icmp(7)

👑 icmp — the Linux IPv4 ICMP kernel module 👑

okay bestie, sit DOWN, because we're about to talk about icmp(7) — a page from the Miscellaneous Information Manual, and honestly? she's doing a lot of quiet work behind the scenes 💅

---

🌸 NAME 🌸

icmp — Linux IPv4 ICMP kernel module. that's it, that's the name, no cap ✨

---

🦋 DESCRIPTION 🦋

This kernel protocol module implements the Internet Control Message Protocol as defined in RFC 792. She's the one who shows up to signal error conditions and to do diagnosis — the group-chat moderator of your network stack 🫶

Here's the tea though: the user doesn't interact directly with this module. Instead she talks to the other protocols in the kernel, and those pass the ICMP errors up to the application layers. She also answers ICMP requests herself, like the responsible main character she is 👑

Now, if you're a user protocol and you want ICMP packets:

• You may receive ICMP packets for all local sockets by opening a raw socket with the protocol IPPROTO_ICMP. Run, don't walk, to raw(7) for more info on that 🏃‍♀️
• The types of ICMP packets passed to the socket can be filtered using the ICMP_FILTER socket option ✨
• And remember: ICMP packets are always processed by the kernel too, even when passed to a user socket. like, she's not gonna not look at them 💀

Linux limits the rate of ICMP error packets to each destination. ICMP_REDIRECT and ICMP_DEST_UNREACH are also limited by the destination route of the incoming packets.

---

🎀 /proc interfaces 🎀

ICMP supports a set of /proc interfaces to configure some global IP parameters. You can access these parameters by reading or writing files in the directory /proc/sys/net/ipv4/. (yes bestie, the whole directory, go wild 🙈)

Most of these parameters are rate limitations for specific ICMP types. Linux 2.2 uses a token bucket filter to limit ICMPs. The value is the timeout in jiffies until the token bucket filter is cleared after a burst. And what is a jiffy, you ask? It's a system dependent unit — usually 10ms on i386, and about 1ms on alpha and ia64. not a vibe, but whatever 🌈

---

• icmp_destunreach_rate (Linux 2.2 to Linux 2.4.9) — Maximum rate to send ICMP Destination Unreachable packets. This limits the rate at which packets are sent to any individual route or destination. Important: this limit does not affect the sending of ICMP_FRAG_NEEDED packets needed for path MTU discovery 🧚
• icmp_echo_ignore_all (since Linux 2.2) — If this value is nonzero, Linux will ignore all ICMP_ECHO requests. straight up ghosting them 💀
• icmp_echo_ignore_broadcasts (since Linux 2.2) — If this value is nonzero, Linux will ignore all ICMP_ECHO packets sent to broadcast addresses.
• icmp_echoreply_rate (Linux 2.2 to Linux 2.4.9) — Maximum rate for sending ICMP_ECHOREPLY packets in response to ICMP_ECHOREQUEST packets.
• icmp_errors_use_inbound_ifaddr (Boolean; default: disabled; since Linux 2.6.12) — If disabled, ICMP error messages are sent with the primary address of the exiting interface.

If enabled, the message will be sent with the primary address of the interface that received the packet that caused the ICMP error. This is the behavior that many network administrators will expect from a router. And it can make debugging complicated network layouts much easier. lowkey a glow-up for your network debugging ✨

Note that if no primary address exists for the interface selected, then the primary address of the first non-loopback interface that has one will be used regardless of this setting. she improvises, okay 🫶

• icmp_ignore_bogus_error_responses (Boolean; default: disabled; since Linux 2.2) — Some routers violate RFC1122 by sending bogus responses to broadcast frames. Such violations are normally logged via a kernel warning. If this parameter is enabled, the kernel will not give such warnings, which will avoid log file clutter. your logs said "I need a break" and honestly, valid 🌸
• icmp_paramprob_rate (Linux 2.2 to Linux 2.4.9) — Maximum rate for sending ICMP_PARAMETERPROB packets. These packets are sent when a packet arrives with an invalid IP header.
• icmp_ratelimit (integer; default: 1000; since Linux 2.4.10) — Limit the maximum rates for sending ICMP packets whose type matches icmp_ratemask (see below) to specific targets. 0 to disable any limiting, otherwise the minimum space between responses in milliseconds. zero means chaos, you've been warned 🔥
• icmp_ratemask (integer; default: see below; since Linux 2.4.10) — Mask made of ICMP types for which rates are being limited.

Significant bits and the default mask, laid out for you exactly:

Significant bits: IHGFEDCBA9876543210
Default mask:     0000001100000011000 (0x1818)

Bit definitions (see the Linux kernel source file include/linux/icmp.h):

• 0 Echo Reply
• 3 Destination Unreachable *
• 4 Source Quench *
• 5 Redirect
• 8 Echo Request
• B Time Exceeded *
• C Parameter Problem *
• D Timestamp Request
• E Timestamp Reply
• F Info Request
• G Info Reply
• H Address Mask Request
• I Address Mask Reply

The bits marked with an asterisk are rate limited by default (see the default mask above). the stars are not decorative bestie, they mean something ⭐

• icmp_timeexceed_rate (Linux 2.2 to Linux 2.4.9) — Maximum rate for sending ICMP_TIME_EXCEEDED packets. These packets are sent to prevent loops when a packet has crossed too many hops.
• ping_group_range (two integers; default: see below; since Linux 2.6.39) — Range of the group IDs (minimum and maximum group IDs, inclusive) that are allowed to create ICMP Echo sockets. The default is "1 0", which means no group is allowed to create ICMP Echo sockets. yeah, the default is a locked door 🚪

---

💜 VERSIONS 💜

• Support for the ICMP_ADDRESS request was removed in Linux 2.2.
• Support for ICMP_SOURCE_QUENCH was removed in Linux 2.2.

they're gone, they're not coming back, we don't talk about them 💀

---

🦋 NOTES 🦋

• As many other implementations don't support IPPROTO_ICMP raw sockets, this feature should not be relied on in portable programs. fr fr, portable code, do not count on it 🙈
• ICMP_REDIRECT packets are not sent when Linux is not acting as a router. They are also accepted only from the old gateway defined in the routing table, and the redirect routes are expired after some time. strict queen 👑
• The 64-bit timestamp returned by ICMP_TIMESTAMP is in milliseconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
• Linux ICMP internally uses a raw socket to send ICMPs. This raw socket may appear in netstat(8) output with a zero inode. sneaky lil thing 🧚

---

🌈 SEE ALSO 🌈

• ip(7)
• rdisc(8)

RFC 792 for a description of the ICMP protocol. go read it, iykyk 📚

---

okay that's the whole page, you ate it, periodt 💅

this little masterpiece came to you from Linux man-pages 6.20, dated 2026-02-08 — treasure her, cite her, and go ping something responsibly 🎀✨🦋