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Fix parsing of pcaps with nested VLAN tags (q-in-q) #564

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Fix parsing of pcaps with nested VLAN tags (q-in-q) #564

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d9987c7
Define VLAN 802.1ad Ethernet type
rsmck Aug 24, 2026
857e542
Correctly skip multiple VLAN tags (Q-in-Q)
rsmck Aug 24, 2026
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28 changes: 21 additions & 7 deletions src/capture.c
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Original file line number Diff line number Diff line change
Expand Up @@ -519,6 +519,23 @@ parse_packet(u_char *info, const struct pcap_pkthdr *header, const u_char *packe
capture_unlock();
}

// Skip any 802.1Q/802.1ad VLAN tags.
// VLAN tags can be stacked (q-in-q) and each one is 4 bytes ending
// with the type of whatever follows, so re-read that type from the
// tag we skipped.
capture_packet_skip_vlan(u_char *packet, uint32_t caplen, uint16_t *link_hl, uint16_t ethertype)
{
while (ethertype == ETHERTYPE_8021Q || ethertype == ETHERTYPE_8021AD) {
// Malformed packet, the tag doesn't fit...
if (*link_hl + 4 > caplen)
return;

*link_hl += 4;
memcpy(&ethertype, packet + *link_hl - 2, sizeof(ethertype));
ethertype = ntohs(ethertype);
}
}

packet_t *
capture_packet_reasm_ip(capture_info_t *capinfo, const struct pcap_pkthdr *header, u_char *packet, uint32_t *size, uint32_t *caplen)
{
Expand Down Expand Up @@ -561,20 +578,17 @@ capture_packet_reasm_ip(capture_info_t *capinfo, const struct pcap_pkthdr *heade
struct ip6_frag *ip6f;
#endif

// Skip VLAN header if present
// Skip VLAN header(s) if present.
// NB: they can be stacked (q-in-q), this is implemented in capture_packet_skip_vlan()
if (capinfo->link == DLT_EN10MB) {
struct ether_header *eth = (struct ether_header *) packet;
if (ntohs(eth->ether_type) == ETHERTYPE_8021Q) {
link_hl += 4;
}
capture_packet_skip_vlan(packet, *caplen, &link_hl, ntohs(eth->ether_type));
}

#ifdef SLL_HDR_LEN
if (capinfo->link == DLT_LINUX_SLL) {
struct sll_header *sll = (struct sll_header *) packet;
if (ntohs(sll->sll_protocol) == ETHERTYPE_8021Q) {
link_hl += 4;
}
capture_packet_skip_vlan(packet, *caplen, &link_hl, ntohs(sll->sll_protocol));
}
#endif

Expand Down
5 changes: 5 additions & 0 deletions src/capture.h
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Original file line number Diff line number Diff line change
Expand Up @@ -83,6 +83,11 @@
#define ETHERTYPE_8021Q 0x8100
#endif

//! Define VLAN 802.1ad (QinQ) Ethernet type
#ifndef ETHERTYPE_8021AD
#define ETHERTYPE_8021AD 0x88a8
#endif

//! NFLOG Support (for libpcap <1.6.0)
#define DLT_NFLOG 239
#define NFULA_PAYLOAD 9
Expand Down