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<!DOCTYPE html>
<html>
<head>
<title>ProFTPD module mod_digest</title>
</head>

<hr>
<center>
<h2>ProFTPD module <code>mod_digest</code></h2>
</center>
<hr>

The <code>mod_digest</code> module offers functionality for calculating the hash
(or digest) value of files. This is particularly useful when verifying
the integrity of files. This functionality is used by the following custom
FTP commands:
<ul>
 <li><code>XCRC</code> (requests CRC32 digest/checksum)
 <li><code>MD5/XMD5</code> (requests MD5 digest/checksum)
 <li><code>XSHA</code>/<code>XSHA1</code> (requests SHA1 digest/checksum)
 <li><code>XSHA256</code> (requests SHA256 digest/checksum)
 <li><code>XSHA512</code> (requests SHA512 digest/checksum)
</ul>
In addition, <code>mod_digest</code> supports the more modern <a href="https://tools.ietf.org/html/draft-bryan-ftpext-hash-02"><code>HASH</code></a> command.

Depending on the file size and the hash function, it takes a fair amount of
CPU and IO resources to calculate the result. Therefore decide wisely where
to enable the features and set the <a href="#DigestMaxSize">DigestMaxSize</a>
configuration directive appropriately.

This module was compiled and tested against ProFTPD 1.3.3 Installation
instructions are discussed <a href="#Installation">here</a>.

The most current version of <code>mod_digest</code> is distributed with the
ProFTPD source code.

<h2>Author</h2>

Please contact TJ Saunders &lt;tj at castaglia.org&gt; with any
questions, concerns, or suggestions regarding this module.

<h2>Thanks</h2>

2016-01-09: Thanks to Mathias Berchtold &lt;mb at
smartftp.com&gt; for his original <code>mod_digest</code>, upon which this
version is based.

<h2>Directives</h2>
<ul>
 <li><a href="#DigestAlgorithms">DigestAlgorithms</a>
 <li><a href="#DigestCache">DigestCache</a>
 <li><a href="#DigestDefaultAlgorithm">DigestDefaultAlgorithm</a>
 <li><a href="#DigestEnable">DigestEnable</a>
 <li><a href="#DigestEngine">DigestEngine</a>
 <li><a href="#DigestMaxSize">DigestMaxSize</a>
 <li><a href="#DigestOptions">DigestOptions</a>
</ul>

<hr>
<h3><a name="DigestAlgorithms">DigestAlgorithms</a></h3>
Syntax: DigestAlgorithms ["crc32"|"md5"|"sha1"|"sha256"|"sha512"|"all"] 
Default: DigestAlgorithms all 
Context: server config, &lt;VirtualHost&gt;, &lt;Global&gt;, &lt;Anonymous&gt; 
Module: mod_digest 
Compatibility: 1.3.6rc2 or later

The <code>DigestAlgorithms</code> directive configures the enabled digest
algorithms. If no <code>DigestAlgorithms</code> directive is configured, then
all supported digest algorithms are enabled.

Enabled digest algorithms are announced/discovered via the <code>FEAT</code>
response.

The following algorithms are currently supported by <code>mod_digest</code>:
<ul>
 <li><code>crc32</code> (e.g. for the <code>XCRC</code> command)
 <li><code>md5</code> (e.g. for the <code>XMD5</code> command)
 <li><code>sha1</code> (e.g. for the <code>XSHA</code>/<code>XSHA1</code> commands)
 <li><code>sha256</code> (e.g. for the <code>XSHA256</code> command)
 <li><code>sha512</code> (e.g. for the <code>XSHA512</code> command)
</ul>

<hr>
<h3><a name="DigestCache">DigestCache</a></h3>
Syntax: DigestCache on|off|"size" count ["maxAge" secs] 
Default: DigestCache size 10000 maxAge 30s 
Context: server config, &lt;VirtualHost&gt;, &lt;Global&gt;, &lt;Anonymous&gt; 
Module: mod_digest 
Compatibility: 1.3.6rc2 or later

The <code>mod_digest</code> module will cache the results of any checksum
command, on a per-file basis. This improves performance, and reduces
computational overhead. To disable this caching for any reason, use this
directive:
<pre>
 # Disable checksum caching
 DigestCache off
</pre>
This is not recommended.

The <code>DigestCache</code> directive can also be used to configure/tune the
max-size of the in-memory cache. Note that once the maximum cache
size is reached, any checksum FTP commands will be temporarily refused:
<pre>
 # Use a smaller cache size
 DigestCache size 100
</pre>
Cached digests will be expired/ignored after 30 seconds, by default. To change
the expiration, you would use:
<pre>
 # Retain cached entries longer
 DigestCache maxAge 60s
</pre>

If on is used, <code>mod_digest</code> will use the default
max-size of 10000:
<pre>
 DigestCache on
</pre>

<hr>
<h3><a name="DigestDefaultAlgorithm">DigestDefaultAlgorithm</a></h3>
Syntax: DigestDefaultAlgorithm algo 
Default: DigestDefaultAlgorithm sha1 
Context: server config, &lt;VirtualHost&gt;, &lt;Global&gt; 
Module: mod_digest 
Compatibility: 1.3.6rc3 or later

The default digest algorithm that the <code>mod_digest</code> module uses,
for e.g. opportunistic digesting of file transfers, is SHA1. For
selecting a different default algorithm, use the
<code>DigestDefaultAlgorithm</code> directive:
<pre>
 # Use MD5 rather than SHA1 as the default algorithm
 DigestDefaultAlgorithm md5
</pre>

Note that the <code>DigestAlgorithms</code> directive takes precedence;
if the <code>DigestDefaultAlgorithm</code> is not included in the
<code>DigestAlgorithms</code>, the default algorithm setting will be ignored.

<hr>
<h3><a name="DigestEnable">DigestEnable</a></h3>
Syntax: DigestEnable on|off 
Default: Non 
Context: <code>&lt;Directory&gt;</code>, <code>.ftpaccess</code> 
Module: mod_digest 
Compatibility: 1.3.6rc2 or later

The <code>DigestEnable</code> directive can be used to block or prevent
checksumming/digests on files in the configured <code>&lt;Directory&gt;</code>.
This can be very useful for preventing checksumming of files located
on network-mounted filesystems, for example.

<hr>
<h3><a name="DigestEngine">DigestEngine</a></h3>
Syntax: DigestEngine on|off 
Default: DigestEngine on 
Context: server config, &lt;VirtualHost&gt;, &lt;Global&gt;, &lt;Anonymous&gt; 
Module: mod_digest 
Compatibility: 1.3.6rc2 or later

The <code>DigestEngine</code> directive enables or disables the handling of
the checksum-related FTP commands by <code>mod_digest</code>, i.e.:
<ul>
 <li><code>XCRC</code>
 <li><code>XMD5</code>
 <li><code>XSHA</code>
 <li><code>XSHA1</code>
 <li><code>XSHA256</code>
 <li><code>XSHA512</code>
</ul>
If the parameter is off, then these commands will be ignored.

<hr>
<h3><a name="DigestMaxSize">DigestMaxSize</a></h3>
Syntax: DigestMaxSize number [units] 
Default: None 
Context: server config, &lt;VirtualHost&gt;, &lt;Global&gt;, &lt;Anonymous&gt; 
Module: mod_digest 
Compatibility: 1.3.6rc2 or later

The <code>DigestMaxSize</code> directive configures the maximum number of bytes
a single hash command is allowed to read from a file. If the number of bytes
to be read from the file is greater than the configured number the
server will refuse that command.

If no <code>DigestMaxSize</code> directive is configured, then there is no
limit. It is highly recommended to set an upper limit.

Example:
<pre>
 # Limit hashing to 1GB of data
 DigestMaxSize 1 GB
</pre>

<hr>
<h3><a name="DigestOptions">DigestOptions</a></h3>
Syntax: DigestOptions opt1 ... 
Default: None 
Context: server config, <code>&lt;VirtualHost&gt;</code>, <code>&lt;Global&gt;</code> 
Module: mod_digest 
Compatibility: 1.3.6rc2 and later

The <code>DigestOptions</code> directive is used to configure various optional
behavior of <code>mod_digest</code>.

The currently implemented options are:
<ul>
 <li><code>NoTransferCache</code> 
 
 The <code>mod_digest</code> module will automatically calculate and
 cache the results of any transferred file, on a per-file basis. This is
 done assuming that many FTP clients will want to verify the integrity of
 the file just uploaded/downloaded.	This improves performance, and
 reduces computational overhead. To disable this caching for any reason,
 use this option. Not recommended.

Note: The <code>NoTransferCache</code> option is
 automatically enabled when using ProFTPD versions before
 1.3.6rc2, due to bugs/missing support in the older versions.
 </li>
</ul>

<hr>
<h2><a name="Installation">Installation</a></h2>
The <code>mod_digest</code> module is distributed with ProFTPD. Follow the
normal steps for using third-party modules in ProFTPD:
<pre>
 $ ./configure --enable-openssl --with-modules=mod_digest
</pre>
To build <code>mod_digest</code> as a shared/DSO module:
<pre>
 $ ./configure --enable-dso --enable-openssl --with-shared=mod_digest
</pre>
Then follow the usual steps:
<pre>
 $ make
 $ make install
</pre>

Alternatively, if your proftpd was compiled with DSO support, you can
use the <code>prxs</code> tool to build <code>mod_digest</code> as a shared
module:
<pre>
 $ prxs -c -i -d mod_digest.c
</pre>

<hr>
<h2>Usage</h2>
Example Configuration
<pre>
 &lt;IfModule mod_digest.c&gt;
 # Set a limit on file sizes that can be digested
 DigestMaxSize 1 GB
 &lt;/IfModule&gt;
</pre>

Recording Uploaded/Downloaded File Checksums 
One particular use case that comes up is whether the <code>mod_digest</code>
can be used to record the digests ("checksums") of uploaded/downloaded files
in e.g. a SQL database. The answer is "yes", with some caveats.

First, here is a configuration excerpt showing show such functionality might
be implemented, using <code>mod_digest</code> and <code>mod_sql</code>:
<pre>
 &lt;IfModule mod_digest.c&gt;
 &lt;/IfModule&gt;

&lt;IfModule mod_sql.c&gt;
 ...
 SQLNamedQuery log-file-checksum FREEFORM "INSERT INTO file_checksums (user, file, algo, checksum) VALUES ('%u', '%f', '%{note:mod_digest.algo}', '%{note:mod_digest.digest}')"
 SQLLog RETR,STOR log-file-checksum
 ...
 &lt;/IfModule&gt;
</pre>
As you can see, this makes use of the <code>%{note:...}</code> syntax of
the <code>SQLLog</code> directive; the same syntax also works for
<code>LogFormat</code> definitions as well. The <code>mod_digest</code> module
uses the following notes:
<ul>
 <li>mod_digest.algo
 
 Name of the digest algorithm used, e.g. "SHA1".
 </li>

<li>mod_digest.digest
 
 Calculated digest of the file as a hex-encoded lowercase string.
 </li>
</ul>

Now, the caveats with this technique:
<ul>
 <li>Does not work if the <code>NoTransferCache</code> <a href="#DigestOption">DigestOption</a> is used.
 <li>Only works for binary, not ASCII, FTP uploads/downloads currently.
 <li>Only works for uploads (<code>STOR</code>) and downloads (<code>RETR</code>), but not for appends (<code>APPE</code>) or resumed uploads/downloads (<code>REST</code> + <code>RETR/STOR</code>).
 <li>Does not work for FTP downloads if <code>UseSendfile</code> is in effect.
</ul>
In addition, the order in which the <code>mod_digest</code> and
<code>mod_sql</code> appear in your build command is important;
<code>mod_digest</code> must come after <code>mod_sql</code>,
otherwise the note values will not be populated properly in the
<code>SQLLog</code> statement. Thus, if you are building static modules,
your <code>--with-modules</code> parameter would look something like:
<pre>
 $ ./configure --with-modules=mod_sql:mod_sql_mysql:mod_digest ...
</pre>
Or, if you are using shared modules, then your <code>LoadModule</code>
directives must look like:
<pre>
 LoadModule mod_sql.c
 LoadModule mod_sql_mysql.c
 LoadModule mod_digest.c
</pre>

<!--
Why?

TCP-level checksums
packet-level checksums
_file_-level checksums (which is really what most people usually have in mind)

transfers interrupted by timeouts

SFTP has different ways of achieving this, via extensions (link to mod_sftp
docs on extensions)

validating uploads AND downloads (did I download everything?  Did the upload
succeed?)

It's also recommended to disable all features within the &lt;Anonymous&gt; context. How?

<Anonymous>
 <IfModule mod_digest.c>
 DigestEngine off
 </IfModule>
 </Anonymous>

Supported FTP Commands 
 cmd path
 cmd path [end]
 cmd path [off] [len]
<pre>
 XCRC "/path/to/file with spaces" 0 100
</pre>
-->

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