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upstream-jdomain: Asynchronous domain name resolution module for NGINX upstream

Installation

You can install this module in any RHEL-based distribution, including, but not limited to:

  • RedHat Enterprise Linux 7, 8, 9
  • CentOS 7, 8, 9
  • AlmaLinux 8, 9
  • Rocky Linux 8, 9
  • Amazon Linux 2 and Amazon Linux 2023
yum -y install https://extras.getpagespeed.com/release-latest.rpm
yum -y install nginx-module-upstream-jdomain

Enable the module by adding the following at the top of /etc/nginx/nginx.conf:

load_module modules/ngx_http_upstream_jdomain_module.so;

This document describes nginx-module-upstream-jdomain v1.4.0 released on May 29 2022.

An asynchronous domain name resolution module for nginx upstream.

This module allows you to use a domain name in an upstream block and expect the domain name to be dynamically resolved so your upstream may be resilient to DNS entry updates.

The module does not perform DNS resolution automatically on some interval. Instead, the DNS resolution needs to be prompted by a request for the given upstream. If nginx serves a connection bound for a jdomain upstream, and the configured interval has elapsed, then the module will perform a DNS lookup.

The module is compatible with other upstream scope directives. This means you may populate an upstream block with multiple jdomain directives, multiple server directives, keepalive, load balancing directives, etc. Note that unless another load balancing method is specified in the upstream block, this module makes use of the default round robin load balancing algorithm built into nginx core.

Important Note: Should an alternate load balancing algorithm be specified, it must come before the jdomain directive in the upstream block! If this is not followed, nginx will crash during runtime! This is because many other load balancing modules explicitly extend the built in round robin, and thus end up clobbering the jdomain initialization handlers, since jdomain is technically a load balancer module as well. While this may not be the case with all load balancer modules, it's better to stay on the safe side and place jdomain after.

Important Note: Due to the non blocking nature of this module and the fact that its DNS resolution is triggered by incoming requests, the request that prompts a lookup will actually still be forwarded to the upstream that was resolved and cached before the DNS lookup happens. Depending on the scenario, this could result in a one off failure when changing the states of upstreams. This is important to keep in mind to ensure graceful transitions of your upstreams.

This repository is a fork of a repository originally authored by wdaike. As that project is no longer maintained, this repository aims to be its successor and is now several features ahead.

Usage

resolver 8.8.8.8; # Your Local DNS Server

## Basic upstream using domain name defaulting to port 80.
upstream backend_01 {
    jdomain example.com;
}

## Basic upstream specifying different port.
upstream backend_02 {
    jdomain example.com port=8080;
}

## Upstream with a backup server to use in case of host not found or format
## errors on DNS resolution.
upstream backend_03 {
    server 127.0.0.2 backup;
    jdomain example.com;
}

## Upstream which will use backup for any and all DNS resolution errors.
upstream backend_04 {
    server 127.0.0.2 backup;
    jdomain example.com strict;
}

server {
    listen 127.0.0.2:80;
    return 502 'An error.';
}

Synopsis

Syntax: jdomain <domain-name> [port=80] [max_ips=4] [interval=1] [strict]
Context: upstream
Attributes:
    port:       Backend's listening port.                                      (Default: 80)
    max_ips:    IP buffer size. Maximum number of resolved IPs to cache.       (Default: 4)
    interval:   How many seconds to resolve domain name.                       (Default: 1)
    ipver:      Only addresses of family IPv4 or IPv6 will be used if defined  (Default: 0)
    strict:     Require the DNS resolution to succeed and return addresses,
                otherwise marks the underlying server and peers as down and
                forces use of other servers in the upstream block if there
                are any present. A failed resolution can be a timeout, DNS
                server failure, connection refusals, response with no
                addresses, etc.

See https://www.nginx.com/resources/wiki/modules/domain_resolve/ for details.

Development

Prerequisites

To facilitate local development and enable you to build and test the module, you'll need some tools.

  • Docker: to provide an environment to easily reproduce ideal conditions for building and testing.
  • act: to simulate executing github actions workflows locally to save you from pushing commits just to watch the CI fail.
  • rust: dependency of cargo-make.
  • cargo-make: to run common development tasks such as building, testing, and formatting code.

Task Runner

cargo-make is an advanced task runner that will enabled you to easily perform common development operations like formatting the code, building the module, running the test suite, and running code analysis. You can see the task definitions in the file Makefile.toml. Installing cargo-make will result in a standalone executable called makers as well as a cargo extension which can be executed via cargo make. As this project is not a rust crate, it is recommended to simply use makers.

Also note that for simplicity's sake, the task runner uses docker to run all tasks. This means the build binary is not targetting your host platform.

Default Task

To add value, the default task (ie. simply running makers alone) will begin an interactive bash session inside the docker container used for this project.

This should help with debugging and general workflow.

Formatting

Incorrectly formatted code will cause the github actions linting job to fail. To avoid this, you can run the format task before pushing new changes, like so:

makers format

This formatting is performed by a tool called clang-format. You can find the config options for this defined in the file ./.clang-format.

Static Code Analysis

You can run a static analysis on the code via the analyse task:

makers analyse

This analysis is performed by a tool called clang-tidy. You can find the config options for this defined in the file ./.clang-tidy.

Testing

You can run the test suite using the test task, like so:

makers test

Debugging

We can use valgrind and gdb on nginx from inside the container.

First open an interactive shell in the container with:

$ makers

We'll use that session to run valgrind:

$ valgrind --vgdb=full --vgdb-error=0 /github/workspace/bin/static/nginx -p/github/workspace/t/servroot -cconf/nginx.conf
==15== Memcheck, a memory error detector
==15== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==15== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==15== Command: /github/workspace/bin/static/nginx -p/github/workspace/t/servroot -cconf/nginx.conf
==15==
==15== (action at startup) vgdb me ...
==15==
==15== TO DEBUG THIS PROCESS USING GDB: start GDB like this
==15==   /path/to/gdb /github/workspace/bin/static/nginx
==15== and then give GDB the following command
==15==   target remote | /usr/lib64/valgrind/../../bin/vgdb --pid=15
==15== --pid is optional if only one valgrind process is running
==15==

Next, find the container identifier so we can open another session inside it:

$ docker ps
CONTAINER ID        IMAGE                                     COMMAND             CREATED             STATUS              PORTS                    NAMES
55fab1e069ba        act-github-actions-nginx-module-toolbox   "bash"              4 seconds ago       Up 3 seconds        0.0.0.0:1984->1984/tcp   serene_newton

Use either the name or ID to execute a bash session inside the container:

$ docker exec -it serene_newton bash

We'll use this session to start gdb and target the valgrind gdb server we started in the other session:

$ gdb /github/workspace/bin/static/nginx
GNU gdb (GDB) Red Hat Enterprise Linux 8.0.1-30.amzn2.0.3
Copyright (C) 2017 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from /github/workspace/bin/static/nginx...done.
(gdb)

From the gdb prompt, target the valgrind process and begin debugging:

(gdb) target remote | /usr/lib64/valgrind/../../bin/vgdb --pid=15
Remote debugging using | /usr/lib64/valgrind/../../bin/vgdb --pid=15
relaying data between gdb and process 15
warning: remote target does not support file transfer, attempting to access files from local filesystem.
Reading symbols from /lib64/ld-linux-x86-64.so.2...(no debugging symbols found)...done.
0x0000000004000ef0 in _start () from /lib64/ld-linux-x86-64.so.2
Missing separate debuginfos, use: debuginfo-install glibc-2.26-35.amzn2.x86_64
(gdb)

Running GitHub Actions

With act, you can simulate the workflow that will run on GitHub servers once you push changes.

There is more than one job in the main workflow, so you need to specify the test job when you run act. For example, you can use this command to run the code format validation:

act -vj lint

Note that the lint job does not format your code, it only checks that the formatting is as expected.

Also note that -v is used to enable verbose mode to give more visibility on everything act is doing.

The jobs you can (and should) run locally are lint, build, analyse, and test. The test job depends on the output from the build job. To keep the output from the build job, you can add the -b flag to act, or you may simply use the task runner to build.

Known Issues

At the moment? None! 🎉

If you discover a bug or have a question to raise, please open an issue.

Original Author

wdaike wdaike@163.com (https://github.com/wdaike), Baidu Inc.

GitHub

You may find additional configuration tips and documentation for this module in the GitHub repository for nginx-module-upstream-jdomain.