Introduction
This documents describes the expected results of configuring corporate L2TP APNs with the authentication imsi-auth or authentication msisdn-auth.
Problem: The msisdn-auth and imsi-auth APN Configuration Options have a Speciffic (non obvious) Result for L2TP based APNs
The official documentation (for release 19) states:
imsi-auth - Configures the APN to attempt to authenticate the subscriber based on their International Mobile Subscriber Identification (IMSI) number.
msisdn-auth - Configures the APN to attempt to authenticate the subscriber based on their Mobile Station International Integrated Services Digital Network (MSISDN) number as described in the Usage section of this command.
Example configuration:
apn ecs-apn
ims-auth-service IMSA
dns primary 192.168.1.128
dns secondary 192.168.1.129
ip access-group CSS_ACL in
ip access-group CSS_ACL out
authentication imsi-auth username-strip-apn prefer-chap-pco <<<<<<<<<<<<<<<<<<<<<<<<<<<
ip context-name Gi
tunnel l2tp peer-address 2.2.2.2 encrypted secret +A3oxne9nnyqmuz16dddqucwcqz92p2hi4t8z21nx3hmmpcgvh4ida preference 1 <<<<<<<<<<<<<<<<<<<<<<<<<<<
tunnel l2tp peer-address 3.3.3.3 encrypted secret +A2dbz9joxajmv80jxmr5aycl1ka2s6nzmu7s2bte3nnz4o2hgkqxn preference 2 <<<<<<<<<<<<<<<<<<<<<<<<<<<
loadbalance-tunnel-peers prioritized <<<<<<<<<<<<<<<<<<<<<<<<<<<
exit
lac-service LAC-SVC <<<<<<<<<<<<<<<<<<<<<<<<<<<
max-retransmission 1
retransmission-timeout-max 1
load-balancing prioritized
allow aaa-assigned-hostname
keepalive-interval 30
peer-lns 2.2.2.2 encrypted secret +A2q4fv7h5tum1a06vc2wblk9l7k3ma98myremkew1552c2vosy2h1
peer-lns 3.3.3.3 encrypted secret +A16gnydsddbqqx3okh7ln6jrwxz3s3u3lzvzo5bz0ccc0ztr0cvsh
bind address 1.1.1.1
#exit
An expected behavior is that, if one of the above options are configured for a L2TP based APN, the Gateway GPRS Support Node/
Packet Data Network (PDN) Gateway (GGSN/PGW) uses the IMSI or MSISDN for PPP authentication with L2TP Network Server (LNS).
The option works as expected in case there is no username provided by the User Equipment (UE).
ii
Friday April 07 2017
INBOUND>>>>> 09:57:08:270 Eventid:141004(3)
[PGW-S5/S2a/S2b]GTPv2C Rx PDU, from 213.151.233.172:35664 to 213.151.233.230:2123 (271)
TEID: 0x00000000, Message type: EGTP_CREATE_SESSION_REQUEST (0x20)
Sequence Number: 0x317962 (3242338)
GTP HEADER
Version number: 2
TEID flag: Present
Piggybacking flag: Not present
Message Length: 0x010B (267)
INFORMATION ELEMENTS
IMSI:
Type: 1 Length: 8 Inst: 0
Value: 231014450903030
Hex: 0100 0800 3201 4154 9030 30F0
MSISDN:
Type: 76 Length: 6 Inst: 0
Value: 421917667546
Hex: 4C00 0600 2491 7166 5764
MOBILE EQUIPMENT IDENTITY:
Type: 75 Length: 8 Inst: 0
Value: 3594050557927001
Hex: 4B00 0800 5349 5050 7529 0710
[…]
ACCESS POINT NAME:
Type: 71 Length: 38 Inst: 0
Value: ltpipsec.corp.test.mnc001.mcc231.gprs
Hex: 4700 2600 086C 7470 6970 7365 6304 636F
7270 0474 6573 7406 6D6E 6330 3031 066D
6363 3233 3104 6770 7273
SELECTION MODE:
Type: 128 Length: 1 Inst: 0
Value: MS provided APN,subscr not verified (0x01)
Hex: 8000 0100 01
PDN TYPE:
Type: 99 Length: 1 Inst: 0
Value: IPV4
Hex: 6300 0100 01
[…]
PCO:
Type: 78 Length: 32 Inst: 0
Container id: 0xC023 (PAP)
Container length: 0x06 (6)
Container content:
Auth-Req(0), Name=, Passwd=
Container id: 0x8021 (IPCP)
Container length: 0x10 (16)
Container content:
Conf-Req(0), Pri-DNS=0.0.0.0, Sec-DNS=0.0.0.0
Container id: 0x000D (IPv4-DNS-Server)
Container length: 0x00 (0)
Container content:
DNS Address: Request for IPv4 DNS Address allocation
Hex: 4E00 2000 80C0 2306 0100 0006 0000 8021
1001 0000 1081 0600 0000 0083 0600 0000
0000 0D00
[…]
Friday April 07 2017
<<<<OUTBOUND 09:57:08:295 Eventid:25001(0)
PPP Tx PDU (20)
PAP 20: Auth-Req(1), Name=421917667546, Passwd= <-- username is replaced with MSISDN as the APN is configured with “msisdn-auth”
The option doesn’t work if there is a username provided by UE. In this case, GGSN/PGW sends the username and password configured inside the APN.
If nothing is configured there
Friday April 07 2017
INBOUND>>>>> 09:47:51:254 Eventid:141004(3)
[PGW-S5/S2a/S2b]GTPv2C Rx PDU, from 213.151.233.172:35824 to 213.151.233.230:2123 (279)
TEID: 0x00000000, Message type: EGTP_CREATE_SESSION_REQUEST (0x20)
Sequence Number: 0x5C4D6C (6049132)
GTP HEADER
Version number: 2
TEID flag: Present
Piggybacking flag: Not present
Message Length: 0x0113 (275)
INFORMATION ELEMENTS
IMSI:
Type: 1 Length: 8 Inst: 0
Value: 231014450903030
Hex: 0100 0800 3201 4154 9030 30F0
MSISDN:
Type: 76 Length: 6 Inst: 0
Value: 421917667546
Hex: 4C00 0600 2491 7166 5764
MOBILE EQUIPMENT IDENTITY:
Type: 75 Length: 8 Inst: 0
Value: 3594050557927001
Hex: 4B00 0800 5349 5050 7529 0710
[..]
PCO:
Type: 78 Length: 40 Inst: 0
Container id: 0xC023 (PAP)
Container length: 0x0E (14)
Container content:
Auth-Req(0), Name=null, Passwd=null
Container id: 0x8021 (IPCP)
Container length: 0x10 (16)
Container content:
Conf-Req(0), Pri-DNS=0.0.0.0, Sec-DNS=0.0.0.0
Container id: 0x000D (IPv4-DNS-Server)
Container length: 0x00 (0)
Container content:
DNS Address: Request for IPv4 DNS Address allocation
Hex: 4E00 2800 80C0 230E 0100 000E 046E 756C
6C04 6E75 6C6C 8021 1001 0000 1081 0600
0000 0083 0600 0000 0000 0D00
[…]
Friday April 07 2017
<<<<OUTBOUND 09:47:51:334 Eventid:25001(0)
PPP Tx PDU (16)
PAP 16: Auth-Req(1), Name=null, Passwd=null <-- username is the same as in the APN
Solution
Observed behavior is expected as per design.
The authentication imsi-auth username-strip-apn prefer-chap-pco (or authentication msisdn-auth username-strip-apn prefer-chap-pco) configuration is used when there is no Protocol Configuration Options (PCO) username coming in.
This is the order of precedence for Network Access Identifier (NAI) construction configuration:
- If outbound username <1-128 char string> is configured inside the APN, this overrides all other configurations/IEs and is sent in PAP/CHAP req.
- If UE sends PCO username/password it is sent from the UE, it is sent to LNS in Password Authentication Protocol/Challenge Handshake Authentication Protocol (PAP/CHAP) req.
- If no username is sent from UE, then msisdn/imsi@APN is sent by default as username in PAP/CHAP req.
- Further this CLI - authentication msisdn/imsi-auth username-strip-apn can be used to strip the APN and send only the msisdn/imsi in PAP/CHAP req.
Note that in case the authentication is done by Radius (localy) the IMSI (or MSISDN) are sent in Access-Request messages as expected.
As well in scenario of L2TP, if authentication is done by RADIUS (on LAC side), the expected username (IMSI or MSISDN) is seen in Access-Request messages, but not in Auth-Req towards LNS.
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