{"id":2449,"date":"2015-07-29T16:39:36","date_gmt":"2015-07-29T13:39:36","guid":{"rendered":"https:\/\/upcloud.com\/global\/us\/resources\/tutorials\/troubleshoot-network-connectivity-linux-server\/"},"modified":"2015-07-29T16:39:36","modified_gmt":"2015-07-29T13:39:36","slug":"troubleshoot-network-connectivity-linux-server","status":"publish","type":"tutorial","link":"https:\/\/upcloud.com\/global\/resources\/tutorials\/troubleshoot-network-connectivity-linux-server\/","title":{"rendered":"Troubleshooting Network Connectivity on Linux Servers: A Complete Guide"},"content":{"rendered":"\n

When your Linux server experiences network connectivity issues, you need reliable tools and techniques to quickly identify and resolve the problem. This guide covers essential Linux commands for troubleshooting, including how to check network interfaces, test internet connectivity, and verify DNS records. Whether you’re using Ubuntu, CentOS, or Debian, these steps will help you diagnose network failures, check server configurations, and restore connectivity.<\/p>\n\n\n\n

When your Linux server<\/a> seems to be offline or otherwise inaccessible, you should always be able to log in with the web\u00a0console at your UpCloud control panel or through a VNC connection. Once logged in, test your server\u2019s internet connection using ping and a public IP address such as Google\u2019s public DNS server, which is most likely to reply, provided your internet connection works.<\/p>\n\n\n\n

ping -c 4 8.8.8.8<\/pre>\n\n\n\n
Your output should show something like<\/p>\n\n\n\n
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.\n64 bytes from 8.8.8.8: icmp_seq=1 ttl=58 time=1.68 ms\n64 bytes from 8.8.8.8: icmp_seq=2 ttl=58 time=1.70 ms\n64 bytes from 8.8.8.8: icmp_seq=3 ttl=58 time=1.71 ms\n64 bytes from 8.8.8.8: icmp_seq=4 ttl=58 time=1.69 ms\n\n--- 8.8.8.8 ping statistics ---\n4 packets transmitted, 4 received, 0% packet loss, time 3005ms\nrtt min\/avg\/max\/mdev = 1.686\/1.699\/1.718\/0.051 ms\n<\/pre>\n\n\n\n
If the server is unable to reach the destination, there may be a problem with your configuration. Follow the steps described here in order to troubleshoot the most common network issues with Linux Cloud Servers.<\/p>\n\n\n\n
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Try UpCloud for free!<\/a><\/div>\n<\/div>\n\n\n\n
\"developer-cloud\"<\/figure>\n\n\n\n
Check your network configuration<\/h2>\n\n\n\n
If your Linux server is unreachable, start by checking your network interfaces. Use the ip addr command to view the status of all interfaces, ensuring critical connections like eth0 are enabled. Should you find any interfaces disabled, activate them with ifup. Troubleshoot persistent issues by restarting interfaces using the ifdown –force and ifup commands.<\/p>\n\n\n\n
Make sure the network interfaces, such as eth0, are enabled. To see all the configured interfaces, use this command.<\/p>\n\n\n\n
ip addr<\/pre>\n\n\n\n
The output of the command will show the status of each network interface on the server with \u201cstate UP\u201d or \u201cstate DOWN\u201d, for example, as below.<\/p>\n\n\n\n
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000<\/pre>\n\n\n\n
Turn on any disabled interfaces with the next command.<\/p>\n\n\n\n
sudo ifup <interface name><\/pre>\n\n\n\n
Here the interface name is one of the names listed in the ip addr<\/tt> command output such as eth0<\/tt>, eth1<\/tt> or eth2<\/tt>.<\/p>\n\n\n\n
When all network interfaces have been enabled, try using the ping<\/tt> command again. If the problem persists, check that the network interfaces have IPs assigned to them, and they match the information in the Network section<\/a> of the UpCloud control panel.<\/p>\n\n\n\n
Try restarting any problematic interface with the following commands.<\/p>\n\n\n\n
sudo ifdown <interface name>\nsudo ifup <interface name><\/pre>\n\n\n\n
If either of these commands fails, the interface may be in a state unknown to the command script. Try the same commands again withthe \u2014-force<\/tt> parameter to resolve any such issues.<\/p>\n\n\n\n
sudo ifdown --force <interface name>\nsudo ifup <interface name><\/pre>\n\n\n\n
If restarting the network interface fixed the issue then great! If not, continue with the troubleshooting.<\/p>\n\n\n\n
Check the network configuration file<\/h2>\n\n\n\n
Linux commonly stores network settings in specific files and reads them, for example, at boot or when using the ifup command. To make changes to the network configuration, you\u2019ll need to open the right file in a text editor. In Debian and Ubuntu-based distributions, this can be done with<\/p>\n\n\n\n
sudo nano \/etc\/network\/interfaces<\/pre>\n\n\n\n
In most cases, the interfaces<\/tt> file should list the following interfaces.<\/p>\n\n\n\n
auto lo\niface lo inet loopback\n\nauto eth0\niface eth0 inet dhcp\n\nauto eth1\niface eth1 inet dhcp<\/pre>\n\n\n\n
In CentOS and other Red Hat variants, these configurations are split into separate files for each network interface and stored in \/etc\/sysconfig\/network-scripts\/<\/tt>. The default interface for internet connection is usually called eth0<\/tt>, which opens the corresponding configuration file.<\/p>\n\n\n\n
sudo vi \/etc\/sysconfig\/network-scripts\/ifcfg-eth0<\/pre>\n\n\n\n
The configuration file for eth0 should look like this.<\/p>\n\n\n\n
DEVICE=eth0\nBOOTPROTO=dhcp\nONBOOT=yes<\/pre>\n\n\n\n
If the interface configuration files do not match the example here, edit the file specific to your system to restore the original functionality. For any changes made to these files to take effect, you must restart the interfaces to which the changes apply as described previously with ifdown<\/tt> and ifup<\/tt> commands.<\/p>\n\n\n\n
Check the server’s DNS records<\/h2>\n\n\n\n
If your server can ping an IP but fails to connect to domains, it may be a DNS issue. Use ping upcloud.com to test domain name resolution, and check DNS configuration with sudo cat \/etc\/resolv.conf. Ensure your DNS servers, such as UpCloud\u2019s resolvers, are properly configured in this file or through your network interfaces file.<\/p>\n\n\n\n
ping upcloud.com<\/pre>\n\n\n\n
If the domain does not reply, the problem is most likely with how your server resolves domain names to IP addresses. Check your server\u2019s DNS records.<\/p>\n\n\n\n
sudo cat \/etc\/resolv.conf<\/pre>\n\n\n\n
The list should contain a minimum of 1 name server. All default DNS resolvers at UpCloud have the same IP addresses regardless of the availability zone. The DNS servers are provided automatically by the DHCP protocol, and there should be no need for manual configurations in the operating system.<\/p>\n\n\n\n
The UpCloud DNS resolver IPv4 addresses are:<\/p>\n\n\n\n