Subject: kern/10319: NFS_BOOTDHCP fails if DHCP relay honors IP TTL
To: None <email@example.com>
From: Scott Reynolds <firstname.lastname@example.org>
Date: 06/07/2000 22:15:12
>Synopsis: NFS_BOOTDHCP fails if DHCP relay honors IP TTL
>Arrival-Date: Wed Jun 07 22:16:00 PDT 2000
>Originator: Scott Reynolds
>Release: NetBSD-current 2000-05-28
System: NetBSD shark 1.4Z NetBSD 1.4Z (SHARK) #21: Sun May 28 20:03:12 CDT 2000 root@shark:/amd/toaster.eew.tgi.plexus.com/netbsd/src/sys/arch/arm32/compile/SHARK
The IP TTL in a DHCPDISCOVER broadcast is set to 1 when getting boot
parameters for a diskless client. If a DHCP relay honors the TTL,
the broadcast will never reach the server. This behavior has been
observed with a Cisco IOS 12.0-based layer 3 switch.
Code inspection reveals the problem fairly quickly. From
src/sys/nfs/nfs_bootdhcp.c, line 482:
* Skip the route table when sending on this socket.
* If this is not done, ip_output finds the loopback
* interface (why?) and then fails because broadcast
* is not supported on that interface...
Setting SO_DONTROUTE on the socket results in the obvious behavior.
1. Set up a diskless system that uses DHCP to get boot parameters on
a network using a Cisco IOS switch/router. Make sure the DHCP
server is on a different IP network.
2. Use "ip helper-address" to configure DHCP relay from the
diskless client to the DHCP server.
3. Try to boot, watch it fail, and note from tcpdump output that
the IP TTL is set to 1 for the first set of DHCPDISCOVER
broadcasts after the kernel is loaded.
None provided. Use dhcrelay(8) on another system to work around