The Problem with Extraneous Newlines

Oct 13, 2025 by wbrum.org 101 views

Introduction

In the world of HTTP/1.1, a protocol that has been widely used for decades, there are certain rules that must be followed to ensure proper communication between clients and servers. One such rule is related to the handling of empty lines, also known as CRLF (Carriage Return and Line Feed), in HTTP requests. In this article, we will delve into the specifics of this rule, explore why it's essential, and discuss a common issue that arises when this rule is not followed.

Understanding the Rule

According to RFC 9112, a server that is expecting to receive and parse a request-line SHOULD ignore at least one empty line (CRLF) received prior to the request-line. This means that if a client sends a request with an empty line before the actual request-line, the server should not throw an error but instead ignore the empty line and proceed with processing the request.

However, there is a catch. The other side of this rule states that an HTTP/1.1 user agent MUST NOT preface or follow a request with an extra CRLF. This means that clients should not send empty lines before or after the request-line.

The Problem with Extraneous Newlines

When a client sends a pair of requests with an extraneous newline, as shown in the example below, it can cause issues with the server.

POST / HTTP/1.1
Host: whatever
Content-Length: 2
aa

POST / HTTP/1.1
Host: whatever
Content-Length: 2
aa

In this example, the client is sending two requests with an empty line between them. This can cause the server to throw a "400 Bad Request" error, as seen in the Envoy proxy.

Why is this a Problem?

While it's true that the client is not violating a "must" rule, but rather a "should" rule, it would be nice to support the "should" as well. This is because ignoring extraneous newlines can help improve the robustness of the HTTP/1.1 protocol.

Is this an Upstream Issue?

The issue of the Envoy proxy throwing a "400 Bad Request" error when encountering an extraneous newline may be related to the upstream codec being used. This suggests that the problem may not be with the HTTP/1.1 protocol itself, but rather with the implementation of the codec.

Conclusion

In conclusion, the issue of the HTTP/1.1 codec responding with a "Bad Request" error to extraneous newlines is a complex one. While it's not a "must" rule, it would be beneficial to support the "should" rule as well. This can help improve the robustness of the HTTP/1.1 protocol and prevent issues like the one seen in the Envoy proxy.

Recommendations

To resolve this issue, the following recommendations can be made:

Implement the "should" rule: The HTTP/1.1 codec should be modified to ignore extraneous newlines, as per the "should" rule in RFC 9112.
Upgrade the upstream codec: If the issue is related to the upstream codec, it may be necessary to upgrade to a newer version that supports the "should" rule.
Test and validate: Thorough testing and validation should be performed to ensure that the modified codec or upgraded upstream codec does not introduce any new issues.

By following these recommendations, it may be possible to resolve the issue of the HTTP/1.1 codec responding with a "Bad Request" error to extraneous newlines.

Future Work

Future work on this issue may involve:

Further investigation: Investigating the root cause of the issue and identifying the specific component or code that is causing the problem.
Code modifications: Modifying the code to implement the "should" rule and ignore extraneous newlines.
Testing and validation: Thoroughly testing and validating the modified code to ensure that it does not introduce any new issues.

Introduction

In our previous article, we discussed the issue of the HTTP/1.1 codec responding with a "Bad Request" error to extraneous newlines. In this article, we will provide a Q&A section to address some of the common questions related to this issue.

Q: What is an extraneous newline?

A: An extraneous newline is an empty line (CRLF) that is sent before or after the actual request-line in an HTTP request. This can cause issues with the server, as it may not be able to parse the request correctly.

Q: Why is the "should" rule important?

A: The "should" rule in RFC 9112 states that a server should ignore at least one empty line (CRLF) received prior to the request-line. This helps to improve the robustness of the HTTP/1.1 protocol and prevent issues like the one seen in the Envoy proxy.

Q: Is this issue related to the upstream codec?

A: Yes, the issue of the Envoy proxy throwing a "400 Bad Request" error when encountering an extraneous newline may be related to the upstream codec being used. This suggests that the problem may not be with the HTTP/1.1 protocol itself, but rather with the implementation of the codec.

Q: How can I resolve this issue?

A: To resolve this issue, you can try the following:

Implement the "should" rule: Modify the HTTP/1.1 codec to ignore extraneous newlines, as per the "should" rule in RFC 9112.
Upgrade the upstream codec: If the issue is related to the upstream codec, it may be necessary to upgrade to a newer version that supports the "should" rule.
Test and validate: Thoroughly test and validate the modified codec or upgraded upstream codec to ensure that it does not introduce any new issues.

Q: What are the benefits of implementing the "should" rule?

A: Implementing the "should" rule can help improve the robustness of the HTTP/1.1 protocol and prevent issues like the one seen in the Envoy proxy. It can also help to ensure that clients and servers can communicate correctly, even in the presence of extraneous newlines.

Q: Are there any potential drawbacks to implementing the "should" rule?

A: While implementing the "should" rule can help improve the robustness of the HTTP/1.1 protocol, it may also introduce some potential drawbacks. For example, it may require modifications to the HTTP/1.1 codec or the upstream codec, which can be time-consuming and complex. Additionally, it may require additional testing and validation to ensure that the modified codec or upgraded upstream codec does not introduce any new issues.

Q: How can I test and validate the modified codec or upgraded upstream codec?

A: To test and validate the modified codec or upgraded upstream codec, you can try the following:

Use a test framework: Use a test framework to simulate different scenarios and test the modified codec or upgraded upstream codec.
Test with different clients and servers: Test the modified codec or upgraded upstream codec with different clients and servers to ensure that it works correctly in different environments.
Validate the output: Validate the output of the modified codec or upgraded upstream codec to ensure that it is correct and consistent.

By following these steps, you can help ensure that the modified codec or upgraded upstream codec works correctly and does not introduce any new issues.

Conclusion

In conclusion, the issue of the HTTP/1.1 codec responding with a "Bad Request" error to extraneous newlines is a complex one. However, by implementing the "should" rule and upgrading the upstream codec, it may be possible to resolve this issue and improve the robustness of the HTTP/1.1 protocol. By following the steps outlined in this article, you can help ensure that the modified codec or upgraded upstream codec works correctly and does not introduce any new issues.