I did install Mosh on (1) a Mac OS X 10.9.5 machine, (2) a Debian 8 VM, and (3) as Entware-ng package on a router that runs an embedded Linux. Mosh connection from Mac (1) to (2) Debian is ok. Mosh connection from (1) Mac to (3) Linux embedded router fails with the message:

/usr/local/bin/mosh: Did not find mosh server startup message.

Environment:

• Mosh version on the Mac is 1.2.5.
• All machines are configured to ssh login with certificates.
• Mac OS X shell is zsh.
• /usr/local/bin is in the $PATH of the Mac OS X
• Mac Terminal application is iTerm 2.1.4
• Debian 8 is running in a 10.x pvm_nat LAN: parallels virtual machine with shared networking, that Parallels runs on the same Mac.
• Command used to connect to Debian $ mosh [email protected]
• Router rt-n56u is running at IP 192.168.x using a cabled LAN
• Command to connnect from Mac to router is: $ mosh [email protected]
• Linux router is running Padavan firmware RT-N56U_3.4.3.9-099_base.trx

Regression

1. I did try to disable the Little Snitch firewall on the Mac with no improvement, still:
   /usr/local/bin/mosh: Did not find mosh server startup message.
2. I did also try to start a debug session using:
   $ ssh -S none -o ProxyCommand='mosh --fake-proxy -- %h %p' -n -tt [email protected] -- 'mosh-server new'
MOSH IP 192.168.x.x
Connection to 192.168.x.x closed.
3. I commented out SendEnv on the Mac:
   $ cat /etc/ssh_config | grep SendEnv
# SendEnv LANG LC_*
4. I have tried to disable the firewall on the router, which doesn't make any difference:
   /usr/local/bin/mosh: Did not find mosh server startup message.

The rt-n56u log displays this:

May  3 00:42:15 dropbear[819]: Child connection from 192.168.x.y:50824
May  3 00:42:17 dropbear[819]: Pubkey auth succeeded for 'username' with key md5 2c:...:94 from 192.168.x.y:50824
May  3 00:42:18 dropbear[820]: ioctl(TIOCSCTTY): Input/output error
May  3 00:42:18 dropbear[820]: /dev/pts/0: No such file or directory
May  3 00:42:18 dropbear[820]: open /dev/tty failed - could not set controlling tty: No such device or address
May  3 00:42:18 dropbear[819]: Exit (username): Disconnect received

On the rt-n56u:

# ls -l /dev/pts/*
crw--w--w-    1 username   root      136,   0 May  3 00:45 /dev/pts/0
# ls -l /dev/tty
crw-rw----    1 username   root        5,   0 Jan  1  2015 /dev/tty

What can be done to fix the mosh connection from this Mac to rt-n56u router?

Check where the mosh-server binary is installed in the embedded Linux, and pass that path to the server option in your mosh command.

Suppose the mosh-server binary is installed in /usr/bin/ directory in the embedded Linux, then run mosh like:

$ mosh --server=/usr/bin/mosh-server [email protected]

I hope this helps, this problem also happened to me with same message, I'm not an expert yet i solved this way:

Analog to your config (3) and (1) my server is running Ubuntu Server 18.04 and my client is an iMac running Mojave.

1. On the server y run:

   $ echo $LANG

   en_US.UTF-8

2. Then search the mosh-server path, it was on /usr/bin/mosh-server

3. On the Mac change the last line in /etc/ssh/ssh_config to SendEnv LANG en_US.UTF-8 like the previus server output.

4. The command i use for mosh is like this indicating the mosh-server path and ssh port:

   $ mosh --server=/usr/bin/mosh-server [email protected] --ssh="ssh -p XXXX"