port yet.)

nwh2• 14 years ago

Panic has mentioned that they will fully implement Mosh into their Prompt iOS client, it's just not high priority at the moment.

http://twitter.com/panic/status/281208183094145024

misleading_name• 14 years ago

yeah, seems pointless without an iOS implementation. (pointless for people on mobile Apple products that is (iPhone, iPad, iPod touch))

saurik7• 14 years ago

If anyone is interested in the downsides:

1) mosh lowers the average latency, but it causes the perceived latency to be highly variable.

The issue is that there are various moments where the algorithm somewhat-correctly decides it cannot predict what will happen due to new incoming keystrokes. An example of this is after you hit enter: you might be running an entirely different program at this point.

In essence, the algorithm "warms up" as it makes correct decisions, and once it decides "ok, my decisions seem accurate: when he types a character they seem to echo back to the terminal" it makes a sudden transition from "slightly slower than raw SSH" to "much faster than SSH and only slightly slower than a local screen session".

These sudden transitions are jarring: when they happen to you with normal connections the expectation is "oh, my connection is failing", a feeling that with mosh you have every time you type a command.

Meanwhile, for the interactive-shell or complex-text-editor use case, these "unpredictable events" always happen at the worst possible moment: when you first start doing something; the result is that the perceived incidence of latency is actually worse than with normal SSH, as every "new event" has a higher latency... by the time it starts prediction, you are probably "in the flow" enough to not even be paying attention.

2) it relies on a specific range of UDP ports

Because it operates over a custom supposedly-mobile-optimized protocol, it tends to end up being inaccessible when you actually need it due to pervasive modern firewalls. You'll start a session at home, go to the office, and... nothing... because you can't communicate over that UDP port. This happened to me quite often while evaluating mosh.

3) it doesn't have a good way to reconnect later

While it supports sitting down at a different computer and taking over your session, it requires you to drop to a rather low-level mechanism based on knowing the secret session key that was established at the time the server was setup. This actually undermines the "mobile" use case, as it only lets the connection be mobile, not the session.

(I also have some specific issues with a library it relies on for spoofing hostnames in wtmp: just having that library installed on your system opens a vulnerability where any non-privileged user can spoof their hostname as anyone else... their opinion differs, but it seems to mostly be due to a knee-jerk reaction of "you don't get fired for avoiding setuid" than "this is actually secure".)

The result is that after spending a bunch of time testing it, working with the codebase, getting frustrated at a bunch of the design decisions, and then having it both totally fail to work in real-world network settings and become yet another source of infuriating latency, I pretty much dropped all interest in the project and replaced it with autossh, which worked great.

    function cysh() {
        AUTOSSH_GATETIME=0 autossh -M 0 -t -e none \
            -o ServerAliveInterval=2 -o ServerAliveCountMax=4 \
            "$1" screen -xRS "$2"
    }

The primary downside of this is that it doesn't handle the packet loss problem well, as you are really just still using normal SSH to do the connections. The session might get weirdly slow and even lock up in these packet-loss situations.

However: it doesn't require any special code on the server, which is a downside of mosh I didn't even list above (as it is fairly obvious and something you know going in): this means you get to use it for every connection to every server you might ever care about.

Otherwise, you get everything you expect from normal ssh and screen: it works everywhere you might theoretically have been able to connect to the server anyway (so no ending up at the client site and finding out that they firewall UDP), it is obvious how to reconnect to (or even share) the screen session from a different computer or a different client without special software, and you don't need to even contemplate "what if this introduces a security vulnerability by running a new daemon on my server?".

The other annoyance of using autossh is it is pretty adamant about doing exponential back-off on reconnect, and in the naive way I have it setup in that function it is also going to be doing slow-to-fail DNS requests as it attempts to do the reconnection sequence, so the result is that the "open laptop, wait for all terminals to connect" sequence doesn't always click perfectly.

However, that is easily fixable by using a simpler wrapper (to be clear: use autossh when autossh is appropriate; it is designed, however, for server-to-server backend links to maintain persistent port forwarding), so I bring you cysh.sh, which is what I actually use (and which I've done some testing with others to verify works on random platforms):

http://test.saurik.com/hackernews/cysh.sh

(Yes: you are now downloading a random shell script from someone on a web forum. However, it's like 35 lines of very easily readable sh: you should just read it; hell, after reading it, you might consider it so simple you just want to rewrite it for yourself. ;P)

You use this in the same way as the function, which I didn't even document earlier anyway as I knew I'd just be replacing it with this shell script by the end of the comment ;P.

    cysh saurik@server.saurik.com stuff

This will reconnect to the existing session named stuff on server.saurik.com with the username saurik. It will disconnect other clients when it connects: if you don't want that, change -dRS to -xRS. It also disables normal screen control with ctrl-A (so it feels more like "just a console"): if you don't want that, remove -e'\0\0'. If you want some other screen setting: just add it. ;P

(BTW, if you are curious why I have it set to do -dRS "by default" as opposed to -xRS, is that if you have multiple clients associated with the session the size of window can't be automatically updated; the result is that you often find yourself connecting back to copies of things like irssi or even bash prompts that are now the wrong width; -dRS works perfectly out of the box. Many people, however, know enough about screen that they love -x: if that includes you, you should definitely just change that.)

So, if you don't need the "severe packet loss" protection, I really think you are better off just passing on mosh and using something infinitely simpler cobbled over ssh. The latency mitigation feature are overrated or even annoying, and it seems many people actually just want mosh for "easily auto-reconnecting mobile shell", something you seriously can solve very well with a very short shell script and no special server-side logic.

(If anyone tries using that script, it doesn't work, and you want help debugging it, I'm happy to do so; that said, it is purposely not "a product" right now because different people seem to want different things, and it is frankly really simple to write this from scratch if you need it, or just kind of pull it apart and fix/change whatever you don't like about it: just attempting to get an automatically-reconnecting shell is a fairly simple problem.)

zobzu6• 14 years ago

While I agree with all your points to some extent, and I don't actually like the way mosh has been made, I don't think your conclusion is fair.

When used on slow mobile networks, where I actually keep the same incoming IP (thus there is no reconnection), mosh works better than raw SSH for me and here's why:

- when switching cellphone towers, mosh's UDP catches up instantly. SSH's TCP waits for some retransmissions so that it knows if it's going to timeout or resume sending packets, and it takes _several_ seconds!

- prediction actually works well most of the time.

saurik7• 14 years ago

I find switching cell phone towers to be something that happens sufficiently rarely that it doesn't matter if it skips for a second. I am willing to believe your mileage may vary. Really, if you are doing tons of work from cell phones, you may even prefer the mosh UDP protocol for its packet loss handling anyway: people should try it.

However, my experience has been that most of the people I have had go "omg, mosh sounds awesome!" are actually trying to use it from their laptop (not the phone) as they move from home to the office: mosh is not just overkill for that use case, it actually makes some "incorrect" tradeoffs. Maybe with an occasional "roaming on 3G" usage, but "from the coffee shop" not "in a train moving at 300km/h.

As for the mis-prediction issues, I guess we'll just have to differ on that one: if you like it, that's great, but the people I've talked to found the weird jerkiness that you get whenever you hit a prediction boundary awkward and jarring, especially as you end up with a boundary whenever you hit enter. I talked to the developer about the issue, and it didn't seem like something that could be worked around (semantically, I'm not even certain it is possible).

(They also may have fixed some of the more insane "visual artifacts" mosh was getting when I was using it: every time you'd hit enter in bash it would do this totally-wrong prediction of "take some characters and move them up a line, but not all of them". I thereby purposely didn't list it in my "downsides" list as the developer seemed to think that was fixable, unlike the mis-prediction boundary problem, but it certainly drove me crazy while using mosh.)

geofft16• 14 years ago

If you're willing to sell "just use screen" as an answer for auto-reconnecting, why not "just use screen" with mosh? That's what I do, and I don't have any trouble with point 3 -- I just open a new mosh connection, and reattach my existing screen session.

(Points 1 and 2, for what they're worth, seem pretty specific to your environment. I've never run into them. So, please bill those as anecdotes.)

saurik7• 14 years ago

One reason is that mosh adds a bunch of latency itself that is at least comparable to (and frankly somewhat heavier than) screen (if you don't believe me, I can find a quote from the primary author), a latency which is itself somewhat noticeable: once you layer screen into mosh, the situation tends to get sufficiently unreasonable that even on perfect connections the jarring latency effect becomes worse (as when a mis-predict occurs you are now going through two layers: mosh and screen; of course for the prediction case, you are still just dealing with the one layer).

(Then, once you are using screen to solve problem #3, my argument is then that unless you need the features from mosh that deal with packet loss, you can start attempting to whittle away at any other things that may have been less-than-perfect regarding mosh, as you might simply not need it anymore.)

As for your contention that #1 and #2 are "anecdotes", it isn't like I'm just saying "engh, didn't work": I'm explaining exactly why these things are issues from first principals so you can evaluate whether they will effect you or not: it isn't like whether mosh requires weird UDP ports is somehow subjective.

I mean, let's say I had said "one downside of deploying a mobile application instead of a website is that if a user doesn't have a smartphone your content might be inaccessible to them"... is that an "anecdote"? Maybe all your customers have smartphones, and if so you can ignore that, but at least you are aware of it.

(BTW, if you want another example of where the prediction goes insane: if you mosh to a server and then SSH from that server to another server, the latency between mosh and the remote client can no longer be measured by the protocol, and the prediction system gets very confused and mostly ends up turning off. I had a long conversation with the mosh developer about this issue when it affected me. However, that's even rarer for people to run into.)

glesica• 14 years ago

Same here. I use tmux + mosh and I've been extremely happy with the results. I put my laptop to sleep, wake it back up and everything is right where I left it. If I need to use a different machine to connect I just take over the tmux session, and it doesn't really matter if I'm using mosh or ssh.

moe2• 14 years ago

0) It sabotages your scrollback (just like screen), no more mouse-wheel scrolling. Showstopper for me.

calpaterson• 14 years ago

Just in case you didn't know, you can access screens scrollback by hitting Control-a [ (exist with escape).

vacri• 14 years ago

Screen doesn't 'sabotage' it. You're running a program called Screen. It has its own scrollback.

geofft16• 14 years ago

That's somewhat on purpose -- terminal rendering is on the server-side, so that if there's a lot of changes to the screen (a long terminal command, a full-screen curses app that did several updates, etc.), and the connection was bad during that, then the client only needs to see what's currently on the screen. If you're ever used a curses app over a bad connection, and seen it slowly update itself over the course of several seconds to catch up, you can understand why this is useful.

As other folks have mentioned, if you run screen on the server-side, then you can ask screen for its scrollback. Also, see this open ticket: https://github.com/keithw/mosh/issues/2

rckrd• 14 years ago

I love the concept, but "Clients may not even know they have roamed".

Can anyone briefly rehash some of the security issues that were brought up with mosh?

geofft16• 14 years ago

I don't understand your first sentence. Are you claiming that that's a security issue, or a problem somehow?

jread• 14 years ago

I've spent the past year using Mosh from high latency, unreliable satellite connections in Afghanistan. I would have been pulling my hair out without it. My only complaint is the scrollback buffer often gets jumbled.

zobzu6• 14 years ago

After using mosh on and off for a year, I realized that what they wrote in their PDF is absolutely true:

mosh latency isn't much different from SSH latency, which is a common misconception. What mosh adds is:

- decent prediction (this is what mitigate most of the latency)

- integrated auto-reconnect (even thus auto-ssh and others do the same thing, they're not built-in)

- mostly asynchronous keystrokes thanks to UDP+stream protocol, so if you hit ^C it will abort stuff even if the screen is scrolling. Note that the way keystrokes are handled makes it susceptible to a timing attack, and there might have been work to mitigate this that i'm unaware of.

geofft16• 14 years ago

One thing that's distinctly better is that the backoff/recovery behavior of SSP (mosh's custom encrypted protocol, over UDP) works a lot better than TCP's. If I'm typing, and there's a network blip for e.g. thirty seconds (cell handoff), SSH will literally take minutes to get back to normal, and it'll be easier for me to kill SSH and restart the connection. mosh will recover much more quickly. TCP was written in the days when congestion on a shared Ethernet segment was a thing people actually ran into; with wireless and cell networks, that's not actually your protocol's biggest concern.

The common-case latency, when there's a good network connection, shouldn't differ much.

saurik7• 14 years ago

> so if you hit ^C it will abort stuff even if the screen is scrolling

I might be wrong, but AFAIK the fact that your terminal isn't handling this correctly already is due to a flow control setup problem somewhere in the chain (probably the terminal itself, as usually the server-side components are the most ancient): if your local console is only able to download and display and scroll data at a certain rate, it should "push back" on the server to get it to send you data at a slower rate, which in turn will push back on the terminal. When setup correctly, this flows all the way back to the original application, which will itself slow down whatever it is that it is doing.

zobzu6• 14 years ago

its tcp congestion issues. ^C will work just fine locally. it would also be ok if there were 2 tcp channels. or. udp. ;)

tquai• 14 years ago

I wrap a normal OpenSSH connection in CurveCP.

zokier• 14 years ago

CurveCP seems interesting (like most of djbs work). Would you get most of its client-side benefits (mobility support etc) if you'd create VPN over CurveCP? That way you wouldn't need to install it to the destination servers, but instead use a server with a stable, fast connection as a gateway.

Or is there some neat VPN solution that has good support for mobility already?

visarga3• 14 years ago

Is there a neat VPN solution that would work through SSH ?

It is much easier to use one of the servers you have than pay for a VPN account to a provider.

leh0n• 14 years ago

I used it for like 30 minutes before I realized there was no way to enable SSH agent forwarding.

jakozaur• 14 years ago

I would love to see mosh-like solution, but client only.

Pretty much a wraper to ssh which will use screen plus some predictions. I guess it could get 80% of mosh benefits without the hassle of deploying new software on servers.

kolev• 14 years ago

I loved Mosh... until I started using it. Latency is definitely a big problem for me - even on fast connections. Often keystrokes arrive in the wrong order if you type too fast, making it useless.

keithwinstein• 14 years ago

Hi, thanks for your feedback! I have heard of this happening with iSSH's clean-room reimplementation of the protocol, but never with Mosh. Please file a bug report at https://github.com/keithw/mosh because we are eager to trace this down. (We have never heard of this happening to anybody, and it should not be possible if all is well.)

doctorpangloss• 14 years ago

Most network programs lose their connections after roaming, including SSH and Web apps like Gmail.

Gmail.com has been a roaming-safe/disconnect-tolerant web app for quite some time now. It is even tolerant of huge disparities in time, as far as SSL will allow it to be.

I wonder what Gmail client they're talking about...

lwf2• 14 years ago

Gmail often says "reconnecting" for me, and this does not always work correctly unless I manually tell it to "reconnect now".

st3fan• 14 years ago

Mosh needs a proper security and crypto review. AFAIK that has not been done.

geofft16• 14 years ago

Has that been done for OpenSSH?

wangweij• 14 years ago

I know this sounds not very secure, but since one of the best features of mosh is that you needn't install it, why not provide a place to download binaries for all platforms?

jiyinyiyong• 14 years ago

I was sad because it was disturbed when I try to login my VPS(Burst) from mainland China with Mosh, after I was so happy to find it and installed it on all my Linux OS.

Mosh: the mobile shell

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