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SL(4) NetBSD Kernel Interfaces Manual SL(4)
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by Kimmo Suominen.
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sl -- Serial Line IP (SLIP) network interface
The sl interface allows asynchronous serial lines to be used as IPv4 net-
work interfaces using the SLIP protocol.
To use the sl interface, the administrator must first create the inter-
face and assign a tty line to it. The sl interface is created using the
ifconfig(8) create subcommand, and slattach(8) is used to assign a tty
line to the interface. Once the interface is attached, network source
and destination addresses and other parameters are configured via
The sl interface can use Van Jacobson TCP header compression and ICMP
filtering. The following flags to ifconfig(8) control these properties
of a SLIP link:
link0 Turn on Van Jacobson header compression.
-link0 Turn off header compression. (default)
link1 Don't pass through ICMP packets.
-link1 Do pass through ICMP packets. (default)
link2 If a packet with a compressed header is received, automati-
cally enable compression of outgoing packets. (default)
-link2 Don't auto-enable compression.
sl%d: af%d not supported . The interface was handed a message with
addresses formatted in an unsuitable address family; the packet was
inet(4), intro(4), ppp(4), strip(4), ifconfig(8), slattach(8),
J. Romkey, A Nonstandard for Transmission of IP Datagrams over Serial
Lines: SLIP, RFC, 1055, June 1988.
Van Jacobson, Compressing TCP/IP Headers for Low-Speed Serial Links, RFC,
1144, February 1990.
The sl device appeared in NetBSD 1.0.
SLIP can only transmit IPv4 packets between preconfigured hosts on an
asynchronous serial link. It has no provision for address negotiation,
carriage of additional protocols (e.g. XNS, AppleTalk, DECNET), and is
not designed for synchronous serial links. This is why SLIP has been
superseded by the Point-to-Point Protocol (PPP), which does all of those
things, and much more.
NetBSD 8.0 July 9, 2006 NetBSD 8.0