Basic Network Troubleshooting -

Basic Network Troubleshooting

Last updated on: 2021-04-28

Authored by: Rackspace Support

This article provides Linux® commands for basic Cloud Server network troubleshooting. Access your server in the Rackspace Cloud Control Panel and use the web console to run the commands.

`ip addr show` command

The ip addr show command shows the Cloud Server’s IP address configuration. In the following example, the eth0 and eth1 interfaces are configured and running:

# ip addr show
eth0      Link encap:Ethernet  HWaddr 40:40:d9:xx:xx:xx
          inet addr:67.23.13.xx  Bcast:67.23.13.255  Mask:255.255.255.0
          inet6 addr: fe80::4240:d9ff:fe05:xxxx/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:36564 errors:0 dropped:0 overruns:0 frame:0
          TX packets:11490 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:48350683 (46.1 MiB)  TX bytes:1456436 (1.3 MiB)

eth1      Link encap:Ethernet  HWaddr 40:40:a5:xx:xx:xx
          inet addr:10.176.44.xx  Bcast:10.176.63.255  Mask:255.255.224.0
          inet6 addr: fe80::4240:a5ff:fe5f:xxxx/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:3 errors:0 dropped:0 overruns:0 frame:0
          TX packets:53 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:230 (230.0 B)  TX bytes:7764 (7.5 KiB)

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

Common resolutions

Follow these steps to fix common problems on the interfaces:

Enter the following command:
```
ifconfig
```
If eth0 does not show an IP address, use the following command:
```
ip addr show eth0 10.10.10.4 netmask 255.255.255.0 up
```
In this case, 10.10.10.4 is the Cloud Server’s initial configuration IP address for eth0.
If eth0 is not present, run the following command to set the interface and to the default configuration:
```
ifup eth0
```

Perform the same steps for eth1 if needed.

iptables command

iptables is an administration tool for configuring a firewall in Linux. By default, Cloud Servers have iptables installed but not configured.

Enter the following command to list the firewall rules:

iptables -L

A server with its default configuration shows the following output:

# iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

Different output might indicate a problem with the firewall.

`route` command

Use the route command to view and edit the routing table. The output for route varies among Linux distributions. The following output is from a Debian® distribution:

# route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
67.23.13.0      *               255.255.255.0   U     0      0        0 eth0
10.176.32.0     *               255.255.224.0   U     0      0        0 eth1
10.191.192.0    10.176.32.1     255.255.192.0   UG    0      0        0 eth1
10.176.0.0      10.176.32.1     255.248.0.0     UG    0      0        0 eth1
default         67.23.13.1      0.0.0.0         UG    0      0        0 eth0

In this example:

The first line is the 67.23.13.0 network.
The next three lines correspond to the internal network.
The last line is the default gateway 67.23.13.1 of the 67.23.13.0 network.

Common resolutions

Enter the following command to change the default gateway where xx.xx.xx.1 is the default gateway:

route add default gw xx.xx.xx.1

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