Troubleshooting and Best Practices (Part 4 of 4)

1. NetworkManager Troubleshooting

1.1 Common NetworkManager Issues

Issue: Connection Fails to Activate

# Check connection status
nmcli connection show "Connection-Name"

# Check device status
nmcli device status

# View detailed device info
nmcli device show eth0

# Check NetworkManager logs
journalctl -u NetworkManager -n 50 -f

# Check system logs
dmesg | tail -50

# Reload connection
nmcli connection reload

# Restart NetworkManager
sudo systemctl restart NetworkManager

# Bring device down and up
nmcli device disconnect eth0
nmcli device connect eth0

# Delete and recreate connection
nmcli connection delete "Connection-Name"
nmcli connection add type ethernet con-name "Connection-Name" ifname eth0

Issue: DNS Not Working

# Check DNS settings
nmcli device show eth0 | grep DNS

# Check resolv.conf
cat /etc/resolv.conf

# Test DNS resolution
nslookup google.com
dig google.com

# Check if systemd-resolved is running
systemctl status systemd-resolved

# Set DNS servers explicitly
nmcli connection modify "Connection-Name" \
  ipv4.dns "8.8.8.8,8.8.4.4"

# Don't use automatic DNS
nmcli connection modify "Connection-Name" \
  ipv4.ignore-auto-dns yes

# Restart NetworkManager
sudo systemctl restart NetworkManager

# Bring connection up
nmcli connection up "Connection-Name"

Issue: Changes Not Taking Effect

# After modifying connection
nmcli connection reload "Connection-Name"

# Or reload all connections
nmcli connection reload

# Then bring connection down and up
nmcli connection down "Connection-Name"
nmcli connection up "Connection-Name"

# Or disconnect/reconnect device
nmcli device disconnect eth0
nmcli device connect eth0

Issue: Can't Modify System Connections

# Use sudo for system-wide connections
sudo nmcli connection modify "Connection-Name" ...

# Check file permissions
ls -l /etc/NetworkManager/system-connections/

# Fix permissions if needed
sudo chmod 600 /etc/NetworkManager/system-connections/*
sudo chown root:root /etc/NetworkManager/system-connections/*

1.2 Network Connectivity Issues

1.3 NetworkManager Debugging

Enable Debug Logging

# Edit NetworkManager.conf
sudo vi /etc/NetworkManager/NetworkManager.conf

# Add or modify:
[logging]
level=DEBUG
domains=ALL

# Restart NetworkManager
sudo systemctl restart NetworkManager

# View logs
journalctl -u NetworkManager -f

# Remember to disable debug logging when done!
[logging]
level=INFO

Connection Troubleshooting Script

#!/bin/bash
# Quick network diagnostics

echo "=== Network Diagnostics ==="
echo ""

echo "1. NetworkManager Status:"
systemctl status NetworkManager --no-pager | head -5
echo ""

echo "2. Devices:"
nmcli device status
echo ""

echo "3. Active Connections:"
nmcli connection show --active
echo ""

echo "4. IP Configuration:"
ip addr show
echo ""

echo "5. Default Route:"
ip route show default
echo ""

echo "6. DNS Servers:"
cat /etc/resolv.conf | grep nameserver
echo ""

echo "7. Gateway Test:"
GATEWAY=$(ip route | grep default | awk '{print $3}')
if [ -n "$GATEWAY" ]; then
    if ping -c 3 -W 2 "$GATEWAY" &>/dev/null; then
        echo "Gateway ($GATEWAY): OK"
    else
        echo "Gateway ($GATEWAY): FAILED"
    fi
else
    echo "No default gateway configured"
fi
echo ""

echo "8. Internet Test:"
if ping -c 3 -W 2 8.8.8.8 &>/dev/null; then
    echo "Internet (8.8.8.8): OK"
else
    echo "Internet (8.8.8.8): FAILED"
fi

echo ""
echo "=== End Diagnostics ==="

2. nm Symbol Table Troubleshooting

2.1 Common nm Issues

Issue: "No Symbols" Error

# Verify if binary is stripped
file myprogram
# Output: "stripped" = no symbols, "not stripped" = has symbols

# Compare sizes
ls -lh myprogram
# Stripped binaries are smaller

Issue: C++ Name Mangling

_Z3addii
_ZN9MyClass10myFunctionEv

# Demangle C++ names
nm -C myprogram

# Compare:
nm myprogram | grep add
# Output: 0000000000001234 T _Z3addii

nm -C myprogram | grep add
# Output: 0000000000001234 T add(int, int)

Issue: Finding Missing Symbols

undefined reference to `some_function'

# 1. Verify the symbol is actually undefined
nm -u myprogram.o | grep some_function

# 2. Search your object files
nm *.o | grep " T some_function"

# 3. Search system libraries
for lib in /usr/lib/x86_64-linux-gnu/*.so; do
    if nm -D "$lib" 2>/dev/null | grep -q " T some_function"; then
        echo "Found in: $lib"
    fi
done

# 4. Use ldd to check dynamic dependencies
ldd myprogram

2.2 Symbol Analysis Best Practices

3. Best Practices Summary

3.1 NetworkManager Best Practices

3.2 nm Best Practices

3.3 General Linux Command Best Practices

4. Quick Reference Cheat Sheet

4.1 NetworkManager Quick Commands

# Status
nmcli general status                    # Overall status
nmcli device status                     # Device list
nmcli connection show                   # Connection list
nmcli connection show --active          # Active only

# Connect/Disconnect
nmcli connection up "Name"              # Activate
nmcli connection down "Name"            # Deactivate
nmcli device disconnect eth0            # Disconnect device

# Modify
nmcli connection modify "Name" \
  ipv4.addresses "192.168.1.100/24"    # Set IP
nmcli connection reload                 # Reload configs

# WiFi
nmcli device wifi list                  # List networks
nmcli device wifi connect SSID \
  password "pass"                       # Connect to WiFi

4.2 nm Quick Commands

# Basic
nm file                                 # List all symbols
nm -u file                             # Undefined symbols
nm -D file                             # Dynamic symbols
nm -g file                             # Global symbols only

# Analysis
nm -C program                          # Demangle C++
nm --size-sort -S file                 # Sort by size
nm -A *.o | grep symbol                # Find in multiple files

# Filtering
nm file | grep " T "                   # Functions only
nm file | grep " [DdBb] "             # Variables only
nm -D lib.so | grep " T func"         # Specific function

5. Recovery Procedures

5.1 Network Connection Recovery

#!/bin/bash
# Emergency network recovery script

echo "=== Network Recovery ==="

# Stop NetworkManager
sudo systemctl stop NetworkManager

# Manual IP configuration (if needed for remote access)
sudo ip addr add 192.168.1.100/24 dev eth0
sudo ip link set eth0 up
sudo ip route add default via 192.168.1.1

# Test connectivity
ping -c 3 192.168.1.1

# Restart NetworkManager
sudo systemctl start NetworkManager

# Reactivate connection
nmcli connection up "Your-Connection"

5.2 Connection File Backup and Restore

# Backup all connections
sudo tar -czf nm-connections-backup-$(date +%Y%m%d).tar.gz \
  /etc/NetworkManager/system-connections/

# Restore from backup
sudo tar -xzf nm-connections-backup-20241025.tar.gz -C /

# Fix permissions
sudo chmod 600 /etc/NetworkManager/system-connections/*
sudo chown root:root /etc/NetworkManager/system-connections/*

# Reload
sudo nmcli connection reload

6. Performance Tips

6.1 NetworkManager Performance

• Disable unused connection types (WiFi if not needed)
• Set appropriate autoconnect priorities
• Use static IPs when DHCP isn't required
• Disable IPv6 if not used: ipv6.method ignore
• Reduce connection timeout values for faster failover

6.2 nm Performance

• Use -g to filter global symbols for faster searches
• Process fewer files at once for large projects
• Use -p (no sort) if order doesn't matter
• Pipe to grep early to reduce processing
• Consider using objdump for complex analysis

7. Additional Resources

7.1 Man Pages

man nmcli
man nmcli-examples
man nm-settings
man NetworkManager.conf
man nm(1)

7.2 Useful Documentation Paths

• /usr/share/doc/NetworkManager/
• /etc/NetworkManager/
• /var/lib/NetworkManager/

7.3 Related Tools to Explore

8. Final Tips