A sequence is following: 1. Match external member against existing trusted domain 2. Find trusted domain's domain controller and preferred GC hosts 3. Fetch trusted domain account auth info 4. Set up ccache in /var/run/ipa_memcached/krb5cc_TD<domain> with principal ourdomain$@trusted.domain 5. Do LDAP SASL interactive bind using the ccache 6. Search for the member's SID 7. Decode SID 8. Replace external member name by SID