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Tham khảo tài liệu 'dns & bind_8', công nghệ thông tin, hệ điều hành phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | DNS BIND 12.3.3 A Successful Lookup with Retransmissions Debug Level 1 Not all lookups are as clean as the last one - sometimes the query must be retransmitted. The user doesn t see any difference so long as the lookup succeeds although the query involving retransmissions will take longer. Following is a trace where there are retransmissions. We converted the IP addresses to names after the trace was done. Notice how much easier it is to read with names 1 Debug turned ON Level 1 2 3 datagram from terminator.movie.edu port 3397 fd 20 len 35 4 req nlookup ucunix.san.uc.edu id 1 type 1 class 1 5 req found ucunix.san.uc.edu as edu cname 0 6 forw forw - i.root-servers.net port 53 ds 4 nsid 2 id 1 0ms retry 4 sec 7 8 datagram from i.root-servers.net port 53 fd 4 len 240 delegation lines removed 9 resp nlookup ucunix.san.uc.edu qtype 1 10 resp found ucunix.san.uc.edu as san.uc.edu cname 0 11 resp forw - uceng.uc.edu port 53 ds 4 nsid 3 id 1 0ms 12 resend addr 1 n 0 - ucbeh.san.uc.edu port 53 ds 4 nsid 3 id 1 0ms 13 14 datagram from terminator.movie.edu port 3397 fd 20 len 35 15 req nlookup ucunix.san.uc.edu id 1 type 1 class 1 16 req found ucunix.san.uc.edu as san.uc.edu cname 0 17 resend addr 2 n 0 - uccba.uc.edu port 53 ds 4 nsid 3 id 1 0ms 18 resend addr 3 n 0 - mail.cis.ohio-state.edu port 53 ds 4 nsid 3 id 1 0ms 19 20 datagram from mail.cis.ohio-state.edu port 53 fd 4 len 51 21 send_msg - terminator.movie.edu UDP 20 3397 id 1 This trace starts out like the last trace lines 1 through 11 the name server receives a query for ucunix.san.uc.edu sends the query to an edu name server i.root-servers.net receives a response that includes a list of name servers for uc.edu and sends the query to one of the uc.edu name servers uceng.uc.edu . What s new in this trace is the resend lines lines 12 17 and 18 . The forw on line 11 counts as resend addr 0 n 0 - CS dweebs always start counting with zero. Since uceng.uc.edu didn t respond the name server went on to try ucbeh line 12 .
