最後更新: 2019-03-22

目錄

• Resource Record (RR)
• SRV records (RFC 2782)
• PTR record
• TXT record
• DNS Packet

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Resource Record (RR)

 

每一隻 DNS record 都叫 RR (

它們會有屬性: type, TTL, data [A, 300, n.n.n.n]

有效的 Domain

• ASCII letters A-Z
• digits 0-9
• hyphen -
• case-insensitive

 

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SRV records (RFC 2782)

 

Format

_service._proto.domain-name. TTL class SRV priority weight port target.

e.g.

# SRV for provision of SIP (VoIP) services

_sip._udp    IN SRV 1 0 5523 sip2.example.com.

# autodiscover

_autodiscover._tcp 3600 IN SRV 0 0 443 autodiscover
autodiscover       3600 IN   A         n.n.n.n

Service

Defines the "symbolic service name"

symbolic service name:

• _http - web service
• _ftp - file transfer service
• _ldap - LDAP service
• _imap - IMAP mail service

Proto

•  _tcp - TCP protocol
• _udp - UDP protocol

owner-name:

Defines the base domain (or zone) name

一般情況不用填 (leaving the entry blank (without a dot) will substitute the current the zone root)

TTL

Optional

class

Optional. Standard CLASS parameter. (IN)

pri

The relative Priority of this service (Lowest is highest priority)

weight

        Used when more than one service has the same priority. (highest is most frequently delivered)
        0 indicates no weighting should be applied.

target

FQDN

 

---

PTR record

 

又稱 rdns record, IP -> domain name 的反解來

Pointer to a canonical name. Unlike a CNAME, DNS processing stops and just the name is returned.

 

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TXT record

 

The size of the TXT record value to 255 characters

 => you can’t enter the DKIM key as a single entry in the DNS records

 => split the key into multiple quoted text strings and enter them together in the TXT record value field.

i.e.

dkim._domainkey 3600 IN TXT (
  "v=DKIM1; p="
  "..."
  "..."
  "..."
  "...")

 

---

DNS Packet

 

response fits into a single UDP datagram

large responses => lookup can be retried over TCP

+------------+
| Header     |
+------------+
| Question   | the question for the name server (QNAME,  QTYPE , QCLASS ...)
+------------+
| Answer     | RRs answering the question (TTL, RDATA ...)
+------------+
| Authority  | RRs pointing toward an authority
+------------+
| Additional | RRs holding additional information
+------------+

Authority section

may contain a list of resource records, pointing to authoritative name servers (某 Somain NS Server)

Additional section

may contain additional information to answers (i.e. NS Record IP)

Header section

+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                      ID                       |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR|   Opcode  |AA|TC|RD|RA| Z|AD|CD|   RCODE   |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    QDCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    ANCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    NSCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    ARCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

ID:

16-bit value, that is kept same in the response.

Flags:

• QR: 0=Query, 1=response
• OPCODE: 0000=query
• AA: Authoritative Answer (set by the server)
• TC: message is TrunCated (set by the server)
• RD: Recursion Desired (set by the client and copied to the response) (Disable: +nordflag)
  if the server is not authoritative, it will do a recursive query to find the answer.
• RA: Recursion Available (set by the server)
• RCODE, 4-bits: 0000=NOERROR.

DNSSEC flag:

• AD: Authenticated Data (set by the server)
  no Authenticated Data (ad) => a validation problem with one or more records returned
• CD: Checking Disabled (set by the client, copied to the response)
  1 => if the RRs cannot be verified, they should still be returned.

XXCOUNT:

number of records in each section