cipso_v4_error(9)

💅 cipso_v4_error — the "send the correct response for a bad packet" girlie

okay so bestie, we're in the Kernel API Manual, section 9 (cipso_v4_error(9)), and this lil function is literally the bouncer at the club door who tells bad packets to go home. Let's get into it ✨

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🌸 SYNOPSIS 🌸

void cipso_v4_error (struct sk_buff *skb , int error , u32 gateway );

no return value, she's a void girlie, she just does things 💅

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🎀 ARGUMENTS 🎀

• skb — the packet 👀 like, the actual packet in question
• error — the error code 🫶 the vibe of what went wrong
• gateway — CIPSO gateway flag ⭐ this one decides which code number we're sending back, more on that in a sec

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🦋 DESCRIPTION 🦋

okay so the whole point: based on the error code you hand her in error, she sends an ICMP error message right back to the originating host. periodt. and honestly the IETF draft says it better than I ever could, so here's the quote:

"If the contents of the CIPSO [option] are valid but the security label is outside of the configured host or port label range, the datagram is discarded and an ICMP 'destination unreachable' (type 3) is generated and returned. The code field of the ICMP is set to 'communication with destination network administratively prohibited' (code 9) or to (code 10). The value of the code is dependent on whether the originator of the ICMP message is acting as a CIPSO host or a CIPSO gateway. The recipient of the ICMP message MUST be able to handle either value. The same procedure is performed if a CIPSO [option] can not be added to an IP packet because it is too large to fit in the IP options area."

and there's a second part, which is lowkey the most important rule here:

"If the error is triggered by receipt of an ICMP message, the message is discarded and no response is permitted (consistent with general ICMP processing rules)."

so yeah — if an ICMP message is what caused the error, she throws it away and sends nothing back. no response permitted, fr fr. that's just how ICMP processing rules work, don't come for her 🙈

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👑 SEE ALSO 👑

Kernel file ./net/ipv4/cipso_ipv4.c — and here's the whole friend group, no cap:

• cipso_v4_cache_entry_free(9)
• cipso_v4_map_cache_hash(9)
• cipso_v4_cache_init(9)
• cipso_v4_cache_invalidate(9)
• cipso_v4_cache_check(9)
• cipso_v4_cache_add(9)
• cipso_v4_doi_search(9)
• cipso_v4_doi_add(9)
• cipso_v4_doi_free(9)
• cipso_v4_doi_free_rcu(9)
• cipso_v4_doi_remove(9)
• cipso_v4_doi_getdef(9)
• cipso_v4_doi_putdef(9)
• cipso_v4_doi_walk(9)
• cipso_v4_map_lvl_valid(9)
• cipso_v4_map_lvl_hton(9)
• cipso_v4_map_lvl_ntoh(9)
• cipso_v4_map_cat_rbm_valid(9)
• cipso_v4_map_cat_rbm_hton(9)
• cipso_v4_map_cat_rbm_ntoh(9)
• cipso_v4_map_cat_enum_valid(9)
• cipso_v4_map_cat_enum_hton(9)
• cipso_v4_map_cat_enum_ntoh(9)
• cipso_v4_map_cat_rng_valid(9)
• cipso_v4_map_cat_rng_hton(9)
• cipso_v4_map_cat_rng_ntoh(9)
• cipso_v4_gentag_hdr(9)
• cipso_v4_gentag_rbm(9)
• cipso_v4_parsetag_rbm(9)
• cipso_v4_gentag_enum(9)
• cipso_v4_parsetag_enum(9)
• cipso_v4_gentag_rng(9)
• cipso_v4_parsetag_rng(9)
• cipso_v4_gentag_loc(9)
• cipso_v4_parsetag_loc(9)
• cipso_v4_optptr(9)
• cipso_v4_validate(9)
• cipso_v4_genopt(9)
• cipso_v4_sock_setattr(9)
• cipso_v4_req_setattr(9)
• cipso_v4_delopt(9)
• cipso_v4_sock_delattr(9)
• cipso_v4_req_delattr(9)
• cipso_v4_getattr(9)
• cipso_v4_sock_getattr(9)
• cipso_v4_skbuff_setattr(9)
• cipso_v4_skbuff_delattr(9)
• cipso_v4_init(9)

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and that's the whole page bestie, no notes 💜 sign-off time: this one lives in ./net/ipv4 and the page is dated October 2026 — ily, go send some correct responses 🎀✨🦋