netstat vs ss

The Unix Way — Episode 16

On a busy Linux load balancer one types netstat -anp and the prompt is in no hurry to come back. On FreeBSD the same workload returns at once. Both tools speak Unix text. Both pipe into grep, awk and sort without fuss. This is not a story about Linux being slow. It is a story about where the Unix text interface lives (in the tool, or in a file the tool must read), and what that choice costs a decade later.

McIlroy's principle, the one that gets quoted at every Unix lecture, says: text streams are the universal interface. The Unix Way is to build tools that answer questions in text, and let pipelines compose them on the fly. The interface lives in the tool's mouth: you ask netstat, it speaks; you pipe its answer into grep or awk, that tool speaks; you compose. The text exists where one program hands its output to another or to the human at the keyboard, and not before.

FreeBSD took that principle and built netstat accordingly. The tool asks the kernel through sysctl, formats one answer at its output, pipes onward. KISS in the classical sense. Linux took the same principle and added something McIlroy never asked for: a file that looks like a text interface and isn't quite one. /proc/net/tcp is a kernel-rendered ASCII dump. It is readable, in the sense that one can cat it. It is not askable: there is no way to query it for "sockets on port 443" or "all UDP listeners". One can only read it whole. So the tool behind it still does all the work, plus the parsing of someone else's text on the way in.

A Short History of netstat

netstat was part of the 4.2BSD networking release of August 1983. The CSRG team at Berkeley, working with the new TCP/IP stack that Bill Joy and Sam Leffler had brought together, needed a tool that could ask the kernel about its sockets, its routing table, its interfaces and its protocol statistics. The result was netstat(1). The command-line vocabulary it established (-a for all sockets, -n for numeric output, -r for the routing table, -i for interfaces, -s for protocol statistics) became part of the shared idiom of every Unix administrator for the next several decades.

For most of its history, netstat was a thin Unix tool: ask the kernel, format the answer, send it to standard output. On BSD it asked through sysctl(3) and kvm(3): interfaces designed to be queried by a tool. On early Linux it asked through /proc/net: files designed to be read whole, in ASCII, by humans and tools alike. The format on the screen looked the same. The shape of the interface underneath did not. The first is a tool asking another tool a question. The second is a tool reading a file that someone else has already laid out and which one cannot, in any meaningful sense, ask.

FreeBSD: One Tool, Still in Base

On FreeBSD in 2026, the answer to "how do I list all the established TCP connections on this host" is the same as the answer in 1996, in 2006, and in 2016:

netstat -an

To get a specific protocol family:

netstat -anp tcp
netstat -anp udp
netstat -an -f unix

For the routing table, the same convention:

netstat -rn

For protocol statistics, the same again:

netstat -s
netstat -s -p tcp

The tool is part of the base system. The man page lives at /usr/share/man/man1/netstat.1.gz. The source lives under /usr/src/usr.bin/netstat/. It is updated as part of the FreeBSD release cycle, by the people who also maintain the kernel that produces the data.

The reason it is still fast on a busy server is the interface underneath. The FreeBSD kernel exposes its in-kernel protocol control blocks through the sysctl tree:

• net.inet.tcp.pcblist (TCP PCBs)
• net.inet.udp.pcblist (UDP PCBs)
• net.inet.raw.pcblist (raw sockets)
• net.local.stream.pcblist (Unix-domain stream)
• net.local.dgram.pcblist (Unix-domain datagram)
• net.local.seqpacket.pcblist (Unix-domain seqpacket)

A single sysctl call returns a structured array of records. The userland netstat formats them into columns, applies the requested filters, and exits. The interface here is netstat itself: one asks netstat, netstat speaks Unix text back, and that text pipes into grep, awk, sort or any other tool one cares to compose it with. There is no intermediate file pretending to be a tool. The cost is linear in the number of sockets, with a small constant; on a server with sixty thousand connections the command still returns in well under a second.

For users who prefer a more focused view of sockets without the routing-and-statistics surface area of netstat, FreeBSD also ships sockstat(1) in base:

sockstat -4l       # IPv4 listeners
sockstat -p 443    # everything on port 443

sockstat reads the same sysctl tree. It is a complementary tool, not a replacement, and it has been in the base system since FreeBSD 3.0 in 1998.

Linux: netstat, Still There, No Longer Maintained

On a Linux system, netstat is also still there, in most cases. It lives in the net-tools package, alongside ifconfig, arp, route and a handful of other tools that the BSD-trained administrator has known since the late 1980s. The Linux versions were written in the early 1990s by Fred N. van Kempen and others, modelled on the BSD originals but implemented from scratch against the Linux /proc filesystem rather than against sysctl and kvm.

For most of Linux's history, this worked. /proc/net/tcp contains one line per active TCP socket, in a well-defined ASCII format. The Linux netstat opens that file, reads the lines, parses each one, looks up process information through /proc/<pid>/fd/, and prints the result. On a small or medium server with a few hundred sockets, the cost was invisible.

The deeper issue is the shape of the interaction, not the byte count. /proc/net/tcp is a file. One can read it. One cannot ask it anything. If one wants only sockets on port 443, one has to read the whole file and filter in userspace. If one wants only established sockets, same thing. If one wants only sockets owned by a particular process, one has to read /proc/net/tcp whole and walk /proc/<pid>/fd/ whole, matching inodes by hand. The Unix Way says "compose tools by piping text"; /proc/net/tcp is not a tool, it is the text pretending to be one, and the actual work still happens in the tool behind it.

On a busy server the pretence becomes expensive. A reverse proxy or a load balancer in 2010 might have sixty thousand simultaneous connections, perhaps far more. Listing them with netstat -anp meant reading sixty thousand lines from /proc/net/tcp, parsing them all, walking the /proc/<pid>/fd/ tree, and filtering in userspace. On the same server, netstat could take ten or fifteen seconds and burn a noticeable amount of CPU during that time. None of that cost was paid by the kernel; all of it was paid by the tool, because the file in front of the tool was not askable.

The net-tools package itself has not had a new release since 2011. The maintainer publicly attempted to deprecate it in 2009 (the LWN article Moving on from net-tools documents the conversation). Most major distributions now recommend iproute2 as the replacement, and several no longer install net-tools by default. The Linux netstat is still there, it still works for small workloads, and it still does the wrong thing on busy ones. The wrong thing is not the tool's parsing; it is being asked to do that parsing because the thing in front of it was a file rather than a queryable interface.

Linux: ss, the Unix Pattern Restored

The replacement is ss(8), short for socket statistics. It was written by Alexey Kuznetsov, the same author who wrote much of the Linux QoS code and started the iproute2 package. ss does not read /proc/net/tcp. It asks netlink instead, and netlink, crucially, is askable.

netlink is a Linux kernel-to-userspace IPC mechanism, also Kuznetsov's work, designed for structured queries against kernel subsystems. In 2005, Linux 2.6.14 added a netlink family called NETLINK_INET_DIAG, which exposes IPv4 and IPv6 socket information as a queryable interface. In 2012, Linux 3.3 generalised it to NETLINK_SOCK_DIAG, adding Unix-domain socket support. The userland tool opens one netlink socket, sends one request specifying exactly what it wants, and receives back only the matching sockets as a stream of inet_diag_msg or unix_diag_msg records. ss formats those records into Unix text, prints them, and exits. The interface here is ss itself: one asks ss, ss speaks, and the answer pipes onward like any other Unix tool's output.

The performance difference is not a constant factor. On a server with a few hundred sockets it is barely measurable; on a server with sixty thousand it is the difference between a snappy tool and a slow one. The structured query also allows filtering in the kernel rather than in userspace: ss -tan state established '( dport = :443 )' translates the bracketed expression into a kernel-side filter, and only the matching records come back. Both gains follow from the same source: the kernel offers a queryable interface, and ss is free to be the Unix text tool one talks to. The middleman that pretended to be one is gone.

In day-to-day use:

ss -tan                     # all TCP, numeric
ss -tanp                    # add process info (needs root)
ss -s                       # summary by protocol
ss -tan state established   # only established TCP
ss -lntp                    # listening TCP with processes

The vocabulary is similar enough to netstat that a long-time BSD administrator can guess most of it on first encounter. The behaviour is fast enough that it survives in production.

The Architectural Point

The Unix Way puts the text interface in the tool's mouth. netstat IS the interface: one asks it, it speaks, the answer pipes into grep, awk, sort, less and whatever else one cares to compose. That is McIlroy's text-streams principle in its proper place. KISS in the classical sense: one tool, one question, one answer.

/proc/net/tcp is a different idea altogether. It is a file shaped like text. One can cat it, one can squint at it, one can grep it in a pinch. It is not a tool one can ask: there is no syntax for "established sockets on port 443 owned by nginx". So the tool behind it has to do that work: reading the whole file, parsing every line, filtering in userspace, walking another /proc tree, and only then formatting the result for the human. The Unix-ness has been pushed one layer out, and the work in front of it has not gone anywhere.

This is not a criticism of /proc as a debugging surface. Being able to grep /proc/net/tcp for one weird socket without writing a tool is a real benefit, and it is the reason /proc is still there in 2026. The trouble is that the same surface was also asked to serve as the production data feed, and a fixed text dump is the wrong material for that role. It can do both jobs, but only badly. It cannot do both well at scale.

ss and the netlink sock_diag family fixed the production case by adding a second interface that one can actually ask. The old surface stays for cat and grep. The new one took over the actual workload. The cost of the transition was a decade of slow netstat on production servers, a long mailing-list conversation, and a generation of administrators learning a new tool name. The shape of the fix, twenty-two years on, looked exactly like sysctl.

FreeBSD never had to choose, because FreeBSD never put a non-askable file between its tools and the kernel in the first place. The tool from 1983 still answers the question on the workloads of 2026. The Linux ecosystem did the right thing by adding ss. The fact that it had to is the architectural lesson.

The Unix text interface is a tool one can question, not a file one can read. netstat and sockstat are the interface; /proc/net/tcp only ever looked like one. KISS, properly applied, knows the difference.

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By Vivian Voss — System Architect & Software Developer. Follow me on LinkedIn for daily technical writing.