I'm running a few Ubuntu 16.04 VMs in the Azure cloud. Whenever I reboot a VM I have to set the default gateway manually.

The gateway should be set by dhclient. I activated the debugging and get this:

root@backend01:/etc/dhcp/dhclient-exit-hooks.d# cat /tmp/dhclient-script.debug

root@backend01:/etc/dhcp/dhclient-exit-hooks.d# cat /tmp/dhclient-script.debug
Thu Jun  2 17:18:33 UTC 2016: entering /etc/dhcp/dhclient-enter-hooks.d, dumping variables.
Thu Jun  2 17:18:33 UTC 2016: entering /etc/dhcp/dhclient-enter-hooks.d, dumping variables.
new_rfc3442_classless_static_routes='0 10 10 0 1 32 168 63 129 16 10 10 0 1'

So the routers option is set correctly. Even the lease file shows the correct value:

lease {
  interface "eth0";
  server-name "RD7CFE90879C98";
  option subnet-mask;
  option dhcp-lease-time 4294967295;
  option routers;
  option dhcp-message-type 5;
  option dhcp-server-identifier;
  option domain-name-servers,;
  option domain-search "azure-prod.";
  option dhcp-renewal-time 4294967295;
  option rfc3442-classless-static-routes 0,10,10,0,1,32,168,63,129,16,10,10,0,1;
  option unknown-245 a8:3f:81:10;
  option dhcp-rebinding-time 4294967295;
  option domain-name "azure-prod";
  renew 0 2152/07/09 23:41:47;
  rebind 0 2152/07/09 23:41:47;
  expire 0 2152/07/09 23:41:47;

This is how my /etc/dhcp/dhclient.conf looks:

option rfc3442-classless-static-routes code 121 = array of unsigned integer 8;

send host-name = gethostname();
request subnet-mask, broadcast-address, time-offset, routers,
    domain-name, domain-name-servers, domain-search, host-name,
    dhcp6.name-servers, dhcp6.domain-search, dhcp6.fqdn, dhcp6.sntp-servers,
    netbios-name-servers, netbios-scope, interface-mtu,
    rfc3442-classless-static-routes, ntp-servers;

timeout 300;

supersede domain-name "azure-prod";
supersede domain-search "azure-prod";

Why doesn't dhclient set my default route?

  • Check your dhclient-enter-hooks.d for any file containing unset new_routers. – rda Jun 2 '16 at 18:46
  • Unfortunately thats not the case. I don't have any "unset" statements in my hooks (enter and exit). – Kai Jun 3 '16 at 7:37

I found the problem. Since the Azure DHCP sets the rfc3442-classless-static-routes option the routers option gets ignored by the dhclient-script.

The rfc3442 gets handled by an exit hook script in /etc/dhcp/dhclient-exit-hooks.d/rfc3442-classless-routes

My problem was that I had another custom exit-hook, which did an exit 0 at the end and therefore ended the execution of dhclient-script which in return never got to execute the rfc3442-classless-routes exit-hook.

This was deeply buried and cost my almost a day of work. So I hope that this will keep someone from having the same problem in the future.

  • 2
    Wanted to add something to your findings. I had the same problem, and it seems the "issue" is with the from RFC3442 If the DHCP server returns both a Classless Static Routes option and a Router option, the DHCP client MUST ignore the Router option. Similarly, if the DHCP server returns both a Classless Static Routes option and a Static Routes option, the DHCP client MUST ignore the Static Routes option. mail-archive.com/debian-bugs-dist@lists.debian.org/… – tore- Sep 25 '16 at 20:04

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