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70 lines
2.9 KiB
Markdown
70 lines
2.9 KiB
Markdown
# 使用workflow实现DNS服务器
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前述文档已经讲解了使用workflow实现服务器的方法，workflow框架贴心地为用户处理了底层逻辑和各种细节，因此本文档主要介绍如何组装DNS消息。
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[tutorial-19-dns_server.cc](/tutorial/tutorial-19-dns_server.cc)
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DNS协议内容中包含三个section，有`DNS_ANSWER_SECTION`、`DNS_AUTHORITY_SECTION`、`DNS_ADDITIONAL_SECTION`，每个section中可包含零或多条资源记录`Resource record`。目前`protocol::DnsResponse`支持添加的资源记录类型有`DNS_TYPE_A`、`DNS_TYPE_AAAA`、`DNS_TYPE_CNAME`、`DNS_TYPE_PTR`、`DNS_TYPE_SOA`、`DNS_TYPE_SRV`、`DNS_TYPE_MX`，其接口如下所示。
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```cpp
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int add_a_record(int section, const char *name,
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uint16_t rclass, uint32_t ttl,
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const void *data);
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int add_aaaa_record(int section, const char *name,
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uint16_t rclass, uint32_t ttl,
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const void *data);
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int add_ns_record(int section, const char *name,
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uint16_t rclass, uint32_t ttl,
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const char *data);
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int add_cname_record(int section, const char *name,
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uint16_t rclass, uint32_t ttl,
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const char *data);
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int add_ptr_record(int section, const char *name,
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uint16_t rclass, uint32_t ttl,
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const char *data);
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int add_soa_record(int section, const char *name,
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uint16_t rclass, uint32_t ttl,
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const char *mname, const char *rname,
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uint32_t serial, int32_t refresh,
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int32_t retry, int32_t expire, uint32_t minimum);
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int add_srv_record(int section, const char *name,
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uint16_t rclass, uint32_t ttl,
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uint16_t priority, uint16_t weight, uint16_t port,
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const char *target);
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int add_mx_record(int section, const char *name,
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uint16_t rclass, uint32_t ttl,
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int16_t preference, const char *exchange);
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int add_raw_record(int section, const char *name, uint16_t type,
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uint16_t rclass, uint32_t ttl,
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const void *data, uint16_t dlen);
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```
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例如要添加一条AAAA记录，可使用下述方式实现
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```cpp
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struct in6_addr addr;
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inet_pton(AF_INET6, "1234:5678:9abc:def0::", (void *)&addr);
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resp->add_aaaa_record(DNS_ANSWER_SECTION,
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name.c_str(), DNS_CLASS_IN, 600, &addr);
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```
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对于未支持的资源记录类型，可通过`add_raw_record`接口添加，例如要添加一条TXT记录，可使用下述方式实现
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```cpp
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const char *raw_txt_data = "\x0dmy dns server\x0fyour dns server";
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uint16_t data_len = 30;
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resp->add_raw_record(DNS_ANSWER_SECTION, name.c_str(), DNS_TYPE_TXT,
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DNS_CLASS_IN, 1200, raw_txt_data, data_len);
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```
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注意，默认情况下`WFDnsServer`会启动一个UDP服务，若需要启动TCP服务，可通过修改WFServerParams中的transport_type字段为`TT_TCP`来实现。DNS客户端通常会优先使用UDP协议发起请求，当要回复的消息过大时，可仅添加部分资源记录，并通过`resp->set_tc(1)`设置截断标记，指示客户端可使用TCP协议重新请求。
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