Fixing Errors After a Linux Update: Common Problems and Practical Solutions in the Terminal

1. Layer 3: Addressing and Routing Logic

Advanced users don’t just check for an IP; they analyze how the kernel handles packet forwarding and routing tables.

• Extended Interface Stats: ip -s -s link show eth0 – Reveals precise counters for discards, collisions, and window errors.
• Route Selection Debugging: ip route get 1.1.1.1 – Shows exactly which gateway, interface, and source IP the kernel will select for a specific destination.
• Event Monitoring: ip monitor all – A live feed of all network configuration changes (links, addresses, and routes).

2. High-Performance Socket Analysis with ss

The ss tool queries the kernel directly, making it significantly faster and more powerful than netstat for high-concurrency servers.

• Filtering by TCP State: ss -t state syn-sent – Identify processes struggling to establish outbound connections.
• Socket Memory Pressure: ss -tmp – Displays the memory buffers used by each socket, essential for tuning high-load web servers.
• Complex Expressions: ss -nt dst 192.168.1.0/24 – View all TCP connections to a specific subnet.

3. DNS Propagation and Path Latency

Beyond simple connectivity, we must identify packet loss at specific hops and verify DNS integrity.

• MTR (My Traceroute): mtr -rw google.com – Provides a wide, comprehensive report of packet loss and jitter across the entire network path.
• Iterative DNS Discovery: dig +trace example.com – Bypasses local resolvers to query the DNS hierarchy from the Root servers down to the Authoritative ones.

4. Deep Packet Inspection: tcpdump

When the application layer fails, you must inspect the raw frames. tcpdump allows for granular filtering using BPF (Berkeley Packet Filter) syntax.

Filtering for specific TCP flags (e.g., ACK/RST):
sudo tcpdump -i eth0 'tcp[tcpflags] & (tcp-ack|tcp-rst) != 0'

Inspecting HTTP Headers in real-time:
sudo tcpdump -A -s 0 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'

Advanced Commands Summary