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# 1  
Old 06-02-2016
Check for valid hostnames
Hello,

I am trying to develop a script to check for valid hostnames. Below are the prerequisites for a valid hostname which I got from wiki :

Hostnames are composed of series of labels concatenated with dots, as are all domain names. For example, "en.wikipedia.org" is a hostname. Each label must be between 1 and 63 characters long, and the entire hostname (including the delimiting dots but not a trailing dot) has a maximum of 253 ASCII characters.

The Internet standards (Requests for Comments) for protocols mandate that component hostname labels may contain only the ASCII letters 'a' through 'z' (in a case-insensitive manner), the digits '0' through '9', and the hyphen ('-'). The original specification of hostnames in RFC 952, mandated that labels could not start with a digit or with a hyphen, and must not end with a hyphen. However, a subsequent specification (RFC 1123) permitted hostname labels to start with digits. No other symbols, punctuation characters, or white space are permitted.

Could someone please help.

Thanks
Rahul
# 2  
Old 06-02-2016
Quote:
Originally Posted by rahul2662
Hello,

I am trying to develop a script to check for valid hostnames. Below are the prerequisites for a valid hostname which I got from wiki :

Hostnames are composed of series of labels concatenated with dots, as are all domain names. For example, "en.wikipedia.org" is a hostname. Each label must be between 1 and 63 characters long, and the entire hostname (including the delimiting dots but not a trailing dot) has a maximum of 253 ASCII characters.

The Internet standards (Requests for Comments) for protocols mandate that component hostname labels may contain only the ASCII letters 'a' through 'z' (in a case-insensitive manner), the digits '0' through '9', and the hyphen ('-'). The original specification of hostnames in RFC 952, mandated that labels could not start with a digit or with a hyphen, and must not end with a hyphen. However, a subsequent specification (RFC 1123) permitted hostname labels to start with digits. No other symbols, punctuation characters, or white space are permitted.

Could someone please help.

Thanks
Rahul
Hello Rahul,

Good to see you in forums Smilie. Regarding above query I would like to say it completely depends on the organization like they are really following the mentioned rules/guidelines or not. Why not do a ping test for each host and find out(means--> if a host is valid it will pass the test), please do let me know your thoughts on same.

Thanks,
R. Singh
# 3  
Old 06-02-2016
Hello Ravinder,

Thanks for your reply. Smilie I am actually developing a script to configure hostname. The script which will ask the user for the hostname. It will check for all the conditions mentioned in my previous mail and validate if the user has entered hostname as per the correct naming convention. If the user has not adhered to the valid hostname convention then script will echo error else script will proceed and make changes specific to the hostname in the RedHat configuration files.

Thanks
Rahul
Last edited by rahul2662; 06-02-2016 at 06:42 AM..
# 4  
Old 06-02-2016
Quote:
Originally Posted by rahul2662
Hello Ravinder,
Thanks for your reply. Smilie I am actually developing a script to configure hostname. The script which will ask the user for the hostname. It will check for all the conditions mentioned in my previous mail and validate if the user has entered hostname as per the correct naming convention. If the user has not adhered to the valid hostname convention then script will echo error else script will proceed and make changes specific to the hostname in the RedHat configuration files.
Thanks
Rahul
Hello rahul2662,

Could you please try following and let me know if this helps and covers your conditions, though I haven't tested it.
Code:
 echo "Please enter the name:"
read NAME
ONE=1
 VALUE=`echo $NAME | awk -F"." '{if(length($0)<=63 && NF==3 && $0 !~ /\.$/ && $0 !~ /^[[:digit:]]/ && $0 !~ /^-/ && $0 !~ /-$/ && $0 !~ /[[:space:]]/ && $0 !~ /[[:punct:]]/ && $0 !~ /[[:cntrl:]]/){print 1}}'`
 if [[ $VALUE == $ONE ]]
then
        echo "You have entered correct name as " $NAME
else
        echo "Please enter correct name as it is matching the standards."
fi

It is taking care of following conditions too.
i- length should be 63 char long.
ii- 3 dots.
iii- NO trailing dot at last of variable.
iv- should not start with a digit and hyphen, should not end with hyphen too.
v- no other symbols, punctuations, white spaces.
Could you please more elaborate about following point.
Quote:
has a maximum of 253 ASCII characters.
Thanks,
R. Singh
Last edited by RavinderSingh13; 06-02-2016 at 07:48 AM..
# 5  
Old 06-02-2016
Hello Ravinder,

Below conditions should be satisfied for a valid hostname :

1) Maximum 3 dots ( can have zero dots as well)
2) maximum 63 characters within each dot

for example below should be a valid hostname

]$ksh hostnamecheck.ksh
Please enter the name:
newhost
Please enter correct name as it is matching the standards.

Thanks
Rahul
# 6  
Old 06-02-2016
Hello rahul2662,

Could you please try following and let me know how it goes then.
Code:
echo "Please enter the name:"
read NAME
ONE=1
VALUE=`echo $NAME | awk '{if(length($0)<=253 && $0 !~ /\.$/ && $0 !~ /^[[:digit:]]/ && $0 !~ /^-/ && $0 !~ /-$/ && $0 !~ /[[:space:]]/ && $0 !~ /[[:punct:]]/ && $0 !~ /[[:cntrl:]]/){num=split($0, A,".");if(num==1){print 1}} else {for(i=1;i<=num;i++){if(length(A[i])<=63){k++}}};if(k==num){print 1}}'`
if [[ $VALUE == $ONE ]]
then
        echo "You have entered correct name as " $NAME
else
        echo "Please enter correct name as it is NOT matching the standards."
fi

You should check all permutations and combinations for above example too. Please let me know if you have any queries on same.

EDIT: I have added logic to check 0 occurrences for DOT too now in above code. Adding a more clarifying one liner form of awkwithin script.
Code:
echo "Please enter the name:"
read NAME
ONE=1
VALUE=`echo $NAME | awk '{
                                if(length($0)<=253 && $0 !~ /\.$/ && $0 !~ /^[[:digit:]]/ && $0 !~ /^-/ && $0 !~ /-$/ && $0 !~ /[[:space:]]/ && $0 !~ /[[:punct:]]/ && $0 !~ /[[:cntrl:]]/)      {
                                num=split($0, A,".");
                                if(num==1){
                                                print 1
                                          }
                                else      {
                                                for(i=1;i<=num;i++){
                                                                        if(length(A[i])<=63){
                                                                                                k++
                                                                                          }
                                                                   };
                                                                   }
                                          }
                                if(k==num){
                                                print 1
                                          }
                          }
                        '`
if [[ $VALUE == $ONE ]]
then
        echo "You have entered correct name as " $NAME
else
        echo "Please enter corect name as it is NOT matching the standards."
fi

Thanks,
R. Singh
Last edited by RavinderSingh13; 06-02-2016 at 08:45 AM.. Reason: Added logic for 0 occurances of DOT in user input now too.
# 7  
Old 06-02-2016
How about (pure recent bash):

Code:
read NAME
IFS="."; NARR=($NAME)
ST=1
for i in ${!NARR[@]}; do [ ${#NARR[$i]} -gt 63 ] && { ST=0; break; }; done
[ ${#NAME} -le 253 ] && [ ${#NARR[@]} -le 4 ] && [[ ! "$NAME" =~ [^[:alnum:].-] ]] && [ $ST -eq 1 ] && echo good || echo bad
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