eclipse-vertx/vert.x · error · io.vertx.core.VertxException

Cannot read hosts config

Error message

Cannot read hosts config 

What it means

Runtime error while parsing the hosts config supplied as an in-memory string (hostsValue): HostsFileParser threw an IOException while reading the StringReader. The message is a generic sentinel for a malformed hosts-format value; the offending input is the hostsValue string itself.

Source

Thrown at vertx-core/src/main/java/io/vertx/core/dns/impl/DnsAddressResolverProvider.java:247

  }

  private void refreshHostsFile() {
    HostsFileEntries entries;
    if (hostsPath != null) {
      File file = vertx.fileResolver().resolve(hostsPath).getAbsoluteFile();
      try {
        if (!file.exists() || !file.isFile()) {
          throw new IOException();
        }
        entries = HostsFileParser.parse(file);
      } catch (IOException e) {
        throw new VertxException("Cannot read hosts file " + file.getAbsolutePath());
      }
    } else if (hostsValue != null) {
      try {
        entries = HostsFileParser.parse(new StringReader(hostsValue.toString()));
      } catch (IOException e) {
        throw new VertxException("Cannot read hosts config ", e);
      }
    } else {
      entries = HostsFileParser.parseSilently();
    }
    parsedHostsFile = entries;
  }

  private static class ResolverRegistration {
    private final io.netty.resolver.AddressResolver<InetSocketAddress> resolver;
    private final EventLoop executor;
    ResolverRegistration(io.netty.resolver.AddressResolver<InetSocketAddress> resolver, EventLoop executor) {
      this.resolver = resolver;
      this.executor = executor;
    }
  }

  // Avoid FastThreadLocal query server address stream default implementation
  private static class ThreadLocalNameServerAddressStream implements DnsServerAddressStream {

View on GitHub (pinned to fb308bd8c3)

Solutions

1. Correct the hosts-value string to follow the hosts file format (IP hostname aliases per line)
2. Remove malformed lines and let the next refresh re-parse the corrected value

When it happens

Trigger: Thrown at vertx-core/src/main/java/io/vertx/core/dns/impl/DnsAddressResolverProvider.java:247 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of eclipse-vertx/vert.x@fb308bd8c3 (2026-09-06). Data as JSON: /api/errors/cb92d8fa4c989610. Report an issue: GitHub.