apache/cassandra · error · IllegalArgumentException

Unknown host specified

Error message

Unknown host specified 

What it means

filterNeighbors parses the user-supplied -hosts list; each entry must resolve via InetAddressAndPort.getByName. When a hostname cannot be resolved or an IP is malformed, UnknownHostException is caught and rethrown as IllegalArgumentException naming the offending host string.

Solutions

1. Correct the hostname/IP in the -hosts argument
2. Verify DNS resolution from the Cassandra node (ping/nslookup the host)
3. Use numeric IP:port pairs instead of hostnames to bypass DNS

Example fix

// before
nodetool repair keyspace1 -hosts node3,ndoe4   // 'ndoe4' typo
// after
nodetool repair keyspace1 -hosts node3,node4

for (String h : hostsArg.split(",")) {
    try { InetAddress.getByName(h.trim()); }
    catch (UnknownHostException e) { throw new IllegalArgumentException("unresolvable host: " + h); }
}

try { repair(...); }
catch (IllegalArgumentException e) {
    if (e.getMessage().startsWith("Unknown host specified")) fixHostList(e);
    throw e;
}

When it happens

Trigger: Running nodetool repair -hosts with a hostname that does not resolve in DNS/hosts file, an IP typo, or a hostname with trailing/odd whitespace beyond trim().

Common situations: DNS outages or stale /etc/hosts entries during repair automation; copy-paste of hostnames including typos; specifying hostnames in a container environment where they don't resolve.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• Unable to look up endpoint
• A repair_session_space of
• A repair_session_space of
• An incremental repair with session id
• Auto-repair scheduler is disabled.

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/4b03548a4021ce0d. Report an issue: GitHub.

Appendix: source

Thrown at src/java/org/apache/cassandra/service/ActiveRepairService.java:644

        {
            Multimap<String, InetAddressAndPort> dcEndpointsMap = metadata.directory.allDatacenterEndpoints();
            Iterable<InetAddressAndPort> dcEndpoints = concat(transform(dataCenters, dcEndpointsMap::get));
            return neighbors.select(dcEndpoints, true);
        }
        else if (hosts != null && !hosts.isEmpty())
        {
            Set<InetAddressAndPort> specifiedHost = new HashSet<>();
            for (final String host : hosts)
            {
                try
                {
                    final InetAddressAndPort endpoint = InetAddressAndPort.getByName(host.trim());
                    if (endpoint.equals(ctx.broadcastAddressAndPort()) || neighbors.endpoints().contains(endpoint))
                        specifiedHost.add(endpoint);
                }
                catch (UnknownHostException e)
                {
                    throw new IllegalArgumentException("Unknown host specified " + host, e);
                }
            }

            if (!specifiedHost.contains(ctx.broadcastAddressAndPort()))
                throw new IllegalArgumentException("The current host must be part of the repair");

            if (specifiedHost.size() <= 1)
            {
                String msg = "Specified hosts %s do not share range %s needed for repair. Either restrict repair ranges " +
                             "with -st/-et options, or specify one of the neighbors that share this range with " +
                             "this node: %s.";
                throw new IllegalArgumentException(String.format(msg, hosts, toRepair, neighbors));
            }

            specifiedHost.remove(ctx.broadcastAddressAndPort());
            return neighbors.keep(specifiedHost);
        }

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