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I'm trying to configure my dorm room network. I'm working on ubuntu. It seems that the dorm building has a host as gateway

My route -n looks like this:

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface   U     0      0        0 eth0     U     1000   0        0 eth0         UG    100    0        0 eth0

I can't ping

$ ping
PING ( 56(84) bytes of data.
From icmp_seq=3 Packet filtered
From icmp_seq=5 Packet filtered
From icmp_seq=7 Packet filtered
--- ping statistics ---
7 packets transmitted, 0 received, +3 errors, 100% packet loss, time 6014ms

But I can ssh to a school server with an ip like 202.XXX.XXX.XXX, and gain internet access from that server.

  • Is this normal?
  • I'm desperate for some basic network knowledge, are there any good books or web resources?


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up vote 6 down vote accepted

1) Not normal, but certainly not very unusual. Many networks don't allow anyone to ping the infrastructure, especially if they service comparitively-untrusted end-users.

A more useful test of whether a host is really available is to try pinging it, and then examine your ARP cache for an entry. To participate in a layer 2 conversation (which acting as your default gateway on an ethernet network definitely counts as), the host must respond to ARP requests, or you can't talk to it. From your ubuntu host, try

arp -a -n|grep 22.254

If you see something like

? ( at 00:c0:f0:3b:4d:eb [ether] on eth0

then it's up and available, even if it's filtering ICMP echo-requests (ping requests). If you get

? ( at <incomplete> on eth0

then it's genuinely down.

2) The absolute bible is Stevens' "TCP/IP Illustrated", but it's not light reading. However, by the time you get to the end of the last volume, you'll know a lot! Others may have recommendations for lighter reading.

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