Networking on a Host

When a service is unreachable, figure out whether the problem is the process, the local host, DNS, or the network path.

Core commands

ip addr                    # show network interfaces and assigned IPs
ip route                   # show the routing table
ss -ltnp                   # show listening TCP ports and owning processes
ss -lunp                   # show listening UDP ports and owning processes
ping <host>
curl -I http://<host>:<port>  # fetch response headers only
dig <name>                 # query DNS records for a name
nslookup <name>            # simple DNS lookup

Fast workflow

1. Is the service listening?

ss -ltnp | grep <port>

2. Is the interface up and does the host have the expected IP?

ip addr

3. Is the route sane?

ip route

4. Does DNS resolve the name you are using?

dig <hostname>

5. Can you reach the endpoint from this host?

curl -v http://<host>:<port>

6. If it is listening but still unreachable, check the local firewall with iptables -L -n, nft list ruleset, or firewall-cmd --list-all depending on the host.

What matters

• 0.0.0.0:<port> means listening on all IPv4 interfaces.
• 127.0.0.1:<port> means local only.
• A healthy process can still be unreachable if it bound to the wrong address.

Common mistake

People often stop at “the service is running.” That is not enough. It also needs to be listening on the right port and bound to the right interface.