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	== Architecture and Internal Design ==		== Architecture and Internal Design ==

Admin: Created page with "Category:Wiki == Architecture and Internal Design == APT-Cacher-NG (ACNG) acts as a specialized HTTP proxy optimized for package managers such as APT. Unlike generic proxies, it understands repository layouts and package metadata patterns. === Request Flow === # Client sends HTTP request for a package or index file # ACNG checks its local cache directory # If present and valid, the cached object is served # If missing or expired, ACNG fetches it from the upstrea..."

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Created page with "Category:Wiki == Architecture and Internal Design == APT-Cacher-NG (ACNG) acts as a specialized HTTP proxy optimized for package managers such as APT. Unlike generic proxies, it understands repository layouts and package metadata patterns. === Request Flow === # Client sends HTTP request for a package or index file # ACNG checks its local cache directory # If present and valid, the cached object is served # If missing or expired, ACNG fetches it from the upstrea..."

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[[Category:Wiki]]

== Architecture and Internal Design ==

APT-Cacher-NG (ACNG) acts as a specialized HTTP proxy optimized for package managers such as APT.
Unlike generic proxies, it understands repository layouts and package metadata patterns.

=== Request Flow ===
# Client sends HTTP request for a package or index file
# ACNG checks its local cache directory
# If present and valid, the cached object is served
# If missing or expired, ACNG fetches it from the upstream repository
# The fetched object is stored and indexed locally

=== Cache Storage Model ===
* Cache root usually located in <code>/var/cache/apt-cacher-ng</code>
* Files are stored using a hashed directory structure
* Metadata files (.db, .head) are maintained for validation and expiration
* Index files are cached separately from binary packages

=== Protocol Support ===
* HTTP (native)
* HTTPS via CONNECT tunneling (limited inspection)
* IPv4 and IPv6

=== Performance Characteristics ===
* Optimized for many small files (package indexes)
* Handles concurrent clients efficiently
* Reduces bandwidth and mirror load significantly in multi-host environments

== Configuration Directives (Advanced) ==

Configuration files are typically located in <code>/etc/apt-cacher-ng</code>.

=== Important Configuration Files ===
* <code>acng.conf</code> – main configuration
* <code>security.conf</code> – access control rules
* <code>backends_*.conf</code> – repository mappings

=== Cache Behavior Tuning ===

<nowiki>
CacheDir: /var/cache/apt-cacher-ng
ExTreshold: 4
MaxSpareThreadCount: 20
MaxThreads: 100
</nowiki>

* <code>ExTreshold</code>: expiration sensitivity for index files
* <code>MaxThreads</code>: controls parallel client handling
* <code>MaxSpareThreadCount</code>: idle worker threads kept alive

=== Repository Remapping ===

ACNG can remap repository URLs to alternative mirrors:

<nowiki>
Remap-debian: file:deb_mirror*.conf
Remap-ubuntu: file:ubuntu_mirror*.conf
</nowiki>

This allows transparent redirection without client-side changes.

== Client Configuration and Usage Examples ==

=== System-Wide APT Proxy Configuration ===

<nowiki>
Acquire::http::Proxy "http://proxy-server:3142";
Acquire::https::Proxy "http://proxy-server:3142";
</nowiki>

=== Per-Command Proxy Usage ===

<nowiki>
apt-get -o Acquire::http::Proxy="http://proxy-server:3142" update
</nowiki>

=== Verifying Proxy Usage ===

<nowiki>
apt-get -o Debug::Acquire::http=true update
</nowiki>

Logs should show requests routed through the ACNG proxy.

== Cache Management and Maintenance ==

=== Manual Cache Cleanup ===

<nowiki>
apt-cacher-ng -c /etc/apt-cacher-ng -s
</nowiki>

* Removes expired and orphaned files
* Rebuilds internal metadata databases

=== Cache Statistics ===

<nowiki>
curl http://localhost:3142/acng-report.html
</nowiki>

Provides:
* Cache hit ratio
* Disk usage
* Traffic savings
* Active connections

=== Pre-Fetching Packages ===

<nowiki>
apt-get --download-only install linux-image-amd64
</nowiki>

Used on one client to populate cache for others.

== Security Concepts ==

=== Access Control ===

Access rules are defined in <code>security.conf</code>:

<nowiki>
Allow: 192.168.1.0/24
Deny: all
</nowiki>

* Supports CIDR notation
* First-match rule evaluation
* Deny rules should always be explicit

=== Authentication Limitations ===
* ACNG does not natively support user authentication
* Intended for trusted networks
* Authentication must be enforced externally (firewall, VPN)

=== HTTPS Considerations ===
* HTTPS repositories are tunneled, not cached at object level
* Index files may still be cached depending on client behavior
* Full HTTPS caching requires repository-specific configuration

=== Network-Level Hardening ===
* Bind to specific interface:

<nowiki>
BindAddress: 192.168.1.10
</nowiki>

* Use firewall rules to restrict access to port 3142
* Recommended to run behind NAT or internal VLAN

=== Privilege Model ===
* Runs as unprivileged user
* No need for root access during normal operation
* Writes only to cache and log directories

== Logging and Monitoring ==

=== Log Files ===
* <code>/var/log/apt-cacher-ng/access.log</code>
* <code>/var/log/apt-cacher-ng/error.log</code>

=== Log Verbosity ===

<nowiki>
VerboseLog: 1
Debug: 7
</nowiki>

Higher debug levels significantly increase log volume.

=== Common Log Patterns ===
* <code>HIT</code> – served from cache
* <code>MISS</code> – fetched from upstream
* <code>EXPIRED</code> – revalidated content

== Troubleshooting ==

=== Clients Bypass the Cache ===
* Verify proxy settings on client
* Ensure no conflicting proxy definitions
* Check access.log for incoming requests

<nowiki>
grep MISS /var/log/apt-cacher-ng/access.log
</nowiki>

=== Permission Errors in Cache Directory ===
* Check ownership of cache directory
* Ensure sufficient disk space

<nowiki>
chown -R apt-cacher-ng:apt-cacher-ng /var/cache/apt-cacher-ng
</nowiki>

=== Slow Package Downloads ===
* Check upstream mirror responsiveness
* Increase thread limits
* Verify DNS resolution

=== Repository Not Cached ===
* Ensure repository uses HTTP
* Verify remap configuration
* Some third-party repositories disable caching headers

=== Service Fails to Start ===
* Check error.log for syntax errors
* Validate configuration files

<nowiki>
apt-cacher-ng -c /etc/apt-cacher-ng -t
</nowiki>

== Useful Links ==

* Official APT-Cacher-NG Homepage
https://www.unix-ag.uni-kl.de/~bloch/acng/

* APT-Cacher-NG Git Repository
https://salsa.debian.org/blade/apt-cacher-ng

* Debian Wiki – APT-Cacher-NG
https://wiki.debian.org/AptCacherNg

* Ubuntu Documentation – Package Caching
https://help.ubuntu.com/community/Apt-Cacher%20NG

* APT Configuration Manual
https://manpages.debian.org/apt.conf

APT-CACHER-NG - Advanced Config - Revision history

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