CentOS Dropped日志分析

CentOS Dropped Logs: Analysis and Troubleshooting Guide
Dropped logs in CentOS record events where the system discards data packets, connections, or processes—often due to network congestion, firewall rules, hardware limitations, or misconfigurations. These logs are critical for identifying performance bottlenecks, security threats, or system errors. Below is a structured approach to accessing, analyzing, and resolving dropped issues using CentOS’s native logging tools.

1. Key Log Files for Dropped Events

CentOS stores dropped-related logs in multiple locations, depending on the system component (network, firewall, security). The most relevant files include:

• /var/log/messages: General system logs covering kernel messages, network errors, and service issues (e.g., packet drops from iptables or network drivers).
• /var/log/syslog: Similar to messages but may contain additional application-level logs (useful for user-space processes that drop connections).
• /var/log/kern.log: Kernel-specific logs (e.g., network interface errors, DMA issues causing packet drops).
• /var/log/secure: Authentication and firewall logs (e.g., firewalld or iptables dropping unauthorized access attempts).
• /var/log/audit/audit.log: Security audit logs (if enabled), recording privileged operations (e.g., users dropping processes via kill).

These logs are the primary sources for investigating dropped events.

2. Tools to View and Filter Dropped Logs

CentOS provides command-line tools to efficiently search and analyze dropped logs:

• journalctl (Recommended for CentOS 7+):

  • View recent system logs: journalctl -n 100 (displays the last 100 entries).
  • Filter for dropped keywords: journalctl -k | grep -i "dropped\|drop" (shows kernel-level dropped events) or journalctl -u firewalld | grep "Dropped" (checks firewalld-specific drops).
  • Real-time monitoring: journalctl -f (tracks new logs as they are generated).

• grep/tail for Text Logs:

  • Search for dropped entries in /var/log/messages: grep -i "dropped" /var/log/messages.
  • Tail real-time updates: tail -f /var/log/syslog | grep "drop".

• ausearch/aureport (for Audit Logs):

  • Search audit logs for dropped processes: ausearch -k process_drop (requires predefined audit rules).
  • Generate a user activity report: aureport -u -ts today (identifies users associated with dropped events).

3. Common Causes of Dropped Events

Dropped logs often indicate underlying issues. Key causes include:

• Network Issues:

  • MTU mismatch: Large packets exceeding the network interface’s MTU are dropped (check with ip link show eth0 or ethtool -S eth0 | grep "rx_oversize_pkts_phy").
  • Interface errors: CRC errors, buffer overflows (view with netstat -i or ethtool -S eth0).

• Firewall/Security Rules:

  • iptables/firewalld blocking legitimate traffic (check rules with iptables -L -n -v or firewall-cmd --list-all).
  • Connection tracking table full (tracked in /proc/sys/net/netfilter/nf_conntrack_count; increase limit with sysctl -w net.netfilter.nf_conntrack_max=100000).

• Hardware/Resource Constraints:

  • Network card failures: Use ethtool -S eth0 to check for errors like rx_errors or tx_dropped.
  • System resource exhaustion: High CPU/memory usage can cause the kernel to drop packets (monitor with top or vmstat 1).

4. Resolving Dropped Issues

Once the root cause is identified, take targeted actions:

• Adjust Network Configuration:

  • Fix MTU issues: Set the correct MTU (e.g., ip link set eth0 mtu 1500) and verify with ip link show eth0.
  • Replace faulty hardware: If ethtool shows persistent errors, replace the network cable or card.

• Modify Firewall Rules:

  • Allow legitimate traffic: Add specific rules to iptables (e.g., iptables -A INPUT -p tcp --dport 80 -j ACCEPT) or adjust firewalld zones.
  • Increase connection tracking limit: Add net.netfilter.nf_conntrack_max=100000 to /etc/sysctl.conf and run sysctl -p.

• Optimize System Resources:

  • Upgrade hardware: Add more RAM or replace an underperforming CPU.
  • Reduce load: Terminate unnecessary processes (use top to identify high-resource consumers) or optimize applications.

By systematically accessing logs, filtering for dropped events, and addressing root causes, you can effectively troubleshoot and resolve dropped issues in CentOS. Regular log monitoring (e.g., using logrotate to manage log size) ensures proactive detection of future problems.