Linux Kernel Bids Farewell to ISDN Subsystem and Legacy Network Drivers
In a significant cleanup effort, the Linux kernel development community has greenlit the removal of the long-obsolete ISDN subsystem along with several other aging network drivers. This decision, detailed in recent pull requests and mailing list discussions, underscores the ongoing evolution of the kernel toward modernity, shedding code that has lingered without active maintenance or users for years.
The primary change targets the net/isdn directory, home to the Integrated Services Digital Network (ISDN) subsystem. ISDN, once a staple for digital telephony and early internet access in the 1990s, provided basic-rate and primary-rate interfaces for voice and data transmission over traditional phone lines. However, with the advent of broadband technologies like DSL, cable, and fiber optics, ISDN faded into obscurity. The subsystem, comprising drivers such as isdn4linux, hysdn, isdn_ppp, and others, has seen no meaningful updates or testing in over a decade. Kernel maintainer David Šterbá, in his pull request to Linux 6.15, cited the lack of userspace testers and maintainers as the key rationale: “No testers or maintainers have come forward in years, and the code poses a maintenance burden with potential security implications.”
This isn’t an isolated purge. Accompanying the ISDN removal are deletions of other archaic networking components:
- Appletalk: Apple’s proprietary protocol suite from the Mac OS era, used for local networking in pre-TCP/IP Apple environments. Drivers like
ltpcandipddpare axed due to zero usage. - IPX: Novell’s Internetwork Packet Exchange, a staple of NetWare networks in the 1980s and 1990s. The
ipxmodule, unmaintained since kernel 2.6 days, is finally retired. - DECnet: Digital Equipment Corporation’s networking protocol, relevant in VMS and Ultrix systems but irrelevant today.
- X.25 and LAPB: Packet-switched network protocols from the pre-Internet age, supported via drivers like
nr(amateur radio AX.25) andwanrouter. These saw their last commits around 2010. - ATM (Asynchronous Transfer Mode): Once hyped for high-speed networking, now supplanted entirely. Drivers such as
zatm,ambassador, andeniare dropped. - Token Ring: IBM’s deterministic LAN technology, with drivers like
ibmtr,smctrgone after decades of dormancy. - Arcnet: An early Ethernet alternative using token-passing, represented by drivers like
arcnetand8390.
These removals were proposed and vetted through the linux-kernel mailing list, where developers emphasized empirical evidence from kernel coverage reports and user surveys. For instance, ISDN-related modules registered zero loads in recent kernel usage statistics. Security concerns also factored in: dormant codebases are prime vectors for vulnerabilities, as seen in past CVEs affecting unpatched legacy drivers.
The pull requests, merged ahead of Linux 6.15’s release cycle, total thousands of lines of code excised. Šterbá’s ISDN patch alone spans over 200 files, while companion changes from Jakub Kicinski and others handle the broader network driver cull. This aligns with Linus Torvalds’ long-standing philosophy of pruning dead weight to enhance kernel stability, auditability, and performance. The kernel’s modular nature ensures removals are non-disruptive; users relying on these drivers—if any exist—can always backport or maintain out-of-tree patches.
For enterprise and embedded users, the impact is negligible. Modern distributions like Ubuntu, Fedora, and RHEL have long disabled these modules by default. Embedded systems using real-time kernels (PREEMPT_RT) benefit most, as slimmer kernels reduce boot times and memory footprints. Developers are encouraged to migrate to contemporary alternatives: Ethernet (with its vast driver ecosystem), Wi-Fi (via nl80211), and virtual interfaces like TUN/TAP or virtio-net.
This cleanup continues a trend. Recent kernels have similarly retired 32-bit USB quirks, old sound drivers, and PowerPC platform code. Looking ahead, expect further scrutiny of subsystems like Infrared (IrDA) and Bluetooth classic modules showing similar neglect. The kernel’s git tree reflects this rigor: commits are gated by rigorous review, with maintainers like Kicinski enforcing a “no dead code” policy in netdev.git.
Community reaction, as gauged from Slashdot and Phoronix comments, is overwhelmingly supportive. Nostalgic users recount ISDN’s role in dial-up glory days—speeds up to 128 kbps bidirectional—but acknowledge its obsolescence. One commenter noted, “ISDN was great for backup links in the '90s; now it’s a museum piece.” Concerns about breaking ancient hardware are minimal, given the kernel’s distro-agnostic modularity.
In summary, Linux 6.15 marks a milestone in codebase hygiene, freeing resources for innovations like eBPF networking, Rust drivers, and AI-accelerated packet processing. By excising these relics, the kernel stays lean, secure, and future-proof, embodying the open-source ethos of relentless improvement.
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