Make the federation homeserver resolve more accurate (on resolve via DNS record check that the certificate present for the original host).

src/main/java/io/kamax/mxisd/matrix/HomeserverFederationResolver.java

@@ -178,26 +178,26 @@ public class HomeserverFederationResolver {
         }
     }
 
-    public URL resolve(String domain) {
+    public HomeserverTarget resolve(String domain) {
         Optional<URL> s1 = resolveOverwrite(domain);
         if (s1.isPresent()) {
             URL dest = s1.get();
             log.info("Resolution of {} via DNS overwrite to {}", domain, dest);
-            return dest;
+            return new HomeserverTarget(dest.getHost(), dest);
         }
 
         Optional<URL> s2 = resolveLiteral(domain);
         if (s2.isPresent()) {
             URL dest = s2.get();
             log.info("Resolution of {} as IP literal or IP/hostname with explicit port to {}", domain, dest);
-            return dest;
+            return new HomeserverTarget(dest.getHost(), dest);
         }
 
         Optional<URL> s3 = resolveWellKnown(domain);
         if (s3.isPresent()) {
             URL dest = s3.get();
             log.info("Resolution of {} via well-known to {}", domain, dest);
-            return dest;
+            return new HomeserverTarget(dest.getHost(), dest);
         }
         // The domain needs to be resolved
 
@@ -205,12 +205,30 @@ public class HomeserverFederationResolver {
         if (s4.isPresent()) {
             URL dest = s4.get();
             log.info("Resolution of {} via DNS SRV record to {}", domain, dest);
-            return dest;
+            return new HomeserverTarget(domain, dest);
         }
 
         URL dest = build(domain + ":" + getDefaultPort());
         log.info("Resolution of {} to {}", domain, dest);
-        return dest;
+        return new HomeserverTarget(dest.getHost(), dest);
     }
 
+    public static class HomeserverTarget {
+
+        private final String domain;
+        private final URL url;
+
+        HomeserverTarget(String domain, URL url) {
+            this.domain = domain;
+            this.url = url;
+        }
+
+        public String getDomain() {
+            return domain;
+        }
+
+        public URL getUrl() {
+            return url;
+        }
+    }
 }

src/main/java/io/kamax/mxisd/matrix/HomeserverVerifier.java

@@ -0,0 +1,85 @@
+package io.kamax.mxisd.matrix;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.security.cert.Certificate;
+import java.security.cert.CertificateParsingException;
+import java.security.cert.X509Certificate;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import javax.net.ssl.HostnameVerifier;
+import javax.net.ssl.SSLPeerUnverifiedException;
+import javax.net.ssl.SSLSession;
+
+public class HomeserverVerifier implements HostnameVerifier {
+
+    private static final Logger LOGGER = LoggerFactory.getLogger(HomeserverVerifier.class);
+    private static final String ALT_DNS_NAME_TYPE = "2";
+    private static final String ALT_IP_ADDRESS_TYPE = "7";
+
+    private final String matrixHostname;
+
+    public HomeserverVerifier(String matrixHostname) {
+        this.matrixHostname = matrixHostname;
+    }
+
+    @Override
+    public boolean verify(String hostname, SSLSession session) {
+        try {
+            Certificate peerCertificate = session.getPeerCertificates()[0];
+            if (peerCertificate instanceof X509Certificate) {
+                X509Certificate x509Certificate = (X509Certificate) peerCertificate;
+                if (x509Certificate.getSubjectAlternativeNames() == null) {
+                    return false;
+                }
+                for (String altSubjectName : getAltSubjectNames(x509Certificate)) {
+                    if (match(altSubjectName)) {
+                        return true;
+                    }
+                }
+            }
+        } catch (SSLPeerUnverifiedException | CertificateParsingException e) {
+            LOGGER.error("Unable to check remote host", e);
+            return false;
+        }
+
+        return false;
+    }
+
+    private List<String> getAltSubjectNames(X509Certificate x509Certificate) {
+        List<String> subjectNames = new ArrayList<>();
+        try {
+            for (List<?> subjectAlternativeNames : x509Certificate.getSubjectAlternativeNames()) {
+                if (subjectAlternativeNames == null
+                    || subjectAlternativeNames.size() < 2
+                    || subjectAlternativeNames.get(0) == null
+                    || subjectAlternativeNames.get(1) == null) {
+                    continue;
+                }
+                String subjectType = subjectAlternativeNames.get(0).toString();
+                switch (subjectType) {
+                    case ALT_DNS_NAME_TYPE:
+                    case ALT_IP_ADDRESS_TYPE:
+                        subjectNames.add(subjectAlternativeNames.get(1).toString());
+                        break;
+                    default:
+                        LOGGER.trace("Unusable subject type: " + subjectType);
+                }
+            }
+        } catch (CertificateParsingException e) {
+            LOGGER.error("Unable to parse the certificate", e);
+            return Collections.emptyList();
+        }
+        return subjectNames;
+    }
+
+    private boolean match(String altSubjectName) {
+        if (altSubjectName.startsWith("*.")) {
+            return altSubjectName.toLowerCase().endsWith(matrixHostname.toLowerCase());
+        } else {
+            return matrixHostname.equalsIgnoreCase(altSubjectName);
+        }
+    }
+}