[rabbitmq-discuss] User/Password maximum lengths

[Tony Garnock-Jones]

Hi Pieter,

Pieter de Zwart wrote:
> Can someone let me know what the maximum allowed length of the username,
> password, queue name and exchange name can be? A cursory search of the
> AMQP spec yielded nothing.

Queue names and exchange names have easy answers: they're limited to 255
*bytes* long, each. (Not characters, encoded bytes.) This stuff is given in the
XML part of the specification rather than in the main PDF, and it can be a
little difficult to piece together the relevant fragments to discover facts
like these.

Usernames and passwords will take a little more text to explain.

All authentication data is sent on the wire as a longstring, which has a
maximum length limit of 2^32-1. However, the longstring has to travel in
an AMQP method frame *before* a maximum frame size has been negotiated,
meaning that while servers *may* accept authenticating frames longer than 4096
bytes, they are not required to do so. RabbitMQ doesn't limit frames to 4096
bytes here, so the full 2^32-1 bytes are in theory available to you.

So if you're thinking about AMQP generally, rather than RabbitMQ in particular,
the "response" longstring in the connection.start_ok method that carries
username and password information should be thought of as limited to roughly
3800 bytes, given a guess at the number of bytes used for other information in
the start_ok method (see below for details).

AMQP delegates to SASL for its authentication. Both the ad-hoc AMQPLAIN
method and the more standard PLAIN authentication methods put both the
username and password (with small constant padding) into that single
longstring, so your constraint is:

   len(username) + len(password) + k < 3800

where k is either 2 or 23 depending on whether your client library uses
PLAIN or AMQPLAIN, respectively.

Here's the connection.start-ok frame as sent by our Java client library. We see
that the frame is 0x122 bytes long after the 7-byte frame header, not including
the 1-byte frame footer, and that username and password information take up 11
bytes of that, meaning that the rest of the frame takes up 279 bytes (this will
vary slightly depending on library version and so forth), which leaves us with
3817, or about 3800, bytes for the authentication information.

Mon Apr 12 08:42:36 2010 (4bc2343c): TCP: fd 5 (I; localhost) sent 298 bytes:
00000000: 01 00 00 00 00 01 22 00 : 0A 00 0B 00 00 00 FE 07  ......".........
00000010: 70 72 6F 64 75 63 74 53 : 00 00 00 08 52 61 62 62  productS....Rabb
00000020: 69 74 4D 51 08 70 6C 61 : 74 66 6F 72 6D 53 00 00  itMQ.platformS..
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00000050: 75 6E 64 65 72 20 74 68 : 65 20 4D 50 4C 2E 20 53  under the MPL. S
00000060: 65 65 20 68 74 74 70 3A : 2F 2F 77 77 77 2E 72 61  ee http://www.ra
00000070: 62 62 69 74 6D 71 2E 63 : 6F 6D 2F 07 76 65 72 73  bbitmq.com/.vers
00000080: 69 6F 6E 53 00 00 00 0F : 24 7B 69 6D 70 6C 2E 76  ionS....${impl.v
00000090: 65 72 73 69 6F 6E 7D 09 : 63 6F 70 79 72 69 67 68  ersion}.copyrigh
000000A0: 74 53 00 00 00 67 43 6F : 70 79 72 69 67 68 74 20  tS...gCopyright
000000B0: 28 43 29 20 32 30 30 37 : 2D 32 30 30 38 20 4C 53  (C) 2007-2008 LS
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000000D0: 69 76 65 20 46 69 6E 61 : 6E 63 69 61 6C 20 54 65  ive Financial Te
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000000F0: 20 61 6E 64 20 52 61 62 : 62 69 74 20 54 65 63 68   and Rabbit Tech
00000100: 6E 6F 6C 6F 67 69 65 73 : 20 4C 74 64 2E 05 50 4C  nologies Ltd..PL
00000110: 41 49 4E 00 00 00 0C 00 : 67 75 65 73 74 00 67 75  AIN.....guest.gu
00000120: 65 73 74 05 65 6E 5F 55 : 53 CE                    est.en_US.
0000012A:

Regards,
  Tony