Apache Crash & Not Working: Complete Troubleshooting Guide (Connection Refused, OOM, Core Dump)

Understanding Apache Crashes and Failures

Apache HTTP Server (apache2 on Debian/Ubuntu, httpd on RHEL/CentOS) is a multi-process/multi-threaded daemon managed by a parent process that spawns workers. When the parent or a worker dies unexpectedly, symptoms range from a full service outage (apache crash, apache not working) to intermittent apache connection refused errors during high traffic. Because workers are forked from the parent, a single misconfigured module or a memory cliff can take down every active connection simultaneously.

The five most common root causes are:

1. OOM kill — the Linux kernel terminates Apache workers to reclaim memory
2. Port binding failure — another process holds port 80 or 443 at startup
3. Configuration syntax error — a bad VirtualHost or SSLCertificateFile path prevents Apache from loading
4. Segfault in a module — a third-party .so (mod_php, mod_security) corrupts memory
5. Worker pool exhaustion — all workers are occupied, new connections are refused at the TCP layer

Step 1: Identify the Failure Mode

Check service status and last journal entries first:

systemctl status apache2          # Debian/Ubuntu
systemctl status httpd            # RHEL/CentOS/Fedora
journalctl -u apache2 -n 100 --no-pager

Look for one of these patterns in the output:

• Main PID: 1234 (code=killed, signal=KILL) → OOM kill by kernel
• Main PID: 1234 (code=dumped, signal=SEGV) → segmentation fault, core dump generated
• AH00016: Configuration Failed → syntax error, check apachectl -t
• (98)Address already in use: AH00072: make_sock: could not bind to address 0.0.0.0:80 → port conflict
• AH00485: scoreboard is full, not at MaxRequestWorkers → worker pool saturated

Check the Apache error log directly:

tail -n 200 /var/log/apache2/error.log    # Debian/Ubuntu
tail -n 200 /var/log/httpd/error_log      # RHEL/CentOS

Step 2: Fix OOM Crashes (apache out of memory)

The Linux OOM killer logs to /var/log/kern.log or the kernel ring buffer:

dmesg | grep -i 'oom\|killed process\|apache\|httpd' | tail -30
grep -i 'oom\|out of memory' /var/log/kern.log | tail -20

You will see lines like:

Out of memory: Kill process 4521 (apache2) score 312 or sacrifice child
Killed process 4521 (apache2) total-vm:1048576kB, anon-rss:524288kB

Calculate the correct MaxRequestWorkers:

# Check current memory per worker
ps -ylC apache2 --sort:rss | awk 'NR>1{sum+=$8; count++} END{print "Avg RSS:", sum/count/1024, "MB; Count:", count}'

# Get available RAM (exclude OS overhead ~256MB)
free -m | awk '/Mem/{print ($2 - 256) " MB available for Apache"}'  

Then set in /etc/apache2/mods-enabled/mpm_prefork.conf (or equivalent):

<IfModule mpm_prefork_module>
    StartServers             5
    MinSpareServers          5
    MaxSpareServers         10
    MaxRequestWorkers       50    # (available_RAM_MB / avg_worker_MB)
    ServerLimit             50
    MaxConnectionsPerChild 1000
</IfModule>

After editing: apachectl configtest && systemctl reload apache2

Step 3: Fix Connection Refused (apache connection refused)

If Apache is not running:

systemctl start apache2
systemctl status apache2   # look for the failure reason

If Apache is running but still refusing connections:

# Confirm what's listening on port 80
ss -tlnp | grep ':80'
lsof -i :80

# Test locally to isolate network vs Apache
curl -v http://127.0.0.1/

If another process owns port 80 (nginx, another Apache instance):

# Identify the conflicting process
ss -tlnp | grep ':80' | awk '{print $NF}'
# Output example: users:(("nginx",pid=1122,fd=6))
systemctl stop nginx   # or reconfigure ports

Step 4: Fix apache service not starting (Configuration Errors)

Always validate before starting:

apachectl -t           # syntax check
apachectl -t -D DUMP_VHOSTS   # list all virtual hosts as Apache sees them
apachectl -t -D DUMP_MODULES  # list loaded modules

Common errors and their fixes:

Step 5: Analyze apache core dump (Segfault Debugging)

Enable core dumps and locate them:

# Check if core dumps are being generated
ulimit -c             # should not be 0
cat /proc/sys/kernel/core_pattern

# Common locations
ls -lh /var/crash/
ls -lh /tmp/core*
ls -lh /etc/apache2/core*
coredumpctl list      # on systemd systems

Analyze with gdb:

apt-get install -y apache2-dbg gdb   # install debug symbols

# Load the core dump
gdb /usr/sbin/apache2 /var/crash/core.1234

# Inside gdb:
(gdb) bt full          # full backtrace
(gdb) info threads     # check all threads
(gdb) thread apply all bt  # backtrace for every thread

A backtrace pointing to mod_php, mod_security2, or a third-party module:

# Temporarily disable the suspect module
a2dismod php8.1
a2dismod security2
apachectl configtest && systemctl restart apache2

Step 6: Fix apache too many connections / apache slow

Tune KeepAlive for high-traffic servers:

# /etc/apache2/apache2.conf
KeepAlive On
KeepAliveTimeout 2        # reduce from default 5
MaxKeepAliveRequests 100

Identify slow requests in real time:

# Enable mod_status
a2enmod status
# Add to a VirtualHost or .htaccess:
# <Location /server-status>
#   SetHandler server-status
#   Require ip 127.0.0.1
# </Location>

curl http://127.0.0.1/server-status?refresh=5

# Find long-running requests
watch -n2 'curl -s http://127.0.0.1/server-status | grep -E "(W|_|R|C|D|K)\s+[0-9]+\s+[0-9]+\s+[0-9]+" | head -20'

Disable reverse DNS lookups (major performance killer):

HostnameLookups Off   # ensure this is Off, not On

Check for a full filesystem causing apache slow / apache failed log writes:

df -h
lsof | grep deleted | grep apache   # large deleted-but-open log files

Step 7: Prevent Future Crashes

Set up automatic restart and memory alerting:

# Systemd restart policy
systemctl edit apache2
# Add:
# [Service]
# Restart=on-failure
# RestartSec=5s
# MemoryMax=2G

# Monitor with a simple cron health check
echo '*/5 * * * * root curl -sf http://127.0.0.1/ > /dev/null || systemctl restart apache2' > /etc/cron.d/apache-watchdog

Enable Apache crash log rotation to prevent log files consuming all disk space:

cat /etc/logrotate.d/apache2   # verify rotation is configured
apachectl -t && logrotate -f /etc/logrotate.d/apache2