Restrictions to File Transfer

If Tectia Server for IBM z/OS is used for file transfer only, it is advisable to disable tunneling and terminal access to the server.

Enabling the SFTP Subsystem

To allow the users to connect with SFTP to Tectia Server for IBM z/OS, the secure file transfer subsystem has to be defined in the sshd2_config file:

subsystem-sftp   /opt/tectia/libexec/sft-server-g3

Restricting Access to User's MVS User Catalog

To restrict the users' access with SFTP, the --attribute=zos-access option can be specified with sft-server-g3:

subsystem-sftp   /opt/tectia/libexec/sft-server-g3 --attribute=zos-access:value

The values for the zos-access attribute are:

mvs: User is allowed access only to the MVS side of the server.
usercatalog: User is allowed access only to the MVS side of the server and only to his/her own catalog.
hfs: User is allowed access only to the HFS side of the server.
mvs,hfs or hfs,mvs or all: User is allowed access to both MVS and HFS sides of the server. No access restrictions are active.
usercatalog,hfs or hfs,usercatalog: User is allowed access to the HFS side of the server and to the MVS side of the server but only to his/her own catalog.

The values are case-insensitive. You can use both "MVS" or "mvs". The values cannot include white spaces. Value "mvs,hfs" works, but "mvs, hfs" does not.

Example 1

To start sft-server-g3 so that user can only access his/her own MVS catalog:

sft-server-g3 --attribute=zos-access:usercatalog

Example 2

To start sft-server-g3 so that only HFS can be accessed:

sft-server-g3 --attribute=zos-access:hfs

Setting Up Security for Processing Offline Data Sets

Tectia Server for IBM z/OS can control who is permitted to request offline data sets to be mounted. Tape data sets are typically offline and a DASD data set is offline if the volume it resides on is not mounted. To control mounting, the System Authorization Facility (RACF, ACF2, or TSS) facility SSZ.MOUNT must be defined. When it is defined Tectia requires that the user has at least READ access before it requests a data set to be mounted. If SSZ.MOUNT is not defined there is no restriction on mounting.

The restriction is enforced both by the server and by the sftpg3 and scpg3 client programs.

In addition to having permission to request mounts, the user must also specify the file transfer attribute automount=yes for Tectia to allocate a data set with mounts allowed.

Tectia first attempts to allocate a data set without allowing the system to do a mount. If this fails because the data set is offline, and the user has the required permission, Tectia repeats the allocation and allows the system to mount the data set. The user can instruct Tectia to omit the first allocation by specifying automount=immed.

Note that a user who can open a shell or issue remote commands with Tectia has other ways of causing tape mount requests. To control tape mounts effectively, do the following steps:

Define SSZ.MOUNT with universal permission NONE:
```
RDEFINE FACILITY (SSZ.MOUNT) UACC(NONE)
```
Set up the users who are to be allowed to mount tapes as file-transfer only users as instructed above (Restricting Access to User's MVS User Catalog ).

Give these users READ access to SSZ.MOUNT:

PERMIT SSZ.MOUNT CLASS(FACILITY) ID(SRVACC1) ACCESS(READ)

After each command refresh the RACLIST:
```
SETROPTS RACLIST(FACILITY) REFRESH
```

Disabling Tunneling

If you are sure you or your users do not need to create tunnels (possibly going around firewall restrictions or such), you can disable tunneling (port forwarding) altogether by adding the following to your sshd2_config:

AllowTcpForwarding       no

Note that this disables also transparent FTP tunneling through the server.

Disabling Terminal Access

The following configuration option of Tectia Server for IBM z/OS will deny the group sftpusers terminal access.

Terminal.DenyGroups       sftpusers