Debugging Squid Proxy Caching Issues: Why All Requests Show TCP_MISS and How to Fix


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When Squid consistently shows TCP_MISS in access logs, it indicates the proxy isn't caching content as expected. Let's analyze the key configuration elements that affect caching behavior:

# Critical caching parameters in squid.conf
cache_dir ufs /var/spool/squid3 100 16 256
refresh_pattern .   0   20% 4320
http_port 3128 accel defaultsite=cona-proxy vhost

From your configuration, several potential issues stand out:

  • The accel mode with vhost requires special handling of Host headers
  • Cache directory permissions (already checked) and available disk space
  • Missing or too restrictive refresh_pattern rules
  • Upstream server sending no-cache headers

First, let's modify your refresh_pattern rules to be more permissive:

refresh_pattern -i \.(gif|png|jpg|jpeg|ico)$ 1440 50% 40320 ignore-reload
refresh_pattern -i \.(css|js|html)$ 60 20% 1440
refresh_pattern . 20 40% 1440 override-expire override-lastmod

Then adjust your cache_peer configuration:

cache_peer 192.168.122.11 parent 80 0 no-query originserver login=PAS name=webserver
cache_peer_access webserver allow all

The HTTP response from your Apache server shows:

HTTP/1.1 202 OK
Date Mon, 02 Jul 2012 05:48:50 GMT
Server: Apache/2.2.22 (Ubuntu)
Last-Modified: Tue, 19 Jun 2012 23:04:25 GMT
ETag: "27389-b1-4c2db4dc2c182"

Add these directives to your Apache configuration to ensure cache-friendly headers:

<IfModule mod_expires.c>
    ExpiresActive On
    ExpiresDefault "access plus 1 hour"
</IfModule>

Use these commands to verify cache operations:

# Check cache manager
squidclient mgr:info

# Verify storage
squidclient mgr:storedir

# Check refresh pattern matching
squidclient mgr:refresh

Since you're using accel mode, these additional settings are crucial:

# Properly handle Host headers
http_port 3128 accel defaultsite=cona-proxy vhost

# Cache even when requests vary by Host header
vary_ignore_expire on

# Special handling for accelerated domains
acl accelerated dstdomain cona-proxy
cache_peer_access webserver allow accelerated

After making these changes, check your access logs for cache hits:

tail -f /var/log/squid/access.log | grep TCP_

You should start seeing TCP_HIT, TCP_MEM_HIT, and TCP_REFRESH_HIT entries alongside TCP_MISS.

When configuring Squid as a reverse proxy (accelerator mode) for an Apache web server, seeing constant TCP_MISS entries in access logs typically indicates one of these fundamental issues:

# Sample problematic log entry
1467342932.005     23 192.168.122.22 TCP_MISS/200 177 GET http://cona-proxy/ - DIRECT/192.168.122.11 text/html

Based on your setup, here are the key elements needing verification:

# Essential Squid directives for caching
http_port 3128 accel defaultsite=cona-proxy vhost
cache_peer 192.168.122.11 parent 80 0 no-query originserver login=PAS name=webserver
cache_dir ufs /var/spool/squid3 100 16 256
refresh_pattern .   0   20% 4320

1. Cache-Control Headers: Your Apache response shows missing Cache-Control headers. Add this to Apache config:

# Apache configuration for cacheable content
<FilesMatch "\.(html|css|js)$">
    Header set Cache-Control "max-age=86400, public"
</FilesMatch>

2. Vary Headers: The "Vary: Accept-Encoding" header might prevent caching unless properly handled:

# Squid configuration addition
acl vary_accept_encoding rep_header Vary Accept-Encoding
storeurl_rewrite_program /usr/lib/squid/storeurl_rewrite

Check Squid's cache manager interface for real-time monitoring:

squidclient -p 3128 mgr:objects | grep cona-proxy
squidclient -p 3128 mgr:store_digest

Verify cache directory initialization:

squid -Nz  # Create cache directories
chown -R squid:squid /var/spool/squid3

Here's a corrected squid.conf configuration:

# Revised cache_peer directive
cache_peer 192.168.122.11 parent 80 0 no-query originserver name=webserver
    login=PAS
    proxy-only=off
    connect-timeout=5
    ttl=3600

# Enhanced refresh patterns
refresh_pattern -i \.(gif|png|jpg|jpeg|ico)$ 1440 50% 10080 override-expire
refresh_pattern -i \.(css|js)$ 1440 40% 40320 override-lastmod
refresh_pattern ^ftp: 1440 20% 10080
refresh_pattern . 0 20% 4320 override-lastmod

Remember to test with cacheable content and verify with:

curl -v -x http://192.168.122.21:3128 http://cona-proxy/test.html