dhcp - Ping retrieves an IP from DNS, but loses all packets

06
2014-04

vermiculus

I'm working with a minimal installation of RHEL 6.

When I first turned this machine on, it couldn't ping anybody. It turned out that I needed to add the GATEWAY under /etc/sysconfig/networking and configure /etc/sysconfig/network-scripts/ifcfg-eth0 to point to the gateway and boot on start using DHCP.

As of now, I can access any other machine on the local network; I can even SSH into the machine I'm working with. What I still cannot do is access anything outside of the local network; I can't access the internet.

Consider:

[root@dbrepa ~]# ping dev-108a -c 4
PING dev-108a.some.domain.net (192.168.108.119) 56(84) bytes of data.
64 bytes from dev-108a.some.domain.net (192.168.108.119): icmp_seq=1 ttl=64 time=0.276 ms
64 bytes from dev-108a.some.domain.net (192.168.108.119): icmp_seq=2 ttl=64 time=0.249 ms
64 bytes from dev-108a.some.domain.net (192.168.108.119): icmp_seq=3 ttl=64 time=0.250 ms
64 bytes from dev-108a.some.domain.net (192.168.108.119): icmp_seq=4 ttl=64 time=0.259 ms

--- dev-108a.some.domain.net ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3000ms
rtt min/avg/max/mdev = 0.249/0.258/0.276/0.019 ms



[root@dbrepa ~]# ping example.com
PING example.com (93.184.216.119) 56(84) bytes of data.
^C
--- example.com ping statistics ---
7 packets transmitted, 0 received, 100% packet loss, time 6741ms



[root@dbrepa ~]# route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.108.0   *               255.255.255.0   U     0      0        0 eth0
link-local      *               255.255.0.0     U     1002   0        0 eth0
default         dc-108a.cool.do 0.0.0.0         UG    0      0        0 eth0

dev-108a is a server on our network, and as you can see I can speak to it with no issues. However, for example.com, I can retrieve the IP address from the DNS for example.com, in my case 93.184.216.119, but then ping just hangs. It's still working---it's still sending out packets, but none of them will get replies and this process will continue until kill.

As per the comments, I've brought the local firewall down (with service iptables stop) to no effect.

What could be going wrong? How am I getting an IP address while not able to access it?

I know that I am missing several core UNIX tools; I am at least missing:

man
dig
traceroute
telnet
wget

Answers

Nikolay

'What could be going wrong? How am I getting an IP address while not able to access it?"

You are getting IP address because your computer can successfully reach the DNS server which your computer was instructed to use.

To make sure you can communicate to the outside of the network you need to make sure you have correct network settings including IP address, subnet mask and router.

If there is firewall operating at the gateway you have to make sure your computer was granted access from inside the network to the outside.

If can provide output of the following commands it will be possible to debug it faster:

ifconfig -a
route

Related Answers

t0mm13b

When ping states 'Destination Host unreachable', that is a likely indication that the router could not forward the ICMP packet across to the specific host across the internet.

Simple steps to check:

ifconfig eth0 - checks to see if the network device is up and running
ping 127.0.0.1 - checks to see if the tcp/ip stack is functioning
ping {your_routers_ip_address - checks to see if the nic can transmit data to your router's ip address (by default, 192.168.1.1) - if it doesn't at this stage, then your nic could be faulty. If it works, then the issue is with the router...

If the NIC appears to be faulty, check the log by issuing this on the command line

less /var/log/messages

  If this fails, you need to use this instead: 

sudo less /var/log/messages 

  then enter your password when prompted to do so.

and look for the specific keyword eth0 followed by the message on the same line - double check it. Maybe an update of the driver might be required or an incorrect configuration that causes your pc to drop connections.

If the issue is the router - there could be a fault on the line itself. As John mentioned - don't be surprised - even the simplest thing as a bad cable can mislead you. Swap the cable for a new one and repeat the steps above. If it still fails, the problem is on your computer's NIC.

Can you please confirm all of this first and let us know what driver you are using, make/manufacturer of the NIC, what kind of router etc..this will help us all in troubleshooting further.

John Gardeniers

Start with some basic network connectivity checks. If it's a cabled network my first guess would be a bad cable. Try another. If it's wireless you may be losing the signal.

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dhcp - Ping retrieves an IP from DNS, but loses all packets