Ошибка empty reply from server

I ran into this error sporadically and could not understand. Googling did not help.

I finally found out. I run a couple of docker containers, among them NGINX and Apache. The command at hand addresses a specific container, running Apache. As it turned out, I also have a cron job doing some heavy lifting at times running on the same container. Depending on the load this cron job puts on this container, it was not able to answer my command in a timely manner, resulting in error 52 empty reply from server or even 502 Bad Gateway.

I discovered and verified this by plain curl when I noticed that the process I investigated took less than 2 seconds and all of a sudden I got a 52 error and then a 502 error and then again less than 2 seconds — so it was definitely not my code which was unchanged. Using ps aux within the container I saw the other process running and understood.

Actually, I was bothered by 502 Bad Gateway from NGINX with long running jobs and could not fix it with the appropriate parameters, so I finally gave up and switched these things to Apache. That’s why I was puzzled even more about these errors.

The remedy is simple. I just fired up some more instances of this container with docker service scale and that was it. docker load balances on its own.

Well, there is more to this as another example showed. This time I did some repetitious jobs.

I found out that after some time I ran out of memory used by PHP which cannot be reclaimed, so the process died.

Why? Having more than a dozen containers on a 8GB RAM machine, I initially thought it would be a good idea to limit RAM usage on PHP containers to 50MB.

Stupid! I forgot about it, but swarmpit gave me a hint. I call ini_set("memory_limit",-1); in the constructor of my class, but that only went as far as those 50MB.

So I removed those restrictions from my compose file. Now those containers may use up to 8GB. The process runs with Apache for hours now and it looks like the problem is solved, memory usage rising to well beyond 100MB.

Another caveat: To easily get and read debug messages, I started said process in Opera under Windows. That is fine with errors appearing soon.

However, if the last one is cared for, quite naturally the process runs and runs and memory usage in the browser builds up, eventually making my local machine unusable. So if that happens, kill this tab and the process keeps running fine.

I ran into this error sporadically and could not understand. Googling did not help.

I finally found out. I run a couple of docker containers, among them NGINX and Apache. The command at hand addresses a specific container, running Apache. As it turned out, I also have a cron job doing some heavy lifting at times running on the same container. Depending on the load this cron job puts on this container, it was not able to answer my command in a timely manner, resulting in error 52 empty reply from server or even 502 Bad Gateway.

I discovered and verified this by plain curl when I noticed that the process I investigated took less than 2 seconds and all of a sudden I got a 52 error and then a 502 error and then again less than 2 seconds — so it was definitely not my code which was unchanged. Using ps aux within the container I saw the other process running and understood.

Actually, I was bothered by 502 Bad Gateway from NGINX with long running jobs and could not fix it with the appropriate parameters, so I finally gave up and switched these things to Apache. That’s why I was puzzled even more about these errors.

The remedy is simple. I just fired up some more instances of this container with docker service scale and that was it. docker load balances on its own.

Well, there is more to this as another example showed. This time I did some repetitious jobs.

I found out that after some time I ran out of memory used by PHP which cannot be reclaimed, so the process died.

Why? Having more than a dozen containers on a 8GB RAM machine, I initially thought it would be a good idea to limit RAM usage on PHP containers to 50MB.

Stupid! I forgot about it, but swarmpit gave me a hint. I call ini_set("memory_limit",-1); in the constructor of my class, but that only went as far as those 50MB.

So I removed those restrictions from my compose file. Now those containers may use up to 8GB. The process runs with Apache for hours now and it looks like the problem is solved, memory usage rising to well beyond 100MB.

Another caveat: To easily get and read debug messages, I started said process in Opera under Windows. That is fine with errors appearing soon.

However, if the last one is cared for, quite naturally the process runs and runs and memory usage in the browser builds up, eventually making my local machine unusable. So if that happens, kill this tab and the process keeps running fine.

Curl 52 empty reply from server is an error message from “cURL” when it encounters a connection breakdown between a client and a server. If you’re seeing this error in your environment, this article is for you because you’ll learn what caused it and how you can fix it.

We’ll explain a lot of technical concepts using simple language, and this ensures that you’ll walk away with more knowledge about “cURL”. Now, launch your terminal (or command prompt) in your environment, and let’s fix the cURL 52 error.

Why Is a Server Returning an Empty Reply to Curl? Main Culprits

A server returned an empty reply to “cURL” because security software blocked your connection or the server was not listening to the host. You’ll also get an empty reply if you switched communication protocol or you have an older version of “libcurl” that had a software bug.

– A Software Security Setting Is Blocking Your Connection

When a software security setting from your firewall or proxy blocks your connection, you can get the “curl (52) empty reply from server kubernetes” error. That’s because the firewall or proxy might not permit the use of “cURL” in your environment, so when it sees your “cURL” request to a URL, it drops the connection.

For example, if you’ve installed Cisco AnyConnect VPN client and all its packages on your computer, it can block your “cURL” request. Behind the scenes, one of these modules will keep dropping your cURL request even if the VPN is not running.

Another example is the “xpack.security.enabled” settings that dictate access in Elasticsearch, and you’ll find it in the “elasticsearch.yml” file. If this setting is set to “true”, it can cause the “curl (52) empty reply from server elasticsearch docker” error.

You’ll notice this error when you query Elasticsearch using Python requests, and that’s because the setting prevents anonymous access to Elasticsearch.

Finally, if you work in an organization, their authentication proxy can stop any “cURL” request or add a timeout that’ll cause the “curl: (52) empty reply from server in odoo” error.

– The Server Is Not Listening on the Host

If a server is not listening to a specific hostname, all “cURL” requests to that hostname can cause the docker curl: (52) empty reply from server error. For example, the following Dockerfile will set up the Apache webserver to only listen to a request within the Docker container.

However, an external “cURL” request from your operating system to the container will result in the “cURL” 52 error. Also, you’ll see the “kong curl: (52) empty reply from server” error when the Kong API Gateway is not listening to outside connection requests.

– You Switched Communication Protocol

A change in a communication protocol that you’re using in “cURL” to connect to a server can cause the “cURL” 52 error. For example, if you’re connecting to a secure server, “cURL” expects you to specify HTTPS as the protocol and the secure port number (port 443 by default).

However, if you change the protocol to HTTP (without the “s”) and ask “cURL” to make a request to the secure server on port 443, you’ll get an empty result. The following cURL session is an example:

From the session above, we asked “cURL” to do an HTTP request to Amazon on port 443. By default, Amazon is a secure website with HTTPS, and here we want Amazon with an HTTP on port 443.

This is impossible, and if you open the URL in your web browser, you’ll get an error message that states, “400 Bad Request The plain HTTP request was sent to HTTPS port.

– You’re Using an Older Version of Libcurl

Older versions of the “libcurl” library contained bugs that can lead to the “cURL” 52 error. So, if you have this old version on your computer, “cURL” requests will fail. For example, at the time of writing, the current version of the “libcurl” library is “7.55.1”, but versions below “7.21.6” will cause an empty result from the server.

How Can a Server Return a Reply When You’re Using Curl? Fixed Properly

A server can return a reply when you’re using “cURL” if you do the following:

• Update your security settings or remove the software
• Allow all connections to the server
• Use the right protocol to connect to the server
• Download the latest version of libcurl



– Update Your Security Settings or Remove the Software

The first thing that you can do when you encounter the “cURL” 52 error is to update your security settings and, if you can, remove the software.

In the context of ElasticSearch, you can change the value of “xpack.security.enabled” in the “elasticsearch.yml” file to “false”, and this will fix the “curl (52) empty reply from server elasticsearch Ubuntu” error.

In addition, if you have the Cisco AnyConnect VPN client and you will see the cURL 52 error, do the following:

1. Uninstall Cisco AnyConnect VPN client from your computer.
2. Download it again from their website.
3. Start the installation, and on the “Installation Type” window, uncheck the box for the “Web Security” module.
4. Complete the installation, and it should not block your cURL request anymore.

What’s more, if you have proxy software that intercepts your network connections, you should switch it off. However, if it’s a firewall, you can configure it to allow “cURL” access to the internet or the remote URL.

– Allow All Connections to the Server

To fix errors like the “curl: (52) empty reply from server redis” error and others that are related to the server not listening on a host, you need to configure the server to listen on all available network interfaces.

Now, The following is the updated version of the previous Dockerfile that caused the “cURL” 52 error. This time, we use “0.0.0.0:80” to tell the server to listen to all network available interfaces, and you can use this address as the binding address for other applications that have the same issue.

Finally, you build an image from this Dockerfile using “docker build -t apache server .” and start the server using the next command.

This command will publish the container’s port 80 to the system’s port 8080, and this will allow a “cURL” request from your system’s operating system.

docker run -p 8080:80 apache-server

– Use the Right Protocol To Connect to the Server

When you’re connecting to a server, use the right protocol that’s supported by the server. For example, the following is the bash session that caused the “cURL” 52 error when trying to connect to Amazon:

– Download the Latest Version of Libcurl

If you’re using an older version of the “libcurl” library, and you’re getting the “cURL” 52 error, you can download the latest version. Or you can use the following steps on Windows to get the curl because it’s powered by “libcurl”:

1. Go to the official download web page of “cURL” at https://curl.se/download.html
2. Scroll down to the bottom of the page, and you’ll find software binaries for “libcurl”.
3. Download the binary for your system architecture (32-bit or 64-bit), or your terminal environment.
4. Open your “C:” drive and create a folder. For convenience, name this folder “Curl”.
5. Extract the zip archive of cURL to your “Curl” folder
6. Copy the path of the “bin” folder within the “Curl” folder.
7. Add this path to the Windows environment variables.
8. Use “curl –version” to verify that you have cURL.

Conclusion

This article explained why “cURL” returns the “cURL” 52 error and how you can fix it without encountering .other errors. From our discussion, the following are the most important thing that you should remember:

• The Cisco AnyConnect VPN client on your computer can cause the cURL 52 error because it can block your request.
• Asking cURL to serve the content of an HTTPS website in HTTP will lead to the cURL 52 error.
• A wrong URL to cURL can cause the “curl (52) empty reply from server node” error.
• To solve the cURL 52 error, ensure that no security software is blocking your connection request to the remote server.

Now, you know the cause of the “cURL” 52 error and how you can solve it. Happy coding, and don’t forget to share our article to help others overcome similar challenges!

You may have encountered the ‘curl (52) empty reply from a server error’ when you try to visit a website. This happens when the HTTP client does not connect to the intended web server. This response indicates that you are not using a secure HTTPS connection. There may be other issues causing the server to not process your request. Let’s look at some common reasons and ways to solve them.

Causes and Solutions

A Mismatched Certificate

You may get an empty response if the SSL certificate is mismatched. The solution to this is to check whether the HTTPS certificate actually works with the simple tools available. Apply the same command to check both servers. After you’ve verified the certificate, consider posting a test request. You ought to be able to send your SSL request if the two certificates are the same. However, if they’re not, you should try a different server.

You may also be getting a mismatch if you have not added your website address as SAN to the certificate or are trying to access a website using its internal name. Websites that share their IP addresses with sites without SSL get the mismatched certificate error. Sometimes the issue is caused by a hosting platform with pre-configured SSL settings. The solution to each of these cases depends on the cause.

Your HTTPS Request Is Targeting a Non-HTTPS Port

Another possible reason why you may be receiving the error is that your HTTPS connection is targeting an HTTP port. No HTTPS client can establish any encrypted session with an HTTP port. In this case, most browsers will indicate that one is also trying to access an unsafe website. There is a simple command that allows you to test if a connection is secure so that you can take the right steps.

Sometimes the reason for this mismatch is firewall rules that are wrongly configured. You may be using the wrong port, or some services that should be enabled to use the HTTPS port are not enabled. Resolve the issue on the user terminal.

The Server May Have Closed the TCP Connection

If your server returns an empty response, it means the server has terminated the TCP connection. This is usually caused by the HTTP request being delivered to an HTTPS port. This error can occur if you try submitting an HTTPs request using an HTTP port. It is the opposite of the case discussed above. Because HTTPS clients cannot create an encrypted session with a plain HTTP port, such a request is not possible.

Start by checking if your HTTPS client uses the right protocol. Only use the HTTPS port for all HTTPS connections. By default, HTTPS connections use TCP port 443. However, the unsecure protocol HTTP uses port 80. You can change ports on the Window Defender Firewall as a user to force connections through TCP port 443. On the other hand, website owners need to configure server applications like Apache to ensure their sites use the same port.

You are Using an Older Version of Libcurl

If your curl is using an outdated version of libcurl, you are bound to get an empty response from the server. This can be resolved by updating the libcurl. If you want to upgrade your curls, you can download the latest official curl release at curl.se and select your OS. If you have the latest version but are still facing issues, check if curl has a port assignment and consider using another port.

When resolving the “curl (52) Empty Reply from Server” error, start with the basics, such as checking ports before looking at certificates and the curl version in use. They are the most common issues where the site is duly updated.

curl (52) empty reply from server occurs when the libcurl didn’t receive any response from the server after it sent off its request.

Here at Bobcares, we have seen several such curl related issues as part of our Server Management Services for web hosts and online service providers.

Today we’ll take a look at the cause for this error and how to fix it.

Know more about curl (52) empty reply from server

The error “empty reply from server” indicates that a zero-length response was received. This means no HTTP headers or content, simply a closed TCP connection with no HTTP payload is transmitted.

curl: (52) Empty reply from the server is a server related issue. However, this happens when libcurl did not receive any response from the server even after it has sent off its request.

For instance, the error appears as below.

Here, we need to troubleshoot this error from the server-side and not from the client-side. Also, ‘Empty response’ is different from ‘no response’. Empty response means you are getting a reply that does not contain any data.

Causes and Fixes for curl (52) empty reply from server

Now, let’s discuss the different causes and fixes provided by our Support Engineers.

1. Cause: Using a very old version of libcurl

Fix: In this case, we suggest customers upgrade the version of libcurl

2. Cause: Something in the network/setup is preventing this from working, like a firewall.

Fix: We check the firewall rules and ensure that HTTP, HTTPS, required ports and services are enabled in the firewall.

3. Cause: WebSite could not complete a loopback request in WordPress

To run scheduled events, we use Loopback requests. Also, it is used by the built-in editors for themes and plugins to verify code stability. When the loopback request fails, it means features relying on them are not currently working as we expect. However, this happens if the loopback request is disabled.

Fix: We suggest adding the below code to the “wp-config.php” file and save it.

define(‘ALTERNATE_WP_CRON’, true);

This code will use the alternative Cron job system that can solve the problem generally.

4. Cause: Using curl with a port assignment in the URL

Fix: We suggest using a different port

5. Cause: If curl is asked to do plain HTTP on a server that does HTTPS.

Fix: In this case, we suggest to try the command using https instead of HTTP.

6. Cause: It can be due to server redirection

Fix: Here, we try executing the command using curl -L

[Need any further assistance in fixing curl errors? – We’re available 24*7]

Conclusion

In short, this error occurs when the libcurl didn’t receive any response from the server after it sent off its request. Today, we saw the resolution to this curl error.

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