Maximum hostname length

This is mainly a Unix kernel related issue related to the length of particular string in one of kernel structures. But applications might also have problem if they reserve only eight character string (applications usually use DNS, not hostname, but Unix scripts often use hostname command and $HOSTNAME variable without full understanding that it can be a long, up to 64 bytes string).

It is very similar to Year 2000 problem: most modern Unixes work OK with long hostnames but risks remains.

Generally it is prudent to limit hostnames to 8 characters if possible and to use DNS aliases with longer names. One exception is Oracle which for some reason prefers fully qualified names as hostname. RHEL supports such behavior by default.

This is mainly a Unix kernel related issue related to the length of particular string in one of kernel structures. But applications might also have problem if they reserve only eight character string.

Same considerations:

• Limits differ from one Unix flavor to another

  • Linux supports up to 64 character hostnames

    • Default hostname on RHEL is fully qualified name (FQN) which is usually longer then 8 characters hostnames on RHEL (for example, Oracle 11 requires fully qualified names as the hostname). This feature broke several legacy monitoring scripts.

    • Suse supports long hostname much like RHEL.

  • HP-UX 11.11 is the most problematic and versions before 11i v2 do not support longer hostnames. For HP-UX 11.11 there was a known problem with ignite if longer then eight characters are used in hostname. Might be resolved in later patches.

    There is progress in 11 v2 and 11 v3 toward at least 64 bytes max length. In earlier versions:

    Versions of HP-UX prior to 11i version 2 May 2005 Update do not support expanded host names. Therefore, various client utilities on prior HP-UX versions might not be able to access any servers that have long host names. For example, utilities such as remsh(1)might not be able to access servers with canonical names (names with domains added) longer than 63 bytes. Similarly, NFS client systems having earlier OS versions installed cannot access NFS servers having names longer than 31 bytes (excluding null terminator). Therefore, HP-UX NFS server systems should not be assigned longer names unless all HP-UX client systems that access that server have been updated.

    There are also some problems with CDE long names can be truncates and displayed incorrectly.

    UUCP does not support extended names in all HP-UX version (we don't use it) VxVM volume manager is limited to node names of 8 bytes There are problems with older libraries compiled without option _HPUX_API_LEVEL set to 20040821. Minimum JDK level for support of extended names is 1.4.1.07

  • AIX supports longer then eight characters hostnames but only up to 32 characters. AIX applications are generally older and are less likely support this extension, but generally should rely on DNS.

  • Solaris supports hostnames up to 255 characters since version 9. Solaris applications typically rely of DNS.

  • Windows uses longer then 8 character hostnames for a while.

  • For VMware long hostnames probably is not be a problem.

• Applications usually handle longer hostname OK as they typically rely of DNS.

  • For Data Protector this is not a problem

  • OpenView allows hostname longer then eight characters (it actually uses DNS names)

• Scripts can be problem.  For example if script implicitly assumes that hostname is a short name and consociates it with domain usage of  qualified name (FQN) as hostname (such as in RHEL) will break the script.

This is like year 2000 problem. It might well be exaggerated but that does not mean that it does not exist. Unix is a legacy system and if you exceed legacy limits you might be punished, no matter what new standard say (POSIX standard specifies 255 characters, but only Solaris currently supports it).

No amount of Internet search will give us correct estimate of the risk. The danger is that such errors are very subtle. Like complex routing problems they can demonstrate itself in a way we will not even understand that this is hostname length related problem.

Unless absolutely necessary you should generally avoid using hostnames longer then eighth characters as a matter of policy. Here is one good advice from the Net

For compatibility sake with other systems, no matter how long the OS states the hostname can be you do not want to exceed 8 characters.

Even some of the newer systems coming out have hard limits built in to help the windows people conform to the standards, instead of defining their own. again........

if you need long names, setup DNS and make good domain/sub-domain names. This way legacy systems and systems that support only 8 characters can still function

Here are a few systems not supporting more than 8 characters.

NEWS CONTENTS

• 200102 : Linux Maximum Length Of A Host Name ( Linux Maximum Length Of A Host Name, )
• 200102 : maximum hostname length in linux - Ars Technica OpenForum ( maximum hostname length in linux - Ars Technica OpenForum, )
• 200102 : Length of hostname ( ‎12-10-2002 , HP Enterprise Business Community )
• 200102 : Unix server hostname length - HP Enterprise Business Community ( Unix server hostname length - HP Enterprise Business Community, )
• 200102 : IBM AIX - How long can hostname be ( IBM AIX - How long can hostname be, )

Old News ;-)

Linux Maximum Length Of A Host Name

Jeff Schroeder

This is one that you are actually wrong on.

The Single Unix Specification version 2 (SuSv2) guarantees 'Host names are limited to 255 bytes'.

POSIX 1003.1-2001 guarantees 'Host names (not including the terminating NULL) are limited to HOST_NAME_MAX bytes'.

$ grep HOST_NAME_MAX /usr/include/bits/local_lim.h
#define HOST_NAME_MAX 64

$ getconf HOST_NAME_MAX
64

This is part of the c library you use and libc defaults to 64. Changing your kernel and recompiling would still give you a max hostname size of 64 characters including the null termination byte at the end.

maximum hostname length in linux - Ars Technica OpenForum

Short answer: 64 bytes.

Long answer: in Linux, the hostname is stored in the nodename field of the utsname structure for the current namespace. For all versions of Linux that I am aware of (say, since 1994 or so) the length of that field is 65 bytes (meaning, 64 chars terminated by a 0). In order to increase it, you will need to, at the very least:
1. change __NEW_UTS_LEN in include/linux/utsname.h to whatever number you need;
2. change the sizes of the fields in the old_utsname and new_utsname structs to __NEW_UTS_LEN + 1
3. recompile the kernel
4. replace /usr/include/linux/utsname.h with the new version
5. recompile glibc to use the modified linux/utsname.h
6. make sure that /usr/include/bits/utsname.h has _UTSNAME_LENGTH set to the right value (recompiling glibc should take care of this, probably)
7. recompile all programs and libraries that include /usr/include/sys/utsname.h, /usr/include/bits/utsname.h, or /usr/include/linux/utsname.h, or that use functions or constants or structs defined in those header files.
8. and if you run closed-source software, you are kinda stuck.

quote:

http://refspecs.freestandards.org/LSB_2.0.1/LSB-Core/LSB-Core.html

Maximum length of a host name (not including the terminating null) as returned from the gethostname function shall be at least 255 bytes.

tetromino

Ars Centurion

quote:

Originally posted by Linux_Biz:
then steps no; 3 to 8 , i am saying sorry, i dont know how to follow.

To be perfectly honest, I don't know what's the best way to follow them either. For one thing, it would probably be dangerous to run a glibc that uses one definition of utsname on a kernel that uses a different definition. The safest approach would be to compile a new livecd that uses the large utsname, using something like Gentoo's catalyst tool, and use that to reinstall your entire operating system. Essentially, you will need to maintain your own fork of some existing distro.

It's not a trivial undertaking. If you aren't comfortable with Linux innards, in particular if you don't know how to compile your own kernel, you really should go for a workaround -- maybe DNS aliases, like kperrier suggested.

And those people that think Linux supports 255 byte hostnames, try the following as root:

#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

char oldname[65];
char name[256];

int main() {
memset(name, 'x', 256);
if(!gethostname(oldname, 65))
printf("Saving your current hostname (%s)...\n",oldname);
if(!sethostname(name, 64))
printf("64 bytes works :)\n");
if(sethostname(name, 65))
perror("65 bytes is too long");
if(sethostname(name, 255))
perror("255 bytes is too long");
if(!gethostname(name, 255))
printf("Hostname successfully set to %s (%d bytes)\n", name, strlen(name));
if(!sethostname(oldname, strlen(oldname)))
printf("Restored your hostname to %s\n", oldname);
return 0;
}

linux_biz

what is the default length in redhat liunx? 65 or 256?

tetromino

@Linux_Biz:
copy the code in my post above, compile with gcc, and run the resulting executable as root. If it says that 64 bytes works but 65 bytes is too long, well, you'll know what length redhat supports.

BajanDude

64. Run tetromino's code, it's fairly safe.

Ta btw tetromino, that's a nice empirical test. I was merely quoting POSIX, but did see mentions that it's only 64 when I was googling.

Length of hostname

‎12-10-2002 | HP Enterprise Business Community

This is one area where Linux is ahead of HP-UX.

My web server for my web hosting biz at home runs Linux. The main server host name is jerusalem, my favorite city. That's nine characters btw

I tried that with an HP-UX box and had issues with swinstall and Ignite. That server is now called Moriah (the Temple Mount).

8 character limit is the way to go with HP-UX and thats 11.11 btw.

Steve
Steven E Protter
Owner of ISN Corporation
http://isnamerica.com
http://hpuxconsulting.com
Sponsor: http://hpux.ws
Twitter: http://twitter.com/hpuxlinux
Founder http://newdatacloud.com

===

Re: Length of hostnamePost options

‎12-10-2002 09:28 AM

For compatability sake with other systems, no matter how long the OS states the hostname can be you do not want to exceed 8 characters.

Even some of the newer systems coming out have hard limite built in to help the windows people conform to the standards, instead of defining their own. again........

if you need long names, setup DNS and make good domain/sub-domain names. This way legacy systems and systems that support only 8 characters can still function.
Here are a few systems not supporting more than 8 characters.

OS/400, MVS, WinNT3.5, Windows 3.1, DOS, MacOS less than 9, SCO Unix, Most Unices older than 4 years.

Regards,
Shannon
Microsoft. When do you want a virus today?

Unix server hostname length - HP Enterprise Business Community

Re: Unix server hostname lengthPost options

‎04-26-2005 03:04 PM

HP-UX is a standards-based system and uname must remain consistent with the standards. uname is defined as an 8-character (max) name as stated in the man page for hostname. Once a standard is changed, all sorts of problems with compatibility start showing up. Each man page documents the applicable standard(s) at the bottom.

NOTE: hostname is limited to 64 characters but nodename (the value reported by uname) is limited to 8 characters. BOTH can be set by editing the /etc/rc.conig.d/netconf file and entering separate values for HOSTNAME and NODENAME. If you must use a long hostname, then set NODENAME *and* HOSTNAME in the netconf file:

NODENAME="shorty"
HOSTNAME="shortstuffismyname"

Now you have both...but read this document:

http://www1.itrc.hp.com/service/cki/docDisplay.do?docLocale=en_US&docId=200000063236997

and then remember that all the sysadmins will have to handle any issues.

Re: Unix server hostname lengthPost options

HP is very much aware of the desire/need to support longer hostnames. However, as noted in some of the previous responses, it's a lot harder than it sounds to make such changes and still keep compatibility with older software (especially since software stacks come from many different vendors, many of which have no relationship with HP).

You might find it informative to look at the patch description text for PHCO_31598.

IBM AIX - How long can hostname be

Blizard

3 Nov 06 10:09

I am trying to find what the maximum length is for an AIX hostname - just the machine name without domain info. We currently use 8 characters, but I am building a new data center and the way we are configuring the machines, an extra character or two would be very helpful.

We chose 8 characters several years ago, but I can't remember why. It might have had something to do with the HP systems we have.

Any info gratefully accepted.

Robert Blizard
National Grid USA Service Co.
An expert is a man who has made all the mistakes which can be made, in a narrow field.
Neils Bohr (1885-1962)

p5wizard (IS/IT--Management)

3 Nov 06 10:14 Help screen in smit (F1 when in "smit mkhostname") talks about 32 chars or less for compatibility with older hosts, so you shouldn't get in any trouble...

HTH,

p5wizard

Recommended Links

• Hostname - Wikipedia, the free encyclopedia
• RFC 1034, Section 3.1 "Name space specifications and terminology"

• Jump uUnderscores in DNS". Retrieved 2011-07-20.

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• Chi Hung Chan What is the Maximum Length for Host Name in Solaris
• gethostname(2) get-set hostname - Linux man page
• Linux World c program in linux to find the maximum length allowed for a host name
• DNS hostname exceeding maximum length (DNS_Hostname_Overflow)
• Linux Maximum Length Of A Host Name

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