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What's the proper way to determine the computer's gateway when going from DHCP to a static IP. Operating system is Mythbunutu with Ububtu desktop installed.

I had previously been editing /etc/network/interfaces, but eventually the computer would stop being able to connect to the Internet (although computers behind the access point could still connect to it).

Now I'm using gnome-network-admin assuming the GUI knows what's going on behind the scenes better than I do.

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Why are you going with a static-ip? ( Edit the question ) –  Brad Gilbert Jul 18 '09 at 21:13
    
MythTV backend server. –  Will Jul 19 '09 at 13:40
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3 Answers

up vote 3 down vote accepted

To get your gateway's IP address, use the route command with the -n flag to translate hostnames into IP addresses (or rather, disable DNS lookups on the IP address).

$ route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
10.10.10.0      0.0.0.0         255.255.255.0   U     0      0        0 eth0
0.0.0.0         10.10.10.1      0.0.0.0         UG    100    0        0 eth0

The default gateway is the one where the destination is 0.0.0.0, in this case, 10.10.10.1.

Though I'm wondering, why are you manually assigning a static IP? Does your access device (router, wifi, whatever) not allow DHCP reservations? Most devices do these days. You just need to provide the MAC address, and many will already know the MAC by hostname. If your's doesn't you can get the MAC from ip, the "link/ether" address:

$ ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN qlen 1000
    link/ether 00:50:56:10.10:21 brd ff:ff:ff:ff:ff:ff
    inet 10.10.10.121/24 brd 10.10.10.255 scope global eth0
    inet6 fe80::250:56ff:fe10.1021/64 scope link
       valid_lft forever preferred_lft forever

or ifconfig, the "HWaddr" address:

$ ifconfig
eth0      Link encap:Ethernet  HWaddr 00:50:56:10.10:21
          inet addr:10.10.10.121  Bcast:10.10.10.255  Mask:255.255.255.0
          inet6 addr: fe80::250:56ff:fe10.1021/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:107139 errors:0 dropped:0 overruns:0 frame:0
          TX packets:12345 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:29375815 (29.3 MB)  TX bytes:1447714 (1.4 MB)
          Interrupt:18 Base address:0x2000
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Why am I manually assigning a static IP? WRT54G2 does not let you assign static IPs in the router as far as I can tell. –  Will Jul 19 '09 at 13:49
    
I have a WRT54G(L) and I replaced the Linksys firmware with a 3rd party - DD-WRT. –  jtimberman Jul 19 '09 at 14:38
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jtimberman and freiheit provided good answers. Let me add one thing: it is not clear if you are the network administrator or not. If you are not, assigning IP addresses statically when the network is supposed to use DHCP is really playing the Sorcerer's Apprentice. You can create a lot of problems (assigning an existing IP address, for instance).

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I am the network administrator, it's home. –  Will Jul 19 '09 at 13:50
    
Then, the advices from jtimberman and freiheit are good. –  bortzmeyer Jul 20 '09 at 10:49
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/etc/network/interfaces is the right thing to use, but the GUI tools are fine as well.

Are you simply statically configuring the same IP that DHCP gave you? If you do that, the DHCP server will eventually lease the same IP out to another computer/device and you'll have an IP conflict. If you give your system an IP via any of those static methods, you need to give it an IP nothing else will use. Either outside the range of DHCP, or one that DHCP knows belongs only to your MAC address. If you configure a static assignment on the DHCP server for your MAC address, you shouldn't have to worry about any of this...

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