Linux – Server not sending ICMP echo reply

centoscentos6linux

I'm having a weird problem with a CentOS 6.6 machine. It is a server running KVM and a few guest VMs using a bridge interface. Basically, my issue is this: The VMs on this host have no network connectivity issues at all, I can ssh to them and ping them just fine. The host, however, can pretty much only communicate with devices in its own subnet (including the VMs running on it) and a few switches…

If I run a tcpdump on it and try pinging from a device that can't communicate with it, I can see the ICMP echo requests coming in, but no echo reply is being sent back out. I've checked everything I can think of: restart networking, restart server, iptables is totally empty, selinux turned off, icmp_echo_ignore_all = 0, all routes are correct and identical to other servers in the same subnet that have no issues.

I'm pretty much out of ideas, any help would be greatly appreciated!

Thanks,

Best Answer

It seems like this could be a routing issue. Since the VMs inside KVM can route independently of the host, the issue is not going to be your physical network connectivity, but rather your logical network connectivity. Check your default gateway and static routes, make sure those are configured properly. If your host cannot find a route back to the devices pinging it, it cannot respond to icmp (ping) requests.

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Linux – Cannot Ssh to VM but can Ping it

If you can ping it it means there is tcp connectivity ; packet can reach the destination and traverse back -- which is good.

Now just test if there is ssh service listening.

telnet server.ip 22

you should get something like

Trying someIPaddress...
Connected to thatIPaddress.
Escape character is '^]'.
SSH-2.0-OpenSSH_X.Y YourDistro

if that does not happen; there is chance that - service is not active on destination host - service is filtered

Nmap can help you to detect if service is filtered; but at this point my bet would be you don't have openssh-server installed on those hosts; just ssh client.

You can also check on those systems with something

netstat -nlp | grep ssh

lsof -i -n  |grep ssh | grep LISTEN

if the service is active and listening...

Centos/OVH: public IP on KVM virtual machine

I have the same issue here. I want to be able to have host and VM guest on public IP. Communication between host and VM seems to work fine, but VM cannot reach the internet or vice versa.

My provider also has rule to have just one MAC address per switchport. Essentially VMAC addresses are used for failover in networking appliances like VRRP and some other protocols, that means if first switch fails the second link to another switch takes over and you get "replicated" switchport with actual MAC address and everything.

I don't see how can you benefit from this.

The best solution is to put 2 public IPs on host Ethernet interface, one for host second for VM. And do some policy routing.

Regards

Best Answer

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Centos/OVH: public IP on KVM virtual machine

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