lnd: add neutrino backend support

Add option to use neutrino instead of bitcoind for fetching blockchain
data. This allows running LND as a lightweight client that connects to
remote Bitcoin full nodes via P2P protocol.

New options:
- services.lnd.backend: choose between "bitcoind" (default) or "neutrino"
- services.lnd.neutrino.addpeers: list of Bitcoin node peers to connect to

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

modules/lnd.nix

@@ -144,6 +144,22 @@ let
       description = "LND TLS certificate path.";
     };
     tor = nbLib.tor;
+    backend = mkOption {
+      type = types.enum [ "bitcoind" "neutrino" ];
+      default = "bitcoind";
+      description = "The backend to use for fetching blockchain data.";
+    };
+    neutrino = {
+      addpeers = mkOption {
+        type = types.listOf types.str;
+        default = [];
+        example = [ "192.168.1.1:8333" "btcd.example.com:8333" ];
+        description = ''
+          List of Bitcoin full node peers to connect to via neutrino.addpeer.
+          Multiple peers provide redundancy for maximum uptime.
+        '';
+      };
+    };
   };
 
   cfg = config.services.lnd;
@@ -170,15 +186,20 @@ let
     restlisten=${cfg.restAddress}:${toString cfg.restPort}
 
     bitcoin.${bitcoind.network}=1
-    bitcoin.node=bitcoind
 
     ${optionalString (cfg.tor.proxy) "tor.active=true"}
     ${optionalString (cfg.tor-socks != null) "tor.socks=${cfg.tor-socks}"}
 
-    bitcoind.rpchost=${bitcoindRpcAddress}:${toString bitcoind.rpc.port}
+    ${if cfg.backend == "bitcoind" then ''
-    bitcoind.rpcuser=${bitcoind.rpc.users.public.name}
+      bitcoin.node=bitcoind
-    bitcoind.zmqpubrawblock=${zmqHandleSpecialAddress bitcoind.zmqpubrawblock}
+      bitcoind.rpchost=${bitcoindRpcAddress}:${toString bitcoind.rpc.port}
-    bitcoind.zmqpubrawtx=${zmqHandleSpecialAddress bitcoind.zmqpubrawtx}
+      bitcoind.rpcuser=${bitcoind.rpc.users.public.name}
+      bitcoind.zmqpubrawblock=${zmqHandleSpecialAddress bitcoind.zmqpubrawblock}
+      bitcoind.zmqpubrawtx=${zmqHandleSpecialAddress bitcoind.zmqpubrawtx}
+    '' else ''
+      bitcoin.node=neutrino
+      ${lib.concatMapStringsSep "\n" (peer: "neutrino.addpeer=${peer}") cfg.neutrino.addpeers}
+    ''}
 
     wallet-unlock-password-file=${secretsDir}/lnd-wallet-password
 
@@ -202,9 +223,15 @@ in {
           services.lnd.port to a port other than 9735.
         '';
       }
+      { assertion = cfg.backend != "neutrino" || cfg.neutrino.addpeers != [];
+        message = ''
+          When using neutrino backend, you must configure at least one peer
+          in services.lnd.neutrino.addpeers.
+        '';
+      }
     ];
 
-    services.bitcoind = {
+    services.bitcoind = mkIf (cfg.backend == "bitcoind") {
       enable = true;
 
       # Increase rpc thread count due to reports that lightning implementations fail
@@ -225,12 +252,14 @@ in {
 
     systemd.services.lnd = {
       wantedBy = [ "multi-user.target" ];
-      requires = [ "bitcoind.service" ];
+      requires = optional (cfg.backend == "bitcoind") "bitcoind.service";
-      after = [ "bitcoind.service" "nix-bitcoin-secrets.target" ];
+      after = optional (cfg.backend == "bitcoind") "bitcoind.service" ++ [ "nix-bitcoin-secrets.target" ];
       preStart = ''
         install -m600 ${configFile} '${cfg.dataDir}/lnd.conf'
         {
-          echo "bitcoind.rpcpass=$(cat ${secretsDir}/bitcoin-rpcpassword-public)"
+          ${optionalString (cfg.backend == "bitcoind") ''
+            echo "bitcoind.rpcpass=$(cat ${secretsDir}/bitcoin-rpcpassword-public)"
+          ''}
           ${optionalString (cfg.getPublicAddressCmd != "") ''
             echo "externalip=$(${cfg.getPublicAddressCmd})"
           ''}
@@ -288,7 +317,7 @@ in {
     users.users.${cfg.user} = {
       isSystemUser = true;
       group = cfg.group;
-      extraGroups = [ "bitcoinrpc-public" ];
+      extraGroups = optional (cfg.backend == "bitcoind") "bitcoinrpc-public";
       home = cfg.dataDir; # lnd creates .lnd dir in HOME
     };
     users.groups.${cfg.group} = {};