CSN(1)
csn - casync-nano
csn INDEX TARGET STORE...++ casync [--store HTTP_URL...] [--seed PATH] [--seed-output YESNO] extract INDEX TARGET
casync-nano is a feature-reduced replacement for casync intended as a building block for implementing image-based differential updates on embedded systems. To that end, the only operation it supports is the extraction of .caibx files that utilize SHA256 hashes. On the other hand, it has some features that help in its intended application compared to the original casync tool:
- Side-loading of index files corresponding to a local source of chunks. This is useful to avoid slow reindexing of a partition in an A/B update scheme (see OPERATION and EXAMPLES below).
- Support for the kernel crypto API to accelerate hashing operations using hardware mechanisms that are present on many modern SoCs.
- Support for encrypting chunks so that they can be stored on untrusted third-party HTTP infrastructure.
- Vastly reduced memory usage.
casync-nano is a multi-call binary and provides a compatibility layer that implements a subset of the casync(1) extract command to the extent necessary to support tools such as RAUC when called as casync and presents its own interface when called as csn.
INDEX Specifies the .caibx file that describes the image to be extracted.
TARGET Specifies where the extracted image will be written to. This will normally be a block device. casync-nano currently does not support writing to MTD devices.
As a safety mechanism, casync-nano will refuse to create _TARGET_ when it
does not exist. To extract an image to a regular file, create an empty file
first.STORE A generic source of chunks. This can either be a HTTP(S) URL pointing towards a casync-style chunk store or a path to a local file or block device that will be broken apart into chunks. (See OPERATION below for more details on how the synchronization process works).
Additional attributes can be set for a HTTP chunk store using URI
fragments, see HTTP CHUNK STORE ATTRIBUTES below for more details.
A local store specification may optionally be followed by a colon and the
path to a .caibx file representing the contents of that local store. If
supplied, this index will be used instead of running the chunking algorithm
on the local store, which is computationally expensive. Such a side-loaded
index will be verified such that if it is incorrect for whatever reason
(e.g. if the local store has bit-rotted or the .caibx simply doesn't match
the contents), it will be discarded and the synchronization process
continues as if it hadn't been specified at all.
Normally, when a chunk appears multiple times in _INDEX_, casync-nano will
query its stores for it multiple times. Thus, if it is not contained in a
local store, it will be downloaded over the network multiple times. While
this sounds problematic, it empirically does not really happen in practice
in the cases which casync-nano is intended for (since such a redundant
chunk will usually already have been redundant in the previous version of
the image and will thus be present in a local store). Still, if you are
concerned about this possibility, you can also supply _TARGET_ as an
additional _STORE_. This triggers some special logic that will prevent this
problem by reusing chunks that have been written to _TARGET_ already. Note
however that because of the data structures involved, this currently
carries a moderate performance penalty.
During synchronization, the specified stores are queried in the order given
on the command line. Thus, local stores should appear first.The casync compatibility layer accepts getopt-style arguments that map to the concepts above: Both --store and --seed are equivalent to specifying a STORE. The --seed-output option is ignored to avoid incurring a warning when used with RAUC.
When the CSN_KCAPI_DRIVER environment variable is set, that particular algorithm specifier/driver of the Linux kernel crypto API is used for calculating SHA256 checksums (via the AF_ALG mechanism). Note that this might actually be slower than using the default user space software implementation depending on your hardware. Please run a benchmark before using this in your application. The source distribution includes the csn-bench tool that can be used for this purpose.
casync and casync-nano use Content-Defined Chunking (CDC) to split large files such as disk images into smaller chunks in way that (statistically) produces the same chunks even when content moves within the file. This affords a way to transfer different versions of the same file efficiently by only transferring a list of the chunks in the new version and their order to the target. If the target has an older version of the same file available locally, it stands to reason that a large percentage of the chunks will have remained the same. Thus, only the missing ones need to be downloaded, saving a significant amount of bandwidth.
The advantage of this approach compared to other binary delta systems is that it is generic and very simple to reason about, does not require generation of deltas between each pair of possible versions and allows archiving of old versions with little marginal storage cost. The downside is that the effective deltas (i.e. the amount of bytes a client has to download) will likely be larger than the ones produced by more specialized binary delta systems.
casync-nano implements the exact same chunking algorithm as casync. The only parameters of that algorithm are the minimum, target average and maximum chunk sizes (in practice, these are always set to the default values of 16, 64 and 256 KiB, respectively). Given the same set of parameters, all implementations of this algorithm will always produce the exact same set of chunks for a given input file.
Chunks are identified by their SHA256 checksum, which provides protection against corruption and also cryptographically binds the output of a synchronization process to the input index that produced it. For each chunk that it retrieves, casync-nano verifies that the contents actually hash to the chunk id. This means that authenticating the .caibx file (for example through some kind of signature) implicitly authenticates the output image. This check is also performed for local stores to prevent bit-rot.
When starting up, casync-nano runs the chunking algorithm using the parameters specified in the .caibx file on all local stores that were supplied on the command line. Since this process is computationally expensive, it also allows the user to supply the (supposed) result of this process externally by passing an auxiliary .caibx file for each store (casync-nano calls this "side-loading").
The idea here is that the most common application of casync-nano will be an OTA update system that uses an A/B partition scheme where one partition is the one the system will currently be executing from and the other one is the one that will contain the new system image at the end of the synchronization process. Crucially, the current system partition usually is read-only for resilience reasons and is thus unmodified from when it was originally created. This means that since the current system image usually also is the result of a previous OTA update, we could have saved the .caibx file that produced it on mutable storage somewhere and supply it for the current synchronization run, avoiding recomputing the chunk boundaries.
HTTP chunk stores are expected to follow the casync store structure, that is:
- Individual chunks correspond to individual files
- Each file is compressed using the zstd algorithm and named "HASH.cacnk" where HASH is the lower-case hexadecimal representation of the SHA256 hash of the uncompressed chunk.
- Each file is placed in a directory below the root that corresponds to the first four characters of its file name.
E.g. the path corresponding to an 256 KiB long all-zero-byte chunk would be /8a39/8a39d2abd3999ab73c34db2476849cddf303ce389b35826850f9a700589b4a90.cacnk.
casync-nano aims to be resilient in face of the various issues that can occur on systems that have unstable or intermittent connectivity:
- HTTP chunk downloads are retried a number of times using exponential backoff if they or the network fail transiently.
- If a given HTTP store has incurred a number of transient failures that couldn't be recovered from, it is disabled to avoid hammering it excessively.
- casync-nano checks the content that is already present on the target and skips any prefix that has already been synchronized, which allows resuming a synchronization process that has been interrupted, ensuring forward progress.
In general, casync-nano does not cache individual chunks in memory or elsewhere to avoid unpredictable memory usage. Chunks are always retrieved from the specified stores on demand. The only exception is that the previously retrieved chunk is reused if it repeats in the .caibx file. This happens during long runs of null bytes, for example. Benchmarking has shown that this is generally sufficient for the intended applications of casync-nano.
To specify additional configuration data for a given HTTP chunk store, URI fragments can be used:
https://example.com#key1=value1,key2=value2Currently, the only valid key is encrypt, the value is a string describing how to retrieve the encryption key. In each case, the 32 byte key is represented in hexadecimal.
- key:HEXSTRING - supplies the key literally through the command line. The usual caveats about this making it visible to other processes on the system apply.
- env:NAME - read the key from the environment variable NAME
- file:PATH - read the key from the file at PATH
If specified, casync-nano expects to find .cacnk.enc files encrypted with the specified key in the respective store instead of regular .cacnk files. csn-tool(1) can be used to convert between .cacnk and .cacnk.enc files. See the chunk encryption design document in the source tree (doc/chunk-encryption.md) for more details.
Extract image.caibx to /dev/mmcblk0p3, downloading all chunks from example.com:
csn image.caibx /dev/mmcblk0p3 https://example.comSame as before, but avoid downloading chunks multiple times by using the partially-written-to target as a cache:
csn image.caibx /dev/mmcblk0p3 /dev/mmcblk0p3 https://example.comExtract image.caibx to /dev/mmcblk0p3, using both example.com and /dev/mmcblk0p2 as sources for chunks, preferring to use /dev/mmcblk0p2 if possible. Furthermore, assume old.caibx was used to create /dev/mmcblk0p2 previously:
csn image.caibx /dev/mmcblk0p3 /dev/mmcblk0p2:old.caibx https://example.comThe latter is the most common application for casync-nano.
Since casync-nano does not provide a mechanism to generate .caibx files or the corresponding chunk stores, the original casync tool has to be used for that purpose:
casync make --digest=sha256 image.caibx image.imgIndices are internally implemented using sorted arrays and binary search. This is fine for static indices, but when using the target as a store (which is continuously updated during synchronization), it causes a certain amount of overhead. However, since this is more of a niche use case, as of now, it does not really justify adding a more complex/expensive data structure for the other cases as well.
Please report bugs in casync-nano or errors in this manual page via GitHub (https://github.com/florolf/casync-nano/issues) or email (fl@n621.de).
The casync compatibility layer only targets RAUC right now. Any incompatibility is considered a bug. If you encounter any problems or use another update orchestration system that requires broader casync emulation, please report a bug.
casync(1)++ csn-tool(1)++ RAUC (https://rauc.io/)