{"id":421,"date":"2013-09-09T11:56:35","date_gmt":"2013-09-09T16:56:35","guid":{"rendered":"https:\/\/www.softwareab.net\/wordpress\/?p=421"},"modified":"2014-06-09T06:36:44","modified_gmt":"2014-06-09T11:36:44","slug":"centosrhel-and-wrong-network-interface","status":"publish","type":"post","link":"https:\/\/www.softwareab.net\/wordpress\/centosrhel-and-wrong-network-interface\/","title":{"rendered":"CentOS\/RHEL and wrong Network Interface"},"content":{"rendered":"

This is a quick entry on a particularly irritating CentOS peculiarity – after you clone a virtual machine, your \/etc\/sysconfig\/network-scripts\/ifcfg-eth0<\/code> no longer gets invoked. This is because CentOS can arbitrarily assign the physical network card to a different logic interface such as eth1<\/code>. Read on for a quick tutorial on how to identify and fix this problem.<\/p>\n

<\/p>\n

Many times after provisioning CentOS box you will notice that ifconfig -a doesn’t show the connected vSphere network adapter:<\/p>\n

\"centos_nic_prob_01\"<\/a><\/p>\n

Take a look closely at the above drawing. The logical eth1<\/code> interface has no network traffic (no RX<\/code> received or TX<\/code> transmitted packets), and there is certainly no IP address. However, I know for a fact that this VM – before cloning – had the logical eth1<\/code> interface properly bound to the physical network card (NIC) presented by VMware to the guest OS (eth0<\/code>).<\/p>\n

So what happened??<\/p>\n

Many of you already know this, but CentOS \/ RHEL can (and do!) arbitrarily reassign a “physical” network adapter (MAC address) to a new interface (“ethX<\/code>“). By default we expect the network adapter to be assigned to eth0<\/code> and we have \/etc\/sysconfig\/network-scripts\/ifcfg-eth0<\/code> – in the above shot, CentOS assigned the VMware-presented “physical” network adapter to eth1. (It’s always best to put quotes around that word “physical” when you’re talking about a virtualized environment!) Thus…no network connectivity!<\/p>\n

There are many ways to fix this problem: create a new file \/etc\/sysconfig\/network-scripts\/ifcfg-eth1<\/code> to match the CentOS values, run ifconfig<\/code> commands manually to set this value, or update the udev<\/code> files to force CentOS to use the desired interface.<\/p>\n

For this blog entry, let’s take the third approach and update the udev<\/code> file to fix this problem: first, locate and edit \/etc\/udev\/rules.d\/70-persistent-net.rules<\/code> and modify as shown in the screenshot below:<\/p>\n

\"centos_nic_prob_02\"<\/a><\/p>\n

Look at the shot above: Do you see the circled area? In the original template VM, this file had eth0<\/code> as the named PCI device (via the NAME<\/code> element from the screenshot). However, the *cloned* VM had that value replaced with eth1<\/code>. (Maybe a CentOS \/ RHEL expert can share with us the logic behind this action…all I know is that it happens, not *why* it happens!)<\/p>\n

To fix this problem: Find the interface in the udev<\/code> file and modify the NAME<\/code> variable to be eth0<\/code> as indicated above. While at it, open \/etc\/sysconfig\/network-scripts\/ifcfg-eth0<\/code> and verify that no MAC address is specified. This allows the system to auto-bind to the correct interface. Then reboot the VM and all should be well:<\/p>\n

\"centos_nic_prob_03\"<\/a><\/p>\n

Ah – now we see that our eth1<\/code> interface has the IP address 172.24.7.249 assigned to it which is exactly what we wanted. Victory!<\/p>\n

Hope you all enjoyed this quick article, let me know if you have any questions or comments.<\/p>\n","protected":false},"excerpt":{"rendered":"

This is a quick entry on a particularly irritating CentOS peculiarity – after you clone a virtual machine, your \/etc\/sysconfig\/network-scripts\/ifcfg-eth0 no longer gets invoked. This is because CentOS can arbitrarily assign the physical network card to a different logic interface …<\/p>\n

CentOS\/RHEL and wrong Network Interface<\/span> Read More »<\/a><\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[42,19],"yoast_head":"\nCentOS\/RHEL and wrong Network Interface - softwareab<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.softwareab.net\/wordpress\/centosrhel-and-wrong-network-interface\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"CentOS\/RHEL and wrong Network Interface - softwareab\" \/>\n<meta property=\"og:description\" content=\"This is a quick entry on a particularly irritating CentOS peculiarity – after you clone a virtual machine, your \/etc\/sysconfig\/network-scripts\/ifcfg-eth0 no longer gets invoked. 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