x/crypto/ssh: possible missing stdout?

[snoowman]

Please answer these questions before submitting your issue. Thanks!

What did you do?

code is here: very basic ssh run command
https://play.golang.org/p/eYDOY1qg6aJ

ssh server is an embed system running dropbear.
SSH-2.0-dropbear_2016.73

Using Python Pamiko and OpenSSH client works just fine.

debug info:
2018/08/15 19:25:23 send global(1): ssh.channelOpenMsg{ChanType:"session", PeersID:0x1, PeersWindow:0x200000, MaxPacketSize:0x8000, TypeSpecificData:[]uint8(nil)}
2018/08/15 19:25:23 decoding(1): 91 &ssh.channelOpenConfirmMsg{PeersID:0x1, MyID:0x0, MyWindow:0x6000, MaxPacketSize:0x7ff7, TypeSpecificData:[]uint8{}} - 17 bytes
2018/08/15 19:25:23 send(1): ssh.channelRequestMsg{PeersID:0x0, Request:"exec", WantReply:true, RequestSpecificData:[]uint8{0x0, 0x0, 0x0, 0x4, 0x68, 0x65, 0x6c, 0x70}}
2018/08/15 19:25:23 decoding(1): 99 &ssh.channelRequestSuccessMsg{PeersID:0x1} - 5 bytes
2018/08/15 19:25:23 send(1): ssh.channelEOFMsg{PeersID:0x0}
2018/08/15 19:25:23 decoding(1): data packet - 12 bytes
2018/08/15 19:25:23 send(1): ssh.windowAdjustMsg{PeersID:0x0, AdditionalBytes:0x3}
2018/08/15 19:25:23 decoding(1): data packet - 55 bytes
2018/08/15 19:25:23 decoding(1): 96 &ssh.channelEOFMsg{PeersID:0x1} - 5 bytes
2018/08/15 19:25:23 send(1): ssh.windowAdjustMsg{PeersID:0x0, AdditionalBytes:0x2e}
2018/08/15 19:25:23 decoding(1): 98 &ssh.channelRequestMsg{PeersID:0x1, Request:"exit-status", WantReply:false, RequestSpecificData:[]uint8{0x0, 0x0, 0x0, 0x0}} - 25 bytes
2018/08/15 19:25:23 decoding(1): 97 &ssh.channelCloseMsg{PeersID:0x1} - 5 bytes
2018/08/15 19:25:23 send(1): ssh.channelCloseMsg{PeersID:0x0}

What did you expect to see?

capture all of the SSH output

What did you see instead?

just "Bye bye. Have a nice day!!!"
Which is what the CLI send when exiting.
It show that ssh is connecting but output for the commands seems to be missing.

System details

go version go1.10.3 darwin/amd64
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/x/Library/Caches/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/x/go"
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/2_/g6jgpj8x36g41j7wm08xtzxw0000gn/T/go-build612508101=/tmp/go-build -gno-record-gcc-switches -fno-common"
GOROOT/bin/go version: go version go1.10.3 darwin/amd64
GOROOT/bin/go tool compile -V: compile version go1.10.3
uname -v: Darwin Kernel Version 15.6.0: Thu Jun 21 20:07:40 PDT 2018; root:xnu-3248.73.11~1/RELEASE_X86_64
ProductName:	Mac OS X
ProductVersion:	10.11.6
BuildVersion:	15G22010
lldb --version: lldb-360.1.70

[agnivade]

@hanwen

[hanwen]

the trace looks perfectly valid. You could try to insert some sleep statements in CombinedOutputs().

I bet dropbear is doing something wrong with their synchronization.

[snoowman]

Thank you. I strongly suspect that dropbear is the culprit as well.
I will try sleep and run some command(ping and tcpdump on the destination host) to see if the commands are being executed at all.

[snoowman]

seems like the command are not even being executed.
It just connect and exit. Hence no output were captured.
This is not a bug on x/crypto/ssh.

Any tip on how to further trouble shoot this only from the client side?
I can't do much debugging on the host side since it's a limited shell.

My naive approached would be to exam each of the ssh layers.
channel/session/pty/shell and etc.

[hanwen]

CombinedOutput() is just shorthand for a couple of other commands. You copy & paste that code, and remove the bits that close channels etc.

[hanwen]

you can also compare with openssh doing

ssh -vvvv

and see how Go and SSH traces are different.

[agnivade]

Does not seem like there is a Go bug here. Closing. Feel free to ask around in the forums on how to debug this better.