KRB5_425_CONV_PRINCIPAL(3)

💅 KRB5_425_CONV_PRINCIPAL — bestie, it's in the Library Functions Manual, and the manual page is literally called KRB5_425_CONV_PRINCIPAL(3)

okay so picture this: you're out here living your life, and suddenly you need to move principals back and forth between Kerberos version 4 and version 5. bestie. I'm gonna hold your hand through this. grab a strawberry 🍓 and let's go.

---

✨ NAME ✨

krb5_425_conv_principal, krb5_425_conv_principal_ext, krb5_524_conv_principal — the three icons that convert to and from version 4 principals 💖

---

📚 LIBRARY 📚

Kerberos 5 Library — that's libkrb5, and you link it with -lkrb5. periodt.

---

🎀 SYNOPSIS 🎀

first, the vibe check — you gotta include the header:

#include <krb5.h>

then here are your three main characters, in all their glory:

krb5_error_code
krb5_425_conv_principal(krb5_context context, const char *name,
    const char *instance, const char *realm, krb5_principal *principal);

krb5_error_code
krb5_425_conv_principal_ext(krb5_context context, const char *name,
    const char *instance, const char *realm,
    krb5_boolean (*func)(krb5_context, krb5_principal),
    krb5_boolean resolve, krb5_principal *principal);

krb5_error_code
krb5_524_conv_principal(krb5_context context,
    const krb5_principal principal, char *name, char *instance,
    char *realm);

---

🌸 DESCRIPTION 🌸

ngl, converting between version 4 and version 5 principals can at best be described as a mess. like, genuinely. that's not me being dramatic, that's just the lore. 💀

so here's the tea. a version 4 principal consists of a name, an instance, and a realm. a version 5 principal consists of one or more components, and a realm. and sometimes the first component/name will differ between version 4 and version 5, because of course it will. 🙈

and it gets messier: the second component of a host principal is the fully qualified domain name of the host in question, while the instance of a version 4 principal only contains the first part (the short hostname). so because of all of these problems, the conversion between principals will have to be site customized. you can't just wing it, bestie. it's giving "we need a config file".

🦋 the extended one 🦋

krb5_425_conv_principal_ext() will try to convert a version 4 principal — given by name, instance, and realm — to a version 5 principal. this can result in several possible principals, and if func is non-NULL, it will be called for each candidate principal. func should return true if the principal was "good".

to accomplish this, krb5_425_conv_principal_ext() will look up the name in krb5.conf. here's the order of operations, and yes you need to remember it:

• it first looks in the v4_name_convert/host subsection, which should contain a list of version 4 names whose instance should be treated as a hostname. this list can be specified for each realm (in the realms section), or in the libdefaults section.
• if the name is found, the resulting name of the principal will be the value of this binding. the instance is then first looked up in v4_instance_convert for the specified realm.
• if found, the resulting value will be used as instance (this can be used for special cases), and no further attempts will be made to find a conversion if this fails (with func).
• if the resolve parameter is true, the instance will be looked up with gethostbyname(). 🚨 heads up bestie: this can be a time consuming, error prone, and unsafe operation. not cute.
• next, a list of hostnames will be created from the instance and the v4_domains variable, which should contain a list of possible domains for the specific realm.

on the other hand, if the name is not found in a host section, it is looked up in a v4_name_convert/plain binding. if found here, the name will be converted, but the instance will be untouched.

👑 the compiled-in defaults 👑

this list of default host-type conversions is compiled-in:

v4_name_convert = {
        host = {
                ftp = ftp
                hprop = hprop
                imap = imap
                pop = pop
                rcmd = host
                smtp = smtp
        }
}

it will only be used if there isn't an entry for these names in the config file, so you can override these defaults. you have the power. 💅

🧚 the other two 🧚

krb5_425_conv_principal() will call krb5_425_conv_principal_ext() with NULL as func, and the value of v4_instance_resolve (from the libdefaults section) as resolve. so it's basically the ext version but with the defaults already picked out for you. convenient!

krb5_524_conv_principal() basically does the opposite of krb5_425_conv_principal(). it just doesn't have to look up any names, but will instead truncate instances found to belong to a host principal.

⚠️ the name, instance, and realm should be at least 40 characters long. fr fr, 40. don't skimp.

---

🌈 EXAMPLES 🌈

since this is confusing (bestie, it IS), an example is in place. let's get into it. ✨

assume that we have the "foo.com" and "bar.com" domains that have shared a single version 4 realm, FOO.COM. the version 4 krb.realms file looked like:

foo.com         FOO.COM
.foo.com        FOO.COM
.bar.com        FOO.COM

a krb5.conf file that covers this case might look like:

[libdefaults]
        v4_instance_resolve = yes
[realms]
        FOO.COM = {
                kdc = kerberos.foo.com
                v4_instance_convert = {
                        foo = foo.com
                }
                v4_domains = foo.com
        }

with this setup and the following host table:

foo.com
a-host.foo.com
b-host.bar.com

the following conversions will be made:

rcmd.a-host     -> host/a-host.foo.com
ftp.b-host      -> ftp/b-host.bar.com
pop.foo         -> pop/foo.com
ftp.other       -> ftp/other.foo.com
other.a-host    -> other/a-host

the first three are what you expect. 💖

if you remove the v4_domains, the fourth entry will result in an error (since the host other can't be found). 😭

even if a-host is a valid host name, the last entry will not be converted, since the other name is not known to represent a host-type principal.

if you turn off v4_instance_resolve, the second example will result in ftp/b-host.foo.com (because of the default domain).

and all of this is of course only valid if you have working name resolving. no resolving, no conversions, that's the deal. 🫶

---

👀 SEE ALSO 👀

• krb5_build_principal(3)
• krb5_free_principal(3)
• krb5_parse_name(3)
• krb5_sname_to_principal(3)
• krb5_unparse_name(3)
• krb5.conf(5)

---

and that's the whole page, bestie!! 🎀 this icon of a manual page came to you from HEIMDAL, dated September 3, 2003 — a true vintage moment, and it still ate. go convert some principals, you've got this. 💖✨🦋👑