Debian: Try DHCP but set specific IP if it fails

[oh_noes]

When networking comes online for a specific group of servers, I need them to try to DHCP, but if that fails I need to set a specific IP address (private net).

How can I do this? Everyone is saying it's not possible, but it's FOSS right, so anything is possible.

I'm thinking I might have to write a udev or if-up script?

[HyRax1]

...why would you expect DHCP to fail?

[f3n1x]

No need. (edit: Whoever told you wasn't possible is talking out their backside).

Here's the way i'd do that.

Edit your /etc/network/interfaces file and create a static ip entry for the desired interface with the private network ip address you wish the machine to use assuming a DHCP lease is not obtained like this:

auto eth0
iface eth0 inet static
address 10.0.94.254
netmask 255.0.0.0
broadcast 10.255.255.255
network 10.0.0.0
post-up /sbin/dhclient eth0 || true

The last line will run dhclient on the eth0 interface and then ensure that even if it fails ifup will continue to consider the interface configured.

Obviously change the details to match your network and system configuration.

[f3n1x]

Originally Posted by HyRax1

...why would you expect DHCP to fail?

Laptop moving between networks where one has a dhcp server and the other has machines configured statically? Some people hate NetworkManager that much.

[cleary]

You could also try using something like guessnet:

Package: guessnet                                                                                       
Priority: optional                                                                                      
Section: net                                                                                            
Installed-Size: 492                                                                                     
Maintainer: Enrico Zini <enrico@debian.org>                                                             
Architecture: amd64                                                                                     
Version: 0.49-1                                                                                         
Depends: libc6 (>= 2.7-1), libgcc1 (>= 1:4.1.1-21), libiw29 (>= 28+29pre7), libnet1 (>= 1.1.2-1), libpcap0.8 (>= 0.9.3-1), libstdc++6 (>= 4.2.1-4)
Suggests: pppoe, waproamd, ifplugd                                                                                                                
Filename: pool/main/g/guessnet/guessnet_0.49-1_amd64.deb                                                                                          
Size: 186516                                                                                                                                      
MD5sum: 51f7c7f1a32c3f0c5ac6910302799496                                                                                                          
SHA1: 5f9bcd01ca092f2d642aa6eff8e0e7ebe111ee8e                                                                                                    
SHA256: e3b074bea69d359d9d5c9487d58e6675df85f2394fb1b53ac6fc8efb53620c03                                                                          
Description: Guess which LAN a network device is connected to                                                                                     
 Guessnet is a non-aggressive network detection tool to use when moving                                                                           
 a machine among networks which don't necessarily provide DHCP.                                                                                   
 .                                                                                                                                                
 Guessnet takes in input a list of candidate network profiles,
 each of which includes a test description; then it runs all
 the tests in parallel and prints the name of the profile whose
 test was the first one to succeed.
 .
 Available tests are:
  * ARP probes to check for known hosts in the network
  * link beat check, to check if the interface is connected to anything
  * Wireless lan checks, to look for access points found by waproamd
  * PPPOE check to see if there is a concentrator accessible via PPPOE
  * Checks provided by custom arbitrary scripts.
 .
 Guessnet can be used in either native mode or "ifupdown mode".
 In the latter case guessnet integrates nicely with ifupdown
 as a "mapping script".
Enhances: ifupdown
Tag: hardware::laptop, implemented-in::c++, interface::commandline, network::scanner, role::program, scope::utility, use::scanning

[oh_noes]

Originally Posted by f3n1x

No need. (edit: Whoever told you wasn't possible is talking out their backside).

Here's the way i'd do that.

Edit your /etc/network/interfaces file and create a static ip entry for the desired interface with the private network ip address you wish the machine to use assuming a DHCP lease is not obtained like this:

auto eth0
iface eth0 inet static
address 10.0.94.254
netmask 255.0.0.0
broadcast 10.255.255.255
network 10.0.0.0
post-up /sbin/dhclient eth0 || true

The last line will run dhclient on the eth0 interface and then ensure that even if it fails ifup will continue to consider the interface configured.

Obviously change the details to match your network and system configuration.

Sorry, major bump as I finally got time to implement this.

Firstly, I like your thinking process. I didn't think of doing it that way.

However, when I tried to implement it I found it didn't work. Once dhclient tried to bind to an interface, it immediately reset that static IP to 'nothing' and once it failed the DHCP, it assigned 'nothing' to that interface.

Any ideas?

[cleary]

you probably want to run the dhclient command as a pre-up command

man interfaces:

pre-up command
              Run  command  before  bringing  the  interface  up.  If this command fails then ifup aborts, refraining from marking the interface as configured, prints an error
              message, and exits with status 0.  This behavior may change in the future.

you need to catch it if it fails though, I'd begin with the OR (||) syntax f3nix used above, but I have no idea how to group/apply the static settings after that

[Crinos]

You're looking in the wrong config file.

Leave /etc/network/interfaces set to 'dhcp', and let dhclient's logic determine how to configure your interface.

Add the following(or similar) block to /etc/dhcp3/dhclient.conf(modified for your network obviously):

timeout 10;
initial-interval 1;
backoff-cutoff 3;

lease {
  interface "eth0";
  fixed-address 192.168.1.2;
  option subnet-mask 255.255.255.0;
  option broadcast-address 192.168.1.255;
  option routers 192.168.1.154;
}

It will try the DHCP server for a lease, then try to use this "local lease" if it fails. I tweaked the timeout options so you don't sit there waiting for over a minute twiddling your thumbs waiting for DHCP to fail.. these settings cause it to give up after about 15 seconds on my home network, but on a congested network(or one where you can't send DHCP requests quite as aggressively as this, every couple of seconds), where DHCP might take longer, you might want to up the times a bit.

As far as "everyone saying it's not possible"... uh, who's everyone? They should be sacked for a lack of thoroughness when it comes to finding solutions to pretty trivial problems