Linux Maximum Length Of A Host Name

by on April 30, 2008 · 3 comments· LAST UPDATED May 1, 2008

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While browsing Linux kernel source code I came across POSIX (Portable Operating System Interface) variable called _POSIX_HOST_NAME_MAX . It defines the maximum length of a host name (not including the terminating null) as returned from the gethostname function.

This variable sets the limit to the number of characters in a hostname under Linux.
$ grep '_POSIX_HOST_NAME_MAX' /usr/include/bits/posix1_lim.h
Output:

#define _POSIX_HOST_NAME_MAX    255

You can change limit and recompile the kernel to get bigger hostname.

See comment below for correct information.

Updated for accuracy.

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{ 3 comments… read them below or add one }

1 Jeff May 1, 2008 at 1:14 am

This is a VERY bad idea. The length of hostnames is a standard defined by the RFC’s.

If anyone adjusts this to excess of the standards and has problems, they are just a idiot to be laughed at.

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2 Jeff Schroeder May 1, 2008 at 12:29 pm

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.

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3 nixCraft May 1, 2008 at 4:54 pm

Thanks for correct information, The post has been updated.

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