apache/hadoop · error · DisallowedDatanodeException

Datanode denied communication with namenode because the host

Error message

Datanode denied communication with namenode because the host is not in the include-list: {nodeReg}

What it means

Thrown as DisallowedDatanodeException from DatanodeManager.registerDatanode when hostConfigManager.isIncluded(nodeReg) returns false: the datanode's address is not in the include list, so the NameNode refuses registration entirely. This is the hosts-exclusion enforcement point for the classic include/exclude files (dfs.hosts / dfs.hosts.exclude) or the JsonServiceFile-based host configuration.

Source

Thrown at hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/DatanodeManager.java:1214

        // Reject registration of unresolved datanode to prevent performance
        // impact of repetitive DNS lookups later.
        final String message = "hostname cannot be resolved (ip="
            + ip + ", hostname=" + hostname + ")";
        LOG.warn("Unresolved datanode registration: " + message);
        throw new DisallowedDatanodeException(nodeReg, message);
      }
      // update node registration with the ip and hostname from rpc request
      nodeReg.setIpAddr(ip);
      nodeReg.setPeerHostName(hostname);
    }
    
    try {
      nodeReg.setExportedKeys(blockManager.getBlockKeys());
  
      // Checks if the node is not on the hosts list.  If it is not, then
      // it will be disallowed from registering. 
      if (!hostConfigManager.isIncluded(nodeReg)) {
        throw new DisallowedDatanodeException(nodeReg);
      }
        
      NameNode.stateChangeLog.info("BLOCK* registerDatanode: from "
          + nodeReg + " storage " + nodeReg.getDatanodeUuid());
  
      DatanodeDescriptor nodeS = getDatanode(nodeReg.getDatanodeUuid());
      DatanodeDescriptor nodeN = host2DatanodeMap.getDatanodeByXferAddr(
          nodeReg.getIpAddr(), nodeReg.getXferPort());
        
      if (nodeN != null && nodeN != nodeS) {
        NameNode.LOG.info("BLOCK* registerDatanode: " + nodeN);
        // nodeN previously served a different data storage, 
        // which is not served by anybody anymore.
        removeDatanode(nodeN);
        // physically remove node from datanodeMap
        wipeDatanode(nodeN);
        nodeN = null;
      }

View on GitHub (pinned to 2add963021)

Solutions

1. Add the datanode (as it reports itself, typically host:transferPort or IP) to the dfs.hosts include file
2. Run `hdfs dfsadmin -refreshNodes` on the NameNode and then restart/re-register the datanode (`hdfs --daemon restart datanode`)
3. If the node should be decommissioned, this denial is intended — retire the DN
4. Check exact matching: use the same hostname form the DN registers with (verify in DN log 'banner' / nodeReg toString)

Example fix

# before: DN not in include list
# NN log: Datanode denied communication with namenode: not in include-list

# after: allow the node
echo 'dn5.example.com:9866' >> /etc/hadoop/dfs.hosts
hdfs dfsadmin -refreshNodes
hdfs --daemon restart datanode

// Ops pre-check: is the DN in the include list before starting it?
String inc = conf.get("dfs.hosts");
String entry = "dn5.example.com:9866";
boolean included = Files.readAllLines(Paths.get(inc)).stream()
    .map(String::trim).filter(s -> !s.isEmpty() && !s.startsWith("#"))
    .anyMatch(l -> l.equals(entry) || l.split(":")[0].equals(entry.split(":")[0]));
if (!included) { /* add entry + refreshNodes before starting the DN */ }

When it happens

Trigger: Datanode starts and registers while its host is absent from dfs.hosts include file (when include-list enforcement is enabled), or it is present only in dfs.hosts.exclude; after editing host files without `hdfs dfsadmin -refreshNodes`, or when DNS/hostname normalization makes the node not match an entry.

Common situations: First-time cluster bring-up without dfs.hosts configured correctly; adding a new DN without updating the include file; hostname vs IP mismatch in the files; decommission config left over excluding the node.

Related errors

• Unable to create the Decommission monitor from {cls}
• Unresolved topology mapping for host {hostname}
• Datanode denied communication with namenode because hostname
• Can not create a Path from a null string
• Can not create a Path from an empty string

AI-assisted analysis of apache/hadoop@2add963021 (2026-08-22). Data as JSON: /api/errors/5c33e290ae66ec2b. Report an issue: GitHub.