You are best off not accepting DHCP leases from such networks. Using a trivial DHCP starvation attack (dhcdrop, dhcpstarv, scapy) anyone can take over and serve leases => route, DNS servers, by running their own DHCP server. Use netdiscover (in e.g. Ubuntu repo) to discover the network range, and OpenDNS/Google DNS, and a static configuration. Add a VPN configuration as well, and use a firewall rule to use only the VPN virtual network (e.g. tun0) interface to route your traffic out. 

Have a look at the attacks made possible with the wifi pineapple.

And additionally, avoid entering any password you use elsewhere into a public wifi's captive portals.