src/main/java/io/github/hectorvent/floci/services/efs/EfsException.java

@@ -29,6 +29,10 @@ public class EfsException extends AwsException {
         return new EfsException(Status.NOT_FOUND, "MountTargetNotFound", "Mount target " + mountTargetId + " does not exist.");
     }
     
+    public static EfsException subnetNotFound(String subnetId) {
+        return new EfsException(Status.BAD_REQUEST, "SubnetNotFound", "The subnet ID '" + subnetId + "' does not exist");
+    }
+
     public static EfsException mountTargetConflict(String message) {
         return new EfsException(Status.CONFLICT, "MountTargetConflict", message);
     }

src/main/java/io/github/hectorvent/floci/services/efs/EfsService.java

@@ -2,7 +2,10 @@ package io.github.hectorvent.floci.services.efs;
 
 import io.github.hectorvent.floci.core.storage.StorageBackend;
 import io.github.hectorvent.floci.core.storage.StorageFactory;
+import io.github.hectorvent.floci.core.common.AwsException;
 import io.github.hectorvent.floci.core.common.Resettable;
+import io.github.hectorvent.floci.services.ec2.Ec2Service;
+import io.github.hectorvent.floci.services.ec2.model.Subnet;
 import io.github.hectorvent.floci.services.efs.model.*;
 import jakarta.enterprise.context.ApplicationScoped;
 import jakarta.inject.Inject;
@@ -33,13 +36,16 @@ public class EfsService implements Resettable {
     private final ConcurrentHashMap<String, Object> syncLocks = new ConcurrentHashMap<>();
     private final RegionResolver regionResolver;
 
+    private final Ec2Service ec2Service;
+
     private Object lockFor(String key) {
         return syncLocks.computeIfAbsent(key, k -> new Object());
     }
 
     @Inject
-    public EfsService(StorageFactory storageFactory, RegionResolver regionResolver) {
+    public EfsService(StorageFactory storageFactory, RegionResolver regionResolver, Ec2Service ec2Service) {
         this.regionResolver = regionResolver;
+        this.ec2Service = ec2Service;
         this.fileSystemStore = storageFactory.create("efs", "efs-filesystems.json",
                 new com.fasterxml.jackson.core.type.TypeReference<java.util.Map<String, FileSystem>>() {});
         this.mountTargetStore = storageFactory.create("efs", "efs-mounttargets.json",
@@ -361,14 +367,24 @@ public class EfsService implements Resettable {
     public MountTarget createMountTarget(CreateMountTargetRequest request, String region) {
         synchronized (lockFor(regionKey(region, request.getFileSystemId()))) {
             FileSystem fs = getFileSystem(region, request.getFileSystemId());
+            Subnet subnet = requireSubnet(region, request.getSubnetId());
 
-            for (MountTarget existing : mountTargetStore.scan(k -> k.startsWith(region + "::"))) {
+            List<MountTarget> regionMountTargets = mountTargetStore.scan(k -> k.startsWith(region + "::"));
+            for (MountTarget existing : regionMountTargets) {
                 if (existing.getFileSystemId().equals(request.getFileSystemId())) {
                     if (existing.getSubnetId().equals(request.getSubnetId())) {
                         throw EfsException.mountTargetConflict("Mount target already exists in this subnet");
                     }
+                    // AWS allows one mount target per Availability Zone, all in one VPC.
+                    if (subnet.getAvailabilityZone().equals(existing.getAvailabilityZoneName())) {
+                        throw EfsException.mountTargetConflict("Mount target already exists in this AZ");
+                    }
+                    if (!subnet.getVpcId().equals(existing.getVpcId())) {
+                        throw EfsException.mountTargetConflict("Mount targets of a file system must be in the same VPC");
                     }
                 }
+            }
+            long inSubnet = regionMountTargets.stream().filter(existing -> request.getSubnetId().equals(existing.getSubnetId())).count();
             
             String mtId = "fsmt-" + UUID.randomUUID().toString().replace("-", "").substring(0, 17);
         
@@ -376,11 +392,11 @@ public class EfsService implements Resettable {
             mt.setMountTargetId(mtId);
             mt.setFileSystemId(request.getFileSystemId());
             mt.setSubnetId(request.getSubnetId());
-            mt.setIpAddress(request.getIpAddress() != null ? request.getIpAddress() : "10.0.0.10");
+            mt.setIpAddress(request.getIpAddress() != null ? request.getIpAddress() : hostAddress(subnet.getCidrBlock(), inSubnet));
             mt.setLifeCycleState(LifeCycleState.available);
-            mt.setVpcId("vpc-12345678");
+            mt.setVpcId(subnet.getVpcId());
-            mt.setAvailabilityZoneName(region + "a");
+            mt.setAvailabilityZoneName(subnet.getAvailabilityZone());
-            mt.setAvailabilityZoneId(generateAzId(region + "a"));
+            mt.setAvailabilityZoneId(generateAzId(subnet.getAvailabilityZone()));
         mt.setNetworkInterfaceId("eni-" + UUID.randomUUID().toString().replace("-", "").substring(0, 17));
         if (request.getSecurityGroups() != null) {
             mt.setSecurityGroups(new ArrayList<>(request.getSecurityGroups()));
@@ -613,6 +629,32 @@ public class EfsService implements Resettable {
         return region + "::" + id;
     }
 
+    private Subnet requireSubnet(String region, String subnetId) {
+        if (subnetId == null || subnetId.isBlank()) {
+            throw EfsException.badRequest("SubnetId is required.");
+        }
+        try {
+            return ec2Service.requireSubnet(region, subnetId);
+        } catch (AwsException e) {
+            throw EfsException.subnetNotFound(subnetId);
+        }
+    }
+
+    /**
+     * An address inside the subnet for its n-th mount target, skipping the first four
+     * addresses AWS reserves in every subnet.
+     */
+    static String hostAddress(String cidrBlock, long index) {
+        String[] parts = cidrBlock.split("/");
+        String[] octets = parts[0].split("\\.");
+        long base = 0;
+        for (String octet : octets) {
+            base = (base << 8) | Long.parseLong(octet);
+        }
+        long address = base + 4 + index;
+        return ((address >> 24) & 0xff) + "." + ((address >> 16) & 0xff) + "." + ((address >> 8) & 0xff) + "." + (address & 0xff);
+    }
+
     private String generateAzId(String azName) {
         if (azName == null || azName.length() < 2) return azName + "-id";
         String region = azName.substring(0, azName.length() - 1);

src/test/java/io/github/hectorvent/floci/services/efs/EfsIntegrationTest.java

@@ -90,7 +90,7 @@ class EfsIntegrationTest {
             .body("""
                 {
                     "FileSystemId": "%s",
-                    "SubnetId": "subnet-12345",
+                    "SubnetId": "subnet-default-us-east-1-a",
                     "SecurityGroups": ["sg-11111"]
                 }
                 """.formatted(fileSystemId))
@@ -100,7 +100,10 @@ class EfsIntegrationTest {
             .statusCode(200)
             .body("MountTargetId", startsWith("fsmt-"))
             .body("FileSystemId", equalTo(fileSystemId))
-            .body("SubnetId", equalTo("subnet-12345"))
+            .body("SubnetId", equalTo("subnet-default-us-east-1-a"))
+            .body("VpcId", equalTo("vpc-default-us-east-1"))
+            .body("AvailabilityZoneName", equalTo("us-east-1a"))
+            .body("IpAddress", equalTo("172.31.0.4"))
             .extract().jsonPath().getString("MountTargetId");
     }
 

src/test/java/io/github/hectorvent/floci/services/efs/EfsMountTargetSubnetTest.java

@@ -0,0 +1,91 @@
+package io.github.hectorvent.floci.services.efs;
+
+import io.github.hectorvent.floci.testing.RestAssuredJsonUtils;
+import io.quarkus.test.junit.QuarkusTest;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+
+import static io.restassured.RestAssured.given;
+import static org.hamcrest.Matchers.equalTo;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/** Mount targets take their VPC, zone and address from the EC2 subnet they are created in. */
+@QuarkusTest
+class EfsMountTargetSubnetTest {
+
+    @BeforeAll
+    static void configureRestAssured() {
+        RestAssuredJsonUtils.configureAwsContentTypes();
+    }
+
+    private String createFileSystem(String token) {
+        return given()
+            .contentType("application/json")
+            .body("{\"CreationToken\": \"" + token + "\"}")
+        .when()
+            .post("/2015-02-01/file-systems")
+        .then()
+            .statusCode(201)
+            .extract().jsonPath().getString("FileSystemId");
+    }
+
+    private io.restassured.response.ValidatableResponse createMountTarget(String fileSystemId, String subnetId) {
+        return given()
+            .contentType("application/json")
+            .body("{\"FileSystemId\": \"" + fileSystemId + "\", \"SubnetId\": \"" + subnetId + "\"}")
+        .when()
+            .post("/2015-02-01/mount-targets")
+        .then();
+    }
+
+    @Test
+    void usesTheZoneAndVpcOfEachSubnet() {
+        String fileSystemId = createFileSystem("subnet-zones");
+
+        createMountTarget(fileSystemId, "subnet-default-us-east-1-b")
+            .statusCode(200)
+            .body("VpcId", equalTo("vpc-default-us-east-1"))
+            .body("AvailabilityZoneName", equalTo("us-east-1b"))
+            .body("IpAddress", equalTo("172.31.16.4"));
+        createMountTarget(fileSystemId, "subnet-default-us-east-1-c")
+            .statusCode(200)
+            .body("AvailabilityZoneName", equalTo("us-east-1c"));
+    }
+
+    @Test
+    void refusesASubnetThatDoesNotExist() {
+        String fileSystemId = createFileSystem("subnet-missing");
+
+        createMountTarget(fileSystemId, "subnet-nope")
+            .statusCode(400)
+            .body("__type", equalTo("SubnetNotFound"));
+    }
+
+    @Test
+    void allowsOneMountTargetPerZone() {
+        String fileSystemId = createFileSystem("subnet-one-per-zone");
+        String vpcId = "vpc-default-us-east-1";
+        String secondSubnet = given()
+            .formParam("Action", "CreateSubnet")
+            .formParam("VpcId", vpcId)
+            .formParam("CidrBlock", "172.31.200.0/24")
+            .formParam("AvailabilityZone", "us-east-1a")
+            .header("Authorization", "AWS4-HMAC-SHA256 Credential=test/20260101/us-east-1/ec2/aws4_request")
+        .when()
+            .post("/")
+        .then()
+            .statusCode(200)
+            .extract().path("CreateSubnetResponse.subnet.subnetId");
+
+        createMountTarget(fileSystemId, "subnet-default-us-east-1-a").statusCode(200);
+        createMountTarget(fileSystemId, secondSubnet)
+            .statusCode(409)
+            .body("__type", equalTo("MountTargetConflict"));
+    }
+
+    @Test
+    void picksAddressesAfterTheFourAwsReserves() {
+        assertEquals("10.0.1.4", EfsService.hostAddress("10.0.1.0/24", 0));
+        assertEquals("10.0.2.1", EfsService.hostAddress("10.0.1.0/24", 253));
+    }
+}