6f8dd9baff
When using libssh2 to perform an SFTP file transfer from the "JSCAPE MFT Server" (http://www.jscape.com) the transfer failed. The default JSCAPE configuration is to enforce zlib compression on SSH2 sessions so the session was compressed. The relevant part of the debug trace contained: [libssh2] 1.052750 Transport: unhandled zlib error -5 [libssh2] 1.052750 Failure Event: -29 - decompression failure The trace comes from comp_method_zlib_decomp() in comp.c. The "unhandled zlib error -5" is the status returned from the zlib function inflate(). The -5 status corresponds to "Z_BUF_ERROR". The inflate() function takes a pointer to a z_stream structure and "inflates" (decompresses) as much as it can. The relevant fields of the z_stream structure are: next_in - pointer to the input buffer containing compressed data avail_in - the number of bytes available at next_in next_out - pointer to the output buffer to be filled with uncompressed data avail_out - how much space available at next_out To decompress data you set up a z_stream struct with the relevant fields filled in and pass it to inflate(). On return the fields will have been updated so next_in and avail_in show how much compressed data is yet to be processed and next_out and avail_out show how much space is left in the output buffer. If the supplied output buffer is too small then on return there will be compressed data yet to be processed (avail_in != 0) and inflate() will return Z_OK. In this case the output buffer must be grown, avail_out updated and inflate() called again. If the supplied output buffer was big enough then on return the compressed data will have been exhausted (avail_in == 0) and inflate() will return Z_OK, so the data has all been uncompressed. There is a corner case where inflate() makes no progress. That is, there may be unprocessed compressed data and space available in the output buffer and yet the function does nothing. In this case inflate() will return Z_BUF_ERROR. From the zlib documentation and the source code it is not clear under what circumstances this happens. It could be that it needs to write multiple bytes (all in one go) from its internal state to the output buffer before processing the next chunk of input but but can't because there is not enough space (though my guesses as to the cause are not really relevant). Recovery from Z_BUF_ERROR is pretty simple - just grow the output buffer, update avail_out and call inflate() again. The comp_method_zlib_decomp() function does not handle the case when inflate() returns Z_BUF_ERROR. It treats it as a non-recoverable error and basically aborts the session. Fixes #240
libssh2 - SSH2 library ====================== libssh2 is a library implementing the SSH2 protocol, available under the revised BSD license. Web site: http://www.libssh2.org/ Mailing list: http://cool.haxx.se/mailman/listinfo/libssh2-devel Generic installation instructions are in INSTALL. Some ./configure options deserve additional comments: * --enable-crypt-none The SSH2 Transport allows for unencrypted data transmission using the "none" cipher. Because this is such a huge security hole, it is typically disabled on SSH2 implementations and is disabled in libssh2 by default as well. Enabling this option will allow for "none" as a negotiable method, however it still requires that the method be advertized by the remote end and that no more-preferable methods are available. * --enable-mac-none The SSH2 Transport also allows implementations to forego a message authentication code. While this is less of a security risk than using a "none" cipher, it is still not recommended as disabling MAC hashes removes a layer of security. Enabling this option will allow for "none" as a negotiable method, however it still requires that the method be advertized by the remote end and that no more-preferable methods are available. * --disable-gex-new The diffie-hellman-group-exchange-sha1 (dh-gex) key exchange method originally defined an exchange negotiation using packet type 30 to request a generation pair based on a single target value. Later refinement of dh-gex provided for range and target values. By default libssh2 will use the newer range method. If you experience trouble connecting to an old SSH server using dh-gex, try this option to fallback on the older more reliable method. * --with-libgcrypt * --without-libgcrypt * --with-libgcrypt-prefix=DIR libssh2 can use the Libgcrypt library (http://www.gnupg.org/) for cryptographic operations. Either Libgcrypt or OpenSSL is required. Configure will attempt to locate Libgcrypt automatically. If your installation of Libgcrypt is in another location, specify it using --with-libgcrypt-prefix. * --with-openssl * --without-openssl * --with-libssl-prefix=[DIR] libssh2 can use the OpenSSL library (http://www.openssl.org) for cryptographic operations. Either Libgcrypt or OpenSSL is required. Configure will attempt to locate OpenSSL in the default location. If your installation of OpenSSL is in another location, specify it using --with-libssl-prefix. * --with-libz * --without-libz * --with-libz-prefix=[DIR] If present, libssh2 will attempt to use the zlib (http://www.zlib.org) for payload compression, however zlib is not required. If your installation of Libz is in another location, specify it using --with-libz-prefix. * --enable-debug Will make the build use more pedantic and strict compiler options as well as enable the libssh2_trace() function (for showing debug traces).