using your example file

awk 'BEGIN {RS="\n\n\n";FS="\n\n";OFS="*";ORS=""} {print $1,$2,$3,$4,$5}' file |awk -F'*' '{for(i=1; i<=NF; i++) c[i]=$i; n=asort(c); for (i=1; i<=n; i++) printf "%s%s*", c[i], (i<n?OFS:RS); delete c}' |sed 's/^*//' |awk -F'*' '{print $5"*"$1"*"$2"*"$3"*"$4}' |sort |awk -F'*' 'BEGIN{OFS="\n\n";ORS="\n\n\n"} {print $1,$2,$3,$4,$5;}'

Convert every block of text to rows and using '*' to separate the fields

awk 'BEGIN {RS="\n\n\n";FS="\n\n";OFS="*";ORS=""} {print $1,$2,$3,$4,$5}' file dn: 2*attr1: b*attr2: a*attr1: a*attr1: c dn: 3*attr2: a*attr1: c*attr1: b*attr1: a dn: 1*attr1: a*attr1: c*attr1: b*attr2: a

Sort fields within the lines and using '*' to separate the fields

awk 'BEGIN {RS="\n\n\n";FS="\n\n";OFS="*";ORS=""} {print $1,$2,$3,$4,$5}' file |awk -F'*' '{for(i=1; i<=NF; i++) c[i]=$i; n=asort(c); for (i=1; i<=n; i++) printf "%s%s*", c[i], (i<n?OFS:RS); delete c}' |sed 's/^*//'

attr1: a *attr1: b *attr1: c *attr2: a *dn: 2 attr1: a *attr1: b *attr1: c *attr2: a *dn: 3 attr1: a *attr1: b *attr1: c *attr2: a *dn: 1

rearrange the fieslds in the lines to 'print dn: x' in first place

awk 'BEGIN {RS="\n\n\n";FS="\n\n";OFS="*";ORS=""} {print $1,$2,$3,$4,$5}' file |awk -F'*' '{for(i=1; i<=NF; i++) c[i]=$i; n=asort(c); for (i=1; i<=n; i++) printf "%s%s*", c[i], (i<n?OFS:RS); delete c}' |sed 's/^*//' |awk -F'*' '{print $5"*"$1"*"$2"*"$3"*"$4}'

dn: 2*attr1: a *attr1: b *attr1: c *attr2: a dn: 3*attr1: a *attr1: b *attr1: c *attr2: a dn: 1*attr1: a *attr1: b *attr1: c *attr2: a

Sorting the lines by the first column or field

awk 'BEGIN {RS="\n\n\n";FS="\n\n";OFS="*";ORS=""} {print $1,$2,$3,$4,$5}' file |awk -F'*' '{for(i=1; i<=NF; i++) c[i]=$i; n=asort(c); for (i=1; i<=n; i++) printf "%s%s*", c[i], (i<n?OFS:RS); delete c}' |sed 's/^*//' |awk -F'*' '{print $5"*"$1"*"$2"*"$3"*"$4}' |sort

dn: 1*attr1: a *attr1: b *attr1: c *attr2: a dn: 2*attr1: a *attr1: b *attr1: c *attr2: a dn: 3*attr1: a *attr1: b *attr1: c *attr2: a

Converting the rows to one column and inserting empty lines

awk 'BEGIN {RS="\n\n\n";FS="\n\n";OFS="*";ORS=""} {print $1,$2,$3,$4,$5}' file |awk -F'*' '{for(i=1; i<=NF; i++) c[i]=$i; n=asort(c); for (i=1; i<=n; i++) printf "%s%s*", c[i], (i<n?OFS:RS); delete c}' |sed 's/^*//' |awk -F'*' '{print $5"*"$1"*"$2"*"$3"*"$4}' |sort |awk -F'*' 'BEGIN{OFS="\n\n";ORS="\n\n\n"} {print $1,$2,$3,$4,$5;}'

dn: 1

attr1: a

attr1: b

attr1: c

attr2: a


dn: 2

attr1: a

attr1: b

attr1: c

attr2: a


dn: 3

attr1: a

attr1: b

attr1: c

attr2: a

I know I'm using too much steps, any help to simplify the code will be welcome