Intermittent failures of mod_krb5

Adam Bishop Adam.Bishop at
Thu Mar 1 19:27:59 CET 2018

On Mar 1, 2018, at 8:56 AM, Brian Candler <b.candler at> wrote:
>> Many thanks for the hints, especially KRB5_TRACE.
>> It certainly looks like the KDC (Samba4) is taking a long time to reply. In the example below, when freeradius gets a UDP response saying the data is too big for UDP, it reconnects over TCP. This happens twice - once to get a TGT for the authenticating user and once to get a service ticket - and these are taking 2.4 and 6.2 seconds respectively. With additional UDP round-trips, the whole thing is taking nearly 12 seconds in the example below.

This is fairly common with Windows Kerberos - the size of the ticket is directly proportional to the the number of times certain changes have been made to the principal, and the number of groups the principal is a member of.

Microsoft's implementation has a setting to allow larger payloads (not sure about Samba), but that can cause other issues if you have e.g. a VPN in play, so the best solution is just to prefer TCP if speed is an issue.

Have you looked at building FreeRADIUS with libwbclient instead, or maybe looking at if you could use sssd to cache?


Adam Bishop

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