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[2016-11-14 20:32 UTC] teo8976 at gmail dot com
Description:
------------
When you call gethostbyaddr($someip), it does 2 DNS lookups:
1 - looks up the IP to find the corresponding domain name, i.e. does its job
2 - then looks up the domain name, as if it needed to find the corresponding IP address
The second is completely unnecessary, especially considering that, if it times out, it nonetheless returns the host name found at step 1 (if, in this case, it returned false or the IP address, one may perhaps argue that it makes sense in order to verify that the host name is correct or something).
Test script:
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<?php
echo gethostbyaddr('95.232.181.106');
Expected result:
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should immediately print:
host106-181-dynamic.232-95-r.retail.telecomitalia.it
At http://mxtoolbox.com/SuperTool.aspx?action=ptr%3a95.232.181.106&run=toolpage# you can verify that it takes a fraction of a second to do the reverse lookup
Actual result:
--------------
From most servers, it takes more than 15 seconds (I guess it depends on how long it takes for the DNS resolver to time out), and then it prints the expected domain name.
Apparently, there is something wrong with this domain which prevents the name from being resolved to an IP:
http://mxtoolbox.com/SuperTool.aspx?action=a%3ahost106-181-dynamic.232-95-r.retail.telecomitalia.it&run=toolpage
However, as explained above, the name->IP lookup is completely unnecessary.
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Last updated: Sun Nov 02 14:00:01 2025 UTC |
It is easy enough to verify that there aren't 2 lookups: strace -Ff -tt -T php -r "echo gethostbyaddr('95.232.181.106');" ... 13:09:27.142617 stat("/etc/resolv.conf", {st_mode=S_IFREG|0644, st_size=55, ...}) = 0 <0.000006> 13:09:27.142644 socket(AF_INET, SOCK_DGRAM|SOCK_NONBLOCK, IPPROTO_IP) = 3 <0.000006> 13:09:27.142660 connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.200.1")}, 16) = 0 <0.000008> 13:09:27.142688 poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}]) <0.000004> 13:09:27.142703 sendto(3, "\224!\1\0\0\1\0\0\0\0\0\0\003106\003181\003232\00295\7in-a"..., 45, MSG_NOSIGNAL, NULL, 0) = 45 <0.000014> 13:09:27.142731 poll([{fd=3, events=POLLIN}], 1, 5000) = 1 ([{fd=3, revents=POLLIN}]) <0.000634> 13:09:27.143377 ioctl(3, FIONREAD, [111]) = 0 <0.000005> 13:09:27.143394 recvfrom(3, "\224!\201\200\0\1\0\1\0\0\0\0\003106\003181\003232\00295\7in-a"..., 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.200.1")}, [28->16]) = 111 <0.000006> 13:09:27.143420 close(3) = 0 <0.000007> 13:09:27.143450 write(1, "host106-181-dynamic.232-95-r.ret"..., 52host106-181-dynamic.232-95-r.retail.telecomitalia.it) = 52 <0.000005> ... PHP calls the standard POSIX library function gethostbyaddr() just once. The above strace shows what that function does below the covers. It checks to see if resolv.conf has changed, then connects to the dns server (192.168.200.1 in my case), sends the query, polls to wait for the reply, receives the reply and writes the result. That's all. If you reverse lookup is taking a long time look at your resolver. It is not PHP getting in the way. You can see from the syscall timing in the strace that in my case it took 0.000833 seconds from the resolv.conf stat to writing the answer. A lot less than 15 seconds.