(PHP 4, PHP 5, PHP 7, PHP 8)
gethostbyname — Get the IPv4 address corresponding to a given Internet host name
Returns the IPv4 address of the Internet host specified by
hostname.
hostnameThe host name.
Returns the IPv4 address or a string containing the unmodified
hostname on failure.
Example #1 A simple gethostbyname() example
<?php
$ip = gethostbyname('www.example.com');
echo $ip;
?>
If you do a gethostbyname() and there is no trailing dot after a domainname that does not resolve, this domainname will ultimately be appended to the server-FQDN by nslookup.
So if you do a lookup for nonexistentdomainname.be your server may return the ip for nonexistentdomainname.be.yourhostname.com, which is the server-ip.
To avoid this behaviour, just add a trailing dot to the domainname; i.e. gethostbyname('nonexistentdomainname.be.')This function says "Returns the IPv4 address or a string containing the unmodified hostname on failure.
This isn't entirely true, any hostname with a null byte in it will only return the characters BEFORE the null byte.
<?php
$hostname = "foo\0bar";
var_dump($hostname );
var_dump(gethostbyname($hostname ));
?>
Results:
string 'foo�bar' (length=7)
string 'foo' (length=3)Important note: You should avoid its use in production.
DNS Resolution may take from 0.5 to 4 seconds, and during this time your script is NOT being executed.
Your customers may think that the server is slow, but actually it is just waiting for the DNS resolution response.
You can use it, but if you want performance, you should avoid it, or schedule it to some CRON script...Options for the underlying resolver functions can be supplied by using the RES_OPTIONS environmental variable. (at least under Linux, see man resolv.conf)
Set timeout and retries to 1 to have a maximum execution time of 1 second for the DNS lookup:
<?php
putenv('RES_OPTIONS=retrans:1 retry:1 timeout:1 attempts:1');
gethostbyname($something);
?>
You should also use fully qualified domain names ending in a dot. This prevents the resolver from walking though all search domains and retrying the domain with the search domain appended.For doing basic RBL (Real Time Blacklist) lookups with this function do:
<?php
$host = '64.53.200.156';
$rbl = 'sbl-xbl.spamhaus.org';
// valid query format is: 156.200.53.64.sbl-xbl.spamhaus.org
$rev = array_reverse(explode('.', $host));
$lookup = implode('.', $rev) . '.' . $rbl;
if ($lookup != gethostbyname($lookup)) {
echo "ip: $host is listed in $rbl\n";
} else {
echo "ip: $host NOT listed in $rbl\n";
}
?>
Tomas V.V.Coxgethostbyname and gethostbynamel does not ask for AAAA records. I have written two functions to implement this. gethostbyname6 and gethostbynamel6. I don't believe this issue has been addressed yet.
They are made to replace gethostbyname[l], in a way that if $try_a is true, if it fails to get AAAA records it will fall back on trying to get A records.
Feel free to correct any errors, I realise that it is asking for *both* A and AAAA records, so this means two DNS calls.. probably would be more efficient if it checked $try_a before making the query, but this works for me so I'll leave that up to someone else to implement in their own work.. the tip is out there now anyway..
Here is the code:
function gethostbyname6($host, $try_a = false) {
// get AAAA record for $host
// if $try_a is true, if AAAA fails, it tries for A
// the first match found is returned
// otherwise returns false
$dns = gethostbynamel6($host, $try_a);
if ($dns == false) { return false; }
else { return $dns[0]; }
}
function gethostbynamel6($host, $try_a = false) {
// get AAAA records for $host,
// if $try_a is true, if AAAA fails, it tries for A
// results are returned in an array of ips found matching type
// otherwise returns false
$dns6 = dns_get_record($host, DNS_AAAA);
if ($try_a == true) {
$dns4 = dns_get_record($host, DNS_A);
$dns = array_merge($dns4, $dns6);
}
else { $dns = $dns6; }
$ip6 = array();
$ip4 = array();
foreach ($dns as $record) {
if ($record["type"] == "A") {
$ip4[] = $record["ip"];
}
if ($record["type"] == "AAAA") {
$ip6[] = $record["ipv6"];
}
}
if (count($ip6) < 1) {
if ($try_a == true) {
if (count($ip4) < 1) {
return false;
}
else {
return $ip4;
}
}
else {
return false;
}
}
else {
return $ip6;
}
}Since 2015, the hostname parameter is arbitrarily limited to 255 characters to address CVE-2015-0235, probably to mirror RFC1035's octet limit on domain names.Note that if you pass an IP address to gethostbyname() it will return that IP address.Grabbing IPs for multiple hosts:
<?php
function getAddrByHost($hosts, $timeout = 3) {
$returnString = '';
foreach ($hosts as $host) {
$query = `nslookup -timeout=$timeout -retry=1 $host`;
if (preg_match('/\nAddress: (.*)\n/', $query, $matches))
$returnString .= trim($matches[1]) . '<br>';
$returnString .= $host . '<br>';
}
return $returnString;
}
$hostArray[] = 'www.domain1.com';
$hostArray[] = 'www.domain2.com';
//$hostArray[] = 'www.domain3.com';
//$hostArray[] = 'www.domain4.com';
$returnString = getAddrByHost($hostArray);
echo $returnString;
?>I had difficulty getting gethostbyname to work under OpenBSD 3.2 and Apache, until I discovered that the default Apache chroot caused the problem.
To get PHP's gethostbyname to work, you need resolv.conf (and possibly hosts) in /var/www/etc (assuming default install dirs).In PHP4 you can use gethostbyname() but I have found this unreliable when doing lookups on entries that return A records on the private network. PHP5 has a much better routine -- dns_get_record(). If you are stuck with PHP4 or don't want to upgrade you can use dig:
<?php
$ip = `/usr/bin/dig $host A +short`;
?>mmucklo raises a valid point, however the function to use would be getmxrr() in the simplest case, or alternatively checkdnsrr() if you so wish; getdnsrr() does not exist.
On another note, it can be a pain (when iterating with while loops for example) that on failure gethostbyname() returns the hostname, rather than FALSE. You could wrap it in a simple user function, to "correct" this:
<?php
function fixed_gethostbyname ($host) {
// Try the lookup as normal...
$ip = gethostbyname($host);
// ...but if it fails, FALSE is returned instead of the unresolved host
if ($ip != $host) { return $ip; } else return false;
}
?><?php
//script to time DNS propagation
//(Above script modified slightly to show micro time)
//seems pretty damn quick to me.. I'm getting .0055 sec worstcase badhost times.
//A known good dns name (my own)
$nametotest = "fuzzygroup.com";
//Call address test function
$time_start = getmicrotime();
testipaddress($nametotest);
$time_end = getmicrotime();
$time = $time_end - $time_start;
echo "Good Host Search took $time seconds<br><br>";
//A known bad name (trust me)
$nametotest = "providence.mascot.com";
$time_start = getmicrotime();
testipaddress($nametotest);
$time_end = getmicrotime();
$time = $time_end - $time_start;
echo "Bad Host Search took $time seconds<br>";
function getmicrotime(){
list($usec, $sec) = explode(" ",microtime());
return ((float)$usec + (float)$sec);
}
//ip address checking function
//for real use should have a return value but example code
function testipaddress ($nametotest) {
$ipaddress = $nametotest;
$ipaddress = gethostbyname($nametotest);
if ($ipaddress == $nametotest) {
echo "No ip address for host<br>";
}
else {
echo "good hostname, $nametotest ipaddress = $ipaddress<br>";
}
}
//Recommended fix for sql applications:
// store url to temporary table
// run second process periodically to
// check urls and update main table
?>referring to ralphbolton at mail2sexy dot com comment:
(at least in 5.2.0 + djbdns-dnscache) gethostbyname does not really seem to cache entries. If somebody notices a speed-up after the second lookup of the same domain - that's most likely your dns-cache itself, not some php-internal dns-cache.
It does cache the entries in your /etc/resolv.conf (e.g. what dns to use) so I agree with him, that stopping and starting apache, will reload the resolv.conf.On a side-note, PHP (5.0.4, but probably other versions too) can cache gethostbyname information.
In short, once PHP looks up an address, it may not actually perform another lookup as you may expect. In my particular case (I think) the problem was a change to resolv.conf didn't take effect inside PHP (although nslookup/ping etc worked fine). Stop/Starting Apache fixed it (although a simple 'restart' (kill -HUP) didn't).
In short, if you change resolv.conf, stop and restart Apache.I was using file_get_contents on a set of URLs. Some of them URLs were invalid (the structure of it was ok but the DNS hosts couldn't resolve them) and I kept getting an annoying warning. I wanted to check the DNS somehow but existing check dns function in php doesn't have one for windows and the one a person supplied there does not work 100% of the time.
Instead use this function to try to resolve a host. This won't throw any warnings, you just need to check the output. You'll get the same warnings with fopen and fsockopen.Just a heads up. I was using this function on my site to verify email host addresses. I thought all was good, until a potential client contacted me and said they couldn't sign up correctly. They have a valid email address and domain, but this function failed.
Good luck.For the "dig" solution, here is a better one:
<?php
$IP = `/usr/bin/dig $host. A +short | /usr/bin/tail -n1`
?>
Using tail because dig, even in the +short form, may return CNAME entries first, and also may return many, many entries (see "dig google.com").
Usig the dot at the end of the host to not perform domain searches.
This works also when $host = IP address already
Will return empty string in case o resolve failure (invalid hostname)This logic solution to problem checking dns reverse name resolution:
<?php
$ip = gethostbyname($host);
if(ip2long($ip) == -1 || ($ip == gethostbyaddr($ip) && preg_match("/.*\.[a-zA-Z]{2,3}$/",$host) == 0) ) {
echo 'Error, incorrect host or ip';
}
else {
echo 'Ok';
}
?>One way to discover your IP address automatically:
<?php
// need to trim() because whitespace will confuse the name lookup
$myIP = gethostbyname(trim(`hostname`));
echo $myIP;
?>When using gethostbynamel() and gethostbyname() together, you must do gethostbynamel() first, otherwise it will always give you one IP (or none) from the cache, and not return the full lookup.<?php
//script to see if host exists on Internet
//following up on the above point about host name
//checking and SQL timeouts, run this test script
//and see how long it takes for 2nd call to
//hostname check to fail
//NOTE -- not PHP's fault -- nature of DNS
//A known good dns name (my own)
$nametotest = "fuzzygroup.com";
//Call address test function
testipaddress($nametotest);
//A known bad name (trust me)
$nametotest = "providence.mascot.com";
//Call address test function
testipaddress($nametotest);
//ip address checking function
//for real use should have a return value but example code
function testipaddress ($nametotest) {
$ipaddress = $nametotest;
$ipaddress = gethostbyname($nametotest);
if ($ipaddress == $nametotest) {
echo "No ip address for host, so host "
. "not currently available in DNS and "
. "probably offline for some time<BR>";
}
else {
echo "good hostname, ipaddress = $ipaddress<BR>";
}
}
//Recommended fix for sql applications:
// store url to temporary table
// run second process periodically to
// check urls and update main table
?>Always add the root . at the end or your server can add it's own suffix.
example: abc.com could become abc.com.yourcompany.com if you don't use abc.com.
I also find this function a litle bit limited because it doesn't tell you if the resolve failed.
TO adress all those issues I use :
function host2ip($host)
{
$host=trim($host.'.'); // clean and add root .
$ip= gethostbyname($host);
if($ip==$host) $ip='';// empty IP if there is no ip
return $ip;
}When using PHP and Apache in a chroot environment on RedHat Linux, I have found that I need to bind-mount /var/run/nscd to get this to work. Apparently, the socket in that directory is needed for all the DNS things.One note about using gethostbyname() for checking email address domains:
If the name doesn't resolve, follow up with getdnsrr() and make sure they don't have an MX entry before returning an error.
It is possible for a domain name not to have an A record, but still have an MX entry.The dns entries get cached, whether they exist or not. Expect really good response times after the first one.If name resolution fails with apache2, mod_chroot and php5, add
LoadFile /lib/libnss_dns.so.2
to the mod_chroot config.