Ubuntu – unable to reach local network computers, but able to browse the web

This question should not have been migrated from serverfault.com, as it is a common system administration issue faced by admins and IT guys every day.

In short, certain router setups/network topologies prevent you from accessing the external address of the network from within the internal network, especially when traffic from the external address is sent back to the internal network anyway. Look at the following topology:

[A] Web --> [b]External ip address ---> [c]Router/firewall/gateway[d] ---> [e]Actual server ip address

The problem is that while users from [A] can see [e] by going to www.imaginaryplace.com, people inside the lan going to that address really want to go straight to [e] - and the router/firewall/gateway setup isn't bright enough to send traffic coming from [e]s local network all the way to [b] and then back to [e], where it would become confused by the [d] to [c] to [d] path and likely drop the traffic.

The fix is to a) use a different URL for internal traffic, like inside.domain.tld or b) use split DNS where the name server knows that requests coming from certain addresses get handed addresses on the [e] network, or using hosts files on the internal workstations which override the external DNS requests. On small windows networks, this is a job for batch files.

In most events, the way to fix it is to a) use split dns, where you hand out a different IP address

Many corporate firewalls block ICMP - which is the protocol is used by ping utility.

Better solution for this is to try TCP connection to google.com:80.

Easiest way to check for basic internet connectivity in shell script is utility nc (it should be easily available on most Linux systems):

nc -w 3 -z google.com 80
echo $?

This means check if port 80 on google.com is open, and timeout after 3 seconds. If connection was successful, it will print 0, and if it failed, it will print 1.

If you want to check internet connectivity without checking DNS (which could be broken in itself), you can use Google's preferred DNS server 8.8.8.8, but the only port that it has open is 53 (aka domain):

nc -w 3 -z 8.8.8.8 53
echo $?

However, port 53 can be also blocked by your corporate firewall (not common, but possible). Ports 80 and 443, on other hand, are almost never firewalled.