SFTP support in midnight commander

This is a translation of a note about my testing of SFTP support in the midnight commander. The original in English is published on my blog .

I continue to follow the development of SFTP support in the midnight commander, this week I talked to the author - authorization via ssh-agent appeared in sftp support. To celebrate, I quickly threw the package and tested it.

And the first disappointment is that midnight cannot log in through an agent. I tried the previous authorization schemes that worked in the previous version:
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• password - works without problems;
• by key it works, but only for keys that are not protected by a password (apparently this is a library restriction).

Having tried different options with the agent - asked the author how he works. It turned out that no problem. Later I found an example of authorization via ssh-agent on the libssh2 developer site and tried to compile it.

$ gcc -o agent_auth -Wall -I/usr/include -I. -lssh2 ssh2_agent.c ssh2_agent.c: In function 'main': ssh2_agent.c:99: warning: implicit declaration of function 'libssh2_session_handshake' /home/andrey/tmp/ccmczn2V.o: In function `main': ssh2_agent.c:(.text+0x1a8): undefined reference to `libssh2_session_handshake' collect2: ld returned 1 exit status 

The error message suggested that the example was designed for a version that is newer than the one in the debian squeeze repository. I tried to compile the current version of libssh2 (1.3.0) and reassemble an example with a new library already.

 $ gcc -o agent_auth -Wall -I$PWD/libssh2/include -I. -L$PWD/libssh2/lib -lssh2 ssh2_agent.c 

After this, the example was assembled without errors, but refused to work, referring to the impossibility of the connection.

 $ LD_LIBRARY_PATH=$PWD/libssh2/lib ./agent-auth localhost andrey failed to connect! zsh: segmentation fault ./agent_auth localhost andrey 

Running under strace showed that, firstly, the connection goes to 255.255.255.255 instead of 127.0.0.1, and secondly, the example code tries to free resources without checking their use. Hence segfault on exit.

 munmap(0xb7813000, 4096) = 0 socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 3 connect(3, {sa_family=AF_INET, sin_port=htons(22), sin_addr=inet_addr("255.255.255.255")}, 16) = -1 ENETUNREACH (Network is unreachable) write(2, "failed to connect!\n", 19failed to connect! ) = 19 --- SIGSEGV (Segmentation fault) @ 0 (0) --- +++ killed by SIGSEGV +++ zsh: segmentation fault strace -f ./agent_auth localhost username 

Added check before freeing resources

 --- ssh2_agent.c.orig 2011-12-08 23:53:41.000000000 +0300 +++ ssh2_agent.c 2011-12-09 00:02:10.000000000 +0300 @@ -231,10 +231,12 @@ int main(int argc, char *argv[]) */ shutdown: - - libssh2_agent_disconnect(agent); - libssh2_agent_free(agent); + if (agent) { + libssh2_agent_disconnect(agent); + + libssh2_agent_free(agent); + } if(session) { 

Then rebuilt and launched.

 $ gcc -o agent_auth -Wall -I$PWD/libssh2/include -I. -L$PWD/libssh2/lib -lssh2 ssh2_agent.c $ LD_LIBRARY_PATH=$PWD/libssh2/lib ./agent_auth localhost andrey failed to connect! 

Already better - the segfault at the output disappeared, but swears at the inability to connect. Looking at the code again. Inet_addr () is used to convert the host to a format understandable connect () function.

  if (argc > 1) { hostaddr = inet_addr(argv[1]); } else { hostaddr = htonl(0x7F000001); } 

I look in man 3 inet_addr and see that the function converts the symbolic entry of the ip address into its binary representation. Bingo! I after all transfer a host name as localhost. Replacing localhost with 127.0.0.1 I get a working example.

 $ LD_LIBRARY_PATH=$PWD/libssh2/lib ./agent_auth 127.0.0.1 andrey Fingerprint: FA F3 92 9E C4 AE 14 B4 FC BE ED 2A E8 33 0C 1E 34 09 9F B3 Authentication methods: publickey,password Authentication with username andrey and public key /home/andrey/.ssh/id_rsa failed! Authentication with username andrey and public key /home/andrey/.ssh/id_dsa succeeded! all done! 

Now it's up to you to build libssh2 (1.3.0) for squeeze and rebuild midnight with the new version of the library. The finished assembly can be picked up here.