[PATCH net-next v2 4/4] ipv6: Document defaults for max_{dst|hbh}_opts_number sysctls
From: Tom Herbert <hidden>
Date: 2026-01-08 17:16:03
Subsystem:
documentation, networking [general], the rest · Maintainers:
Jonathan Corbet, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds
In the descriptions of max_dst_opts_number and max_hbh_opts_number sysctls add text about how a zero setting means that a packet with any Destination or Hop-by-Hop options is dropped. Report the default for max_dst_opts_number to be zero (i.e. packets with Destination options are dropped by default), and add a justification. Report the default for max_hbh_opts_number to be one. Signed-off-by: Tom Herbert <redacted> --- Documentation/networking/ip-sysctl.rst | 38 ++++++++++++++++++-------- 1 file changed, 27 insertions(+), 11 deletions(-)
diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index bc9a01606daf..de078f7f6a17 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst@@ -2474,20 +2474,36 @@ mld_qrv - INTEGER Minimum: 1 (as specified by RFC6636 4.5) max_dst_opts_number - INTEGER - Maximum number of non-padding TLVs allowed in a Destination - options extension header. If this value is less than zero - then unknown options are disallowed and the number of known - TLVs allowed is the absolute value of this number. - - Default: 8 + Maximum number of non-padding TLVs allowed in a Destination + options extension header. If this value is zero then receive + Destination Options processing is disabled in which case packets + with the Destination Options extension header are dropped. If + this value is less than zero then unknown options are disallowed + and the number of known TLVs allowed is the absolute value of + this number. + + The default is zero which means the all received packets with + Destination Options extension header are dropped. The rationale is that + for the vast majority of hosts, Destination Options serve no purpose. + In the thirty years of IPv6 no broadly useful IPv6 Destination options + have been defined, they have no security or even checksum protection, + latest data shows the Destination have drop rates on the Internet + from ten percent to more than thirty percent (depending on the size of + the extension header). They also have the potential to be used as a + Denial of Service attack. + + Default: 0 max_hbh_opts_number - INTEGER Maximum number of non-padding TLVs allowed in a Hop-by-Hop - options extension header. If this value is less than zero - then unknown options are disallowed and the number of known - TLVs allowed is the absolute value of this number. - - Default: 8 + options extension header. If this value is zero then receive + Hop-by-Hop Options processing is disabled in which case packets + with the Hop-by-Hop Options extension header are dropped. + If this value is less than zero then unknown options are disallowed + and the number of known TLVs allowed is the absolute value of this + number. + + Default: 1 max_dst_opts_length - INTEGER Maximum length allowed for a Destination options extension
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2.43.0