nmap扫描端口导致线上大量Java服务FullGC甚至OOM

nmap扫描端口导致线上大量Java服务FullGC甚至OOM

最近公司遇到了一次诡异的线上FullGC保障，多个服务几乎所有的实例集中报FullGC，个别实例甚至出现了OOM，直接被docker杀掉。

观察报警服务的log，均有大量的此log

*TNonblockingServer [ERROR] Read a frame size of ****, which is bigger than the maximum allowable buffer size for ALL connections.

注意到报警的几个服务都是net in流量突然有峰值，但是对应时间的http和rpc请求数没有增加。

此时已经开始怀疑是否有人恶意的调用接口。分析log应该与rpc接口有关，rpc端口外网访问不了，请求肯定来自内网。

后查明是公司内有人在用nmap扫描端口，nmap会向端口发送随机的数据包。公司内的JavaRpc采用的是Thrift，Thrift在解析异常数据包时有概率会申请大量内存，进而导致服务OOM。

分析了一波Thrift的源码，主要关注AbstractNonblockingServer类，以及其中的FrameBuffer。这两个类主要与Thrift解析数据流有关。

thrift采用NIO进行网络数据处理，NIO将数据放到buffer中，thrift再通过socketChannel读取buffer

thrift先从socketChannel中读取4个字节数据，4个字节转成int frameSize ，thrift就认作这个frameSize是整个数据包的size了，下一步就去申请对应大小的内存，再做进一步读取。

问题就出在了这里，如果是符合thrift IDL的数据包应该就没有问题，可是nmap发送的tcp包里的数据是随机的，而且我测试发现nmap一下会发几十个tcp请求过来。

thrift估计是为了避免这个问题，在拿到frameSize后，申请内存前，会有一个判断，当frameSize > MAX_READ_BUFFER_BYTES时，会把请求认作异常，不再进行处理。

if (frameSize > MAX_READ_BUFFER_BYTES) {
            LOGGER.error("Read a frame size of " + frameSize
                + ", which is bigger than the maximum allowable buffer size for ALL connections.");
            return false;
          }

MAX_READ_BUFFER_BYTES默认是Long.MAX_VALUE，可以在thrift的TThreadedSelectorServer构造函数中进行指定，公司指定的值为100M。

这个100M仍然过大了，当frameSize小于100M，thrift仍然会申请内存的，而nmap会几秒钟发几十个请求过来，极端情况下这几十个请求均会申请<=100M的内存，最终导致服务的OOM

一个治标不治本的解决方法是调小MAX_READ_BUFFER_BYTES。然而将MAX_READ_BUFFER_BYTES调整的过小的话也要考虑是否影响到正常的RPC请求。

总之，考虑到thrift如此迷惑的设计，运维应该在网络上做更多的限制可能会更好一点，首先RPC端口外网禁止访问，避免心怀不轨的人来搞破坏，其次内网也应该建立规范，比如扫描时尽量有意的避开绕过RPC端口。

核心代码

public boolean read() {
      if (state_ == FrameBufferState.READING_FRAME_SIZE) {
        // try to read the frame size completely
        if (!internalRead()) {
          return false;
        }

        // if the frame size has been read completely, then prepare to read the
        // actual frame.
        if (buffer_.remaining() == 0) {
          // pull out the frame size as an integer.
          int frameSize = buffer_.getInt(0);
          if (frameSize <= 0) {
            LOGGER.error("Read an invalid frame size of " + frameSize
                + ". Are you using TFramedTransport on the client side?");
            return false;
          }

          // if this frame will always be too large for this server, log the
          // error and close the connection.
          if (frameSize > MAX_READ_BUFFER_BYTES) {
            LOGGER.error("Read a frame size of " + frameSize
                + ", which is bigger than the maximum allowable buffer size for ALL connections.");
            return false;
          }

          // if this frame will push us over the memory limit, then return.
          // with luck, more memory will free up the next time around.
          if (readBufferBytesAllocated.get() + frameSize > MAX_READ_BUFFER_BYTES) {
            return true;
          }

          // increment the amount of memory allocated to read buffers
          readBufferBytesAllocated.addAndGet(frameSize + 4);

          // reallocate the readbuffer as a frame-sized buffer
          buffer_ = ByteBuffer.allocate(frameSize + 4);
          buffer_.putInt(frameSize);

          state_ = FrameBufferState.READING_FRAME;
        } else {
          // this skips the check of READING_FRAME state below, since we can't
          // possibly go on to that state if there's data left to be read at
          // this one.
          return true;
        }
      }

      // it is possible to fall through from the READING_FRAME_SIZE section
      // to READING_FRAME if there's already some frame data available once
      // READING_FRAME_SIZE is complete.

      if (state_ == FrameBufferState.READING_FRAME) {
        if (!internalRead()) {
          return false;
        }

        // since we're already in the select loop here for sure, we can just
        // modify our selection key directly.
        if (buffer_.remaining() == 0) {
          // get rid of the read select interests
          selectionKey_.interestOps(0);
          state_ = FrameBufferState.READ_FRAME_COMPLETE;
        }

        return true;
      }

      // if we fall through to this point, then the state must be invalid.
      LOGGER.error("Read was called but state is invalid (" + state_ + ")");
      return false;
    }

参考：https://my.oschina.net/shipley/blog/422204