> ## Documentation Index > Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt > Use this file to discover all available pages before exploring further. # Demo Checking Network Configuration > Step by step Linux network troubleshooting guide covering interface and IP inspection, routing checks, connectivity tests, packet tracing, and listing open ports and listening services. In this guide you'll verify a Linux system's network configuration step by step. We'll inspect interfaces and IP addresses, review the routing table, test connectivity (including DNS resolution), trace a packet path, and list open ports and listening services. Follow the sequence below as a practical troubleshooting workflow. Some commands (for example `ss -p` or certain `ip` operations) require root privileges to show process or full interface details. Prefix them with `sudo` or run as root when necessary. ## Quick command reference | Command | Purpose | Example / Notes | | -: | - | - | | `ip link` | List network interfaces and link/admin state | Shows interfaces even if they have no IP address | | `ip addr` | Show IP addresses assigned to interfaces | Use to find `inet` (IPv4) and `inet6` (IPv6) addresses | | `ip -brief address` | Compact, one-line-per-interface summary | Good for quick inspection | | `ip route` | Display kernel routing table (default gateway & routes) | Look for `default via ...` for outbound access | | `ping -c ` | Test ICMP reachability to gateway, IP or hostname | Use to verify network and DNS | | `tracepath ` | Trace hop-by-hop path to a destination | Helps locate where packets are dropped | | `ss -tulnp` | List listening TCP/UDP sockets and their processes | `-p` requires root to show processes | ## 1) Show interfaces and their status List interfaces and their administrative/link state: ```bash theme={null} ip link ``` Show IP addresses assigned to each interface (IPv4 and IPv6): ```bash theme={null} ip addr ``` Example combined output demonstrating link-level state and assigned addresses. If an interface lacks `inet` or `inet6`, it cannot exchange IP traffic: ```bash theme={null} root@control:~# ip link 1: lo: mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 2: enp0s3: mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000 link/ether 08:00:27:f0:46:06 brd ff:ff:ff:ff:ff:ff altname enx080027f04606 root@control:~# ip addr 1: lo: mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 inet 127.0.0.1/8 scope host lo valid_lft forever preferred_lft forever inet6 ::1/128 scope host noprefixroute valid_lft forever preferred_lft forever 2: enp0s3: mtu 1500 qdisc fq_codel state UP group default qlen 1000 link/ether 08:00:27:f0:46:06 brd ff:ff:ff:ff:ff:ff altname enx080027f04606 inet 10.0.2.3/24 brd 10.0.2.255 scope global dynamic noprefixroute enp0s3 valid_lft 524sec preferred_lft 524sec inet6 fe80::a00:27ff:fef0:4606/64 scope link noprefixroute valid_lft forever preferred_lft forever ``` Tip: An interface can be administratively `UP` but still lack an IP address (no `inet`/`inet6`). That interface will not participate in IP networking until an address is assigned. ## 2) Check the routing table Display the kernel routing table to find the default gateway and any specific routes: ```bash theme={null} ip route ``` Interpretation notes: * Look for a line beginning with `default via` — that is the system's default route for outbound traffic. * If the default route is missing, the system likely cannot reach external networks beyond its local subnets. ## 3) Test connectivity with ping Use `ping` to test reachability at different layers: * Ping the local gateway (e.g., `10.0.2.1`) — verifies local network and L2/L3 connectivity. * Ping an external IP (e.g., `8.8.8.8`) — verifies upstream routing and Internet access. * Ping a hostname (e.g., `google.com`) — verifies DNS resolution and downstream reachability. Example session: ```bash theme={null} root@control:~# ping -c 4 10.0.2.1 PING 10.0.2.1 (10.0.2.1) 56(84) bytes of data. 64 bytes from 10.0.2.1: icmp_seq=1 ttl=255 time=0.309 ms 64 bytes from 10.0.2.1: icmp_seq=2 ttl=255 time=1.160 ms 64 bytes from 10.0.2.1: icmp_seq=3 ttl=255 time=0.285 ms 64 bytes from 10.0.2.1: icmp_seq=4 ttl=255 time=0.346 ms --- 10.0.2.1 ping statistics --- 4 packets transmitted, 4 received, 0% packet loss, time 3062ms rtt min/avg/max/mdev = 0.285/0.526/1.164/0.368 ms root@control:~# ping -c 4 8.8.8.8 PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data. 64 bytes from 8.8.8.8: icmp_seq=1 ttl=255 time=18.1 ms 64 bytes from 8.8.8.8: icmp_seq=2 ttl=255 time=16.8 ms 64 bytes from 8.8.8.8: icmp_seq=3 ttl=255 time=18.0 ms 64 bytes from 8.8.8.8: icmp_seq=4 ttl=255 time=18.4 ms --- 8.8.8.8 ping statistics --- 4 packets transmitted, 4 received, 0% packet loss, time 3005ms rtt min/avg/max/mdev = 16.812/17.823/18.429/0.607 ms root@control:~# ping -c 5 google.com PING google.com (172.217.23.46) 56(84) bytes of data. 64 bytes from lcbuda-an-in-f14.1e100.net (172.217.23.46): icmp_seq=1 ttl=255 time=21.5 ms 64 bytes from lcbuda-an-in-f14.1e100.net (172.217.23.46): icmp_seq=2 ttl=255 time=19.5 ms 64 bytes from lcbuda-an-in-f14.1e100.net (172.217.23.46): icmp_seq=3 ttl=255 time=21.2 ms 64 bytes from lcbuda-an-in-f14.1e100.net (172.217.23.46): icmp_seq=4 ttl=255 time=21.0 ms 64 bytes from lcbuda-an-in-f14.1e100.net (172.217.23.46): icmp_seq=5 ttl=255 time=21.7 ms ``` Troubleshooting hints: * If you can ping an IP (e.g., `8.8.8.8`) but not a hostname, DNS is likely the issue. * If you can ping the gateway but not an external IP, check the default route or upstream connectivity. * Some networks block or rate-limit ICMP. A lack of responses is not definitive proof of total unreachability. ## 4) Trace the path to a destination Use `tracepath` to reveal each hop packets take and identify where packets are delayed or dropped: ```bash theme={null} tracepath google.com ``` Interpreting `tracepath` output: * Each line represents a hop (router) along the path. * Increasing latencies are expected; repeated `*` or timeouts show where responses are blocked or lost. * Use this to narrow the location of failures (local router, ISP, or remote network). Example invocation (output varies by network): ```bash theme={null} root@control:~# tracepath google.com ``` ## 5) Show open ports and listening services To see which sockets are listening and which processes own them, use `ss`: ```bash theme={null} ss -tulnp ``` Options explained: * `-t` : TCP sockets * `-u` : UDP sockets * `-l` : only listening sockets * `-n` : numeric addresses and ports (no name resolution) * `-p` : show process using the socket (requires root) `ss -p` requires root privileges to display processes. Run as `sudo ss -tulnp` to see process names/PIDs owning the sockets. Misinterpreting listening addresses can cause services to be bound only to localhost (127.0.0.1) and appear unreachable remotely. Use the output to confirm which services are bound to which interfaces and ports, and whether they are accepting connections from other hosts. ## 6) Show interfaces in a brief, summarized form For a compact view (one line per interface) with primary addresses: ```bash theme={null} ip -brief address ``` Example: ```bash theme={null} root@control:~# ip -brief address lo UNKNOWN 127.0.0.1/8 ::1/128 enp0s3 UP 10.0.2.3/24 fe80::a00:27ff:fef0:4606/64 ``` This command is useful for quick status checks or scripts that parse interface state. *** Conclusion and practical workflow A reliable sequence for troubleshooting network issues is: 1. Inspect link state and interfaces (`ip link`, `ip addr`, or `ip -brief address`) 2. Verify routing (`ip route`) 3. Test layer-3 reachability (`ping` to gateway, external IP, and hostname) 4. Trace the path to the destination (`tracepath`) 5. Inspect listening sockets and services (`ss -tulnp`) Following these steps helps you pinpoint where configuration or connectivity breaks occur — whether local interface, routing, DNS, upstream connectivity, or service binding. Links and references * [Linux ip(8) manual](https://man7.org/linux/man-pages/man8/ip.8.html) * [ss(8) — socket statistics](https://man7.org/linux/man-pages/man8/ss.8.html) * [tracepath(8) manual page](https://man7.org/linux/man-pages/man8/tracepath.8.html) * [Ping command reference](https://man7.org/linux/man-pages/man8/ping.8.html) This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.