iptfs_encap_add_ipv4(9)

🦋 iptfs_encap_add_ipv4(9) — like, the whole glow-up, bestie 🦋

okay so this is a page from the Kernel API Manual, and the function we're obsessing over today is iptfs_encap_add_ipv4. yes it's a 9-section manual page, yes it's a kernel function, and yes we're gonna make it make sense. periodt. 💅

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✨ NAME ✨

iptfs_encap_add_ipv4 — adds the outer encaps. that's it, that's the whole vibe. 🎀

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✨ SYNOPSIS ✨

int iptfs_encap_add_ipv4 (struct xfrm_state *x , struct sk_buff *skb );

like, it takes two things and hands you back an int. iconic signature tbh. 🫶

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✨ ARGUMENTS ✨

• x — the xfrm state. your main character struct, fr.
• skb — the packet. the actual little guy we're working with. 💌

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✨ DESCRIPTION ✨

okay so storytime 🧚 — this function was originally taken from xfrm4_tunnel_encap_add. and the reason for the copy? literally because IP-TFS/AGGFRAG can have different functionality for how to set the TOS/DSCP bits. different needs, different function, no cap.

so what does it actually do? it sets the protocol to a different value, and it doesn't do anything with inner headers — because those inner headers aren't pointing into a normal IP singleton inner packet. so like… there's nothing there to touch, bestie. not applicable. we leave them alone. 🙈

lowkey important: the "doesn't do anything with inner headers" part is by design, not a bug. don't come for her.

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✨ RETURN ✨

• 0 on success 🎉
• a negative error code on failure 😭

that's the whole return contract. zero is the slay outcome.

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✨ SEE ALSO ✨

okay so the related-reading list is long and honestly kind of iconic. buckle up 💖

• Kernel file ./net/xfrm/xfrm_iptfs.c
• IPTFS_DEFAULT_DROP_TIME_USECS(9)
• IPTFS_DEFAULT_REORDER_WINDOW(9)
• IPTFS_DEFAULT_INIT_DELAY_USECS(9)
• IPTFS_DEFAULT_MAX_QUEUE_SIZE(9)
• struct xfrm_iptfs_config(9)
• struct xfrm_iptfs_data(9)
• iptfs_alloc_skb(9)
• iptfs_skb_head_to_frag(9)
• struct iptfs_skb_frag_walk(9)
• iptfs_skb_prepare_frag_walk(9)
• iptfs_skb_can_add_frags(9)
• iptfs_skb_add_frags(9)
• iptfs_pskb_add_frags(9)
• iptfs_pskb_extract_seq(9)
• iptfs_input_save_runt(9)
• __iptfs_iphlen(9)
• __iptfs_iplen(9)
• iptfs_complete_inner_skb(9)
• iptfs_reassem_abort(9)
• iptfs_reassem_done(9)
• iptfs_reassem_cont(9)
• iptfs_input_ordered(9)
• iptfs_drop_timer(9)
• iptfs_input(9)
• iptfs_enqueue(9)
• iptfs_copy_create_frag(9)
• iptfs_copy_create_frags(9)
• iptfs_first_skb(9)
• iptfs_encap_add_ipv6(9)
• iptfs_prepare_output(9)
• __iptfs_get_inner_mtu(9)
• iptfs_get_inner_mtu(9)
• iptfs_user_init(9)

like, go read them. iykyk. 📚✨

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and that's the page!! 🎀 this little beauty lives in ./net/xfrm and the version/date stamp on it is October 2026 — so yeah, she's from the future, we don't ask questions 💀✨ mwah, go forth and encapsulate responsibly 🦋💜