10

I'm having problems getting sFTP working while there are no problems with ssh. I'm basically building zlib, openssl, and openssh for an ARM processor using an existing embedded Linux filesystem. After searching for ideas, this seemed liked a common problem, but I haven't made any progress. I only have one user defined, which is root with a empty password.

I'm using openssh version 4.7p1, and I modified sshd_config with the following settings:

PermitRootLogin yes
PermitEmptyPasswords yes
UseDNS yes
UsePrivilegeSeparation no
SyslogFacility AUTH
LogLevel DEBUG3
Subsystem sftp /usr/local/libexec/sftp-server -f AUTH -l DEBUG3

The sftp-server is located in /usr/local/libexec and has the following permissions:

root@arm:/usr/local/libexec# ls -l
-rwxr-xr-x    1 root     root         65533 Oct  3 22:12 sftp-server
-rwx--x--x    1 root     root        233539 Oct  3 22:12 ssh-keysign

I know sftp-server is being found (path is set in sshd_config) because if I rename the sftp_server executable, I get the following error:

auth.err sshd[1698]: error: subsystem: cannot stat /usr/local/libexec/sftp-server: No such file or directory                                                        
auth.info sshd[1698]: subsystem request for sftp failed, subsystem not found   

Also, the target's login init-scripts are very simple and consists of a single file (etc/profile.d/local.sh), which only contain definitions for LD_LIBRARY_PATH, PATH and PYTHONPATH as shown below:

#!/bin/sh
export LD_LIBRARY_PATH="/usr/local/lib"
export PATH="/usr/local/bin:/usr/local/libexec:${PATH}"
export PYTHONPATH="/home/root/python"

As you can see .bashrc, .profile, etc do not exist in root's home directory:

root@arm:~# ls -la                                                                                                                                                                      
drwxr-xr-x    2 root     root          4096 Oct  4 14:57 .                                                                                                                                    
drwxr-xr-x    3 root     root          4096 Oct  4 01:11 ..                                                                                                                                   
-rw-------    1 root     root           120 Oct  4 01:21 .bash_history

Here is the system log output when using FileZilla to connect to the sftp server on the target. From the log it seems that the sftp-server executable is found, but the child processes is exited immediately. I am using debug arguments when calling sftp-server in sshd_config (Subsystem sftp /usr/local/libexec/sftp-server -f AUTH -l DEBUG3), but no logs were captured.

Oct  4 14:29:45 arm auth.info sshd[2070]: Connection from 192.168.1.12 port 45888                                                                                                       
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug1: Client protocol version 2.0; client software version PuTTY_Local:_Mar_28_2012_12:33:05                                               
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug1: no match: PuTTY_Local:_Mar_28_2012_12:33:05                                                                                          
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug1: Enabling compatibility mode for protocol 2.0                                                                                         
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug1: Local version string SSH-2.0-OpenSSH_4.7                                                                                             
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: fd 3 setting O_NONBLOCK                                                                                                              
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug1: list_hostkey_types: ssh-rsa,ssh-dss                                                                                                  
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug1: SSH2_MSG_KEXINIT sent                                                                                                                
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug1: SSH2_MSG_KEXINIT received                                                                                                            
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellma1
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: ssh-rsa,ssh-dss                                                                                                   
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,arcfour256,arcfour,aes192-cbc,aes256-cbc,rijndael-cbc@lysr
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,arcfour256,arcfour,aes192-cbc,aes256-cbc,rijndael-cbc@lysr
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96         
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96         
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: none,zlib@openssh.com                                                                                             
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: none,zlib@openssh.com                                                                                             
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit:                                                                                                                   
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit:                                                                                                                   
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: first_kex_follows 0                                                                                               
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: reserved 0                                                                                                        
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellma1
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: ssh-rsa,ssh-dss                                                                                                   
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: aes256-ctr,aes256-cbc,rijndael-cbc@lysator.liu.se,aes192-ctr,aes192-cbc,aes128-ctr,aes128-cbc,blowfish-ctr,blowfi8
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: aes256-ctr,aes256-cbc,rijndael-cbc@lysator.liu.se,aes192-ctr,aes192-cbc,aes128-ctr,aes128-cbc,blowfish-ctr,blowfi8
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: hmac-sha1,hmac-sha1-96,hmac-md5                                                                                   
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: hmac-sha1,hmac-sha1-96,hmac-md5                                                                                   
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: none,zlib                                                                                                         
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: none,zlib                                                                                                         
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit:                                                                                                                   
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit:                                                                                                                   
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: first_kex_follows 0                                                                                               
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_parse_kexinit: reserved 0                                                                                                        
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: mac_setup: found hmac-sha1                                                                                                           
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug1: kex: client->server aes256-ctr hmac-sha1 none                                                                                        
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: mac_setup: found hmac-sha1                                                                                                           
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug1: kex: server->client aes256-ctr hmac-sha1 none                                                                                        
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug1: SSH2_MSG_KEX_DH_GEX_REQUEST_OLD received                                                                                             
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug1: SSH2_MSG_KEX_DH_GEX_GROUP sent                                                                                                       
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: dh_gen_key: priv key bits set: 277/512                                                                                               
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: bits set: 2052/4096                                                                                                                  
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug1: expecting SSH2_MSG_KEX_DH_GEX_INIT                                                                                                   
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: bits set: 2036/4096                                                                                                                  
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug1: SSH2_MSG_KEX_DH_GEX_REPLY sent                                                                                                       
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: kex_derive_keys                                                                                                                      
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: set_newkeys: mode 1                                                                                                                  
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug2: cipher_init: set keylen (16 -> 32)                                                                                                   
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug1: SSH2_MSG_NEWKEYS sent                                                                                                                
Oct  4 14:29:45 arm auth.debug sshd[2070]: debug1: expecting SSH2_MSG_NEWKEYS                                                                                                           
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: set_newkeys: mode 0                                                                                                                  
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: cipher_init: set keylen (16 -> 32)                                                                                                   
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: SSH2_MSG_NEWKEYS received                                                                                                            
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: KEX done                                                                                                                             
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: userauth-request for user root service ssh-connection method none                                                                    
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: attempt 0 failures 0                                                                                                                 
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug3: Trying to reverse map address 192.168.1.12.                                                                                          
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: parse_server_config: config reprocess config len 302                                                                                 
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: input_userauth_request: setting up authctxt for root                                                                                 
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: input_userauth_request: try method none                                                                                              
Oct  4 14:29:46 arm auth.info sshd[2070]: Accepted none for root from 192.168.1.12 port 45888 ssh2                                                                                      
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: Entering interactive session for SSH2.                                                                                               
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: fd 4 setting O_NONBLOCK                                                                                                              
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: fd 5 setting O_NONBLOCK                                                                                                              
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: server_init_dispatch_20                                                                                                              
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: server_input_channel_open: ctype session rchan 256 win 2147483647 max 16384                                                          
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: input_session_request                                                                                                                
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: channel 0: new [server-session]                                                                                                      
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: session_new: init                                                                                                                    
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: session_new: session 0                                                                                                               
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: session_open: channel 0                                                                                                              
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: session_open: session 0: link with channel 0                                                                                         
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: server_input_channel_open: confirm session                                                                                           
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: server_input_channel_req: channel 0 request simple@putty.projects.tartarus.org reply 0                                               
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: session_by_channel: session 0 channel 0                                                                                              
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: session_input_channel_req: session 0 req simple@putty.projects.tartarus.org                                                          
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: server_input_channel_req: channel 0 request subsystem reply 1                                                                        
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: session_by_channel: session 0 channel 0                                                                                              
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: session_input_channel_req: session 0 req subsystem                                                                                   
Oct  4 14:29:46 arm auth.info sshd[2070]: subsystem request for sftp                                                                                                                    
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: subsystem: exec() /usr/local/libexec/sftp-server -f AUTH -l DEBUG3                                                                   
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: fd 3 setting TCP_NODELAY                                                                                                             
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: fd 7 setting O_NONBLOCK                                                                                                              
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug3: fd 7 is O_NONBLOCK                                                                                                                   
Oct  4 14:29:46 arm auth.debug sshd[2073]: debug1: permanently_set_uid: 0/0                                                                                                             
Oct  4 14:29:46 arm auth.debug sshd[2073]: debug3: channel 0: close_fds r -1 w -1 e -1 c -1                                                                                             
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: channel 0: read<=0 rfd 7 len -1                                                                                                      
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: channel 0: read failed                                                                                                               
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: channel 0: close_read                                                                                                                
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: channel 0: input open -> drain                                                                                                       
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: channel 0: ibuf empty                                                                                                                
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: channel 0: send eof                                                                                                                  
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: channel 0: input drain -> closed                                                                                                     
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: notify_done: reading                                                                                                                 
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: Received SIGCHLD.                                                                                                                    
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: session_by_pid: pid 2073                                                                                                             
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: session_exit_message: session 0 channel 0 pid 2073                                                                                   
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: channel 0: request exit-status confirm 0                                                                                             
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: session_exit_message: release channel 0                                                                                              
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: channel 0: write failed                                                                                                              
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: channel 0: close_write                                                                                                               
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: channel 0: output open -> closed                                                                                                     
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: channel 0: send close                                                                                                                
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug3: channel 0: will not send data after close                                                                                            
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: channel 0: rcvd close                                                                                                                
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug3: channel 0: will not send data after close                                                                                            
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: channel 0: is dead                                                                                                                   
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: channel 0: gc: notify user                                                                                                           
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: session_by_channel: session 0 channel 0                                                                                              
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: session_close_by_channel: channel 0 child 0                                                                                          
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: session_close: session 0 pid 0                                                                                                       
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: channel 0: gc: user detached                                                                                                         
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: channel 0: is dead                                                                                                                   
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug2: channel 0: garbage collecting                                                                                                        
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: channel 0: free: server-session, nchannels 1                                                                                         
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug3: channel 0: status: The following connections are open:\r\n  #0 server-session (t4 r256 i3/0 o3/0 fd 7/7 cfd -1)\r\n                  
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug3: channel 0: close_fds r 7 w 7 e -1 c -1                                                                                               
Oct  4 14:29:46 arm auth.info sshd[2070]: Connection closed by 192.168.1.12                                                                                                             
Oct  4 14:29:46 arm auth.debug sshd[2070]: debug1: do_cleanup                                                                                                                           
Oct  4 14:29:46 arm auth.info sshd[2070]: Closing connection to 192.168.1.12 
  • I'd rule out FileZilla as a problem. Try with the sftp command (sftp host.name.here). If something fails it might also give you better info. – Patrick Oct 4 '13 at 15:45
  • slightly unrelated: OpenSSH 4.7p1 is 6 years old - for security's sake please use something newer. As for the question: your sftp-server seems to be exiting prematurely: Oct 4 14:29:46 arm auth.debug sshd[2070]: debug1: Received SIGCHLD. – peterph Oct 4 '13 at 15:52
  • @peterph - I updated to the lastest openssh (6.3p1) yesterday and experienced the same problems. – Michael Oct 4 '13 at 15:54
  • @Patrick - I've tried the sftp command from the host-to-target and the results are the same. The server closes the connection after authentication. – Michael Oct 4 '13 at 16:00
  • @Michael could you possibly run sshd in strace -f? – peterph Oct 4 '13 at 16:38
9

While this is more of an alternate solution than a direct answer to your issue, I would try using the internal sftp server instead of an external one. Since this is an embedded system, this probably makes more sense to do anyway.

In your sshd_config, just add:

Subsystem sftp internal-sftp

That way you can leave out the sftp binary and save some space.

  • Didn't seem to work "error: subsystem: cannot stat internal-sftp: No such file or directory." Not sure why this happened, but it doesn't seem to be a compile/configure option... – Michael Oct 4 '13 at 15:41
  • @Michael hrm, thats odd. Perhaps it's a compile time option. Never seen an OpenSSH without it. – Patrick Oct 4 '13 at 15:47
  • I updated to the lastest openssh (6.3p1) and internal-sftp is working as expected. – Michael Oct 4 '13 at 16:39
  • A possible reason for sftp-server failing to start is when the users do not have a shell, e.g. if it is set to /bin/false. In this case you have to use "internal-sftp". – Zrin Oct 26 '14 at 22:32

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