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      HAProxy Network Error: cannot bind socket



      Part of the Series:
      Common HAProxy Errors

      This tutorial series explains how to troubleshoot and fix some of the most common errors that you may encounter when using the HAProxy TCP and HTTP proxy server.

      Each tutorial in this series includes descriptions of common HAProxy configuration, network, filesystem, or permission errors. The series begins with an overview of the commands and log files that you can use to troubleshoot HAProxy. Subsequent tutorials examine specific errors in detail.

      Introduction

      An HAProxy cannot bind socket error message is generated when there is another process listening on the same interface and TCP port combination that HAProxy is configured to use, or when HAProxy attempts to use an IP address that is not assigned to a network interface. Both error conditions derive from the underlying operating system’s network stack.

      In the first case, when there is another process that is already using an interface and port that HAProxy is attempting to bind to, the underlying error on Linux is EADDRINUSE. The issue is that only a single process can be bound to an IP address and port combination at any given time.

      In the second case, when HAProxy is attempting to use an IP address that is not assigned to an interface on the system, the underlying error on Linux is EADDRNOTAVAIL. The issue here is that an IP socket cannot be created using an address that is not available to the operating system.

      However, both underlying errors generate the same HAProxy error message, so troubleshooting a cannot bind socket error requires examining the list of currently used sockets and IP addresses on a Linx system.

      To detect a cannot bind socket error message, you will need to examine systemctl and journalctl output to determine the IP address and port combination that are causing the error. Then you can inspect other running processes and network interfaces and decide how to resolve the issue, whether it is by switching servers, changing the IP address or port that HAProxy uses, or any combination of these options.

      Troubleshooting with systemctl

      Following the troubleshooting steps from the How to Troubleshoot Common HAProxy Errors tutorial at the beginning of this series, the first step when you are troubleshooting an cannot bind socket error message is to check HAProxy’s status with systemctl.

      The output from systemctl status will in many cases contain all the diagnostic information that you need to resolve the error. It may include the IP address that HAProxy is using, as well as the port that it is attempting to bind to. The output will also indicate how long HAProxy has been unable to start so that you can determine how long the issue has been affecting HAProxy.

      Note: If you are using Ubuntu or a Debian-derived Linux distribution, systemctl does not include output from HAProxy with a cannot bind socket error message that describes the problem. Skip to the the next section of this tutorial, Troubleshooting Using journalctl Logs to learn how to examine the systemd logs to find the conflicting IP address or port.

      On CentOS, Fedora and RedHat-derived systems, use this systemctl command to examine HAProxy’s status:

      CentOS and Fedora Systems

      • sudo systemctl status haproxy.service -l --no-pager

      The -l flag will ensure that systemctl outputs the entire contents of a line, instead of substituting in ellipses () for long lines. The --no-pager flag will output the entire log to your screen without invoking a tool like less that only shows a screen of content at a time.

      Since you are troubleshooting a cannot bind socket error message, you should receive output that is similar to the following:

      Output

      ● haproxy.service - HAProxy Load Balancer Loaded: loaded (/usr/lib/systemd/system/haproxy.service; disabled; vendor preset: disabled) Active: failed (Result: exit-code) since Wed 2020-08-19 14:57:05 UTC; 3s ago Process: 138738 ExecStart=/usr/sbin/haproxy -Ws -f $CONFIG -p $PIDFILE (code=exited, status=1/FAILURE) Process: 138736 ExecStartPre=/usr/sbin/haproxy -f $CONFIG -c -q (code=exited, status=0/SUCCESS) Main PID: 138738 (code=exited, status=1/FAILURE) Aug 19 14:57:05 92214d8ff5e2 systemd[1]: Starting HAProxy Load Balancer... Aug 19 14:57:05 92214d8ff5e2 haproxy[138738]: [ALERT] 231/145705 (138738) : Starting frontend main: cannot bind socket [0.0.0.0:80] . . . Aug 19 14:57:05 92214d8ff5e2 systemd[1]: Failed to start HAProxy Load Balancer.

      This example systemctl output includes some highlighted lines from the systemd journal that describes the error. These lines give you all the information about the error that you need to troubleshoot it further. Specifically, the line cannot bind socket [0.0.0.0:80] describes the socket that HAProxy is trying to use (0.0.0.0:80), so you can skip the following journalctl steps and instead proceed to the Troubleshooting with ss and ps Utilities section at the end of this tutorial. The other highlighted line indicates the status of the HAProxy process, which in the case of a cannot bind socket error will show Failed to start HAProxy Load Balancer.

      If your systemctl output does not give specific information about the IP address and port or ports that are causing the error (if you are using Ubuntu or Debian then this applies), then you will need to examine journalctl output from the systemd logs. The following section explains how to use journalctl to troubleshoot a cannot bind socket error.

      Troubleshooting Using journalctl Logs

      If your systemctl output does not include specifics about a cannot bind socket error, you should proceed with using the journalctl command to examine systemd logs for HAProxy.

      On Ubuntu and Debian-derived systems, run the following command:

      • sudo journalctl -u haproxy.service --since today --no-pager

      On CentOS, Fedora, and RedHat-derived systems, use this command to inspect the logs:

      • sudo journalctl -u haproxy.service --since today --no-pager

      The --since today flag will limit the output of the command to log entries beginning at 00:00:00 of the current day only. Using this option will help restrict the volume of log entries that you need to examine when checking for errors.

      If HAProxy is unable to bind to a port that is in use, search through the output for lines that are similar to the following log entries, specifically lines that contain the cannot bind socket error message as highlighted in this example:

      Output

      -- Logs begin at Wed 2020-08-19 19:38:12 UTC, end at Wed 2020-08-19 19:53:53 UTC. -- . . . Aug 19 19:39:21 92214d8ff5e2 systemd[1]: Starting HAProxy Load Balancer... Aug 19 19:39:21 92214d8ff5e2 haproxy[135]: [ALERT] 231/193921 (135) : Starting frontend main: cannot bind socket [0.0.0.0:80] Aug 19 19:39:21 92214d8ff5e2 haproxy[135]: [ALERT] 231/193921 (135) : Starting frontend main: cannot bind socket [:::80] Aug 19 19:39:21 92214d8ff5e2 systemd[1]: haproxy.service: Main process exited, code=exited, status=1/FAILURE Aug 19 19:39:21 92214d8ff5e2 systemd[1]: haproxy.service: Failed with result 'exit-code'. Aug 19 19:39:21 92214d8ff5e2 systemd[1]: Failed to start HAProxy Load Balancer. . . .

      The first highlighted line of output indicates that HAProxy cannot bind to port 80 on all available IPv4 interfaces (denoted by the 0.0.0.0 IP address). Depending on your system’s configuration, the IP addresses may be different and only show individual IPs.

      If you are using HAProxy with IPv6, then the output may also include a line like the second one that is highlighted with an IPv6 specific interface and port error, in this case :::80. The first two :: characters indicate all available IPv6 interfaces, while the trailing :80 indicates the port.

      Even though your own system may have different conflicting interfaces and ports, the errors will be similar to the output shown here. With this output from journalctl you will be able to diagnose the issue using ss, ps, and ip commands in the following section of this tutorial.

      Troubleshooting with ss and ps Utilities

      To troubleshoot a cannot bind socket error you need to determine what other process is listening on the IP address and port that HAProxy is attempting to use, or if the IP address is available to HAProxy.

      For example, if another server like Nginx is configured to listen on port 8080 on all available IPv4 network interfaces, the full socket would be 0.0.0.0:8080. If HAProxy is also configured to use 0.0.0.0:8080 then the operating system will throw an EADDRINUSE error, and HAProxy will show a cannot bind socket error message, since it cannot claim the socket for itself.

      In the previous journalctl section, something was already bound to all the available IPv4 addresses (denoted by 0.0.0.0:80). Most modern Linux distributions include a utility called ss which can be used to gather information about the state of a system’s network sockets.

      The following command will determine the name of the process that is already bound to an IPv4 interface on port 80. Ensure that you substitute the port from the error message if it is different from 80 in the following command:

      • sudo ss -4 -tlnp | grep 80

      The flags to the ss command alter its default output in the following ways:

      • -4 restricts ss to only display IPv4-related socket information.
      • -t restricts the output to tcp sockets only.
      • -l displays all listening sockets with the -4 and -t restrictions taken into account.
      • -n ensures that port numbers are displayed, as opposed to protocol names like ‘httporhttps`. This is important since HAProxy may be attempting to bind to a non-standard port and a service name can be confusing as opposed to the actual port number.
      • -p outputs information about the process that is bound to a port.
      • | grep 80 limits the output to lines that contain the characters 80 so there are fewer lines that you have to examine

      Note: in this IPv4 and the following IPv6 example, if you do not have a line in your output with a matching port, then your cannot bind socket error may be derived from an EADDRNOTAVAIL error. Skip to the next section Troubleshooting with the ip Utility to examine the available IP addresses on your system.

      With all of those flags, you should receive output like the following:

      Output

      LISTEN 0 511 0.0.0.0:80 0.0.0.0:* users:(("nginx",pid=40,fd=6))

      The first three fields are not important when troubleshooting a cannot bind socket error so they can be ignored. The important fields are the fourth (0.0.0.0:80), which matches the journalctl error that you discovered earlier, along with the last users:(("nginx",pid=40,fd=6)), specifically the pid=40 portion.

      If you have a cannot bind socket error that is related to an IPv6 interface, repeat the ss invocation, this time using the -6 flag to restrict the interfaces to the IPv6 network stack like this:

      • sudo ss -6 -tlnp |grep 80

      The -6 flag limits the ip command to IPv6 interfaces. If HAProxy is unable to bind to an IPv6 socket, you should have output like the following:

      Output

      LISTEN 0 511 [::]:80 [::]:* users:(("nginx",pid=40,fd=7))

      Again, substitute the port number in question from your journalctl output if it is different from the highlighted 80 given here.

      In both these cases of IPv4 and IPv6 errors, the ss output indicates that there is a program with process ID 40 (the pid=40 in the output) that is bound to the 0.0.0.0:80 and [::]:80 interfaces respectively. This process is preventing HAProxy from starting since it already owns the port. To determine the name of the program, use the ps utility like this, substituting the process ID from your output in place of the highlighted 40 value in this example:

      You will receive output that is similar to the following:

      Output

      PID TTY TIME CMD 40 ? 00:00:00 nginx

      The highlighted nginx in the output is the name of the process that is listening on the interfaces. Now that you have the name of the program that is preventing HAProxy from starting, you can decide how to resolve the error. You could stop the nginx process, reconfigure nginx to listen on a different interface and port, or reconfigure HAProxy to avoid the port collision.

      It is important to note that the process may be different from nginx and the port and IP addresses may not always be 0.0.0.0 or [::] if you are diagnosing a cannot bind socket error. Oftentimes, different web servers and proxies will be in use on the same server. Each may be attempting to bind to different IPv4 ports and IPv6 interfaces to handle different web traffic. For example, a server that is configured with HAProxy listening on the IPv4 loopback address (also referred to as localhost) on port 8080 will show ss output like this:

      Output

      LISTEN 0 2000 127.0.0.1:8080 0.0.0.0:* users:(("haproxy",pid=545,fd=7))

      It is important to combine systemctl output, or journalctl output that indicates specific IP addresses and ports, with diagnostic data from ss, and then ps to narrow down the process that is causing HAProxy to fail to start.

      Sometimes when you are troubleshooting a cannot bind socket error message with ss and ps there will not be any output at all, which means that the error may not be caused by a socket conflict. The next section of this tutorial explains how to troubleshoot a cannot bind socket error using the ip utility.

      Troubleshooting with the ip Utility

      The previous section explained how an EADDRINUSE operating system error could cause a cannot bind socket error message. However, if you have examined ss and ps output and there is no socket conflict on your system, the issue may be caused by an EADDRNOTAVAIL operating system error instead. In this case HAProxy may be trying to bind to a socket that is not available to your operating system.

      To determine whether a cannot bind socket error is caused by an EADDRNOTAVAIL, examine both the IPv4 and IPv6 network interfaces on your system using the ip command.

      • sudo ip -4 -c address show
      • -4 restricts ip to only display IPv4-related interface information.
      • -c adds color coding to the output so that it is easier to parse visually.
      • address show displays the IP address for an interface, with the -4 and -c flags taken into account.

      You should receive output that looks similar to the following on any Linux distribution that includes the ip tool:

      Output

      1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000 inet 127.0.0.1/8 scope host lo valid_lft forever preferred_lft forever 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000 inet 203.0.113.1/24 brd 203.0.113.255 scope global eth0 valid_lft forever preferred_lft forever inet 192.0.2.1/24 brd 192.0.2.255 scope global eth0 valid_lft forever preferred_lft forever 3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000 inet 198.51.100.1/24 brd 198.51.100.255 scope global eth1 valid_lft forever preferred_lft forever

      Make a note of your IP addresses that correspond to the highlighted examples in this output. Your IP addresses and network interfaces will be different than the examples shown here. You may have more or fewer interfaces, and each may have more or fewer addresses assigned to them. The important part is to note the IP addresses from ip.

      To examine IPv6 addresses that are assigned to your system, use the ip command with the -6 flag like this:

      • sudo ip -6 -c address show

      You should receive output like the following:

      Output

      1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 state UNKNOWN qlen 1000 inet6 ::1/128 scope host valid_lft forever preferred_lft forever 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000 inet6 2604:a880:400:d1::3d3:6001/64 scope global valid_lft forever preferred_lft forever inet6 fe80::a4ff:aaff:fec9:24f8/64 scope link valid_lft forever preferred_lft forever

      Again note the highlighted values in this example output and look for the corresponding IPv6 addresses in your output.

      Once you have a list of addresses that are assigned to your system, you can try to find a matching IP address that corresponds to the cannot bind socket [x.x.x.x:80] error. If there is no IP address that matches, then HAProxy is likely configured to use an IP address that is not available to your system and the cannot bind socket error is being caused by the operating system throwing an EADDRNOTAVAIL error.

      To resolve the error you will need to edit your /etc/haproxy/haproxy.cfg file and change the bind address or addresses to an IP address that is available to your system based on the output of the ip command.

      For example, if /etc/haproxy/haproxy.cfg contained a bind line like the following using 198.51.100.123 as the IP address, but your system has 198.51.100.1 assigned based on the example output above, you will need to edit the bind line.

      Following this hypothetical example, this haproxy.cfg snippet shows the invalid IP address:

      /etc/haproxy/haproxy.cfg

      . . .
      frontend main
              bind 198.51.100.123:80
      

      A correct bind line that matches the IP address in the example ip output would look like this:

      /etc/haproxy/haproxy.cfg

      . . .
      frontend main
              bind 198.51.100.1:80
      

      Once you have edited /etc/haproxy/haproxy.cfg with the correct IP address, restart it using the systemctl command:

      • sudo systemctl restart haproxy.service

      Now examine HAProxy’s status and make sure that the output shows an active (running) line:

      • sudo systemctl status haproxy.service

      Output

      ● haproxy.service - HAProxy Load Balancer Loaded: loaded (/lib/systemd/system/haproxy.service; enabled; vendor preset: enabled) Active: active (running) since Wed 2020-08-19 21:31:46 UTC; 17h ago Docs: man:haproxy(1) file:/usr/share/doc/haproxy/configuration.txt.gz Process: 487 ExecStartPre=/usr/sbin/haproxy -f $CONFIG -c -q $EXTRAOPTS (code=exited, status=0/SUCCESS) . . . Aug 19 21:31:46 d6cdd0c71489 systemd[1]: Started HAProxy Load Balancer.

      If you have resolved the cannot bind socket error your output should be similar to this example output. The highlighted lines that show HAProxy is active, and that the process was started successfully.

      Conclusion

      In this tutorial you learned how to troubleshoot an HAProxy cannot bind socket error message on both IPv4 and IPv6 interfaces. You learned how to use systemctl to examine the status of the HAProxy server and try to find error messages. You also learned how to use journalctl to examine the systemd logs for specific information about a cannot bind socket error.

      With the appropriate error messages from the systemd logs, you then learned about the ss utility and how to use it to examine the state of a system’s network sockets. After that you learned how to combine process ID information from ss with the ps utility to find the name of the process that is causing HAProxy to be unable to start.

      Finally, in the case of a cannot bind socket error that is related to an unavailable IPv4 or IPv6 address, you learned how to use the ip utility to examine available network interfaces on your system.



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      Apache Configuration Error AH00526: Syntax error



      Part of the Series:
      Common Apache Errors

      This tutorial series explains how to troubleshoot and fix some of the most common errors that you may encounter when using the Apache web server.

      Each tutorial in this series includes descriptions of common Apache configuration, network, filesystem, or permission errors. The series begins with an overview of the commands and log files that you can use to troubleshoot Apache. Subsequent tutorials examine specific errors in detail.

      Introduction

      An Apache AH00526: Syntax error message occurs when there is a typo or misconfigured setting somewhere in your Apache configuration files. It is a generic error that can be indicative of a number of underlying problems.

      The error can be detected using apachectl configtest before an invalid configuration is loaded. It can also be found using the systemctl and journalctl commands. In the latter two cases, Apache will be unable to run because of the error.

      If you have detected the error using apachectl then skip to the Troubleshooting Using the Built in apachectl Command section of this tutorial. Otherwise, the next section will explain how to use systemctl to troubleshoot the error.

      Troubleshooting with systemctl

      Following the troubleshooting steps from the How to Troubleshoot Common Apache Errors tutorial at the beginning of this series, the first step when you are troubleshooting an AH00526 error is to check Apache’s status with systemctl. It is important to understand if the error affects the running process, or if it is preventing Apache from starting up.

      On Ubuntu and Debian derived Linux distributions, run the following to check Apache’s status:

      Ubuntu and Debian Systems

      • sudo systemctl status apache2.service -l --no-pager

      On CentOS and Fedora systems, use this command to examine Apache’s status:

      CentOS and Fedora Systems

      • sudo systemctl status httpd.service -l --no-pager

      The -l flag will ensure that systemctl outputs the entire contents of a line, instead of substituting in ellipses () for long lines. The --no-pager flag will output the entire log to your screen without invoking a tool like less that only shows a screen of content at a time.

      Since you are troubleshooting an AH00526: Syntax error message, you should receive output that is similar to the following:

      Output

      ● apache2.service - The Apache HTTP Server Loaded: loaded (/lib/systemd/system/apache2.service; enabled; vendor preset: enabled) Drop-In: /lib/systemd/system/apache2.service.d └─apache2-systemd.conf Active: failed (Result: exit-code) since Wed 2020-07-15 13:45:49 UTC; 1min 37s ago . . . Jul 15 13:45:49 f17f01056c5b systemd[1]: Starting The Apache HTTP Server... Jul 15 13:45:49 f17f01056c5b apachectl[15860]: AH00526: Syntax error on line 2 of /etc/apache2/sites-enabled/000-default.conf: Jul 15 13:45:49 f17f01056c5b apachectl[15860]: Invalid command 'SSSLCertificateFile', perhaps misspelled or defined by a module not included in the server configuration Jul 15 13:45:49 f17f01056c5b apachectl[15860]: Action 'start' failed. Jul 15 13:45:49 f17f01056c5b apachectl[15860]: The Apache error log may have more information. Jul 15 13:45:49 f17f01056c5b systemd[1]: apache2.service: Control process exited, code=exited status=1 Jul 15 13:45:49 f17f01056c5b systemd[1]: apache2.service: Failed with result 'exit-code'. Jul 15 13:45:49 f17f01056c5b systemd[1]: Failed to start The Apache HTTP Server.

      In this case, Apache is not running because of the syntax error. The error is caused by an extra S character at the beginning of the SSSLCertificateFile line in the /etc/apache2/sites-enabled/000-default.conf file. The correct directive should be SSLCertificateFile, so editing the file to fix the directive name in this example would resolve the error and allow Apache to start.

      The systemctl output in this example also includes some lines from the systemd journal. If your output indicates a specific line in your configuration file is generating the syntax error, you can skip the journalctl and apachectl configtest troubleshooting steps. Instead, you can go directly to the file to inspect and edit the erroneous line to resolve the error.

      If your output does not give specific information about the error location in Apache’s configuration files, you will need to examine journalctl output from the systemd logs. The following section explains how to use journalctl to troubleshoot an AH00526 error.

      Troubleshooting with journalctl logs

      If your systemctl output does not include specifics about an AH00526 syntax error, you can proceed with using the journalctl command to examine systemd logs for Apache.

      On Ubuntu and Debian-derived systems, run the following command:

      • sudo journalctl -u apache2.service --since today --no-pager

      On CentOS, Fedora, and RedHat-derived systems, use this command to inspect the logs:

      • sudo journalctl -u httpd.service --since today --no-pager

      The --since today flag will limit the output of the command to log entries beginning at 00:00:00 of the current day only. Using this option will help restrict the volume of log entries that you need to examine when checking for errors.

      If you have an AH00526 error in your Apache configuration, look through the journalctl command output for lines like the following:

      Output

      -- Logs begin at Tue 2019-11-05 21:26:44 UTC, end at Tue 2020-06-09 15:13:01 UTC. -- . . . Jun 09 15:12:28 f17f01056c5b apachectl[3157]: AH00526: Syntax error on line 3 of /etc/apache2/sites-enabled/000-default.conf: Jun 09 15:12:28 f17f01056c5b apachectl[3157]: Invalid command 'SSLCertificateFile', perhaps misspelled or defined by a module not included in the server configuration . . .

      The first line of output is the AH00526 error. Since this error is a general error related to an invalid setting or a typo in a configuration file, the next line explains what caused the error. In this case it is a directive called SSLCertificateFile, which will only be valid if the ssl module is enabled.

      If you encounter an AH00526 error that is related to an invalid SSLCertificateFile directive, you can resolve it by enabling the ssl module and then restarting Apache to make the error go away.

      For Ubuntu and Debian systems, run the following to enable the module:

      • sudo a2enmod ssl
      • sudo systemctl restart apache2.service

      On CentOS and Fedora systems, ensure that the mod_ssl package is installed, and then load the module by adding it to Apache’s /etc/httpd/conf.modules.d directory in a file like this:

      • sudo yum install mod_ssl
      • echo "LoadModule ssl_module modules/mod_ssl.so" | sudo tee > /etc/httpd/conf.modules.d/00-ssl.conf
      • sudo systemctl restart httpd.service

      Once the module is referenced by Apache and you restart it using the command that is appropriate to your Linux distribution, the server will start up if there are no more errors in the configuration.

      However, if there are more errors, Apache and systemctl status will continue to report them and attempt to explain why the server cannot be started. systemctl will output failure messages like this on Ubuntu and Debian systems:

      Ubuntu & Debian Output

      Job for apache2.service failed because the control process exited with error code.
      See "systemctl status apache2.service" and "journalctl -xe" for details
      

      And on CentOS, Fedora, and RedHat derived systems, a failed startup message will be similar to the following:

      CentOS and Fedora Output

      Job for httpd.service failed because the control process exited with error code.
      See "systemctl status httpd.service" and "journalctl -xe" for details.
      

      When Apache will still not start because of errors, using the apachectl configtest command can be the most efficient and effective way to diagnose issues. The next section will explain how to use the utility to resolve an AH00526 error that is again related to an invalid SSLCertificateFile directive.

      Troubleshooting with apachectl

      To troubleshoot an AH00526 error with Apache’s apachectl utility, you can test your Apache configuration using the configtest sub-command. This tool will parse your Apache files to determine whether it’s valid and, if not, locate incorrect settings in the Apache configuration.

      The apachectl configtest command is useful for catching syntax errors before reloading apache with a new configuration. This test can help you to avoid service outages in the event of a misconfigured setting in your Apache files.

      The following example configuration test command will return an AH00526 Syntax error message, and explains that the likely problem is that Apache is referencing an empty SSLCertificateFile:

      • sudo apachectl configtest

      Output

      AH00526: Syntax error on line 3 of /etc/apache2/sites-enabled/000-default.conf: SSLCertificateFile: file '/etc/ssl/certs/example.com.pem' does not exist or is empty

      In this example output, the /etc/ssl/certs/example.com.pem file does not exist as the error message notes. Adding an SSL/TLS certificate to the file, or removing the directive will resolve the issue.

      A successful apachectl configtest invocation should result in output like this:

      Output

      Syntax OK

      Conclusion

      In this tutorial you learned how to troubleshoot an Apache AH00526 syntax error. The first step when investigating any Apache error is to examine the server’s status with systemctl status apache2, or systemctl status httpd depending on your Linux distribution. From there, you can determine whether Apache is running correctly, or if it is unable to start because of the error.

      After you have determined Apache’s status, you can diagnose it further using journalctl to examine the systemd logs for the process. You can also use the apachectl configtest command to check the configuration files for errors directly.



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      Apache Network Error AH00072: make_sock: could not bind to address



      Part of the Series:
      Common Apache Errors

      This tutorial series explains how to troubleshoot and fix some of the most common errors that you may encounter when using the Apache web server.

      Each tutorial in this series includes descriptions of common Apache configuration, network, filesystem, or permission errors. The series begins with an overview of the commands and log files that you can use to troubleshoot Apache. Subsequent tutorials examine specific errors in detail.

      Introduction

      An Apache AH00072: make_sock: could not bind to address error message is generated when there is another process listening on the same port that Apache is configured to use. Typically the port will be the standard port 80 for HTTP connections, or port 443 for HTTPS connections. However, any port conflict with another process can cause an AH00072 error.

      The error is derived from the underlying operating system system’s network stack. The issue is that only a single process can be bound to a port at any given time. If another web server like Nginx is configured to listen on port 80 and it is running, then Apache will not be able to claim the port for itself.

      To detect a port conflict with Apache, you will need to examine systemctl and journalctl output to determine the IP address and port that are causing the error. Then you can decide how to resolve the issue, whether it is by switching web servers, changing the IP address that Apache uses, the port, or any combination of these options.

      Troubleshooting with systemctl

      Following the troubleshooting steps from the How to Troubleshoot Common Apache Errors tutorial at the beginning of this series, the first step when you are troubleshooting an AH00072: make_sock: could not bind to address error message is to check Apache’s status with systemctl.

      If systemctl does not include output that describes the problem, then the last section of this tutorial, Troubleshooting Using journalctl Logs explains how to examine the systemd logs to find the conflicting port.

      The output from systemctl status will in many cases contain all the diagnostic information that you need to resolve the error. It will include the IP address that Apache is using, as well as the port that it is attempting to bind to. The output will also indicate how long Apache has been unable to start so that you can determine how long the issue has been affecting Apache.

      On Ubuntu and Debian-derived Linux distributions, run the following to check Apache’s status:

      Ubuntu and Debian Systems

      • sudo systemctl status apache2.service -l --no-pager

      On CentOS and Fedora systems, use this command to examine Apache’s status:

      CentOS and Fedora Systems

      • sudo systemctl status httpd.service -l --no-pager

      The -l flag will ensure that systemctl outputs the entire contents of a line, instead of substituting in ellipses () for long lines. The --no-pager flag will output the entire log to your screen without invoking a tool like less that only shows a screen of content at a time.

      Since you are troubleshooting an AH00072: make_sock error message, you should receive output that is similar to the following:

      Output

      ● httpd.service - The Apache HTTP Server Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled) Active: failed (Result: exit-code) since Tue 2020-07-28 13:58:40 UTC; 8s ago Docs: man:httpd.service(8) Process: 69 ExecStart=/usr/sbin/httpd $OPTIONS -DFOREGROUND (code=exited, status=1/FAILURE) Main PID: 69 (code=exited, status=1/FAILURE) Status: "Reading configuration..." Tasks: 213 (limit: 205060) Memory: 25.9M CGroup: /system.slice/containerd.service/system.slice/httpd.service Jul 28 13:58:40 e3633cbfc65e systemd[1]: Starting The Apache HTTP Server… Jul 28 13:58:40 e3633cbfc65e httpd[69]: (98)Address already in use: AH00072: make_sock: could not bind to address [::]:80 Jul 28 13:58:40 e3633cbfc65e httpd[69]: (98)Address already in use: AH00072: make_sock: could not bind to address 0.0.0.0:80 Jul 28 13:58:40 e3633cbfc65e httpd[69]: no listening sockets available, shutting down Jul 28 13:58:40 e3633cbfc65e httpd[69]: AH00015: Unable to open logs Jul 28 13:58:40 e3633cbfc65e systemd[1]: httpd.service: Main process exited, code=exited, status=1/FAILURE Jul 28 13:58:40 e3633cbfc65e systemd[1]: httpd.service: Failed with result 'exit-code'. Jul 28 13:58:40 e3633cbfc65e systemd[1]: Failed to start The Apache HTTP Server.

      Note that your output may be slightly different if you are using an Ubuntu or Debian-derived distribution, where the name of the Apache process is not httpd but is apache2.

      This example systemctl output includes some highlighted lines from the systemd journal that describes the AH00072 error. These lines, both of which begin with (98)Address already in use: AH00072: make_sock: could not bind to address, give you all the information about the AH00072 error that you need to troubleshoot it further, so you can skip the following journalctl steps and instead proceed to the Troubleshooting with ss and ps Utilities section at the end of this tutorial.

      If your systemctl output does not give specific information about the IP address and port or ports that are causing the AH00072 error, you will need to examine journalctl output from the systemd logs. The following section explains how to use journalctl to troubleshoot an AH00072 error.

      Troubleshooting Using journalctl Logs

      If your systemctl output does not include specifics about an AH00072 error, you should proceed with using the journalctl command to examine systemd logs for Apache.

      On Ubuntu and Debian-derived systems, run the following command:

      • sudo journalctl -u apache2.service --since today --no-pager

      On CentOS, Fedora, and RedHat-derived systems, use this command to inspect the logs:

      • sudo journalctl -u httpd.service --since today --no-pager

      The --since today flag will limit the output of the command to log entries beginning at 00:00:00 of the current day only. Using this option will help restrict the volume of log entries that you need to examine when checking for errors.

      If Apache is unable to bind to a port that is in use, search through the output for lines that are similar to the following log entries, specifically lines that contain the AH00072 error code as highlighted in this example:

      Output

      -- Logs begin at Tue 2020-07-14 20:10:37 UTC, end at Tue 2020-07-28 14:01:40 UTC. -- . . . Jul 28 14:03:01 b06f9c91975d apachectl[71]: (98)Address already in use: AH00072: make_sock: could not bind to address [::]:80 Jul 28 14:03:01 b06f9c91975d apachectl[71]: (98)Address already in use: AH00072: make_sock: could not bind to address 0.0.0.0:80 Jul 28 14:03:01 b06f9c91975d apachectl[71]: no listening sockets available, shutting down

      This output indicates two AH00072 errors. The first of these explains that Apache cannot bind to the [::]:80 address, which is port 80 on all available IPv6 interfaces. The next line, with the address 0.0.0.0:80, indicates Apache cannot bind to port 80 on all available IPv4 interfaces. Depending on your system’s configuration, the IP addresses may be different and only show individual IPs, and may only include IPv4 or IPv6 errors.

      Even though your own system may have different conflicting interfaces and ports, the errors will be similar to the output shown here. With output from journalctl you will be able to diagnose the issue using ss in the following section of this tutorial.

      Troubleshooting with ss and ps Utilities

      To troubleshoot an AH00072 error you need to determine what other process is listening on the IP address and port that Apache is attempting to use. Most modern Linux distributions include a utility called ss which can be used to gather information about the state of a system’s network sockets.

      In the previous journalctl section, something was already bound to the IPv4 and IPv6 addresses on port 80. The following command will determine the name of the process that is already bound to an IPv4 interface on port 80. Ensure that you substitute the port from the error message if it is different from 80 in the following command:

      • sudo ss -4 -tlnp | grep 80

      The flags to the ss command alter its default output in the following ways:

      • -4 restricts ss to only display IPv4-related socket information.
      • -t restricts the output to tcp sockets only.
      • -l displays all listening sockets with the -4 and -t restrictions taken into account.
      • -n ensures that port numbers are displayed, as opposed to protocol names like ‘httporhttps`. This is important since Apache may be attempting to bind to a non-standard port and a service name can be confusing as opposed to the actual port number.
      • -p outputs information about the process that is bound to a port.

      With all of those flags, you will receive output like the following:

      Output

      LISTEN 0 511 0.0.0.0:80 0.0.0.0:* users:(("nginx",pid=40,fd=6))

      The first three fields are not important when troubleshooting an AH00072 error so they can be ignored. The important fields are the fourth (0.0.0.0:80), which matches the journalctl error that you discovered earlier, along with the last users:(("nginx",pid=40,fd=6)), specifically the pid=40 portion.

      If you have an AH00072 error that is related to an IPv6 interface, repeat the ss invocation, this time using the -6 flag to restrict the interfaces to the IPv6 network stack like this:

      • sudo ss -6 -tlnp |grep 80

      Output

      LISTEN 0 511 [::]:80 [::]:* users:(("nginx",pid=40,fd=7))

      Again, substitute the port number in question from your journalctl output if it is different from the highlighted 80 given here.

      In both these cases of IPv4 and IPv6 errors, the ss output indicates that there is a program with process ID 40 (the pid=40 in the output) that is bound to the 0.0.0.0:80 and [::]:80 interfaces respectively. This process is preventing Apache from starting since it already owns the port. To determine the name of the program, use the ps utility like this, substituting the process ID from your output in place of the highlighted 40 value in this example:

      You will receive output that is similar to the following:

      Output

      PID TTY TIME CMD 40 ? 00:00:00 nginx

      The highlighted nginx in the output is the name of the process that is listening on the interfaces. Now that you have the name of the program that is preventing Apache from starting, you can decide how to resolve the error. You could stop the nginx process, reconfigure nginx to listen on a different interface and port, or reconfigure Apache to avoid the port collision.

      It is important to note that the process may be different from nginx and the port and IP addresses may not always be 0.0.0.0 or [::] if you are diagnosing an AH00072 error. Oftentimes, different web servers and proxies will be in use on the same server. Each may be attempting to bind to different IPv4 ports and IPv6 interfaces to handle different web traffic. For example, a server that is configured with HAProxy listening on the IPv4 loopback address (also referred to as localhost) on port 8080 will show ss output like this:

      Output

      LISTEN 0 2000 127.0.0.1:8080 0.0.0.0:* users:(("haproxy",pid=545,fd=7))

      It is important to combine systemctl output, or journalctl output that indicates specific IP addresses and ports, with diagnostic data from ss, and then ps to narrow down the process that is causing Apache to fail to start.

      Conclusion

      In this tutorial you learned how to troubleshoot an Apache AH00072 make_sock: could not bind to address error message on both IPv4 and IPv6 interfaces. You learned how to use systemctl to examine the status of the Apache server and try to find error messages. You also learned how to use journalctl to examine the systemd logs for specific information about an AH00072 error.

      With the appropriate error messages from the logs, you then learned about the ss utility and how to use it to examine the state of a system’s network sockets. After that you learned how to combine process ID information from ss with the ps utility to find the name of the process that is causing Apache to be unable to start.



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