Host name syntax

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(Redirected from L prefix)

Note: This special syntax is generally not used from the user interfaces of wc3270 or wx3270, except in scripts or at the x3270> prompt. Those user interfaces allow these options to be specified separately, and construct a properly-formatted host name from them.

Basic syntax

The basic syntax is:

[prefix:...] host [:port] [=accept-name]


A prefix sets an option for a single host session, rather than for all sessions created by the emulator. A prefix is a single letter, separated from the host name by a colon (:), e.g.:

To specify multiple prefixes, separate each of them with a colon, e.g.:
A - NVT-only session
Marks the host as supporting NVT mode only. The emulator does not attempt to negotiate 3270 mode, and defaults the terminal name to xterm (if a monochrome display) or xterm-color (if a color display, New in 4.2).
B - No-op
Formerly caused a change in bind-unlock behavior, but this has now been subsumed by the default value of the bindUnlock resource.
C - CICS host
Disables the automatic keyboard lock after connecting to a host, waiting for the host to format the screen. This is needed for CICS hosts, for example, which do not initially format the screen.
L - Use a TLS tunnel
Causes the emulator to set up a TLS tunnel for the host connection. TLS negotiation happens before TELNET negotiation, so the entire session is encrypted.
N - Disable TN3270E
Disables TN3270E mode. If the host attempts to negotiate TN3270E when this prefix is in use, the emulator will reject it.
T - No-TELNET host
Disables the TELNET protocol for the session. The emulator will no longer treat the TELNET IAC character (0xff) specially.
Y - Disable TLS host certificate checking
Disables TLS host certificate checking. It overrides the value of the verifyHostCert resource.


The host is either a fully-qualified domain name (e.g., or a numeric address in either IPv4 dotted-decimal or IPv6 hexadecimal format. A numeric IPv6 addres must be surrounded by square-bracket characters, e.g., [100:200::1], to keep the colon characters from being misinterpreted.


The optional port is a number separated from the host name by a colon. It overrides the default value of 23 (the TELNET port).


The optional accept-name is separated from the host and optional port by an equals sign (=). It overrides the value of the acceptHostname resource, specifying a name to match against the host's TLS certificate.


Connect to without a TLS tunnel, on port 23.

Connect to with a TLS tunnel but not verifying the host certificate, on port 9100.

Connect to IPv4 address on port 23.

Connect to IPv6 address 100:200::1 on port 23.


Connect to IPv6 address 100:200::1 on port 9100 with a TLS tunnel, using myhost to match the name from the host's TLS certificate.