According to Alien Bob's WIKI page on Configuring your network in Slackware
, you can set DHCP_KEEPGW in /etc/rc.d/rc.inet1.conf to "yes" if you don't want the DHCP server to change your default gateway.
Used in conjunction with GATEWAY in the same file, this would provide a workaround.
Would be nice, of course, to find out why Slackware is not picking up the gateway from the (DDWRT v24) DHCP server, especially when a variety of other OSes do so. It is unlikely to be a VM networking issue. Have you tried setting DHCP_DEBUG="yes" in /etc/rc.d/rc.inet1.conf? Did you try DEBUG_ETH_UP="yes"?
I haven't been able to capture packets to/from the guest using the host. I tried using tcpdump on eth0 while a (VirtualBox) VM was generating network traffic but saw only traffic to/from the host and no packets to/from the guest. [EDIT: but I'm a tcpdump lightweight so that doesn't mean it's not possible]
It may be possible to run tcpdump on the guest, by modifying the etc/rc.d/rc.inet1 script, to start tcpdump after eth0 is brought up, and stop it a few seconds after the /sbin/dhcpcd -t command.