Why there is a time difference between 'traceroute <hostname>' and 'traceroute -4 <hostname>' command ?

Solution Verified - Updated -

Environment

  • Red Hat Enterprise Linux(RHEL).
  • traceroute.

Issue

  • Why there is a time difference between traceroute <hostname> and traceroute -4 <hostname> command?

Resolution

  • This is an expected behavior. When traceroute <hostname> command is executed it will check via a DNS resolver for both IPV4(A) and IPV6(AAAA) address. Whereas with -4 option traceroute will only resolve for IPv4(A).
  • For more information refer #man traceroute.

Root Cause

Diagnostic Steps

  • From the strace we can see process is holding for 2 seconds at a poll trace. 
<snip>
23704 08:45:20 connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.1.1")}, 28) = 0
23704 08:45:20 fcntl(3, F_GETFL)        = 0x2 (flags O_RDWR)
23704 08:45:20 fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0
23704 08:45:20 poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}])
23704 08:45:20 sendto(3, "\xf3\xb5\x01\x00\x00\x01\x00\x00\x00\x00\x00\x00\x09\x65\x70\x73\x76\x61\x31\x37\x30\x32\x0a\x77\x65\x6c\x6c\x73\x66\x61\x72\x67"..., 42, MSG_NOSIGNAL, NULL, 0) = 42
23704 08:45:20 poll([{fd=3, events=POLLIN}], 1, 5000) = 1 ([{fd=3, revents=POLLIN}]) ====>>> 2 second delay 
23704 08:45:22 ioctl(3, FIONREAD, [42]) = 0
<snip>
  • Again there is 2 seconds delay with the DNS Ip and poll. poll is delaying means it is checking for IPV6 DNS lookups from the DNS server.
<snip>
23704 08:45:22 connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.1.1")}, 28) = 0
23704 08:45:22 fcntl(3, F_GETFL)        = 0x2 (flags O_RDWR)
23704 08:45:22 fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0
23704 08:45:22 poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}])
23704 08:45:22 sendto(3, "\xf3\xb5\x01\x00\x00\x01\x00\x00\x00\x00\x00\x00\x09\x65\x70\x73\x76\x61\x31\x37\x30\x32\x0a\x77\x65\x6c\x6c\x73\x66\x61\x72\x67"..., 42, MSG_NOSIGNAL, NULL, 0) = 42
23704 08:45:22 poll([{fd=3, events=POLLIN}], 1, 5000) = 1 ([{fd=3, revents=POLLIN}])
23704 08:45:24 ioctl(3, FIONREAD, [42]) = 0
23704 08:45:24 recvfrom(3, "\xf3\xb5\x81\x82\x00\x01\x00\x00\x00\x00\x00\x00\x09\x65\x70\x73\x76\x61\x31\x37\x30\x32\x0a\x77\x65\x6c\x6c\x73\x66\x61\x72\x67"..., 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.2.4")}, [16]) = 42 ======>>> 2 second delay
23704 08:45:24 close(3)                 = 0
</snip>
  • This can be confirmed by using captured tcpdump.
400 2017-12-04 00:35:02.302430  192.168.1.2   192.168.1.1 DNS 84  Standard query AAAA server.example.com
626 2017-12-04 00:35:04.887139  192.168.1.1 192.168.1.2   DNS 84  Standard query response, Server failure      ===>>>
853 2017-12-04 00:35:07.982493  192.168.1.2   192.168.1.1 DNS 84  Standard query AAAA server.example.com
998 2017-12-04 00:35:09.751530  192.168.1.2 192.168.1.2   DNS 84  Standard query response, Server failure      ===>>>

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