boot.initrd.network: Support DHCP

This allows us to use it for EC2 instances.

nixos/modules/system/boot/initrd-network.nix

@@ -6,6 +6,23 @@ let
 
   cfg = config.boot.initrd.network;
 
+  udhcpcScript = pkgs.writeScript "udhcp-script"
+    ''
+      #! /bin/sh
+      if [ "$1" = bound ]; then
+        ip address add "$ip/$mask" dev "$interface"
+        if [ -n "$router" ]; then
+          ip route add default via "$router" dev "$interface"
+        fi
+        if [ -n "$dns" ]; then
+          rm -f /etc/resolv.conf
+          for i in $dns; do
+            echo "nameserver $dns" >> /etc/resolv.conf
+          done
+        fi
+      fi
+    '';
+
 in
 
 {
@@ -16,10 +33,13 @@ in
       type = types.bool;
       default = false;
       description = ''
-        Add network connectivity support to initrd.
-
-        Network options are configured via <literal>ip</literal> kernel
-        option, according to the kernel documentation.
+        Add network connectivity support to initrd. The network may be
+        configured using the <literal>ip</literal> kernel parameter,
+        as described in <link
+        xlink:href="https://www.kernel.org/doc/Documentation/filesystems/nfs/nfsroot.txt">the
+        kernel documentation</link>.  Otherwise, if
+        <option>networking.useDHCP</option> is enabled, an IP address
+        is acquired using DHCP.
       '';
     };
 
@@ -43,18 +63,35 @@ in
       copy_bin_and_libs ${pkgs.mkinitcpio-nfs-utils}/bin/ipconfig
     '';
 
-    boot.initrd.preLVMCommands = ''
-      # Search for interface definitions in command line
-      for o in $(cat /proc/cmdline); do
-        case $o in
-          ip=*)
-            ipconfig $o && hasNetwork=1
-            ;;
-        esac
-      done
+    boot.initrd.preLVMCommands =
+      # Search for interface definitions in command line.
+      ''
+        for o in $(cat /proc/cmdline); do
+          case $o in
+            ip=*)
+              ipconfig $o && hasNetwork=1
+              ;;
+          esac
+        done
+      ''
 
-      ${cfg.postCommands}
-    '';
+      # Otherwise, use DHCP.
+      + optionalString config.networking.useDHCP ''
+        if [ -z "$hasNetwork" ]; then
+
+          # Bring up all interfaces.
+          for iface in $(cd /sys/class/net && ls); do
+            echo "bringing up network interface $iface..."
+            ip link set "$iface" up
+          done
+
+          # Acquire a DHCP lease.
+          echo "acquiring IP address via DHCP..."
+          udhcpc --quit --now --script ${udhcpcScript}
+        fi
+      ''
+
+      + cfg.postCommands;
 
   };