wpa_supplicant doesn't get new IP when re-associate?

Question

I am struggling to setup my wlan so it can re-associate to another hotspot and renew its IP automatically. I have found a similar question but nobody responded.

I am using Debian stretch. The first association works fine but when the WiFi re-associates with another hotspot (for whatever reason (e.g. first hotspot gets shutdown)) the IP stays the same. If I manually run dhclient wlan0 the IP will refresh. But I want it to be automatic.

Here is my /etc/network/interfaces:

allow-hotplug eth0

auto wlan0
allow-hotplug wlan0
iface wlan0 inet dhcp
wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf
iface default inet dhcp

/etc/wpa_supplicant/wpa_supplicant.conf contains:

ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1

network={
    ssid="FrstHotspot"
    psk="some password"
}

network={
    ssid="SecondHotspot"
    psk="some password"
}

Answer 1

As for your /etc/network/interfaces, I think it has to be something similar to this to work with your current configuration:

allow-hotplug wlan0
iface wlan0 inet manual
wpa-driver wext
wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf
iface default inet dhcp

You are defining the interface has dhcp, which then enters in conflict with the wpa_supplicant that invokes DHCP. In that way, wpa_supplicant loses the ability to signal when the ssid/network was changed, and so the IP address does not change when changing networks/ssids.

You can also define priorities in the wpa_supplicant.conf on the ssid to use as in:

ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev  
update_config=1

network={  
ssid="FrstHotspot"  
psk="some password" 
priority = 10  
}

network={  
ssid="SecondHotspot"  
psk="some password" 
priority = 20  
}

The higher the priority, that will be the ssid that will be tried first.

Some content of https://www.raspberrypi.org/documentation/configuration/wireless/wireless-cli.md states:

If you have two networks in range, you can add the priority option to choose between them. The network in range, with the highest priority, will be the one that is connected.

Answer 2

Partial answer: wpa_supplicant by itself only manages authentication on WLAN interfaces. dhclient is run by another layer, either the traditional ifup/ifdown or others like network manager. wpa_supplicant can send a signal to the other layer, but the other layer must be configured for that.

So the first step is to find out what kind of network management you have installed (Debian supports several). Then you need to configure it.

The wpa_supplicant documentation also to have a document on roaming access, this may also contain hints how to correctly setup ifup/ifdown for automatic reconnect. Look in /usr/share/doc/.