# Apache

Apache (opens new window) is an established web server that provides a variety of related tasks to serving pages for a web site including SSL termination, request rate-limiting, proxying, forwarding, document root resolving, and redirection. ApisCP uses Event MPM, a threaded, event-driven processing module that reduces overhead and puts performance on par with NGINX in most workloads. NGINX is slightly faster by 1-2 ms from separate testing, but represents less than 1% of typical processing overhead.

# Configuration

A variety of locations exist under /etc/httpd. Some locations may be freely altered. The table summarizes locations within /etc/httpd, whether they may be modified, and intention.


After making changes run htrebuild to rebuild and test configuration. Apache will restart once the configuration is validated.

Modifiable? File Purpose
No conf/httpd.conf Standardized system configuration.
Yes conf.d/ Addin modules. See separate table below for additional information.
Yes conf/httpd-custom.conf User overrides. See special remarks below for specific stanzas set by Bootstrapper.
No conf/apnscp-httpd-rewrite.conf Inheritable base rules applied to webmail redirections, addon domains, and subdomains.
Maybe siteXX/ Per-module configurations. If a module name exists now, do not touch. If a file name does not exist now, it may exist in the future.
Yes siteXX/custom A reserved location for per-site overrides. This will never be overwritten.
Yes siteXX/custom.raw A reserved location for per-site overrides. This will never be overwritten. Does not substitute fst/ with shadow/.
Maybe siteXX.ssl/ Similar to siteXX/. Only applies when site is served over SSL.
Yes siteXX.ssl/custom Similar to siteXX/custom. Only applies when site is served over SSL.
No conf/virtual-* Monolithic files compiled down by htrebuild command. Used by Apache to improve recovery time.
No conf/virtual-*.bad If a failed rebuild occurs, the offending file is saved to this location.
No conf.d/mod_security.conf mod_security configuration distributed as part of ApisCP distribution.

Shadow-layer substitutions

ApisCP will replace all occurrences of fst/ with the direct data layer shadow/ when including drop-in configuration under siteXX during htrebuild. Suffix a file with .raw to prevent this substitution if for example matching a path verbatim as with <Directory /home/virtual/siteXX/fst/var/www/html>.

# Scopes

Scope provide a generalized interface to manage system internals. Apache ships with several Scopes to allow adjusting Apache configuration without getting lost in internals.

# List all Apache Scopes
cpcmd scope:list 'apache:*'

We'll cover a few below.

# Tolerating invalid configuration

Invalid configuration may occur when transitioning between Apache platforms with different modules. Default behavior is to fail, returning a 550 Internal Server Error. This behavior can be changed using apache.strict-mode.


Strict mode does not change behavior of invalid directives inside /etc/httpd, only in .htaccess files. An invalid directive in httpd.conf or siteXX/ will cause any reload requests to fail as well as prevent Apache from starting up.

# Allow invalid directives to exist in .htaccess
cpcmd scope:set apache.strict-mode false

# Changing ports

A Scope is provided to facilitate changing the ports SSL and non-SSL communication reside on. A non-SSL port must always be available, but SSL may be turned off for example if haproxy or Varnish were to sit in front of Apache to terminate SSL requests.

# Move HTTP to port 81, disable SSL
cpcmd scope:set apache.ports '[81,false]'
# Update configuration on all accounts
EditDomain --reconfig --all

# Setting upstream cache

Apache's integrated cache can be activated on the fly using apache.cache.

# Enable memory-backed caching in Apache
cpcmd scope:set apache.cache memory
# Require sites to opt-in with "UnsetEnv no-cache"
cpcmd scope:set apache.cachetype explicit

# Site templates

Each site created generates its HTTP configuration from the template service path, resources/templates/apache/. Templates may be overrode by creating the corresponding structure in config/custom (see Customization.md).

Run EditDomain --rebuild --all after making changes to update configuration for all sites.

# Reload delays

Apache configuration is delayed by 2 minutes to allow for successive calls to *Domain helpers to merge into a single call. Doing so reduces the risk of a self-induced denial of service attack when performing a bulk edit. Behavior may be adjusted via [httpd] => reload_delay Tuneable.

cpcmd scope:set cp.config httpd reload_delay now disables this elision window.


Reloads collapse all configuration into a monolithic file to drastically improve responsiveness. Instead of reading each modular configuration, a monolithic configuration, /etc/httpd/conf/virtual-httpd-built, is read whenever the web server must reload or restart.

# Threat deterrence

Several features are provided to discourage abusive bots. All behaviors may be toggled either globally on an account-by-account basis.

# HTTP/1.0

Bots are sloppy and do not implement appropriate protocol standards. One such standard is HTTP/1.1 usage. In its absence, HTTP/1.0 is assumed, which allows very poor protocol implementations to return a response. For example,

nc google.com 80

No browser sends a protocol-less request, especially those behind SSL. A typical, appropriate request would be,

nc google.com 80
GET / HTTP/1.1
Host: google.com

Denying HTTP/1.0 requests provided added protection against low-hanging fruit.

# Disabling HTTP/1.0 blocks

Blocks may be disabled per hostname in the panel.

  1. Visit Web > Web Apps
  2. Select the hostname
  3. Under Options > Disable HTTP/1.0 Blocks

Alternatively HTTP/1.0 blocks may be removed server-wide via cpcmd scope:set apache.block10 false.

# Request throttling

mod_evasive monitors requests per IP address over a window. Requests beyond the threshold emit a log entry to Rampart, which bans the IP address. Specifics, including customizing, is discussed in Evasive.md.

# Slowloris mitigation

Slowloris is of a family of low bandwidth starvation attacks that slowly send a HTTP request over an extended duration, For example, instead of sending "GET / HTTP/1.1" at once, a Slowloris attack sends G followed by a 5 second wait, then E followed by another 5 second wait... These attacks are highly effective when not mitigated.

mod_reqtimeout (opens new window) is enabled to mandate bandwidth minimums. Requests that do not complete within the window are closed thus freeing up a connection slot.

Limits may be overriden on a per-site basis by setting RequestReadTimeout in /etc/httpd/conf/siteXX/ then running htrebuild.

# Resource monopolization

cgroups are an optional service to enforce resource limits. Limits may be imposed by setting cgroup,enabled=1. CPU, memory, PID, and IO limits are tracked. Resource limits provide a secondary effect by stymying DDoS attacks that may fall under the threshold of mod_evasive. For example, consider a distributed attack of over 10,000 machines each sending 2 requests per second - 20,000 requests per second will quickly swamp a server without necessarily triggering Evasive limits.

Resource enforcement, when applied, prevent a site from creating excessive PHP-FPM workers to cope with swells or monopolizing CPU time slices thus making the server more egalitarian and sometimes too, allowing other reactive protection to act.

# Security changes

Symbolic links redirect one file resource to another. Three behaviors exist for following redirections:

  • Do not follow
  • Follow unconditionally when FollowSymLinks is present
  • Follow conditionally if the link and referent have same ownership

Symlink traversal is necessary for mod_rewrite (opens new window) to operate, a cornerstone of site redirections and pretty-print URLs. Disabling symlink traversal would create catastrophic consequences for many Web Apps, WordPress included, by disabling mod_rewrite usage. Symlinks are always followed, but an extra check is performed unconditionally to ensure the link and referent have the same owner.


Internally, FollowSymLinks is aliased to SymLinksIfOwnerMatch through a patch (opens new window) to Apache's engine. This results in one additional stat() syscall that is cached by the operating system with negligible overhead.

# Removal mod_includes

Server-side includes were a feature in the early web to allow static HTML pages to include other static resources in a crude form of templating. Languages like PHP, Node, Python, Ruby can template much more efficiently and thus there is little need for server-side includes ("SSI") in mod_includes. mod_includes permits a very ugly deflection attack on protected assets if the attacker knows the file name (for example, "wp-config.php"). This attack is discussed in detail in SECURITY.md.

In general there is little benefit from enabling mod_includes other than supporting old, insecure web sites.

# App compatibility

# Non-standard index pages

An abbreviated list of index names are checked when a location is accessed without specifying a filename. For example,

Accessing https://example.com queries all DirectoryIndex files within the document root for example.com returning the first found file in the set index.html, index.php, index.cgi. If other index names are desired, such as index.htm, add to httpd-custom.conf:

DirectoryIndex index.htm - this will append to the list of directory indexes. Before returning a 403 Forbidden error (or listing directory contents), all filenames are tried.

# Default index page order

Setting DirectoryIndex index.htm will set the order in a .htaccess file but append to the list if added to httpd-custom.conf due to an unknown context rule in DirectoryIndex parsing (opens new window):

Note: Multiple DirectoryIndex directives within the same context will add to the list of resources to look for rather than replace:

If the default order of index.html index.php index.cgi is to be changed, perhaps placing index.php in front (this order is determined for historical reasons), then the index list must first be disabled, then set in httpd-custom.conf:

DirectoryIndex disabled
DirectoryIndex index.php index.html index.php4 index.php3 default.html

Run htrebuild after making changes.

# Disabling index negotiation

DirectoryIndex disabled disables index negotiation for a directory. Disabling negotiation is necessary when passing all content to a backend proxy, such as with Passenger.

# Proxying content

Apache's ProxyPass directive sends requests to a standalone service, e.g. a PowerDNS API service or even Node web service. Placing a service behind a proxy protects it from direct, outside access through firewalld and confers benefits of typical website protection in ApisCP: end-to-end SSL encryption, brute-force protection by mod_evasive (opens new window), request logging, and connection/resource limiting.

Two alternatives exist for proxying content, either as a standalone site or a location in an existing site.

# Standalone proxy

A standalone setup maps all requests for a domain to the proxy service. For example, say a service, Jenkins (opens new window), is running on port 8080 and we'd like to place it on a subdomain called "pipeline.mydomain.com".

Create a new account named "pipeline.mydomain.com". If mydomain.com also exists on the server, relocate "pipeline.mydomain.com" it to a higher priority (see Troubleshooting => "Stacking domains" section below). From there, add the following to siteXX/custom in /etc/httpd/conf.



Trailing "/" is significant when setting this configuration.

Run htrebuild and now all requests for pipeline.mydomain.com will be sent to

# Location-based proxy

Taking the above example, what if we want to make the Jenkins service available on mydomain.com/ci? A <Location> block can accomplish this.

In siteXX/custom, the setup is the similar as with Standalone proxy above:

<Location /ci>

# Technical details

Apache is a custom build available through apisnetworks/httpd-apache (opens new window). Nonportable atomics are enabled as well as mod_systemd backported from Apache 2.5 development to facilitate lightweight service reports. Latest APR and APR Utility releases are bundled to maximize efficiency. Compilation targets x86-64 machines using default compile flags.

Overrides are disabled below typical document root locations, siteXX/fst/home/user and siteXX/fst/var/www to reduce the number of recursive htaccess checks. Apache walks the filesystem when SymLinksIfOwnerMatch is enabled, which requires Apache to walk the filesystem to ensure all components are owned by the same user to defeat symlink deflection attacks.

[pid 19214] lstat("/home", {st_mode=S_IFDIR|0755, st_size=48, ...}) = 0
[pid 19214] lstat("/home/virtual", {st_mode=S_IFDIR|0711, st_size=12288, ...}) = 0
[pid 19214] lstat("/home/virtual/site32", {st_mode=S_IFDIR|0711, st_size=43, ...}) = 0
[pid 19214] lstat("/home/virtual/site32/fst", {st_mode=S_IFDIR|0751, st_size=95, ...}) = 0
[pid 19214] lstat("/home/virtual/site32/fst/var", {st_mode=S_IFDIR|S_ISGID|S_ISVTX|0777, st_size=104, ...}) = 0
[pid 19214] lstat("/home/virtual/site32/fst/var/www", {st_mode=S_IFDIR|0755, st_size=270, ...}) = 0
[pid 19214] open("/home/virtual/site32/fst/var/www/.htaccess", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
[pid 19214] lstat("/home/virtual/site32/fst/var/www/domain.com", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
[pid 19214] open("/home/virtual/site32/fst/var/www/domain.com/.htaccess", O_RDONLY|O_CLOEXEC) = 421

Failure to enable SymLinksIfOwnerMatch opens a system to attack. Details of the attacks are covered in SECURITY.md. This behavior is mandatory, but ameliorated greatly by SSD and NVMe-backed storage.

A posixsem mutex is used to serialize accept() requests. This has been determined to result in the fewest zombie/deadlock processes in the event of a service disruption.

Log buffering is enabled to reduce the amount of I/O requests. Buffered logs (opens new window) will enqueue full records up to the in-memory limit, 4 KB. If a record exceeds the buffer length, the buffer is flushed prior to recording the new entry. Buffering may be disabled using the apache.buffered-logs Scope.

# Tuning

In Event MPM, a single child process can handle multiple requests concurrently. By default, a very conservative number of threads (20) is created per process. At most a child process can handle 20 concurrent connections. Active requests beyond this threshold spawn a new child process.

Summarizing parameter relationship

The above scenario would be represented in the following configuration.

MaxRequestWorkers 12
ServerLimit 2
ThreadsPerChild 6

Threads may be tuned to decrease the number of processes spawned without changing the maximum number of concurrent connections (MaxRequestWorkers).

ThreadLimit 128
MaxRequestsPerChild 10240
ThreadsPerChild 100
ThreadStackSize 2097152

Tuning threads will reduce memory overhead a variable amount, but a good rule of thumb is each "VirtualHost" container is 256 KB. A site that has an SSL and non-SSL version thus would require 512 KB to operate in each Apache process. 5 Apache processes at 20 ThreadsPerChild with a 100 connection ServerLimit.

Increasing ThreadsPerChild, which decreases ServerLimit decreases the memory burden, but at the potential risk of segmentation faults.

Check /var/log/httpd/error_log to assess stability.

# Check both error_log and error_log.1
grep 'Segmentation' /var/log/httpd/error_log{,.1}
# 1 segmentation fault - instability!
[core:notice] [pid 11468:tid 140192329971584] AH00052: child pid 18271 exit signal Segmentation fault (11)

# PHP tuning

See PHP-FPM.md.

# Troubleshooting

# Stacking domains and subdomains as separate accounts

Given domain.com and sub.domain.com as 2 separate accounts with domain.com assigned site10 and sub.domain.com assigned site12, when configuration is compiled into a single file, /etc/httpd/conf/virtual-httpd-built, the domain/subdomain configuration for domain.com will take precedence as it is lexicographically lower.

Configuration for site12 will not show as the ServerAlias (opens new window) setting, *.domain.com matches first. A quick fix is to relocate site12 configuration to higher priority by renaming such that it is ordered before site10.

cd /etc/httpd/conf/virtual
mv site12 priority-site12
ln -s priority-site12 site12

site12 is renamed to priority-site12 thus ensuring it comes site10. A symbolic link is created to site12 to allow future panel edits to update the configuration now in priority-site12. htrebuild will skip any symbolic links in /etc/httpd/conf/virtual-built as well. Deletion will delete both the regular file and symbolic link if encountered.

# AH03490: scoreboard is full, not at MaxRequestWorkers.Increase ServerLimit.

Apache uses a mutex to serialize requests when polling with select() (there are intractable scalability problems with select() vs epoll()/kqueue() beyond the scope of this document). posixsem is used by default, which has the highest throughput, ~15% more than the next best option: pthread. Likewise pthread is approximately 12% faster than sysvsem. Depending upon architecture, posixsem may cause spurious lockups evidenced in /var/log/httpd/error_log. If this is the case, then change the mutex model to pthread or sysvsem by using the apache.mutex Scope:

cpcmd scope:set apache.mutex pthread

Apache will automatically reload after 2 minutes.

# BDB0004 fop_read_meta: /etc/httpd/conf/http10: unexpected file type of format

Map files are stored as sdbm, an open implementation of ndbm (opens new window). When no HTTP/1.0 bypasses are configured and a client accesses the web server using HTTP/1.0 protcol - almost always a bot - Apache logs this informative message in the error log.

To bypass this message, add at least 1 hostname to the map file. This can be accomplished from the command-line helper as cpcmd -d domain.com web:allow-protocol domain.com http10. Otherwise the message is purely diagnostic and has no effect on service.

# See also

For supporting documentation, see also