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[#]: subject: "Using Postfix DNS SRV record resolution feature"
[#]: via: "https://fedoramagazine.org/using-postfix-dns-srv-record-resolution-feature/"
[#]: author: "Tomáš Korbař https://fedoramagazine.org/author/tkorbar/"
[#]: collector: "lujun9972"
[#]: translator: "ChatGPT"
[#]: reviewer: "wxy"
[#]: publisher: "wxy"
[#]: url: "https://linux.cn/article-16474-1.html"
利用 DNS SRV 记录为 Postfix 提供负载平衡
====
![][0]
2011 年 3 月，苹果公司提出 [RFC 6186][4]，描述了如何利用域名系统服务（DNS SRV）记录来查找电子邮件的提交以及访问服务。现在 Postfix 从 3.8.0 版本开始支持 RFC 中提出的设计。这个新增功能让你可以使用 DNS SRV 记录进行负载分配和自动配置。
### DNS SRV 记录的形态
DNS SRV 记录定义在 [RFC 2782][5] 中，它指定在区域文件中，并且包含了服务名称、传输协议规范、优先级、权重、端口，以及提供该服务的主机。
```
_submission._tcp SRV 5 10 50 bruce.my-domain.com.
```
字段 | 值 | 意义
---|---|---
服务名称 | `submission` | 服务名为 submission
传输协议规范 | `tcp` | 本服务使用 TCP 协议
优先级 | `5` | 服务器优先级设为 5（数值越小，优先级越高）
权重 | `10` | 服务器应承担的负载部分
端口 | `50` | 服务器监听连接的端口
目标 |` bruce.my-domain.com.` | 提供此服务的服务器名称
*记录解释*
### 服务器选择算法
客户端应该按照 [RFC 2782][5] 中描述的方式解析 SRV 记录。这意味着，首先尝试联系拥有最高优先级（最小的优先级数字）的服务器。如果该服务器无回应，那么重试联系拥有同样或者更低优先级的下一台服务器。当有多台服务器拥有同样优先级的时候，应随机选择其中一台，但是必须确保选择记录的概率符合下列公式：
![][6]
其中 `i` 是 SRV 记录的标识，`k` 是具有相同优先级的 SRV 记录的数量。
在现实中，这意味着如果你有两台服务器，其中一台的处理能力是另一台的三倍，那么你应该给第一台服务器的权重赋于另一台三倍的值。这样就能保证更强大的服务器会接收到大约 75% 的客户端请求，而另一台接收大约 25% 的请求。
这些原则使得 SRV 记录能够同时作为客户端自动配置及在服务器之间分配工作负载的工具。
看看以下这个记录的例子：
```
_submission._tcp SRV 0 0 2525 server-one
_submission._tcp SRV 1 75 2625 server-two
_submission._tcp SRV 1 25 2625 server-three
```
这里 `server-one` 总是会被首选来进行联系。如果 `server-one` 无回应，客户端就会将剩下优先级为 1 的两个记录顺序打乱，生成一个从 0 到 100 的随机数，如果第一条记录的运行总和大于或者等于这个随机数，它就会尝试去联系这个记录。否则，客户端会倒序联系所有服务器。注意，客户端会向它优先成功连接的服务器发送请求。
### 示例配置
请考虑以下这种情况。你想为大量的电脑配置 Postfix，使其通过公司的邮件服务器利用 SRV 记录转发外部电邮。为了达到这个目标，你需要在每台电脑的 Postfix 中配置 `relayhost` 参数，即邮件用户代理（MUA）。如果将 `relayhost` 参数的值设置为 `$mydomain`，你的机器将开始为你的域名查找 MX 记录，并尝试按照它们的优先级顺序发送邮件。这种方法虽然有效，但是可能会遇到负载平衡问题。Postfix 会使用优先级最高的服务器，直到其变为无响应才会联系其他备用服务器。此外，如果你在环境中使用了动态分配的端口，你无法指明哪个端口正在被特定的服务器使用。使用 SRV 记录，你可以应对这些挑战，并在需要改变服务器端口的时候维持服务器的平滑运行。
#### 区域文件
为了使得 DNS 服务器提供信息给客户端，可以参考以下使用服务器 `server-one`、`server-two`、`server-three` 作为中继，并把服务器 `server-four` 配置为接收测试邮件的区域文件示例。
```
$TTL 3600
@ IN SOA example-domain.com. root.example-domain.com. (
1571655122 ; 区域文件的序列号
1200 ; 刷新时间
180 ; 发生问题时的重试时间
1209600 ; 过期时间
10800 ) ; 查询失败时的最大缓存时间
;
IN NS ns1
IN A 192.168.2.0
;
ns1 IN A 192.168.2.2
server-one IN A 192.168.2.4
server-two IN A 192.168.2.5
server-three IN A 192.168.2.6
server-four IN A 192.168.2.7
_submission._tcp SRV 0 0 2525 server-one
_submission._tcp SRV 1 50 2625 server-two
_submission._tcp SRV 1 50 2625 server-three
@ MX 0 server-four
```
#### Postfix MUA 配置
设置客户端机器去查找 SRV 记录：
```
use_srv_lookup = submission
relayhost = example-domain.com:submission
```
通过这个配置，你的客户端机器上的 Postfix 实例会联络到 `example-domain` 的 DNS 服务器，然后获取邮件提交的 SRV 记录。在这个例子中，`server-one` 有最高的优先级，Postfix 会先试图连接它。然后，Postfix 随机的选择剩下的两个服务器其中一个去尝试连接。这个配置确保了大约有 50% 的机会会优先联系到服务器一。注意，SRV 记录的权重值并不等同于百分比。你也可以用 1 和 1 这样的值达到同样的目标。
同时，Postfix 也知道 `server-one` 在监听 2525 端口，而 `server-two` 在监听 2625 端口。如果你正在缓存检索到的 DNS 记录，并且你动态改变 SRV 记录，那么设置一个低的生存时间（TTL）对你的记录是很重要的。
#### 整套设置
![][7]
你可以通过下面的方式尝试这个配置，包含 podman 和在此处提供的 compose 文件：
```
$ git clone https://github.com/TomasKorbar/srv_article
$ cd srv_article/environment
$ podman-compose up
$ podman exec -it article_client /bin/bash
root@client # ./senddummy.sh
root@client # exit
```
完成配置之后，你可以检查日志，查看邮件是否经过 `server-one` 并最终投递到 `server-four`。
```
$ podman stop article_server1
$ podman exec -it article_client /bin/bash
root@client # ./senddummy.sh
root@client # ./senddummy.sh
root@client # ./senddummy.sh
root@client # ./senddummy.sh
root@client # ./senddummy.sh
root@client # ./senddummy.sh
root@client # exit
```
现在 `server-one` 已经关闭了，这六封邮件将会由 `server-two` 或者 `server-three` 中转发出去。
仔细看一下 Dockerfiles 以更深地理解这个配置。
通过执行：`$ podman-compose down` 完成示例的操作。
*（题图：DA/241079fe-58d6-4dc6-8801-f0fd19dfd64b）*
--------------------------------------------------------------------------------
via: https://fedoramagazine.org/using-postfix-dns-srv-record-resolution-feature/
作者：[Tomáš Korbař][a]
选题：[lujun9972][b]
译者：[ChatGPT](https://linux.cn/lctt/ChatGPT)
校对：[wxy](https://github.com/wxy)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
[a]: https://fedoramagazine.org/author/tkorbar/
[b]: https://github.com/lujun9972
[1]: https://fedoramagazine.org/wp-content/uploads/2023/06/DNS_SRV_resolution-816x345.jpg
[2]: https://unsplash.com/ja/@scottrodgerson?utm_source=unsplash&utm_medium=referral&utm_content=creditCopyText
[3]: https://unsplash.com/s/photos/server-routing?orientation=landscape&utm_source=unsplash&utm_medium=referral&utm_content=creditCopyText
[4]: https://www.ietf.org/rfc/rfc6186.txt
[5]: https://www.ietf.org/rfc/rfc2782.txt
[6]: https://fedoramagazine.org/wp-content/uploads/2023/06/probability_equation.png
[7]: https://fedoramagazine.org/wp-content/uploads/2023/06/srv_article.png
[0]: https://img.linux.net.cn/data/attachment/album/202312/15/173522hpze31p331ppljrj.jpg

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[#]: subject: "Using Postfix DNS SRV record resolution feature"
[#]: via: "https://fedoramagazine.org/using-postfix-dns-srv-record-resolution-feature/"
[#]: author: "Tomáš Korbař https://fedoramagazine.org/author/tkorbar/"
[#]: collector: "lujun9972"
[#]: translator: " "
[#]: reviewer: " "
[#]: publisher: " "
[#]: url: " "
Using Postfix DNS SRV record resolution feature
======
![][1]
Photo by [Scott Rodgerson][2] on [Unsplash][3] (cropped)
In March 2011 Apple Inc. proposed [RFC 6186][4] that describes how domain name system service (DNS SRV) records should be used for locating email submission and accessing services. The design presented in the RFC is now supported by Postfix since version 3.8.0. With the new functionality, you can now use DNS SRV records for load distribution and auto-configuration.
### What does the DNS SRV record look like
The DNS SRV records were defined in [RFC 2782][5] and are specified in zone files and contain the service name, transport protocol specification, priority, weight, port, and the host that provides the service.
_submission._tcp SRV 5 10 50 bruce.my-domain.com.
**Field** | **Value** | **Meaning**
---|---|---
service name | submission | service is named submission
transport protocol specification | tcp | service is using TCP
priority | 5 | servers priority is 5 (lower gets tried first)
weight | 10 | portion of load the server should handle
port | 50 | port where server listens for connections
target | bruce.my-domain.com. | name of server providing this service
Record explanation
### Server selection algorithm
Clients should implement the resolution of SRV records as described in [RFC 2782][5]. That means, first contact the server with the lowest priority. If the server does not respond, try to contact the next server with either the same or lower priority. If there are multiple servers with the same priority, choose one randomly, but ensure the probability of choosing records conforms to the equation:
![][6]
where _i_ is the identification of SRV record and _k_ is the count of SRV records with the same priority.
In practice, this means that if you have two servers and one is 3 times as powerful as the other one, then you should give the first weight a value 3 times higher than the other one. This ensures the more powerful server will receive ~75% of client requests and the other one ~25%.
These principles allow SRV records to work as tools to both autoconfigure clients and distribute the workload among servers.
Consider the following example of such a set of records:
```
_submission._tcp SRV 0 0 2525 server-one
_submission._tcp SRV 1 75 2625 server-two
_submission._tcp SRV 1 25 2625 server-three
```
Here **server-one** would always be contacted first. If **server-one** does not respond, the client will shuffle the two remaining records with priority 1, generate a random number from 0 to 100 and if the running sum of the first record is greater or equal, then try to contact it. Otherwise, the client contacts the servers in reverse order. Note that the client submits the request to the first server it successfully connects to.
### Configuration example
Consider the following situation. You want to configure Postfix to relay outgoing emails through a company mail server by using SRV records for a large number of computers. To achieve this, you can configure the **relayhost** parameter in Postfix, which acts as a Mail User Agent (MUA) for each computer. If you set the value of the **relayhost** parameter to **$mydomain** , your machines start to look up MX records for your domain and attempt to submit mail in the order based on their priorities. While this approach works, you can encounter a problem with load balancing. Postfix uses the server with the highest priority until it becomes unresponsive and only then contacts any secondary servers. Additionally, if your environment uses dynamically assigned ports, you are not able to notify the clients what port is a particular server using. With SRV records, you can address these challenges and keep the servers running smoothly without peaks while changing the server’s port as needed.
#### Zone file
To configure a DNS server to provide information to clients, see the following example zone file with servers one, two and three configured as relays and server four for receiving test mail.
```
$TTL 3600
@ IN SOA example-domain.com. root.example-domain.com. (
1571655122 ; Serial number of zone file
1200 ; Refresh time
180 ; Retry time in case of problem
1209600 ; Expiry time
10800 ) ; Maximum caching time in case of failed lookups
;
IN NS ns1
IN A 192.168.2.0
;
ns1 IN A 192.168.2.2
server-one IN A 192.168.2.4
server-two IN A 192.168.2.5
server-three IN A 192.168.2.6
server-four IN A 192.168.2.7
_submission._tcp SRV 0 0 2525 server-one
_submission._tcp SRV 1 50 2625 server-two
_submission._tcp SRV 1 50 2625 server-three
@ MX 0 server-four
```
#### Postfix MUA configuration
Configure client machines to look for SRV records:
```
use_srv_lookup = submission
relayhost = example-domain.com:submission
```
With this configuration, Postfix instances on your client machines contact the DNS server for the **example-domain** and request the SRV records for mail submission. In this example, **server-one** has the highest priority and Postfix tries it first. Postfix then randomly selects one of the two remaining servers to try. The configuration ensures that server one will be contacted first approximately 50% of the time. Note that the weight value in the SRV records does not correspond with the percentage. You can achieve the same goal with the values 1 and 1.
Postfix also has the information that **server-one** listens on port 2525 and **server-two** on port 2625. If you are caching retrieved DNS records and you change the SRV records dynamically, it is important to set a low time to live (TTL) for your records.
#### Complete setup
![][7]
You can try this configuration with podman and compose file included here:
```
$ git clone https://github.com/TomasKorbar/srv_article
$ cd srv_article/environment
$ podman-compose up
$ podman exec -it article_client /bin/bash
root@client # ./senddummy.sh
root@client # exit
```
After completing the configuration steps, you can check the logs to monitor, that the mail passes server one and is delivered to server four.
```
$ podman stop article_server1
$ podman exec -it article_client /bin/bash
root@client # ./senddummy.sh
root@client # ./senddummy.sh
root@client # ./senddummy.sh
root@client # ./senddummy.sh
root@client # ./senddummy.sh
root@client # ./senddummy.sh
root@client # exit
```
Now that the first server is down, these six mails will be relayed by either server two or three.
Take a closer look at the Dockerfiles to understand the configuration more deeply.
Finish working with the example by executing: $ podman-compose down
--------------------------------------------------------------------------------
via: https://fedoramagazine.org/using-postfix-dns-srv-record-resolution-feature/
作者：[Tomáš Korbař][a]
选题：[lujun9972][b]
译者：[译者ID](https://github.com/译者ID)
校对：[校对者ID](https://github.com/校对者ID)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
[a]: https://fedoramagazine.org/author/tkorbar/
[b]: https://github.com/lujun9972
[1]: https://fedoramagazine.org/wp-content/uploads/2023/06/DNS_SRV_resolution-816x345.jpg
[2]: https://unsplash.com/ja/@scottrodgerson?utm_source=unsplash&utm_medium=referral&utm_content=creditCopyText
[3]: https://unsplash.com/s/photos/server-routing?orientation=landscape&utm_source=unsplash&utm_medium=referral&utm_content=creditCopyText
[4]: https://www.ietf.org/rfc/rfc6186.txt
[5]: https://www.ietf.org/rfc/rfc2782.txt
[6]: https://fedoramagazine.org/wp-content/uploads/2023/06/probability_equation.png
[7]: https://fedoramagazine.org/wp-content/uploads/2023/06/srv_article.png