Provided by: merecat_2.31+git20220513+ds-4_amd64 
NAME
merecat — Simple, small and fast HTTP server
SYNOPSIS
merecat [-ghnrsSvV] [-c CGI] [-d PATH] [-f FILE] [-I IDENT] [-l LEVEL] [-p PORT] [-P PIDFN]
[-t FILE] [-u USER] [WEBDIR] [HOSTNAME]
DESCRIPTION
merecat started out as a pun at Mongoose, which is another great web server, but is now
useful for actual web serving purposes. It is however not a real Meerkat, merely yet
another copycat, forked from the great thttpd created by Jef Poskanzer.
The limited feature set makes Merecat very quick:
- Virtual hosts
- URL-traffic-based throttling
- CGI/1.1
- HTTP/1.1 Keep-alive
- Built-in gzip deflate using zlib
- HTTPS support using OpenSSL/LibreSSL
- Dual server support, both HTTP/HTTPS from one process
- HTTP redirect support, per server. E.g., possible to redirect from HTTP to HTTPS.
Limited set of Nginx style environment variables supported.
- Native PHP support, using php-cgi, if enabled in merecat.conf(5)
The resulting footprint (~140 kiB) makes Merecat suitable for small and embedded systems.
OPTIONS
This program follows the usual UNIX command line syntax. Some options are, however, not
available when merecat is built with support for /etc/merecat.conf. The distributed archive
comes with an example configuration file, which should be fairly straightforward to
comprehend. For details on the available configuration directives, see merecat.conf(5).
The options, in their entirety, are as follows:
-c CGI Wildcard pattern for CGI programs. The config file setting for this flag is
cgi-pattern = PATTERN. The default is "**.cgi|/cgi-bin/*".
For more details, see below.
-d PATH
Directory to chdir() to after chrooting. If you are not chrooting use the WEBDIR to
do a single chdir(). If you are chrooting, this lets you put the web files in a
subdirectory of the chroot tree, instead of in the top level mixed in with the
chroot files. The config file setting for this flag is data-directory = DIR.
-f FILE
The config file to read. By default merecat looks for /etc/merecat.conf, unless the
software has been configured to use a different prefix.
If the default system coonfiguration file is missing, or if merecat is started with
-f none, merecat will run in stand-alone mode using only command line options like
listening port and server root directory.
-g Use global .htpasswd and .htaccess files. This means that every file in the entire
document tree is protected by a single .htpasswd or .htaccess file at the top of the
tree. Otherwise the semantics of the .htpasswd and .htaccess files are the same.
If this option is set but there is no .htpasswd or .htaccess files in the top-level
directory, then merecat proceeds as if the option was not set — first looking for
local .htpasswd and .htaccess files, and if they do not exist either then serving
the requested file without any password or access restriction.
The config file setting for this flag is global-passwd = <true | false>.
-h Show summary of command line options and exit.
-I IDENT
The syslog(3) identity to use for all log messages. Useful when running multiple
servers. Defaults to use the program name, i.e. "merecat".
-l LEVEL
Set log level: none, err, info, notice, debug
-n Runs merecat in the foreground like a regular program. Required when when running
in a process monitor like Finit or systemd. This also enables logging of errors and
warnings to stderr, which can be disabled with -s.
-p PORT
Alternate TCP port number to listen on. The default is 80. The config file setting
for this flag is port = PORT.
-P PIDFN
Optional PID file name. By default the IDENT option, or its default value, is used
to construct the PID file name. Usually this results in /var/run/merecat.pid. If
the argument to this option is an absolute path it will be used as-is. Otherwise
the argument will be used as the basename for the PID file.
-r Do a chroot() at initialization time, restricting file access to the program's
current directory. The config file setting for this flag is chroot = <true |
false>.
-s Use syslog, even though running in foreground, -n. merecat uses syslog by default,
this option is only relevant when running in the foreground to prevent warning and
error messages to be printed to stderr.
-S Do explicit symbolic link checking. Normally, merecat does not expand any symbolic
links in filenames. For increased security this option can be enabled to check that
the resulting path stays within the original document tree. Note, that if you are
using the chroot option, the symlink checking is unnecessary and is turned off, so
the safe way to save those CPU cycles is to use chroot. The config file setting for
this is check-symlinks = <true | false>.
-t FILE
Enable throttling using this file with throttle settings. See below for details.
-u USERNAME
User to drop privileges to to after initialization when started as root. The
default is nobody, on some systems www-data is preferred. The config file setting
for this flag is username = USER.
-v Do el-cheapo virtual hosting. The config file setting for this flag is virtual-host
= <true | false>.
-V Shows the current version info.
WEBDIR This optional argument is provided as a convenience — by default merecat serves
files from the current directory. The config file setting for this is directory =
DIR.
HOSTNAME
A second optional command line argument can be given to specify the hostname to bind
to, for multihoming. The default is to bind to all hostnames supported on the local
machine. See below for details. The config file setting for this flag is hostname
= HOSTNAME.
CHROOT
chroot() is a system call that restricts the program's view of the filesystem to the current
directory and directories below it. It becomes impossible for remote users to access any
file outside of the initial directory. The restriction is inherited by child processes, so
CGI programs get it too. This is a very strong security measure, and is recommended. The
only downside is that only root can call chroot(), so this means the program must be started
as root. However, the last thing it does during initialization is to give up root access by
becoming another user, so this is safe.
The program can also be compile-time configured to always do a chroot(), without needing the
-r flag.
Note that with some other web servers, such as NCSA httpd, setting up a directory tree for
use with chroot() is complicated, involving creating a bunch of special directories and
copying in various files. With merecat it's a lot easier, all you have to do is make sure
any shells, utilities, and config files used by your CGI programs and scripts are available.
If you have CGI disabled, or if you make a policy that all CGI programs must be written in a
compiled language such as C and statically linked, then you probably don't have to do any
setup at all.
However, one thing you should do is tell syslogd about the chroot tree, so that merecat can
still generate syslog messages. Check your system's syslogd man page for how to do this.
In FreeBSD you would put something like this in /etc/rc.conf:
syslogd_flags="-l /usr/local/www/data/dev/log"
Substitute in your own chroot tree's pathname, of course. Don't worry about creating the
log socket, syslogd wants to do that itself. (You may need to create the dev directory.)
In Linux the flag is -a instead of -l, and there may be other differences.
CGI
Merecat httpd supports the CGI 1.1 spec., https://tools.ietf.org/html/rfc3875.
In order for a CGI program to be allowed to run, its name must match the pattern specified
either at compile time, on the command line, or in the config file. This is a simple shell-
style filename pattern. Use * to match any string not including a slash, or ** to match any
string including slashes, or ? to match any single character. Multiple patterns separated
by | can also be used. The patterns get checked against the filename part of the incoming
URL. Remember to quote any wildcard characters so that the shell doesn't mess with them.
Restricting CGI programs to a single directory lets the site admin review them for security
holes, and is strongly recommended. If there are individual users that you trust, you can
enable their directories too using the pipe syntax, e.g. "|/jef/**".
To disable CGI as a security measure, either disable the default CGI_PATTERN in merecat.h,
or set the configuration file option to the empty string, like this: cgi-pattern = ""
Note: the current working directory when a CGI program gets run is the directory that the
CGI program lives in. This isn't in the CGI 1.1 spec, but it's what most other HTTP servers
do.
Relevant merecat.h defines: CGI_PATTERN, CGI_TIMELIMIT, CGI_NICE, CGI_PATH,
CGI_LD_LIBRARY_PATH, CGIBINDIR.
BASIC AUTHENTICATION
Basic authentication is available as an option at compile time. See the included configure
script for details. When enabled, it uses a password file in the directory to be protected,
called .htpasswd by default. This file is formatted as the familiar colon-separated
username/encrypted-password pair, records delimited by newlines. The utility program
htpasswd(1) is included to help create and modify .htpasswd files.
merecat can use a global .htpasswd file if started with the -g switch, or you can rely on a
per directory file which also protects subdirectories.
Relevant merecat.h define: AUTH_FILE
ACCESS RESTRICTION
Access restriction is available as an option at compile time. If enabled, it uses an access
file in the directory to be protected, called .htaccess by default. This file consists of a
rule and a host address or a network range per line. Valid rules are:
allow from
The following host address or network range is allowed to access the requested
directory and its files.
deny from
The following host address or network range is not allowed to access the requested
directory and its files.
There are three ways to specify a valid host address or network range:
IPv4 host address,
e.g. 10.2.3.4
IPv4 network with subnet mask,
e.g. 10.0.0.0/255.255.0.0
IPv4 network using CIDR notation,
e.g. 10.0.0.0/16
merecat can use a global .htaccess file if started with the -g switch, or you can rely on a
per directory file which also protects subdirectories.
Note that rules are processed in the same order as they are listed in the access file and
that the first rule which matches the client's address is applied (there is no order
clause).
So if there is no allow from 0.0.0.0/0 at the end of the file the default action is to deny
access.
Relevant merecat.h define: ACCESS_FILE
THROTTLING
The throttle file lets you set maximum byte rates on URLs or URL groups. You can optionally
set a minimum rate too. The format of the throttle file is very simple. A # starts a
comment, and the rest of the line is ignored. Blank lines are ignored. The rest of the
lines should consist of a pattern, whitespace, and a number. The pattern is a simple shell-
style filename pattern, using ?/**/*, or multiple such patterns separated by |.
The numbers in the file are byte rates, specified in units of bytes per second. For
comparison, a v.90 modem gives about 5000 B/s depending on compression, a double-B-channel
ISDN line about 12800 B/s, and a T1 line is about 150000 B/s. If you want to set a minimum
rate as well, use number-number.
Example:
# throttle file for www.acme.com
** 2000-100000 # limit total web usage to 2/3 of our T1,
# but never go below 2000 B/s
**.jpg|**.gif 50000 # limit images to 1/3 of our T1
**.mpg 20000 # and movies to even less
jef/** 20000 # jef's pages are too popular
Throttling is implemented by checking each incoming URL filename against all of the patterns
in the throttle file. The server accumulates statistics on how much bandwidth each pattern
has accounted for recently (via a rolling average). If a URL matches a pattern that has
been exceeding its specified limit, then the data returned is actually slowed down, with
pauses between each block. If that's not possible (e.g. for CGI programs) or if the
bandwidth has gotten way larger than the limit, then the server returns a special code
saying "try again later".
The minimum rates are implemented similarly. If too many people are trying to fetch
something at the same time, throttling may slow down each connection so much that it's not
really useable. Furthermore, all those slow connections clog up the server, using up file
handles and connection slots. Setting a minimum rate says that past a certain point you
should not even bother — the server returns the "try again later" code and the connection is
not even started.
There is no provision for setting a maximum connections/second throttle, because throttling
a request uses as much CPU as handling it, so there would be no point. There is also no
provision for throttling the number of simultaneous connections on a per-URL basis. However
you can control the overall number of connections for the whole server very simply, by
setting the operating system's per-process file descriptor limit before starting merecat.
Be sure to set the hard limit, not the soft limit.
MULTIHOMING
Multihoming means using one machine to serve multiple hostnames. For instance, if you're an
internet provider and you want to let all of your customers have customized web addresses,
you might have www.joe.acme.com, www.jane.acme.com, and your own www.acme.com, all running
on the same physical hardware. This feature is also known as virtual hosts. There are
three steps to setting this up.
One, make DNS entries for all of the hostnames. The current way to do this, allowed by
HTTP/1.1, is to use CNAME aliases, like so:
www.acme.com IN A 192.100.66.1
www.joe.acme.com IN CNAME www.acme.com
www.jane.acme.com IN CNAME www.acme.com
However, this is incompatible with older HTTP/1.0 browsers. If you want to stay compatible,
there is a different way - use A records instead, each with a different IP address, like so:
www.acme.com IN A 192.100.66.1
www.joe.acme.com IN A 192.100.66.200
www.jane.acme.com IN A 192.100.66.201
This is bad because it uses extra IP addresses, a somewhat scarce resource. But if you want
people with older browsers to be able to visit your sites, you still have to do it this way.
Step two. If you're using the modern CNAME method of multihoming, then you can skip this
step. Otherwise, using the older multiple-IP-address method you must set up IP aliases or
multiple interfaces for the extra addresses. You can use ifconfig(8)'s alias command to
tell the machine to answer to all of the different IP addresses. Example:
ifconfig le0 www.acme.com
ifconfig le0 www.joe.acme.com alias
ifconfig le0 www.jane.acme.com alias
If your OS's version of ifconfig doesn't have an alias command, you're probably out of luck
(but see http://www.acme.com/software/thttpd/notes.html for more info).
Third and last, you must set up merecat to handle the multiple hosts. The easiest way is
with the -v flag. This works with either CNAME multihosting or multiple-IP multihosting.
What it does is send each incoming request to a subdirectory based on the hostname it's
intended for. All you have to do in order to set things up is to create those
subdirectories in the directory where merecat will run. With the example above, you'd do
like so:
mkdir www.acme.com www.joe.acme.com www.jane.acme.com
If you're using old-style multiple-IP multihosting, you should also create symbolic links
from the numeric addresses to the names, like so:
ln -s www.acme.com 192.100.66.1
ln -s www.joe.acme.com 192.100.66.200
ln -s www.jane.acme.com 192.100.66.201
This lets the older HTTP/1.0 browsers find the right subdirectory.
There is an optional alternate step three if you're using multiple-IP multihosting: run a
separate merecat process for each hostname This gives you more flexibility, since you can
run each of these processes in separate directories, with different throttle files, etc.
Example:
merecat -r /usr/www www.acme.com
merecat -r -u joe /usr/www/joe www.joe.acme.com
merecat -r -u jane /usr/www/jane www.jane.acme.com
Remember, this multiple-process method does not work with CNAME multihosting — for that, you
must use a single merecat process with the -v flag.
CUSTOM ERRORS
merecat lets you define your own custom error pages for the various HTTP errors. There is a
separate file for each error number, all stored in one special directory. The directory
name is errors/, at the top of the web directory tree. The error files should be named
errNNN.html, where NNN is the error number. So for example, to make a custom error page for
the authentication failure error, which is number 401, you would put your HTML into the file
errors/err401.html. If no custom error file is found for a given error number, then the
usual built-in error page is generated.
In a virtual hosts setup you can also have different custom error pages for each host. In
this case you put another errors/ directory in the top of that virtual host's web tree.
merecat will look first in the virtual host errors directory, and then in the server-wide
errors directory, and if neither of those has an appropriate error file then it will
generate the built-in error.
NON-LOCAL REFERERS
Sometimes another site on the net will embed your image files in their HTML files, which
basically means they're stealing your bandwidth. You can prevent them from doing this by
using non-local referer filtering. With this option, certain files can only be fetched via
a local referer. The files have to be referenced by a local web page. If a web page on
some other site references the files, that fetch will be blocked. There are three config
file variables for this feature:
url-pattern = "**.jpg|**.gif|**.au|**.wav"
A wildcard pattern for the URLs that should require a local referer. This is
typically just image files, sound files, and so on. For example:
urlpat = "**.jpg|**.gif|**.au|**.wav"
For most sites, that one setting is all you need to enable referer filtering.
check-referer = <true | false>
By default, requests with no referer at all, or a null referer, or a referer with no
apparent hostname, are allowed. With this variable set, such requests are
disallowed.
local-pattern = "PATTERN"
A wildcard pattern that specifies the local host or hosts. This is used to
determine if the host in the referer is local or not. If not specified it defaults
to the actual local hostname.
SYMLINKS
merecat is very picky about symbolic links. Before delivering any file, it first checks
each element in the path to see if it is a symbolic link, and expands them all out to get
the final actual filename.
Along the way it checks for things like links with ".." that go above the server's
directory, and absolute symlinks (ones that start with a /). These are prohibited as
security holes, so the server returns an error page for them.
This means you cannot set up your web directory with a bunch of symlinks pointing to
individual users' home web directories. Instead you do it the other way around — the user
web directories are real subdirectories of the main web directory, and in each user's home
directory there is a symlink pointing to their actual web directory.
The CGI pattern is also affected — it gets matched against the fully-expanded filename. So,
if you have a single CGI directory but then put a symbolic link in it pointing somewhere
else, that will not work. The CGI program will be treated as a regular file and returned to
the client, instead of getting run. This could be confusing.
PERMISSIONS
merecat is also picky about file permissions. It wants data files (HTML, images) to be
world readable. Readable by the group that the merecat process runs as is not enough —
merecat checks explicitly for the world-readable bit. This is so that no one ever gets
surprised by a file that's not set world-readable and yet somehow is readable by the HTTP
server and therefore the *whole* world.
The same logic applies to directories. As with the standard UNIX ls program, merecat will
only let you look at the contents of a directory if its read bit is on; but as with data
files, this must be the world-read bit, not just the group-read bit.
merecat also wants the execute bit to be *off* for data files. A file that is marked
executable but doesn't match the CGI pattern might be a script or program that got
accidentally left in the wrong directory. Allowing people to fetch the contents of the file
might be a security breach, so this is prohibited. Of course if an executable file *does*
match the CGI pattern, then it just gets run as a CGI.
In summary, data files should be mode 644 (rw-r--r--), directories should be 755 (rwxr-xr-x)
if you want to allow indexing and 711 (rwx--x--x) to disallow it, and CGI programs should be
mode 755 (rwxr-xr-x) or 711 (rwx--x--x).
LOGS
merecat does all of its logging via syslog(3). All log messages are prepended with the
program name, unless the command line option -I IDENT is used. The facility defaults to
LOG_DAEMON. Aside from error messages, there are only a few log entry types of interest,
all fairly similar to CERN Common Log Format:
Aug 6 15:40:34 acme merecat[583]: 165.113.207.103 - - "GET /file" 200 357
Aug 6 15:40:43 acme merecat[583]: 165.113.207.103 - - "HEAD /file" 200 0
Aug 6 15:41:16 acme merecat[583]: referer http://www.acme.com/ -> /dir
Aug 6 15:41:16 acme merecat[583]: user-agent Mozilla/1.1N
Note that merecat does not translate numeric IP addresses into domain names. This is both
to save time and as a minor security measure (the numeric address is harder to spoof).
If started in the foreground, -n, and with debug log level, -l debug, logs will also be
printed on stderr, unless the user also requested -s. However, not all systems support the
LOG_PERROR option to openlog().
Relevant merecat.h define: LOG_FACILITY.
SIGNALS
merecat handles a couple of signals, which you can send via the standard UNIX kill(1)
command:
INT,TERM These signals tell merecat to shut down immediately.
USR1 This signal tells merecat to toggle log level, between current log level and
LOG_DEBUG. If merecat was started with LOG_DEBUG the toggle will be to
LOG_NOTICE, which is the default log level.
USR2 This signal tells merecat to generate the statistics syslog messages immediately,
instead of waiting for the regular hourly update.
SEE ALSO
merecat.conf(5), ssi(8), htpasswd(1)
AUTHORS
Jef Poskanzer <jef@mail.acme.com> wrote the famous thttpd which merecat is based on.
Joachim Wiberg <troglobit@gmail.com> introduced all new shiny bugs.
THANKS
merecat is a fork of sthttpd, which in turn is a fork of thttpd. So first and foremost, a
huge thanks to Jef Poskanzer for creating thttpd and making it open source under the
simplified 2-clause BSD license! Anthony G. Basile deserves another thank you, for merging
Gentoo patches and refactoring the build system in sthttpd.
Also, many thanks to contributors, reviewers, testers: John LoVerso, Jordan Hayes, Chris
Torek, Jim Thompson, Barton Schaffer, Geoff Adams, Dan Kegel, John Hascall, Bennett Todd,
KIKUCHI Takahiro, Catalin Ionescu, Anders Bornäs, and Martin Olsson. Special thanks to
Craig Leres for substantial debugging and development during the early days of thttpd.